Concept of Hybrid Drugs and Recent Advancements in Anticancer Hybrids

Abstract

Cancer is a complex disease, and its treatment is a big challenge, with variable efficacy of conventional anticancer drugs. A two-drug cocktail hybrid approach is a potential strategy in recent drug discovery that involves the combination of two drug pharmacophores into a single molecule. The hybrid molecule acts through distinct modes of action on several targets at a given time with more efficacy and less susceptibility to resistance. Thus, there is a huge scope for using hybrid compounds to tackle the present difficulties in cancer medicine. Recent work has applied this technique to uncover some interesting molecules with substantial anticancer properties. In this study, we report data on numerous promising hybrid anti-proliferative/anti-tumor agents developed over the previous 10 years (2011–2021). It includes quinazoline, indole, carbazole, pyrimidine, quinoline, quinone, imidazole, selenium, platinum, hydroxamic acid, ferrocene, curcumin, triazole, benzimidazole, isatin, pyrrolo benzodiazepine (PBD), chalcone, coumarin, nitrogen mustard, pyrazole, and pyridine-based anticancer hybrids produced via molecular hybridization techniques. Overall, this review offers a clear indication of the potential benefits of merging pharmacophoric subunits from multiple different known chemical prototypes to produce more potent and precise hybrid compounds. This provides valuable knowledge for researchers working on complex diseases such as cancer.

1. Introduction

Cancer is a complex group of multiple diseases characterized by inappropriately controlled cell proliferation and replication eventually resulting in disruption of normal physiology, metabolism, or structure. Benign tumors are self-limited and do not invade or metastasize, but in complex stages groups of cells display uncontrolled growth, invasion and metastasis [1]. In the metastasis stage, cancer cells migrate from one organ (the original tumor site) to another organ of the body through the circulatory and lymphatic systems [2]. Cancer occurs by a series of successive deleterious mutations that change cell functions. These mutations often cause aberrant proliferation [3].

Cancer is a major global health care problem that continues to remain a leading cause of morbidity and mortality, with >277 different cancer types. According to estimates from the World Health Organization (WHO) in 2019, in 112 of 183 countries, cancer is the first or second major cause of death before the age of 70 and ranks third or fourth in a further 23 countries. In 2020, for all cancers, 19,292,789 new cases were estimated globally, with a total of 9,958,133 cancer deaths [4]. According to global demographic trends, 420 million new cancer cases are expected annually by 2025 [5].

Over the past 20 years, cancer treatments have improved significantly, and more potent medications have better safety profiles and more precise molecular targeting. Drug resistance is a major challenge with cancer treatment. During clinical usage, almost all targeted anticancer medications encounter resistance. Numerous processes have been related to drug resistance, including genetic and/or epigenetic mutation, amplification, cancer stem cells (CSCs), efflux transporters, apoptotic dysregulation, and autophagy, among others [6,7,8,9].

Cancer chemotherapy with single-agent or single mono functional ‘targeted’ drugs has limited rates of success due to resistance and lack of selectivity. To tackle this limitation, combination therapy (multi-component drugs to treat cancer), was developed [10]. Three alternative combination strategies are used: (a) two or more medicines that operate on distinct sites simultaneously or concurrently; (b) multi-targeting or promiscuous drugs treatment; and (c) hybridization drugs [11].

Tumor heterogeneity, drug-drug interactions, unpredictable pharmacokinetic (PK) safety profiles, and poor patient compliance presents a problem for cancer treatment despite the use of drug-combination medicines. Hence, improving drug selectivity while eliminating drug resistance has become crucial for the successful treatment of cancer patients [10].

There are unique challenges to cancer care in the developing as well as developed countries. These include issues with healthcare financing, patient awareness, and treatment delivery [6,7].

To improve the efficiency of using a two-drug cocktail, one approach involves so-called hybrid drugs [12]. The hybridization of biologically active molecules is a new concept and a powerful tool in drug design and development, used to target a variety of diseases [13]. It is a strategy of rational design of such ligands or prototypes based on the recognition of pharmacophoric sub-units that maintain pre-selected characteristics of the original templates [14].

Hybrid drugs are also termed “single molecule multiple targets” or “multiple ligands”. Hybrid molecules imply that one molecule shows structural features of two “parent” molecules. Two parent biologically active molecules (pharmacophores) that independently act at two distinct pharmacological targets. Molecular hybrids designed in a manner to the maintain their activities [15] by merging or blending of two or more bioactive compounds or their pharmacophoric subunits into in a new molecular structure with a dual mode of action. The presence of two or more pharmacophores in a single unit leads to a pharmacological potency greater than the sum of each individual moiety’s potencies [16,17].

Additionally, it is important to note that hybrid drugs usually have high molecular mass and lipophilicity; they violate Lipinski’s and Veber’s rules [17]. However, hybrid anticancer drugs have remarkable advantages over conventional anticancer drugs because they are designed to act on a different bio target or interact with numerous targets simultaneously, reducing the likelihood of drug-drug interactions, with reduced side effects and reduced propensity to elicit resistance relative to the parent drugs. These novel hybrid molecules have improved affinity, enhanced efficacy and improved safety [14,18].

Nowadays, hybrid drugs have drawn interest in the purposeful and logical design of ligands functioning selectively on multiple targets, and this has been reflected by an increase in the number of relevant publications in the field. In this review, we have compiled recent findings from 2011 to 2021 on novel hybrid compounds for different drug classes that exhibit promising anticancer activities. This analysis highlights in vitro anticancer activity of synthesized anticancer hybrids on different cell lines.

2. The Concept of Hybrid Drugs in Anticancer Agent Development

Hybrid drugs are categorized based on the manner in which they are connected to each other. The concept of design and preparation of hybrid molecules is achieved using two strategies, which are described as follows:

1. Combining drug pharmacophoric moieties with: (a) two pharmacophoric groups directly linked; (b) two pharmacophoric groups linked by a spacer.

These approaches are achieved by: (i) the merging of two pharmacophoric groups from two different drugs acting through the same mechanism of action; (ii) the merging of pharmacophoric groups from two drugs acting through different mechanisms of action.

This is the integration of multiple pharmacophores in a single molecule. In this scenario, the starting point is to choose two established pharmacophores with high selectivity for their respective targets and a proper linker is selected to connect these two pharmacophores. This strategy is used to design new anticancer hybrids and is based on the ability of a combination of pharmacophoric moieties on a new molecular structure to retain their affinity and activity for the biological targets (Figure 1).

2. Combining two or more entire drugs: the second approach combines two or more entire medications that can be connected either directly or indirectly using a spacer or linker. The following categories apply:

(a) Directly linked hybrid drugs: each molecule is connected via a functional group. Notable examples are mostly enzymatically hydrolysable esters, and carbamateoramide.

(b) Merged or overlapped hybrid drugs: these types of hybrid agents are obtained by overlapping structural motifs or pharmacophores of two drugs. These hybrid agents differ significantly in their structures compared to the drugs from which they were designed. The hybrid agents may retain the functional properties of either or both of the overlapping drugs.

(c) Spacer linked hybrid drugs: the main purpose of using a linker or spacer is to provide a bridge to connect two drugs and modulate the release of individual drugs in vivo. These molecules can be classified as cleavable and non-cleavable.

(1) Non-cleavable hybrid drugs: non-cleavable linkers are connected with non-hydrolysable chemical bonds to create chemically as well as enzymatically stable linkers. This strategy is also based on the ability of the different molecules to retain their biological activity, specificity and respective affinity for their biological targets.

(2) Cleavable hybrid drugs: a cleavable linker is based on the release of two parental molecular structures under physiological or enzymatic conditions that prevail at the site of activity. The majority of cleavable conjugates have an ester linkage that plasma esterases can cleave to release two separate medicines with independent actions (Figure 1).

The purpose of cleavable hybrid drugs is to either improve poor pharmacokinetic properties and slowly deliver the two therapeutic entities in the body (e.g., ester, amide or carbamate), or to improve the selectivity and the antineoplastic activity of the drugs and release the two drugs directly in the targeted tissues (e.g., phosphorylated DES (Diethylstilbestrol) prodrugs for prostate cancer).

Hybrids drugs have been formed by connecting two drugs with the same mechanism of action or by connecting two drugs with different mechanisms of action. The aim of connecting two drugs is to target specific biological tissues [12,19,20,21,22].

3. Recent Advances in Anticancer Hybrids

3.1. Quinazoline Based Hybrids

Quinazoline is a heterocyclic compound and potent bioactive scaffold that has been associated with anticonvulsant, anticancer, analgesic, sedative, anti-hypertensive, anti-inflammatory, anti-histaminic, antimicrobial, anti-viral and anti-tubercular properties. Quinazoline containing compounds were investigated for their inhibition of kinases, which are major explored targets of cancer medicine development [23,24].

Cheng et al. (2015) synthesized and explored quinazoline based imidazole hybrids and evaluated their anticancer activity against Epidermal Growth Factor Receptor (EGFR) and HT-29 cells (in normoxic and hypoxic conditions). Most of the synthesized compounds displayed potent anticancer activity. Among them, compound 1(a) showed excellent activity with an IC₅₀ of 0.47 nM, 2.21 µM and 1.61 µM, respectively compared to the gefitinib control with an IC₅₀ of 0.45 nM, 3.63 and 5.21 µM, respectively. The structure of quinazoline based imidazole hybrids is given in Figure 2.

Table 1. In vitro cytotoxic activities of hybrid compounds 1(b–e).

Compound No.	R1	R2	R3	R4	n	EGFR (IC50 nM)	HT-29 (IC50 µM)
							Normoxia	Hypoxia
1b	Cl	F	NO2	H	5	0.32	12.89	9.81
1c	Br	H	NO2	H	2	0.66	4.48	4.01
1d	ethynyl	H	NO2	H	3	0.56	10.08	5.96
1e	ethynyl	H	NO2	H	5	0.50	2.93	3.46
Gefitinib						0.45	3.63	5.21

shows in vitro antiproliferative activity against the human cancer cell line HT-29 and EGFR inhibitory activity (nM) of compounds 1(b–e) [25].

Zhang Y et al. (2017) synthesized and evaluated anticancer activity of quinazolinebased deoxynojirimycin hybrids against Epidermal Growth Factor Receptor (EGFR) and α-glucosidase. Most of the synthesized compounds displayed potent anticancer activity. Among them, compound 2(a) showed excellent activity with an IC₅₀ 1.79 nM and 0.39 µM, respectively (the control gefitinib had an (−IC₅₀ of = 3.32 nM and >100 µM, respectively). The structure of a quinazoline-based deoxynojirimycin hybrid is given in Figure 3 and in vitro EGFR and α-glucosidase inhibitory activity of compounds 2(b–e) against human cancer cell lines is shown in

Table 2. In vitro cytotoxic activities of hybrid compounds 2(b–e).

Compound No.	R1	R2	R3	EGFR (IC50 nM)	α-Glucosidase (IC50 µM)
2b	3-Cl, 4-(3-fluorobenzyloxy)			4.53	0.14
2c	3-ethynyl			4.87	0.09
2d	3-ethynyl			ND *	6.25
2e	3-Cl, 4-F			10.71	4.34
Gefitinib				3.32	≥100

* Not determined.

[26].

Quinazoline based urea hybrids were synthesized by Zhang et al. (2016) and evaluated for their ability to inhibit EGFR and Vascular Endothelial Growth Factor Receptor-2 (VEGFR-2). Most of the synthesized compounds displayed potent anticancer activity. Among them, compound 3(a) showed excellent activity with an IC₅₀ of 1.0 nM and 79 nM, respectively, when the control vandetanib had an IC₅₀ of 11 nM and >15 nM, respectively. The structure of a quinazolinebased urea hybrid is given in Figure 4 and in vitro EGFR and VEGFR-2 inhibitory activity of compounds 3(b–e) against human cancer cell lines is shown in

Table 3. In vitro cytotoxic activities of hybrid compounds 3(b–e).

Compound No.	R1	R2	X	EGFR (IC50 nM)	VEGFR-2 (IC50 nM)
3b		m-Cl, p-F	Cl	14	14
3c		m-CH3, p-CH3	Cl	78	51
3d		o-CH3	Cl	15	178
3e		H	Cl	14	261
vandetanib				11	15

[27].

Yadav et al. (2016) synthesized substituted quinazoline based aryl hybrids and evaluated their anticancer activity. Most of the synthesized compounds showed excellent activity against different isoforms of PI3K, but compound 4(a) showed 3.7 times more potent activity against Phosphoinositide 3-Kinase(PI3K) α (IC₅₀ = 0.201 μM) than γ isoform (IC₅₀ = 0.75 μM), and was a selective inhibitor of MCF-7 cells (human breast adenocarcinoma) with GI50 7 μM. Compound 4(a) also did not show any cytotoxicity to normal human cells. It inhibited 37% and 62% triglyceride (TGI) at 25 mg/kg dose in Ehrlich solid tumor and Ehrlich ascites carcinoma tumor models, respectively, with a control of 5-flurouracil at 22 and 20 mg/kg dose with TGI inhibition 50 and 96%, respectively. The structure of quinazoline-based aryl hybrids is given in Figure 5 and the in vitro cytotoxicity of compounds 4(b–e) against human cancer cell lines is shown in

Table 4. In vitro cytotoxic activities of hybrid compounds 4 (b–e).

Compound No.	Ar.	MCF-7 (GI50 μM)	PI3K (IC50 μM)
			A	β	γ
4b		15	5.3	16	2.3
4c		12	22.2	1.9	22.2
4d		12	133	56	5.9
4e		32	14.2	0.5	14.2

[28].

Ding et al. (2018) synthesized and reported aminoquinazoline-sulphonamide based hybrids and evaluated their anticancer activity. Most of these compounds showed dual inhibitory activity against EGFR and PI3K α. Compound 5(a) showed excellent activity with an IC₅₀ of 2.4 and 317 nM, compared to controls gefitinib (IC₅₀ 2.4 nM) and dactolisib (IC₅₀ 16.4 nM). Compound 5(a) exhibited potent activity against a panel of cell lines including adenocarcinomic human alveolar basal epithelial cells (A549 IC₅₀ = 8.23 µM), epithelial cells (BT549, IC₅₀ = 1.02 µM), human colon cancer cells (HCT-116, IC₅₀ = 5.60 µM), breast cancer cells (MCF-7, IC₅₀ = 5.59 µM), human hepatic adenocarcinoma cells (SK-HEP-1, IC₅₀ = 6.10 µM), and gastric carcinoma cells (SNU638, IC₅₀ = 4.10 µM). For comparison, the controls gefitinib and dactolisib had IC₅₀ doses of 8.27, 6.56, 5.98, 26.7, 10.1, and 7.56 µM or 0.62, 0.74, 0.84, 1.33, 1.82,and 1.24 µM in the same cell lines), respectively. The structure of aminoquinazoline-sulphonamide based hybrids is given in Figure 6 and in vitro cytotoxicity (IC₅₀, μM) of compounds 5(b–e) against human cancer cell lines is shown in

Table 5. In vitro cytotoxic activities of hybrid compounds 5(b–e).

Compounds No.	R1	R2	X	A549 (µM)	BT549 (µM)	HCT-116 (µM)	MCF-7 (µM)	SK-HEP-1 (µM)	SNU638 (µM)
5b	3-COOCH		CH	1.10	1.08	0.40	10.1	2.40	1.12
5c	3-COOCH3-4-Cl		CH	2.13	2.36	3.13	1.43	7.06	2.20
5d	4-OCH3		CH	3.58	1.88	3.49	1.79	1.61	3.97
5e	2,4-diF		CH	4.71	2.48	4.01	1.61	2.49	2.05
Gefitinib				8.27	6.56	5.98	26.7	10.1	7.56
Dactolisib				0.62	0.74	0.84	1.33	1.82	1.24

[29].

Continuing the work of Ding et al. (2018), Fan et al. synthesized amino quinazoline based hybrids and evaluated their anticancer activity. Most of these compounds showed inhibitory activity against PI3K. Compound 6(a) showed excellent activity towards PI3Kα with an IC₅₀ 13.6 nM in comparison to other isoforms of PI3K including PI3Kβ, PI3Kγ and PI3Kδ, which had IC₅₀ of396.2, 117.5, and 101.8 nM. Compound 6(a) exibited potent activity against a panel of cell lines including HCT-116, SK-HEP1, MDA-MB-231 (epithelial, human breast cancer cells), SNU638, A549, and MCF-7 with IC₅₀ values 0.16, 0.28, 0.28, 0.48, 1.32, and 3.24 µM. Control BEZ235 had IC₅₀ values of 0.84, 1.82, 0.18, 1.24, 0.62, and 1.33 µM. The structure of quinazoline-amino sulphonamide based hybrid is given in Figure 7 and in vitro cytotoxicity (IC₅₀, μM) of compounds 6(b–e) against human cancer cell lines is shown in

Table 6. In vitro cytotoxic activities of hybrid compounds 6(b–e).

Compound No.	R1	R2	HCT-116 (µM)	SK-HEP1 (µM)	MDA-MB-231 (µM)	SNU638 (µM)	A549 (µM)	MCF-7 (µM)
6b			1.44	4.72	0.71	0.62	0.94	1.02
6c			0.49	0.86	0.88	1.26	3.52	4.73
6d			0.59	0.44	0.42	0.61	1.56	10.8
6e			1.74	1.14	2.58	0.98	3.14	4.59
BEZ235			0.84	1.82	0.18	1.24	0.62	1.33

[30].

Frohlich et al. (2017) synthesized hybrids of quinazoline with artemisinin and evaluated their in vitro anticancer activity on CCRF-CEM (lymphoblastoid cell) and CEM/ADR5000 (leukemia cells). Most of the synthesized compounds showed potent anticancer activity; out of them, compound 7 (Figure 8) showed excellent activity with an EC₅₀ of 2.8 and 0.6 µM. Control doxorubicin had an EC₅₀ of 0.009 and 23.27 µM, respectively [31].

Yang et al. (2018) synthesized quinazoline based hybrids and evaluated them for their Bromodomain-containing protein 4 (BRD4) inhibitory activity. SAR studies revealed that the phynylmorpholine along with the pyrazole skeleton showed potent activity against BRD4. Compound 8(a) showed potent activity against human AML cells (MV4-11) with an IC₅₀ value of 1.10 µM (K_d 66 nM) when control BET760 had an IC₅₀ 0.80 µM (K_d 37 nM). The structure of quinazoline-phenyl morpholine based hybrids is given in Figure 9 and in vitro cytotoxicity of compounds 8(b–e) against human cancer cell lines is shown in

Table 7. In vitro cytotoxic activities of hybrid compounds 8(b–e).

Compound No.	R1	BRD4 Kd (nM)	MV4-11, IC50 (µM)
8b		480	4.88
8c		250	2.54
8d		60	5.07
8e		28	1.83
BET760	-	37	0.80

[32].

Quinazoline-Based Hybrids That Are FDA Approved or under Clinical Trial

The hybrids of quinazoline have been evaluated in various clinical trials in recent years, with several of them showing promising results. In addition, the Food and Drug Administration (FDA) approved certain quinazoline-based enzyme inhibitors for the management of various malignancies. Dacomitinib, erlotinib, gefitinib, afatinib, and lapatinib are the quinazoline based hybrid drugs approved by the FDA for management of different types of cancers (Figure 10). Additionally, in

Table 8. Quinazoline-based compounds approved/under clinical trial with their current status.

Company Name	Compound Name	Drug Target	Type of Cancer	Status	References
AstraZeneca	Vandetanib	Kinase inhibitor	Medullary thyroid cancer	Approved	[33]
Boehringer Ingelheim	Afatinib	Tyrosine kinase	Non-small cell lung Carcinoma	Approved	[34]
Pfizer	Dacomitinib	EGFR inhibitor	Non-small cell lung carcinoma	Approved	[10]
AstraZeneca and Teva	Gefitinib	EGFR inhibitor	Breast and Lung cancer	Approved	[35]
Roche Pharmaceuticals	Erlotinib	EGFR inhibitor	pancreatic cancer and non-small cell lung cancer	Approved	[36]
GlaxoSmith Kline (GSK)	Lapatinib	Dual tyrosine kinase inhibitor	solid tumors and Breast cancer	Approved	[37]
AstraZeneca	Sapitinib (AZD 8931)	Erb8 receptor tyrosine kinase	Breast cancer and metastatic cancer	Clinical trials	[10]
Array Biopharma	Tucatinib (ARRY 380)	Kinase inhibitor	Breast cancer	Approved	[10]
Selleck chemicals	Barasertib (AZD 1152)	Aurora Kinase	Tumor lymphoma, solid tumors and myeloid leukemia	Clinical trials	[38]
Spectrum Pharmaceuticals	Poziotinib	Tyrosine kinase	Breast cancer	Clinical trials	[10]
AstraZeneca	AZD 3759	EGFR antagonist	Non-small cell lung Cancer	Clinical trials	[10]
Curis Inc.	CUDC-101	By inhibiting Histone deacetylase, EGFR and HER2	Advanced /Liver/Neck/Gastric/Head/non-small cell lung cancer and Breast	Clinical trials	[39]
Beta-Phama	Icotinib	EGFR-TK1 inhibitor	Non-small cell lung cancer	Approved	[40]

, we have summarized quinazoline-based hybrid molecules under clinical trials for the treatment of different types of cancer.

3.2. Indole-Based Hybrids

Indole is a remarkable and adaptable heterocycle that has been used to create important biological scaffolds in pharmaceutical research. Indole/indole derivatives have many reported therapeutic properties, such as anti-virals, anti-convulsants, antibacterials, anti-microbials, anticancer agents, anti-malarials, anti-inflammatories, anti-oxidants, and anti-diabetics. Different natural and synthesized indole motifs have demonstrated considerable anticancer potential. Such substances have been found to act on a variety of protein targets, including sirtuins, PIM (proviral integration site for Moloney murine leukemia virus) kinases, HDACs (histone deacetylases) and DNA topoisomerase [41,42].

Zhang et al. (2013) synthesized hybrids of indole with hydroxycinnamamide, and evaluated their anticancer activity against different cell lines. Most of the synthesized compounds showed potent anticancer activity. Among them, compound 9(a) exhibited excellent activity against various cell lines including human myeloid leukemia (U937), prostate adenocarcinoma (PC-3), A549, ovarian carcinoma (ES-2), MDA-MB-231, and HCT116 at IC₅₀ 1.8, 3.7, 4.4, 5.4, 3.1, and 5.5 µM, respectively with control suberoylanilide hydroxamic acid (SAHA) had IC₅₀ values of 2.3, 9.9, 3.8, 12.7, 5.6, and 6.0 µM, respectively. Furthermore, compound 9(a) showed excellent HDAC (histone deacetylase) inhibitory activity against different isoforms of HDAC including HDAC1, HDAC2, HDAC3, and HDAC6 with IC₅₀ values of 0.39, 1.42, 0.28, and 0.94 µM compared to control SAHA at IC₅₀ values of 0.076, 0.256, 0.028, and 0.118 µM, respectively. The structure of indole with hydroxycinnamamide hybrids is shown in Figure 11 and in vitro cytotoxicity of compounds 9(b–e) against human cancer cell lines is shown in

Table 9. In vitro cytotoxicity (IC₅₀) of hybrid compounds 9(b–e).

Compound No.	R	U937 (µM)	PC-3 (µM)	A549 (µM)	ES-2 (µM)	MDA-MB-231 (µM)	HCT116 (µM)
9b		3.1	10.5	11.8	29.2	7.2	6.0
9c		2.2	10.4	4.2	25.1	4.5	3.8
9d		2.2	5.8	1.6	4.4	6.8	5.9
9e		2.7	5.4	7.0	8.9	7.2	2.4
SAHA	-	2.3	9.9	3.8	12.7	5.6	6.0

[43].

Zhang et al. (2013) synthesized and explored hybrids of indole with hydroxycinnamamide, evaluating their anticancer activity against different cell lines. Most of the synthesized compounds showed potent anticancer activity. Among them, compound 10(a) exhibited excellent activity against various cell lines including U937, K562 (myelogenous leukemia cells), HEL (human erythroleukemia cells), KG1(myeloid leukemia cells), HL60 (promyelocytic leukemia cells), MDA-MB-231, PC-3, MCF-7, HCT116, and A549 with IC₅₀ values 0.16, 0.51, 0.19, 0.22, 1.69, 0.22, 0.46, 2.68, 0.52, and 2.74 µM, respectively. Furthermore, compound 10a showed excellent HDAC selectivity against different isoforms of HDAC including HDAC1, HDAC2, HDAC3, and HDAC6 with IC₅₀ values 11.8, 498.1, 3.9, and 308.2 nM with control SAHA had IC₅₀ values of 34.6, 184.7, 90.1, and 63.0 nM, respectively. The T structure of indole and hydroxycinnamamide hybrids is shown in Figure 12 and the in vitro cytotoxicity of compounds 10(b–e) against human cancer cell lines is shown in

Table 10. In vitro cytotoxic activities of hybrid compounds 10(b–e).

Compound No.	R	U937 (µM)	K562 (µM)	HEL (µM)	KG1 (µM)	HL60 (µM)	MDA-MB-231 (µM)	PC-3 (µM)	MCF-7 (µM)	HCT116 (µM)	A549 (µM)
10b		0.33	0.79	0.20	0.39	2.11	0.24	0.33	3.47	0.37	3.39
10c		0.32	0.68	0.27	0.72	1.59	0.41	0.53	2.95	0.57	3.91
10d		0.18	1.01	0.19	0.24	1.04	0.27	0.51	2.7	0.37	2.96
10e		0.34	0.89	0.16	0.47	1.68	0.15	0.29	2.32	0.22	3.27
SAHA		1.45	3.24	0.49	1.59	4.26	1.72	3.57	3.78	2.81	3.9

[44].

Mehndiratta et al. (2014) synthesized hybrids of indole with sulphonamides, and evaluated their anticancer and anti-inflammatory activity. Most of the synthesized compounds showed potent anticancer activity. Among them, compound 11(a) showed excellent activity against HeLa nuclear HDAC enzyme with an IC₅₀ of 7.9 nM. The structure of indole and sulphonamide hybrids is shown in Figure 13 and the in vitro cytotoxicity of compounds 11(b–e) against human cancer cell lines is shown in

Table 11. In vitro cytotoxic activities of hybrid compounds 11(b–e).

Compound No.	R1	R2	R3	HeLa Nuclear HDAC (nM)
11b	H	H	4′-(N-3-hydroxyacrylamide)	2.8
11c	CH3	H	4′-(N-3-hydroxyacrylamide)	3.3
11d	CH2CH3	H	4′-(N-3-hydroxyacrylamide)	3.4
11e	H	CH3	4′-(N-3-hydroxyacrylamide)	47.4
LBH589.HCl				7.5

[45].

Panathur et al. (2013) synthesized indole based hybrids, and evaluated them for anticancer activity against three cancer cell lines including K562, MDA-MB 231 and LNCaP (androgen-sensitive human prostate adenocarcinoma cells). Most of the synthesized compounds showed potent anticancer activity; among them, 9 compounds showed excellent activity against MDA-MB 231, with three compounds inhibiting growth of LNCaP cell up to 50% at 10 µM. Furthermore, the lead molecule 12(a) showed SIRT1 (Sirtuin 1) inhibitory activity up to 70% with control finasteride (growth inhibition 56%) at 40 µM. The structure of indole-triazole based hybrids is given in Figure 14 and the in vitro cytotoxicity of compounds 12(b–e) against human cancer cell lines is shown in

Table 12. In vitro cytotoxic activities of indole-triazole based hybrid compounds 12(b–e).

Compound No.	R1	R2	K562 (Upto %)	MDA-MB 231 (µM)	LNCaP (µM)
12b	H		88	2	32
12c	H		87	28	38
12d	F		64	58	24
12e	F		67	68	35

[46].

Lee et al. (2014) synthesized indole based hybrids and evaluated their proto-oncogene serine/threonine-protein kinase (PIM) kinase selectivity against different forms of PIM kinase including PIM1, PIM2, and PIM3. Compound 13(a) showed excellent selectivity towards PIM1 with IC₅₀ values of 0.058, 0.52, and 0.16 µM. Furthermore, the lead molecule 13(a) was tested against different cell lines, including MV-4-11 (acute myeloid leukemia cell), Jurkat (T lymphocyte cells), and K562, and was found to be very selective towards MV-4-11 in a cell viability assay. The lead molecule also showed binding interaction with Lys67 and Glu89 in the active site of PIM1 (ATP-binding). The structure of an indole-pyrimidine based hybrid is shown in Figure 15, and the in vitro cytotoxicity of compounds 13(b–e) against human cancer cell lines is shown in

Table 13. In vitro cytotoxic activities of indole-pyrimidine based hybrid compounds 13(b–e).

Compound No.	R	PIM (µM)
		PIM1	PIM2	PIM3
13b	Me2N(CH2)2O	0.30	1.40	0.50
13c	Et2N(CH2)2O	0.14	0.84	0.27
13d	Et2N(CH2)3O	0.11	0.38	0.081
13e	Et2N(CH2)3NH	0.067	3.16	0.61

[47].

Mirzaei et al. (2017) synthesized and explored indole-chalcone based hybrids and evaluated their anticancer activity against various cell lines including A549, MCF7, SKOV3 (human ovarian cancer cell), and NIH3T3 (embryonic fibroblast cells), finding IC₅₀ values of 4.3, 100, 20.2, and 154.6 µg/mL with control etoposide IC₅₀ of 7.8, 9.9, 8.5, and 118.0 µg/mL, respectively. Furthermore, the lead molecule 14(a) was subjected to a tubulin polymerization inhibitory assay, with results revealing that the lead molecule showed excellent inhibitory activity, having an IC₅₀ of 17.8 µM with control colchicine, having an IC₅₀ of 2.3 µM. The structure of indole-chalcone based hybrids is shown in Figure 16 and the in vitro cytotoxicity of compounds 14(b–e) against human cancer cell lines is shown in

Table 14. In vitro cytotoxic activities of indole-chalcone based hybrid compounds 14(b–e).

Compound No.	R1	R2	R3	A549 (µg/mL)	MCF7 (µg/mL)	SKOV3 (µg/mL)	NIH3T3 (µg/mL)
14b	C2H5	Br	Me	4.9	50.1	68.8	52.0
14c	H	Br	n-Bu	8.0	54.0	74.1	141.3
14d	Me	OMe	Me	5.1	36.2	53.0	_
14e	H	Br	Me	27.9	28.0	52.0	_
Etoposide				7.8	9.9	8.5	118.0

[48].

Zhou et al. (2016) synthesized and explored hybrids of indole with pyrrole and evaluated their anticancer activity against various cell lines including HL-60, SMMC-7721 (hepatocarcinoma cells), A-549, MCF-7, and SW480 (colon carcinoma cell). Most of the synthesized compounds showed potent anticancer activity. Among them, compound 15(a) showed excellent activity having IC₅₀ values of 1.27, 1.72, 2.68, 1.78, and 1.44 µM with control cisplatin (DDP) having IC₅₀ values of 1.16, 8.08, 7.10, 10.45, and 8.88 µM, respectively. The structure of indole-pyrole based hybrids is shown in Figure 17 and the in vitro cytotoxicity of compounds 15(b–e) against human cancer cell lines is shown in

Table 15. In vitro cytotoxic activities of indole-pyrole based hybrid compounds 15(b–e).

Compound No.	R1	R2	R3	HL-60 (µM)	SMMC-7721 (µM)	A-549 (µM)	MCF-7 (µM)	SW480 (µM)
15b	Bn	Benzimidazole	2-Bromobenzyl	1.21	4.69	6.76	2.23	6.35
15c	Bn	Benzimidazole	4-Methylbenzyl	1.20	4.98	6.23	2.72	6.57
15d	Bn	5,6-Dimethyl-benzimidazole	2-Naphthylmethyl	1.21	2.27	4.80	1.68	1.76
15e	Me	5,6-Dimethyl-benzimidazole	2-Naphthylmethyl	1.35	4.03	6.18	1.84	4.5
DPP	-	-	-	1.16	8.08	7.10	10.45	8.88

[49].

Kumar et al. (2014) synthesized indole based chalcone hybrids and evaluated their antiproliferative activity against A549, PC3 and PaCa2 (pancreatic cancer) cell lines. Among the synthesized derivatives, compound 16(a) showed potent activity having IC₅₀ values of 2.4 and 0.8, 36.0 and 22.5, and >50 µM, respectively (with control mitomycin C having an (IC₅₀ = of 0.45 µM against A549 at 24h). The structure of indole-chalcone based hybrids is shown in Figure 18 and the in vitro cytotoxicity of compounds 16(b–e) against human cancer cell lines is shown in

Table 16. In vitro cytotoxic activities of indole-chalcone based hybrid compounds 16(b–e).

Compound No.	R1	R2	R3	R4	R5	R6	A549 (µM)		PC3 (µM)		PaCa2 (µM)
							24 h	48 h	24 h	48 h	24 h	48 h
16b	H	H	H	OCH3	H	H	9.6	5.8	33.3	12.5	27.5	>50
16c	H	OCH3	H	H	H	H	6.4	7.5	>50	12.0	13.5	>50
16d	H	OCH3	OCH3	H	H	H	3.7	5.5	31.1	37.1	>50	>50
16e							4.9	3.0	17.2	8.1	24.0	>50
Mitomycin C							0.45

[50].

Kumar et al. (2018) synthesized and explored indole-ospemifene-triazole based hybrids and evaluated their anticancer activity against MCF-7 and MDA-MB-231. Most of the synthesized compounds showed good anticancer activity, compound 17(a) exhibited excellent activity at IC₅₀ 1.56 and 48.46 µM controls included ospemifene (IC₅₀ 55 and 50 µM), tamoxifen (IC₅₀ 3.5 and >100 µM), and plumbagin (IC₅₀ 75 and 4.4 µM). The structure of the indole-ospemifene-triazole based hybrid is shown in Figure 19 and the in vitro cytotoxicity of compounds 17(b–e) against human cancer cell lines is shown in

Table 17. In vitro cytotoxic activities of indole-ospemifene-triazole based hybrid compounds 17(b–e).

Compound No.	R	MCF-7 (µM)	MDA-MB-231 (µM)
17b		≥100	71.40
17c		16.50	≥100
17d		10.99	71.40
17e		≥100	≥100
Ospemifene		55	50
Tamoxifen		3.5	≥100
Plumbagin		75	4.4

[51].

Kumar et al. (2018), and Sharma et al. (2019) synthesized indole-isatin based triazole hybrids and evaluated their anticancer activity. Among the synthesized compounds, compound 18(a) showed excellent activity against MCF-7 and MDA-MB-231 cell lines withIC₅₀ values of 37.42 and >100 µM Control included plumbagin (IC₅₀ 3.5 4.4 µM), peganumine A (IC₅₀ 38.5 µM and not observed) and tamoxifen (IC₅₀ 50 and 75 µM), respectively. Furthermore, the biological activity was validated by docking studies. The structure of the indole-isatin-triazole based hybrid is shown in Figure 20, and the in vitro cytotoxicity (µM) of compounds 18(b–c) against human cancer cell lines is shown in

Table 18. In vitro cytotoxic activities of indole-isatin-triazole based hybrid compounds 18(b–c).

Compound No.	R	MCF-7 (µM)	MDA-MB-231 (µM)
18b		50	>100
18c		>100	>100
Plumbagin		3.5	4.4
Peganumine A		38.5	Not observed
Tamoxifen		50	75

[52].

3.3. Indole-Based Hybrids That Are FDA Approved or under Clinical Trial

Dacinostat (LAQ824), an indole-based hybrid molecule, is approved for the management of breast and prostate cancer, whereas panobinostat (LBH-589) is a marketed medicine for numerous malignancies. Quisinostat (JNJ-26481585), a synthetic indole-hydroxamic acid molecule (hybrid) with putative antitumor action is an orally available 2nd generation molecule to inhibit HDAC. Cediranib is a tyrosine kinase inhibitor which affects the function and development of endothelial cells in human kidney tumors. Anlotinib (AL3818) is a potent kinase inhibitor with potential antitumor as well as antiproliferative efficacy currently in clinical trials. Figure 21, shows chemical structures of indole-based FDA approved/clinical trial drugs and

Table 19. Current status of indole-based hybrids that are approved or/-under clinical trials.

Company Name	Compound Name	Drug Target	Type of Cancer	Status	Reference
AstraZeneca	Cediranib	VEGFR tyrosine kinases	Glioblastoma	Approved	[53]
Chia-tai Tianqing Pharmaceutical Co.	AL 3818 (Anlotinib)	Tyrosine kinase	Synovial sarcoma, Advanced alveolar soft part sarcoma	Clinical trials	[54]
Janssen pharmaceuticals	Quisinostat	HDAC inhibitor	Multiple myeloma	Approved	[55]
Novartis	Panobinostat (LBH-589)	Non-selective HDAC inhibitor	Multiple myeloma	Approved	[56]
Selleck	Dacinostat (LAQ824)	Histone deacetylase inhibitor	Breast and Prostate cancer	Approved	[57]

gives descriptions of them.

3.4. Carbazole Based Hybrids

Carbazole is a key scaffold found in a wide range of physiologically potent chemicals, including natural and synthetic analogues. Anticancer, antifungal, antibacterial, anti-HIV, anti-inflammatory, anti-protozoan, anti-psychotic, antidiabetic and anticonvulsant properties are found in molecules with r chemically modified carbazole moiety. The first carbazole compounds, celiptium and ellipticine, targeted topoisomerase II and cytochrome P450, respectively, and are used to combat cancer (metastatic breast cancer). An investigation of the carbazole nucleus for the production of novel structural scaffolds has led to the development of a number of available anti-tumor hybrids [58,59].

Liu et al. (2015) synthesized and evaluated the anticancer activity of carbazole derivatives against various cell lines including HL-60, SMMC-7721, A549, MCF-7 and SW480. Among the synthesized carbazole derivatives, compound 19(a) showed potent activity with IC₅₀ values of 0.51, 2.38, 3.12, 1.40, and 2.48 µM, where control cisplatin (DDP) hadIC₅₀ values of 1.32, 6.24, 11.83, 15.17, and 12.95 µM, respectively. Compound 19(a) showed cell cycle arrest in SMMC-7721 cells. The structure of carbazole-imidazole based hybrids is shown in Figure 22 and the in vitro cytotoxicity of compounds 20(b–e) against human cancer cell lines is shown in

Table 20. In vitro cytotoxic activities of carbazole-imidazole based hybrid compounds 19(b–e).

Compound No.	n	R	R’	HL-60 (µM)	SMMC-7721 (µM)	A549 (µM)	MCF-7 (µM)	SW480 (µM)
19b	2	Benzimidazole	-	3.11	3.21	12.36	5.06	18.25
19c	2	imidazole	4-methylbenzyl	0.84	5.74	3.92	2.24	9.56
19d	2	benzimidazole	2-bromobenzyl	0.71	3.66	3.58	2.14	3.08
19e	3	benzimidazole	4-methylbenzyl	0.57	2.55	2.65	2.82	3.19
DDP				1.32	6.24	11.83	15.17	12.95

[60].

Mongre et al. (2019) synthesized a potent novel hybrid (20) of carbazole and piperazine and evaluated its anticancer activity against various cell lines including A549, NCI-H1299 (non-small cell lung carcinoma cells), HT-29, MCF-7, Hela (cervical carcinoma), and U2OS (osteosarcoma cells). The IC₅₀ values in these cell lined were 1.779, 2.270, 2.20, 2.637, 3.072 and 2.739 µM, respectively. The molecule also showed potent cell cycle arresting activity at concentrations of 0.5, 1.0 and 2.0 µM by affecting G2/M cell cycle transition. Hybrid (20) also inhibited tumor progression in a xenograft model (BALB/c-nu nude mouse) at a dose of 3 mg/kg body weight without any toxicity. A carbzole-piperazine hybrid is depicted in Figure 23 [61].

Carbazole-Based Hybrids That Are FDA Approved or under Clinical Trial

Some well-known carbazole-based hybrids with potential anticancer effects have been published [59,62,63,64]. Some carbazole-based hybrids are FDA approved medications or in clinical studies for cancer treatment. Alectinib was licensed by the US Food and Drug Administration (FDA) and the European Medicines Agency (EMA) in 2015 for the management of anaplastic lymphoma kinase-positive progressive non-small cell lung cancer (NSCLC). Midostaurin is a carbazole hybrid that was approved by the FDA and the European Medicines Agency (EMA) in 2017. It is used to treat recently diagnosed advanced systemic mastocytosis and acute myeloid leukemia [65,66].

Currently, four hybrids of carbzole are being tested in clinical studies including becatecarin and edotecarin, which have been proven to intercalate DNA and maintain the DNA-topo I complex, and are currently in Phase II and Phase III clinical studies, respectively. CEP-2563 is a strong inhibitor of platelet derived growth factor (PDGF) and tyrosine kinase that is now in a Phase I clinical development for medullary thyroid cancer. UCN-01 is now being tested in a Phase II clinical study for breast cancer, lymphoma, and pancreatic by acting upon protein kinases. The structure of FDA approved/clinical trials drugs with carbazole hybrids is given in Figure 24 and their current status is shown in

Table 21. Current status of carbazole-based hybrid drugs that are approved/or under clinical trials.

Company Name	Compound Name	Drug Target	Type of Cancer	Status	References
Novartis Pharmaceutical Corporation	Midostaurin	Kinase inhibitor	Advanced systemic mastocytosis, myelodysplastic syndrome	Approved	[67]
Chugai Pharmaceuticals Co.	Alectinib	Tyrosine kinase	Non-small cell lung cancer	Approved	[68]
Schwarz Pharma	CEP-2563	Tyrosine kinase	Solid tumors	Clinical trials	[10]
Cayman Chemicals	UCN-01	Tyrosine kinase	Pancreatic, malignant melanoma, ovarian Cancer and small cell lung	Clinical trials	[69]
Helsinn Healthcare	Becatecarin	Topoisomerase-I	Leukemia and gastric cancer	Clinical Trials	[10]
Pfizer Pharmaceutical Co.	Edotecarin	Topoisomerase-I	Oesophageal cancer and solid tumors	Clinical trials	[10]

.

3.5. Pyrimidine-Based Hybrids

The basic structure of RNA, DNA, and nucleic acids is the heterocyclic pyrimidine ring. Pyrimidine and its conjugated counterparts possess antiviral, antibacterial, anticancer, analgesic, anti-inflammatory, antimalarial and antioxidant activities. Anticancer action is the most often described therapeutic property of pyrimidine because it interacts with a variety of targets, as well as receptors to cause cell death. Multiple studies on the anticancer efficacy of pyrimidine compounds, as well as their therapeutic use, have supported their position as a prospective drug development nucleus [70,71].

Combs et al. (2015) synthesized and patented aminopyrimidine hybrids and evaluated their P13K enzyme as well as cell proliferation inhibitory potential in 96-well plates using scintillation counting. Compound 21 (Figure 25) showed potent activity with IC₅₀ < 20 nM [72].

Boloor et al. (2012) synthesized and patented pyrimidine-indazole based hybrids as VEGFR2 inhibitors. Compound 22 showed potent activity with an IC₅₀ < 50 μM. Furthermore, the lead compound 22 (Figure 26) was evaluated in an endothilian cell proliferation assay where it displayed activity withIC_50s of 1-200 nM [73].

Hogberg et al. (2012) developed and patented indole based pyrimidine hybrids and evaluated their anticancer activity as tubulin inhibitors. Among the synthesized compounds, compound 23 (Figure 27) showed potent activity on CCRFCEM (T lymphoblastoid cells) with an EC₅₀ value of 0.015 µM [74].

Mao et al. (2012) synthesized and patented pyrimidine-pyrazole based hybrid molecules and evaluated their anticancer activity against H1993 cells. The synthesized compounds did not show desirable activity on representative cell line. Furthermore, potent compound 24 (Figure 28) was subjected to enzymatic assay on cMet protein and displayed excellent activity, having an IC₅₀ in the nM range [75].

Tanaka et al. (2012) synthesized and patented pyrimidine-pyrazole based hybrid molecules and evaluated their anticancer activity as Fyn inhibitors. Among the synthesized compounds, compound 25 (Figure 29) showed excellent activity with an IC₅₀ of 3 nM. Furthermore, 26 (Figure 29) showed excellent activity on carbonyl reductase 1 with an IC₅₀ of 28 nM [76].

Liang et al. (2013) synthesized and patented pyrimidine-pyrazole based hybrid molecules, and evaluated their anticancer activity as mammalian target of rapamycin mTOR inhibitors. Compound 27 (Figure 30) showed potent activity on A549 and U-87MG (glioma cells) with an IC₅₀ < 1 µM [77].

Dorsch et al. (2013) synthesized and patented pyrimidine-triazole based hybrids and evaluated their anticancer activity as general control non-derepressible 2 (GCN2) inhibitors of U20S human cells. Among the synthesized hybrids, the compound 28 (Figure 31) showed excellent activity with IC₅₀ ≤ 0.3µM [78].

El-Sayed et al. (2011) synthesized sulfonamide-thiazole fused pyrimidine derivatives, and evaluated their anticancer activity in amethyl green/DNA displacement assay. Among the synthesized compounds, compound 29a showed potent activity with an IC₅₀ of 40 µg/mL and high DNA binding affinity. Furthermore, in vivo studies showed that the compound 29a increased the % lifespan of mice by 42.86% over standard fluorouracil. The structure of sulfonamide-thiazole fused pyrimidine derivatives is shown in Figure 32 and the in vitro cytotoxicity of compounds 29(b–e) against human cancer cell lines is shown in

Table 22. In vitro cytotoxic activities of sulfonamide-thiazole fused pyrimidine hybrid compounds 29(b–e).

Compound No.	R	R1	DNA Displacement Assay (µg/mL)	DNA Binding Affinity
29b	4-H	4-Br	74	High
29c	4-H	4-NO2	81	weak
29d	4-H	4-OCH3	81	Moderate
29e	4-H	3,4-diOMe	62	High
Ethidium bromide	-	-	1.4	-

[79].

Shao et al. (2013) synthesized tri-substituted pyrimidine based hybrid derivatives, and evaluated their anticancer activity as cyclin-dependent kinase (CDK) inhibitors. Most of the synthesized compounds showed potent anticancer activity. Among them, compound 30a showed potent kinase inhibition against CDK9T1, CDK1B, CDK2A, CDK7H withK_i of 14, 262, 316, and 163 nM, and cell toxicity activity against HCT-116, and MCF-7 cell lines with IC₅₀ value of 0.79, and 0.64 µM, respectively. Furthermore, compound 30a showed potent activity of Mal-1 and caspase-3 inhibition by down regulation of apoptotic protein. The structure of substituted pyrimidine derivatives is shown in Figure 33 and the in vitro cytotoxicity of compounds 30(b–e) against human cancer cell lines is shown in

Table 23. In vitro cytotoxic activities of tri-substituted pyrimidine hybrid compounds 30(b–e).

Compound No.	R1	R2	R3	CDK9T1 (µM)	CDK1B (µM)	CDK2A (µM)	CDK7H (µM)	HCT-116 (µM)	MCF-7 (µM)
30b	NH2	F	m-SO2NH2	3	7	3	252	0.05	0.41
30c	NHMe	CN	m-SO2Me	5	19	43	110	0.20	0.43
30d	NHMe	F	m-SO2NH(CH2)2OCH3	3	10	6	30	0.30	0.72
30e	NHMe	CN	p-CO-N-(1-methylpiperidin-4-yl)	8	43	32	304	0.18	0.5

[80].

Fares et al. (2014) synthesized pyrimidine-based triazole hybrid derivatives and evaluated their anticancer activity. Most of the synthesized compounds showed potent anticancer activity. Among them, compound 31(a) showed excellent activity against various cell lines including MCF-7, human hepatoma carcinoma cells (HEPG2), A-549, and (PC-3) with IC₅₀ of 37.96, 56.65, 0.41, and 0.36 µM. Furthermore, compound 31a was screened in a CDK4 and CDK6 inhibition assay and compound showed 0 and 2% inhibition at 1 µM, 5 and 1% inhibition at 10 µM and 21 and 17% inhibition at 100 µM with control staurosporine having 93 and 90% inhibition at 1 µM. The structure of pyrimidine-triazole based hybrid derivatives is shown in Figure 34 and the in vitro cytotoxicity (µM) of compounds 31(b–e) against human cancer cell lines is shown in

Table 24. In vitro cytotoxic activities of pyrimidine-triazole hybrid compounds 31(b–e).

Compound No.	Ar	R	A-549 (IC50 (µM)	PC-3
31b	4-MeC6H4	COCH3	19.33	16.92
31c	C6H5	COOC2H5	26.64	33.56
31d	4-SO2NH2C6H4	COOC2H5	16.42	7.15
31d	4-ClC6H4	COCH3	30.56	22.90
5-FU			4.21	12.00

[81].

Kurumurthy et al. (2019) synthesized pyrimidine based triazole hybrids and evaluated their anticancer activity. Most of the synthesized compounds showed potent anticancer activity. Among them, compound 32(a) showed excellent activity against various cell lines including U₉₃₇, THP-1 and Colo205 cells having IC₅₀ values of 6.20, 11.27, and 15.01µg/mL with control etoposide having IC₅₀ values of 17.94, 2.16, 7.24, and 1.26 µg/mL, respectively. The structure of pyrimidine-triazole based hybrid derivatives is shown in Figure 35 and the in vitro cytotoxicity of compounds 32(b–e) against human cancer cell lines is shown in

Table 25. In vitro cytotoxic activities of pyrimidine-triazole hybrid compounds 32(b–e).

Compound No.	R1	R2	U937 (µg/mL)	THP-1 (µg/mL)	Colo205 (µg/mL)
32b	H	CH3-(CH2)8-CH2-	8.16	16.91	19.25
32c	CH3-(CH2)4-CH2-	6F13-CH2-CH2-	7.56	-	132.42
32c	(CH3)2CH-	8F17-CH2-CH2-	8.35	142.23	-
32d	C2H5	CH3-(CH2)8-CH2-	17.83	82.65	-
Etoposide			17.94	2.16	7.24

[82].

Nagendra et al. (2014) synthesized pyrimidine based pyrazole hybrids and evaluated their anticancer activity. Most of the synthesized compounds showed potent anticancer activity. Among them, compound 33(a) showed excellent activity against various cell lines including A549, MCF7, DU145 (prostate cancer cell) and HeLa with IC₅₀ values of 4.2, 37.2, 5.8, and 34.3 µM and control 5-FU having IC₅₀ values of 1.3, 1.4, 1.5, and 1.3 µM, respectively. The structure of pyrimidine-pyrazole based hybrid derivatives is shown in Figure 36 and the in vitro cytotoxicity (µM) of compounds 33(b–e) against human cancer cell lines is shown in

Table 26. In vitro cytotoxic activities of pyrimidine-pyrazole based hybrid compounds 33(b–e).

Compound No.	R	A549 (µM)	MCF7 (µM)	DU145 (µM)	HeLa (µM)
33b	CH3CH2OC(O)CH2-	16.3	12.4	18.2	9.8
33c	CH3(CH2)6CH2-	5.7	24.7	6.3	22.7
33d	CF3(CF2)7CH2CH2-	33.7	-	37.7	-
33e		4.1	-	4.7	-
5-FU	-	1.3	1.4	1.5	1.3

[83].

Huang et al. (2012) synthesized pyrimidine based pyrazole hybrids and evaluated their anticancer activity. Most of the synthesized compounds showed potent anticancer activity. Among them, compound 34(a) showed excellent activity against various cell lines including NCI-H226 (human lung squamous carcinoma cells), NPC-TW01 (nasopharyngeal carcinoma), and Jurkat having GI₅₀ 29, 30, and 54 µM, respectively. The structure of pyrimidine-pyrazole based hybrid derivatives is shown in Figure 37, and the in vitro cytotoxicity of compounds 34(b–e) against human cancer cell lines is shown in

Table 27. In vitro cytotoxic activities of pyrimidine-pyrazole based hybrid compounds 34(b–e).

Compound No.	R1	R2	NCI-H226 (µM)	NPC-TW01 (µM)	Jurkat
34b	Ph	p-Me-Ph	35	49	48
34c	2-Quinolinyl	p-Cl-Ph	39	35	69
34d	2-Quinolinyl	p-OMe-Ph	37	36	>100
34e	Ph	p-Cl-Ph	18	23	36
N0-(4-formyl-1,3-diphenyl-1H-pyrazol-5-yl)-N,N-dimethyl-methanimidamide			9.3	31.4	23.5

[84].

FDA Approved Pyrimidine Based Hybrids

Ceritinib, palbociclib, and ibrutinib are three hybrids which contain pyrimidine scaffold that have received US-FDA approval for their antitumor effects against non-small cell lung cancer, advanced breast cancer and leukemia, respectively. Pyrimidine-based FDA approved drugs are depicted in Figure 38 and their details are shown in

Table 28. FDA approved pyrimidine based hybrids.

Company Name	Compound Name	Drug Target	Type of Cancer	Status	Reference
Novartis	Ceritinib	Abnormal ALK-gene	Non-Small cell lung Cancer	Approved	[85]
Pfizer Pharmaceutical company	Palbociclib	CDK4/6 inhibitor	Breast cancer	Approved	[86]
AbbVie Pharmaceuticals	Ibrutinib	Tyrosine kinase	Mantle cell lymphoma	Approved	[87,88]

.

3.6. Quinoline-Based Hybrids

Quinoline has been identified as a major scaffold with huge therapeutic potential, including antibacterial, anti-viral, anti-helmintic, anti-malarial and anti-prozoal activities. Due to its derivatives having demonstrated outstanding effects against cancerous cells via various mechanisms, the quinoline nucleus has played a crucial role in the research and development of chemotherapeutic agents. Camptothecin is a natural anticancer agent with the capacity to obstruct DNA topoisomerase [89,90,91].

Sidoryk et al. (2015), synthesized quinoline based guanidine hybrids and evaluated their anticancer activity. Most of the synthesized compounds showed potent anticancer activity. Among them, compound 35 showed excellent activity against BALB/3T3, A549, MCF-7, LoVo, and KB cell lines (

Table 29. In vitro cytotoxic activities of quinoline-pyrimidine based hybrid compounds 35.

Compounds No.	BALB/3T3 (µM)	A549 (µM)	MCF-7 (µM)	LoVo (µM)	KB (µM)
35	30.04	3.24	0.81	9.38	0.87
Doxorubicin	1.08	0.33	0.44	0.11	0.84
DiMIQ	5.77	2.19	1.54	0.20	1.14

). Furthermore, compound 35 (Figure 39) showed excellent activity in a DNA displacement assay and potent activity in G2/M phase of the cell cycle [92].

Gedawy et al. (2015) synthesized tetrahydro-pyrimido-quinoline based hybrids and evaluated their anticancer activity. Most of the synthesized compounds showed potent anticancer activity. Among them, compound 36 (Figure 40) showed excellent activity against HCT-116, and MCF-7 (

Table 30. In vitro cytotoxic activities of tetrahydro-pyrimido-quinoline based hybrid compound 36.

Compound No.	HCT116 (µM)	MCF7 (µM)
36	16.33	27.26
Imatinib	34.40	-
Tamoxifien	--	34.30

) [93].

Sanchez et al. (2011) synthesized quinoline-based thiazole hybrids based on the structure of m-Amsacrine (37, Figure 41), and evaluated their anticancer activity against cancerous (K-562) and non-cancerous (PMBCs) cells. Compound 37a showed excellent activity with IC₅₀ values of 28.7 and 7.82 µM, respectively, compared to a control of Paclitaxel (IC₅₀ 0.25 µM). Mechanistically, compound 36(a) induced appototic cell death via caspase activation with an IC₅₀ of 7.8 µM [94].

Luniewski et al. (2012) synthesized quinoline based indole hybrids and evaluated their anticancer activity. Most of the synthesized compounds showed potent anticancer activity. Among them, compound 38(a) showed excellent activity against KB, A-549, MCF-7, Hs294T, and BALB/3T3 cell lines having IC₅₀ values of 0.15, 0.24, 0.38, 0.62, and 0.31 µM with control 5,11-dimethyl-5H-indolo [2,3-b]quinoline (DIMIQ) at IC₅₀ values 1.14, 2.19, 1.50, 9.70, and 5.70 µM, respectively. Furthermore, compound 38a showed excellent topo II inhibitory activity at concentration of 0.05 µM with control DIMIQ (Conc. 0.5 µM), m-AMSA (Conc. 0.05 µM), and daunorubicind (Conc. 0.5 µM) and cell cycle inhibitory activity at a concentration of 0.10 µM with control DIMIQ (Conc. 1.02 µM). Additionally, it showed positive inhibitory results in the G2M phase of the cell cycle. The structure of quinoline-indole based hybrid derivatives is given in Figure 42 and the in vitro cytotoxicity of compounds 38(b–e) against human cancer cell lines is shown in

Table 31. In vitro cytotoxic activities of quinoline-indole based hybrid compounds 38(b–e).

Compound No.	R1	R2	KB (µM)	A-549 (µM)	MCF-7 (µM)	Hs294T (µM)	BALB/3T3 (µM)
38b		H	0.15	0.81	0.79	0.64	0.67
38c		H	0.36	0.29	0.99	0.72	0.60
38d	H		0.64	0.17	0.47	0.35	0.34
38e	H		0.08	0.19	0.66	0.76	0.57
DIMIQ			1.14	2.19	1.50	9.70	5.70

[95].

Jin et al. (2019) synthesized quinoline-based ursolic acid hybrids and evaluated their anticancer activity. Most of the synthesized compounds showed potent anticancer activity. Among them, compound 39(a) showed excellent activity against MDA-MB-231, HeLa, SMMC-7721, and QSG-7701 (hepatocyte cell line) at IC₅₀ 0.12, 0.08, 0.34, and 10.76 µM with control Etoposide with IC₅₀ values of 5.26, 2.98, 3.48, and 28.75 µM, respectively. Furthermore, compound 39(a) showed excellent activity apoptosis-inducing activity in on HeLa cell lines (for 48 h). The structure of quinolinebased ursolic acid hybrids derivatives is shown in Figure 43, and the in vitro cytotoxicity (µM) of compounds 39(b–e) against human cancer cell lines is shown in

Table 32. In vitro cytotoxic activities of quinoline based ursolic acid hybrids compounds 39(b–e).

Compound No.	R1	R2	MDA-MB-231(µM)	HeLa (µM)	SMMC-7721(µM)	QSG-7701(µM)
39b	H	CH3	1.84	1.18	17.48	40.59
39c	OCH3	CH3	1.42	0.83	17.65	45.20
39d	F	CH3	1.16	0.99	19.41	>50
39e	H	n-C4H9	>50	>50	>50	Not tested
Etoposide	-	-	5.26	2.98	3.48	28.75

[96].

Solomon et al. (2019) synthesized quinoline based sulphonamide/piperazine hybrids and evaluated their anticancer activity. Most of the synthesized compounds showed potent anticancer activity. Among them, compound 40(a) showed excellent activity against MB231, MB468, MCF7, 184B5 (normal mammary tissue), and MCF10A cell lines, having GI₅₀ values of 3.4, 0.7, 2.3, 9.0, and 12.3 µM. Control chloroquine, had GI₅₀ values of 22.5, 28.6, 38.4, 76.1, and 81.26 µM and Cisplatin having GI₅₀ value 23.7, 31.0, 25.8, 25.5, and 51.51µM, respectively. Furthermore, compound 40(a) showed cell cycle interruption at the meta phase, and also increased the liposamal volume in cancerous cells which led to cell death. The structure of quinoline based piperazine hybrids is given in Figure 44 and the in vitro cytotoxicity (µM) of compounds 40(b–e) against human cancer cell lines is shown in

Table 33. In vitro cytotoxic activities of quinoline based piperazine hybrids compounds 40(b–e).

Compound No.	R1	MB231 (GI50 µM)	MB468 (GI50 µM)	MCF-7 (GI50 µM)	184B5 (GI50 µM)	MCF10A (GI50 µM)
40b	2,4-Dinitrophenyl	24.3	19.2	10.8	37.8	35.4
40c	3-Nitrophenyl	32.2	18.6	9.4	17.7	15.4
40d	2,4-Dichlorophenyl	20.3	18.6	16.7	20.4	15.6
40e	Biphenyl	27.2	20.5	14.8	19.1	15.5
Chloroquine	-	22.5	28.6	38.4	76.1	81.26
Cisplatin	-	23.7	31.0	25.8	25.5	51.51

[97].

Kumar et al. (2014) synthesized quinoline-based gallium(III) hybrids and evaluated their anticancer activity. Synthesized hybrid 41 showed potent activity against HCT-116, Caco-2 (human colon cancer cell), HT-29, and CCD-18C (colonic fibroblasts), having IC₅₀ values of 14.26, 19.56, 19.66, and 28.28 µM, respectively. Control etoposide had comparative values of 38.10, 32.90, 35.10, and 58.90 µM, respectively. Furthermore, hybrid 41 was evaluated for anti-malarial activity, showing more potent activity than lumefantrine on the 3D7 strain of Plasmodium falciparum. A quinoline-based gallium(III) hybrid is depicted in Figure 45 [98].

Quinoline-Based FDA Approved Drugs

Quinoline containing drugs that have received FDA approval include lenvatinib, cabozantinib, and bosutinib which are protein kinase inhibitors and are used to cure medullary thyroid cancer and chronic myelogenous leukemia accordingly. FDA approved drugs with quinoline hybrids are depicted in Figure 46 and their details are shown in

Table 34. Quinoline based FDA approved hybrids.

Company Name	Compound Name	Drug Target	Type of Cancer	Status	Reference
Eisai Co.	Lenvatinib	Kinase inhibitor	Thyroid cancer	Approved	[99]
Exelixis Inc.	Cabozantinib	Tyrosine-kinase	Thyroid cancer and renal carcinoma	Approved	[68]
Wyeth and Pfizer	Bosutinib	BCR and Src tyrosine kinase	myelogenous leukemia	Approved	[100]

.

3.7. Quinone Hybrids

Quinones are found in all living beings; particularly animals, plant and microbes. Through serving as crucial links in the cell nucleus respiratory cycle, they play a vital role in the power generation of such species. Numerous distinct medicinal uses of quinones have been noted, such as antiviral, antithrombotic, antiplatelet, antibacterial, antifungal, anti-inflammatory, and antiallergic properties [101,102].

Markovic et al. (2015) synthesized quinone based chalcone hybrids and evaluated their anticancer activity. Most of the synthesized compounds showed potent anticancer activity. Among them, compound 42(a) showed excellent activity against HeLa, LS174 (colorectal cancer cells), A549, and MRC-5 (multipotent stem cells) with IC₅₀ values of 2.73, 6.44, 28.84, and 48.76, respectively. Furthermore, compound 42(a) showed caspase based apoptosis in G2/M and S phase of cell division. The structure of quinone-based chalcone hybrids derivatives is given in Figure 47 and the in vitro cytotoxicity of compounds 42(b–e) against human cancer cell lines is shown in

Table 35. In vitro cytotoxic activities of quinone based chalcone hybrid compounds 42(b–e).

Compound No.	R	HeLa (µM)	LS174 (µM)	A549 (µM)	MRC-5 (µM)
42b	H	2.41	4.56	26.20	33.57
42c	2-CH3	2.36	3.13	29.05	41.87
42d	3-CH3	2.45	11.79	33.70	52.00
42e	4-CH3	2.64	22.63	24.15	38.49
cisplatin	-	2.10	5.54	11.92	14.21

[103].

Jiang et al. (2015) synthesized quinone based pyran hybrids and evaluated their anticancer activity. Most of the synthesized compounds showed potent anticancer activity. Among them, compound 43(a) showed excellent activity against KB (throat cancer cells), KB/VCR (colon cancer cells), A549, and HL60, with IC₅₀ values 4.05, 1.28, 0.62, and 1.73 µg/mL. A control vincristine had IC₅₀ values of 0.46, 0.26, and 12.09 µg/mL (for KB, KB/VCR, A549), respectively, and adriamycin had an IC₅₀ value 0.02 µg/mL (HL60). The structure of quinone-based pyran hybrid derivatives is given in Figure 48 and the in vitro cytotoxicity (µg/mL) of compounds 43(b–e) against human cancer cell lines is shown in

Table 36. In vitro cytotoxic activities of quinone based pyran hybrid compounds 43(b–e).

Compound No.	R	KB (µg/mL)	KB/VCR (µg/mL)	A549 (µg/mL)	HL60 (µg/mL)
43b		4.31	2.21	6.58	4.45
43c		>8.00	>8.00	>8.00	>8.00
43d		>8.60	8.52	4.49	4.46
Vincristine	-	0.46	0.26	12.09	-
Adriamycin I	-	-	-	-	0.02

[104].

Quinone Containing FDA Approved Drugs

Doxorubicin, daunorubicin, and mitoxantrone drugs are used to treat several types of cancer. These drugs are shown in Figure 49 [101].

3.8. Imidazole Based Hybrids

The imidazole derivatives have recently gained a lot of interest and are already present in many existing treatments. Imidazole was first synthesized in 1858 using glyoxal and formaldehyde by Heinrich Debus [105]. Various medicinal properties of imidazole based hybrid molecules have been reported; especially as antitumor [106], anti-diabetic, anti-HIV, anti-protozoal, anti-mycobacterial, anti-inflammatory, analgesic, and anti-protozoal agents [107,108].

Xiao-Dong Yang et al. (2012) synthesized different derivatives of novel hybrid compounds between 2-phenylbenzofuran and imidazole. Myeloid liver carcinoma (SMMC-7721), colon carcinoma (SW480), breast carcinoma (MCF-7), lung carcinoma (A549) and leukemia (HL-60) cell lines were used for in vitro study of the cytotoxic effects of synthesized hybrids with cisplatin (DPP) as the standard drug. Five derivatives showed more potent cytotoxic activity than standard DPP. The structure of imidazole based benzofuran hybrid derivatives are given in Figure 50 and the in vitro cytotoxic activities of hybrid compounds 44(a–e) (IC₅₀, µM) are listed in

Table 37. In vitro cytotoxic activities of imidazole based benzofuran hybrid compounds 44(a–e).

Compound No.	R	X	SMMC-7721 (µM)	SW480 (µM)	MCF-7 (µM)	A549 (µM)	HL-60 (µM)
44a	2-Bromobenzyl	Br	4.38	12.71	14.29	9.77	1.97
44b	Phenacyl	Br	3.71	10.34	11.90	12.94	2.61
44c	4-Bromophenacyl	Br	3.39	2.85	2.84	8.46	3.15
44d	Naphthyl acyl	Br	1.65	3.38	5.87	10.93	2.49
44e	2′-Phenyl-phenacyl	Br	3.31	6.93	6.90	6.79	2.70
DPP	-	-	8.86	15.92	16.65	11.68	1.81

[109].

Wen Chen et al. (2013) synthesized numerous derivatives of novel hybrid 2-phenyl-3-alkylbenzofuran and imidazole compounds. Myeloid liver carcinoma (SMMC-7721), colon carcinoma (SW480), breast carcinoma (MCF-7), lung carcinoma (A549) and leukemia (HL-60) cell lines were used for in vitro cytotoxic testing of synthesized hybrids, with DPP as the standard drug. They found that five derivatives showed more potent cytotoxic activity than standard DPP. The structure of imidazole based benzofuran hybrid derivatives is given in Figure 51 and the in vitro cytotoxic activities of hybrid compounds 45(a–e) (IC₅₀, µM) are listed in

Table 38. In vitro cytotoxic activities of imidazole based benzofuran hybrid compounds 45(a–e).

Compound No.	R1	R2	SMMC-7721 (µM)	SW480 (µM)	MCF-7 (µM)	A549 (µM)	HL-60 (µM)
45a	Benzimidazole	2-Bromobenzyl	2.10	5.56	4.78	3.34	0.64
45b	2-Ethylimidazole	4-Hydroxyphenacyl	11.81	5.69	3.17	12.90	0.58
45c	2-Ethylimidazole	4-Bromophenacyl	6.07	3.58	2.89	12.76	0.72
45d	2-Ethylimidazole	Naphthylacyl	2.30	3.14	3.03	5.35	0.61
45e	2-Ethylimidazole	2-Bromobenzyl	0.52	0.47	0.51	0.55	0.08
DPP			1.69	12.49	14.09	20.82	18.85

[110].

Al-blewi et al. (2013) synthesized numerous derivatives of novel imidazole-1,2,3-triazole hybrids. The synthesized compounds were screened for their anticancer activity against three different types of cancer, namely human colon carcinoma (Caco2 and HCT116), human cervical carcinoma (HeLa) and human breast adenocarcinoma (MCF-7) cancer cells-, using doxorubicin as a standard drug. They found that these derivatives showed potent more cytotoxic activity than standard doxorubicin. The structure of imidazole based triazole hybrid derivatives is given in Figure 52 and in vitro cytotoxic activities of hybrid compounds 46(a–e) are listed in

Table 39. In vitro cytotoxic activities of imidazole based triazole hybrid compounds 46(a–e).

Compound No.	R	Caco2 (µM)	HCT116 (µM)	HeLa (µM)	MCF-7 (µM)
46a		6.31	12.04	7.91	3.80
46b		8.45	18.32	9.45	4.45
46c		4.67	16.78	6.87	0.38
46d		5.22	18.70	8.42	3.87
46e		10.87	30.98	20.34	15.56
Doxorubicin	-	5.17	5.64	1.25	0.65

[111].

Yanping Hu et al. (2019) designed and synthesized a series of artemisinin-imidazole hybrids derivatives with multidrug resistance (MDR) reversal activity. All hybrids were screened in vitro for anticancer activities against four human cancer cell lines, human breast cancer (MCF-7), human non-small-cell lung (A549), (HEPG-2), breast cancer (MDA-MB-231) and normal human hepatic cells (L02). Adriamycin was used as reference drug. They found that most of the synthesized compounds showed higher anticancer activities than artemisinin. The structure of imidazole based artemisinin hybrid derivatives is given in Figure 53 and the in vitro cytotoxic activities of hybrid compounds 47(a–e) are listed in

Table 40. In vitro cytotoxic activities of imidazole-based artemisinin hybrid compounds 47(a–e).

Compound No.	R1	R2	R3	n	MCF-7 (µM)	A549 (µM)	HEPG-2 (µM)	MDA-MB-231 (µM)	LO2 (µM)
47a	H	CN	CN	1	10.75	12.36	25.59	>100	49.05
47b	H	NO2	H	2	12.86	20.95	39.30	>100	46.37
47c	H	H	Br	2	12.40	26.02	41.59	80.92	>100
47d	H	H	NO2	2	12.69	19.97	33.33	80.92	34.32
47e	H	NO2	H	3	9.78	16.53	21.25	85.20	45.74
Andriamycin	-	-	-		0.67	1.13	0.85	2.94	0.79

[112].

Wen-Jian Song et al. (2012) designed and synthesized a series of novel hybrid compounds of 2-substituted benzofuran and imidazole. In vitro anticancer activity of synthesized hybrids against a panel of human tumor cell lines i.e., ovarian carcinoma cell line (Skov-3), leukemia (HL-60) and breast carcinoma (MCF-7) was evaluated. They found that the hybrid compounds were more selective than standard DPP. The structure of imidazole based benzofuran hybrid derivatives is given in Figure 54 and the in vitro cytotoxic activities of hybrid compounds 48(a–e) are listed in

Table 41. In vitro cytotoxic activities of imidazole-based benzofuran hybrid compounds 48(a–e).

Compounds No.	R1	R2	R3 (Imidazole)	SKOV-3 (µM)	HL-60 (µM)	MCF-7 (µM)
48a	H	H	Benzimidazole	9.5	8.4	11.8
48b	H	Allyl	Benzimidazole	7.9	>40	>40
48c	OMe	Allyl	Imidazole	36.2	>40	>40
48d	OMe	Allyl	Benzimidazole	9.3	>40	>40
48e	H	H	Imidazole	>40	>40	>40
DDP	-	-	-	8.9	5.5	13.0

[113].

Imidazole Based Hybrids That Are FDA Approved or under clinical Trial

Different imidazole-based anticancer hybrid drugs that are FDA approved/or under clinical trials are shown in (Figure 55), and their chemical structures brand/company name and targets are in

Table 42. Imidazole based anticancer drugs with brand/company name, specific cancer types and targets.

Brand/Company Name	Compound Name	Drug Target	Type of Cancer	Status	References
AdisInsight-springer/Ligand pharmaceuticals	Acadesine	AMP-activated protein kinase	Acute lymphoblastic leukemia	Phase-III	[105]
Tasigna/ Novartis	Nilotinib	BCR-ABL	Leukemia	FDA approved	[114]
Hikma Pharmaceuticals	Dacarbazine	DNA synthesis	Melanoma; Lymphoma	FDA approved	[115]
Janssen Pharmaceutical	Tipifarnib	Farnesyltransferase inhibitors (FTIs)	Breast cancer	phase II trials	[116]
Treanda Astellas Pharma	Bendamustine hydrochloride	DNA synthesis	Leukemia; Lymphoma	FDA approved	[115]
Oforta/ Sanofi pharma	Fludarabine phosphate	DNA synthesis	Leukemia	FDA approved Discontinued	[115]
Nova Laboratories, Ltd.	Azathioprine prodrug of mercaptopurine	Thiopurine S-methyltransferase (TPMT)	Childhood acute lymphoblastic leukemia	FDA approved	[115]
Iclusig/ Otsuka Pharmaceutical	Ponatinib	BCR-ABL	Leukemia	FDA approved	[115]
Temodar/ Ranbaxy (UK)	Temozolomide	DNA synthesis	Brain cancer	FDA approved	[115]
Puri-nethol/ German Remedies Limited	Mercaptopurine	HPRT1	Leukemia	FDA approved	[115]
GlaxoSmithKline (GSK)	SB-431542	Activin receptor-like kinase (ALK) receptors	Childhood acute lymphoblastic leukemia	No clinical trials	[105]

.

3.9. Selenium-Based Hybrids

Selenium (Se) is a unique trace element. Se directly or indirectly exerts antioxidant functions in the human body. However, during recent years, researchers reported that Se containing compounds exhibit superior anticancer effects, with high efficacy and selectivity [117]. It has been reported that a variety of organic selenium compounds, including selenoesters, selenocyanates, methylseleninic acid, isoselenocyanates, diselenides, and endocyclic selenium, have anticancer properties [118].

Organic Se compounds have lower systemic effects, fewer side effects, and strong anti-tumor action. They also have a greater ability to inhibit metastasis. Several novel organic Se compounds have been synthesized in order to further improve the selectivity, specificity and efficacy and to lower the toxicity [119].

Xianran He et al. (2020) synthesized organoselenium (SeCF₃) derivatives as potential anticancer agents. The anticancer activity of the synthesized compounds was assessed using human cancer cell lines, human colon adenocarcinoma cells (SW480), human cervical cancer cells (HeLa), human lung carcinoma cells (A549), human breast adenocarcinoma cells (MCF-7). The in vitro biological evaluation was conducted at 24, 48 and 72 h intervals, and it was found that the organoselenium hybrid compounds were more selective than standard Fluorouracil (5FU). The structure of organoselenium hybrid derivatives is given in Figure 56 and the in vitro cytotoxic activities of hybrid compounds 49(a–c) are listed in

Table 43. In vitro cytotoxic activities of organoselenium hybrid compounds 49(a–c).

Compound No.	Structure	h	SW480 (µM)	HeLa (µM)	A549 (µM)	MCF-7 (µM)
49a		24	13.4	24.3	9.4	8.2
		48	11.4	15.1	11.3	6.4
		72	10.1	19.4	7.4	10.4
49b		24	8.2	19.6	13.1	8.6
		48	7.4	17.5	18.4	9.3
		72	6.5	28.7	22.6	9.5
49c		24	4.9	11.5	9.4	3.4
		48	3.3	17.4	15.2	4.3
		72	4.2	19.7	18.5	2.8
Fluorouracil (5FU)	-	24	15.3	20.6	25.3	8.5
		48	12.4	15.5	22.5	10.4
		72	13.1	12.7	17.3	12.6

[118].

Guilherme A. Jardim et al. (2017) synthesized selenium-containing quinone-based 1,2,3-triazoles with potential antitumor activity via rhodium-catalyzed C-H bond activation and click reactions. All compounds were evaluated against five types of cancer cell lines: human promyelocytic leukemia cells (HL-60), human colon carcinoma cells (HCT-116), human glioblastoma cells (SF295), human lung cells (NCIH-460) and human prostate cancer cells (PC3), using paclitaxel as a positive control. L929 cells were also used to test the cytotoxic potential of the naphthoquinoidal derivatives in non-tumor cells. Overall, these compounds represented promising new lead derivatives with potential antitumor activity. The structure of selenium based quinone triazole hybrids is given in Figure 57 and in vitro cytotoxic activities of hybrid compounds 50(a–e) are listed in

Table 44. In vitro cytotoxic activities of selenium based quinone triazole hybrid compounds 50(a–e).

Compound No.	R	HL-60 (µM)	HCT-116 (µM)	SF295 (µM)	NCIH-460 (µM)	PC3 (µM)	L929 (µM)
50a		0.81	78.0	2.60	2.06	2.03	0.52
50b		0.59	0.37	1.48	1.32	1.06	0.36
50c		1.0	2.03	3.12	3.26	2.70	0.61
50d		0.53	78.0	2.13	2.75	2.47	3.16
50e		0.71	0.97	3.43	2.64	1.64	2.12
DOXO	-	0.02	0.21	0.41	0.15	0.76	1.72

[120].

An B et al. (2018) synthesized selenium-containing 4-anilinoquinazoline hybrids and evaluated them as tubulin polymerization inhibitors. All the synthesized compounds were screened against a panel of six human tumor cell lines, human colon cancer cells (RKO), HEPG2, breast adenocarcinoma (MCF-7), human epithelial cervical cancer cells (HeLa), human colon cancer cells (HCT116), and human gastric cancer cells (MGC803). An antiproliferative activity assay showed that most of compounds inhibited human cancer cells at low nano-molar concentrations. These compounds disturbed microtubule dynamics, lowered mitochondrial membrane potential, and stopped Hela cells in the G2/M phase, ultimately leading to apoptosis. The structure of selenium based anilino quinazoline hybrids is given in Figure 58 and the in vitro cytotoxic activities of hybrid compounds 51(a–e) are listed in

Table 45. In vitro cytotoxic activities of selenium based anilino quinazoline hybrid compounds 51(a–e).

Compound No.	R1	R2	R3	R4	RKO (nM)	HGPG2 (nM)	MCF7 (nM)	HELA (nM)	HCT116 (nM)	MGC803 (nM)
51a	H	H	-CN	Cl	3.39	5.24	9.98	2.09	4.97	3.54
51b	CH3	H	-CN	Cl	6.01	6.79	18.9	8.73	22.7	16.5
51c	Cl	H	-CN	Cl	3.42	6.78	10.6	6.78	9.17	9.67
51d	F	H	-Se-CH3	Cl	7.87	13.2	22.5	9.28	12.3	14.8
51e	H	H	-Se-CH3	-OCH3	7.85	8.92	22.7	9.15	11.4	4.78
EP128495	-	-	-	-	4.22	6.47	5.61	5.11	8.27	6.23

[121].

Hairong Tang et al. (2021) synthesized novel selenium-containing chiral 1,4-diarylazetidin-2-ones and biologically evaluated for antitumor activities. All the newly synthesized selenium-containing compounds were screened for their antiproliferative activity against four human cancer cell lines, human epithelial cervical cancer cells (HeLa), human hepatoma cells (HUH-7), ovarian carcinoma cells (Skov-3) and human ovarian cancer cells (A2780), using paclitaxel as the positive control. The structure of Selenium based arylazetidin hybrids is shown in Figure 59 and the in vitro cytotoxic activities of hybrid compounds 51(a–d) are listed in

Table 46. In vitro cytotoxic activities of selenium based anilino quinazoline hybrid compounds 52(a–d).

Comound No.	R	HeLa (nM)	HUH-7 (nM)	SKOV3 (nM)	A2780 (nM)
52a	OH	3.3	2.3	1.6	2.0
52b	F	2.0	6.7	8.5	7.9
52c	Cl	6.1	3.7	1.0	5.9
52d	I	9.2	17.1	9.4	8.2
Paclitaxol	-	2.1	3.2	2.2	3.2

[122].

Sheng Huang et al. (2021) designed and synthesized fourteen novel selenium N-heterocyclic carbene (Se-NHC) compounds derived from 4,5-diarylimidazole and evaluated their antiproliferative activity towards ovarian cancer cells (A2780) and normal ovarian epithelial cells (IOSE80). Most of them were more effective towards ovarian cancer cells (A2780) than HepG2 hepatocellular carcinoma (HCC) cells. In addition, compound 53 displayed more than two-fold higher cytotoxicity to A2780 cells than to normal ovarian epithelial cells (IOSE80). Further research demonstrated that these inhibitors generated reactive oxygen species (ROS), harmed mitochondrial membrane potential (MMP), stopped cells from entering G0/G1 phase, and ultimately promoted apoptosis in the A2780 cells. The structure of selenium-based diarylimidazole hybrids is shown in Figure 60 and the in vitro cytotoxic activities of hybrid compounds 53(a–e) are listed in

Table 47. In vitro cytotoxic activities of selenium based diaryl imidazole hybrid compounds 53(a–e).

Compound No.	R1	R2	A 2780 (nM)	IOSE 80(nM)
53a	OMe	Et	9.7	51.3
53b	OMe		6.4	11.0
53c	F		19.1	34.8
53d	F		13.0	29.4
53e	Br	Et	21.1	43.7
Ebselen	-	-	25.4	55.4

[123].

Selenium-Based Hybrids That Are FDA Approved or under Clinical Trials

Different selenium-based potent anticancer drugs their chemical structure (Figure 61), mode of action and doses are shown in

Table 48. Selenium-based potent compounds with mode of action, specific cancer types and doses.

Compound Name	Mode of Action	Type of Cancer	Dose (Conc.) *	References
Methylseleninic Acid (MSA)	Apoptosis mediated by caspases, ER stress, UPR, mitochondrial dysfunction/signaling and PARP cleavage Anoikis, whereby cell detachment is a prerequisite for caspase activation and PARP cleavage	Breast, Colon Lung, Lymphoma Pancreatic	Medium to Low In pancreatic Very low-Low	[119]
Ebselen and corresponding Analogues	Not determined. Compounds have antioxidant activity	Breast, Liver, Promyelocytic Leukemia, Prostate	Medium-high	[119]
Benzoselenazole- stilbene hybrids	Apoptosis mediated by thioredoxin reductase inhibition and oxidative stress	Breast, Cervical, Liver, Lung	Very low-low	[119]

* Effective Dose in vitro (48–72 h IC₅₀) low (0.1–2 µM) (1–20 µM) Medium (10–100 µM) High (100+ µM).

.

3.10. Platinum-Based Hybrids

Platinum (Pt) medicines are still among the most often used anticancer treatments after more than 40 years of use. It is not unexpected that new research into changes in DNA repair pathways provides a reasonable explanation for Pt medicines’ efficacy since they primarily target DNA [124]. The first platinum drug, cisplatin, was discovered by Barnett Rosenberg in 1960 [125] and received FDA approval in 1978 for the treatment of advanced ovarian, bladder and testicular cancer [126]. Oxaliplatin and carboplatin are also platinum containing clinical drugs. Despite the widespread use of platinum medicines in cancer treatment regimens, there are several associated drawbacks. It is associated with severe side effects such as nephrotoxicity, neurotoxicity, and ototoxicity [127]. Additionally, using platinum medications has a number of adverse effects that range in intensity from mild to toxic (at high doses). In an attempt to circumvent these problems, a large number of platinum complexes have been prepared and tested for anticancer activity [128].

A. Graham et al. (2012) synthesized platinum−acridine hybrid anticancer agents. In vitro cytotoxicity activity was evaluated on different cell lines, ovarian cancer (OVCAR-3), breast cancer (MCF-7, MDA-MB231), pancreatic (PANC1) and non-small cell lung cancer cells (NCI-H460) using cisplatin as the positive control. The structure of Platinum−acridine hybrids is given in Figure 62 and in vitro cytotoxic activities of hybrid compounds 54(a–e) are listed in

Table 49. In vitro cytotoxic activities of Platinum−acridine hybrid compounds 54(a–e).

Compound No.	R	m	OVCAR-3 (µM)	MCF-7 (µM)	MDA-MB231 (µM)	PANC1 (µM)	NCI-H460 (µM)
54a	CH3	2	1.1	2.5	15	0.09	0.008
54b	CH3	3	33	11	37	4.4	0.052
54c	(CH2)(CO)OCH3	2	-	-	-	-	0.036
54d	(CH2)2(CO)OCH3	2	1.9	3.6	9.9	0.086	0.011
54e	(CH2)2(CO)OCH3	3	150	19	36	2.2	0.065
Cisplatin	-	-	3.3	12	60	6.6	1.2

[129].

Jian Zhao et al. (2012) designed and synthesized six novel platinum (II) complexes 1−6 bearing different furoxan moieties as nitric oxide (NO) donors. The furoxan groups were introduced to the platinum complexes to release NO, which may have synergisticaction with the platinum-based moieties on the tumor cells. It was found that all compounds exhibited higher cytotoxicity against human cancer cell lines HCT-116 and SGC-7901 compared to standard carboplatin, and oxaliplatin. The structure of platinum hybrids is shown in Figure 63 and the in vitro cytotoxic activities of hybrid compounds (55–60) are listed in

Table 50. In vitro cytotoxic activities of Platinum hybrid compounds (55–60).

Compounds No.	HCT-116 (µM)	SGC (µM)
55	64.06	217.93
56	57.21	94.23
57	39.43	34.64
58	111.91	248.07
59	142.15	59.10
60	87.06	70.83
carboplatin	273.05	58.11
oxaliplatin	57.04	17.35

[130].

Liu Z et al. (2021) prepared dihydro-2-quinolone (DHQLO) platinum (IV) compounds. Cytotoxic profiles of DHQLO platinum (IV) complexes were tested against five carcinoma cell lines including, ovarian cancer (SKOV-3), human cervical cancer cell (HeLa), human lung cancer (A549), murine colon cancer (CT-26), a cisplatin resistant cell line (A549R), and one normal human embryonic kidney cell line (293T). The antitumor activities of DHQLO platinum (IV) compounds were tested using the MTT assay with cisplatin and oxaliplatin as reference drugs. Cells were treated with drugs at different concentrations for 48 h, and the results were given as IC₅₀ values. The structure of Platinum (iv) dihydro-2-quinolone hybrids is given in Figure 64 and the in vitro cytotoxic activities of hybrid compounds 61(a–e) are listed in

Table 51. In vitro cytotoxic activities of Platinum (iv) dihydro-2-quinolone hybrid compounds 61(a–c).

Compound No.	n	CT26 (μM)	SKOV-3 (μM)	HeLa (μM)	A549 (μM)	A549R (μM)
61a	01	5.0	4.0	3.2	9.8	23.4
61b	03	11.7	2.7	2.6	8.6	8.3
61c	04	9.2	3.5	2.5	6.1	8.9
Cisplatin	-	5.3	2.4	2.4	13.5	22.6
Oxaliplatin	-	3.9	5.3	7.4	26.8	22.2

[131].

Yan Chen et al. (2020) synthesized naproxen platinum(IV) hybrids (62−66, Figure 65) that inhibited, matrix metalloproteinases and caused DNA damage. The antitumor activities of naproxen platinum(IV) compounds (62−66) were tested against four tumor cell lines including human lung cancer (A549), human ovarian cancer (SKOV-3), murine colon cancer (CT-26) and a cisplatin resistant cell line (A549R).A human normal liver cell LO-2 was also evaluated. The results are given in

Table 52. In vitro cytotoxic activities of naproxen platinum (IV) hybrid compounds (62–66).

Compound No.	A549 (μM)	A549R (μM)	SKOV-3 (μM)	CT-26 (μM)	LO-2 (μM)
62	2.2	19.7	14.4	0.2	1.9
63	5.2	4.8	8.5	2.9	4.8
64	47.2	62.2	26.0	26.1	39.9
65	10.2	12.0	11.3	8.2	15.3
66	27.3	83.0	48.9	48.9	26.3
Cisplatin	4.8	15.1	2.5	0.3	3.0
Oxaliplatin	8.4	7.3	9.4	2.3	3.6
Carboplatin	79.6	60.6	38.1	46.2	70.7

. It was observed that compounds 62–66 displayed moderate to effective antitumor activities against the tested tumor cell lines [132].

Raffaella Cincinelli et al. (2013) designed and synthesized camptothecin-linked platinum anticancer agents. Biological activity was tested on different cell lines, including non-small cell lung cancer (H460), osteosarcoma (U2OS), cell carcinoma cells (A431), and ovarian carcinoma (IGROV-1, A2780). These compounds showed growth inhibitory activity against a panel of human tumor cell lines, including sublines resistant to topotecan and platinum compounds. A general reduced potency with respect to TPT was observed in ovarian carcinoma IGROV-1 and A2780 cell lines. The structure of camptothecin-linked platinum anticancer hybrids is given in Figure 66 and the in vitro cytotoxic activities of hybrid compounds (67–69) are listed in

Table 53. In vitro cytotoxic activities of camptothecin-linked platinum hybrid compounds (67–69).

Compound No.	H460 (μM)	U20S (μM)	A431 (μM)	IGROV-1 (μM)	A2780 (μM)
67	1.37	0.48	0.075	1.03	0.036
68	0.4	1.09	0.185	1.24	0.22
69	0.44	1.36	0.28	2.24	0.08
Topotecan (TPT)	1.37	0.48	0.075	1.03	0.036
cDDP	22	20.5	19.57	14.8	4.35

[133].

Platinum Based Drugs That Are FDA Approved or under Clinical Trial

Different platinum-based potent anticancer drugs their chemical structure, specific cancer types and current status are shown in

Table 54. Platinum-based anticancer drugs with chemical structure, specific cancer types and current status.

Compound Name	Chemical Structure	Type of Cancer	Status	References
Cisplatin		solid neoplasms, including ovarian, testicular, bladder, colorectal, lung and head and neck cancers	FDA-approved	[134]
Carboplatin		Retinoblastoma Lung cancer	FDA-approved	[134]
Oxaliplatin		Medulloblastoma Non-small cell lung cancer	FDA-approved	[134]
Nedaplatin		solid neoplasms, including ovarian, testicular	FDA-approved	[134]
Lobaplatin		metastatic breast cancer, chronic myelogenous leukemia and SCLC	Regional approval CHINA	[135]
Heptaplatin		gastric cancer	Regional approval (Republic of Korea)	[135]

.

3.11. Hydroxamic Acid Hybrids

Xing Yan et al. (2016) synthesized hybrids of hydroxamic acid with artemisinin and evaluated their anticancer activity against different cell lines. Most of the synthesized compounds showed potent anticancer activity. Among them, compound 70(a) exhibited excellent activity against various cell lines including HepG2, MCF-7 and HL-60 (human leukemia cell) with IC₅₀ values of 2.50, 2.62, and 1.28 µM, respectively, and control suberoylanilide hydroxamic acid (SAHA) IC₅₀ values of 0.31, 1.90, and 0.18 µM, respectively. Furthermore, they synthesized 14 compounds. Amongst them, only two compounds 70(a), and 70(b), showed excellent HDAC (histone deacetylases) selectivity with IC₅₀ values of 29.31 and 22.7 µM, respectively. The structure of a hydroxamic acid artemisinin hybrid is shown in Figure 67 and the in vitro cytotoxic activities of hybrid compounds 70(a–c) are listed in

Table 55. In vitro cytotoxic activities of hydroxamic acid with artemisinin hybrid compounds 70(a–c).

Compound No.	n	HDAC Inhibiting	Cytotoxicity (µM)
			HepG2	MCF-2	HC-60
70a	(-CH2)6	2.50	2.50	2.62	1.28
70b	(-CH2)2	14.17	14.17	10.98	6.41
70c	(-CH2)3	>125	84.80	71.64	21.84
SAHA	-	0.35	0.31	1.90	0.18

[136].

Yong Ling et al. (2018) synthesized hydroxamate-β-carboline based novel hybrids and evaluated their antiproliferative activity against different cell lines. Most of the synthesized compounds displayed potent anticancer activity. Among them, compound 71(a) exhibited potent activity against panel of cell lines including HCT116 (human colon cancer cell), SUMM-7721(human hepatocellular carcinoma cells), HepG2, MCF-7and Huh-7(human hepatocellular carcinoma cells) with IC₅₀ values of 0.82, 1.06, 0.65, 2.25 and 1.52 µM, respectively, and control SAHA IC₅₀ values of 5.53, 5.61, 6.27, 4.48 and 4.95 µM, respectively. The structure of novel hydroxamate-β-carboline based derivatives is given in Figure 68 and the in vitro cytotoxic activities of hybrid compounds 71(a–d) are listed in

Table 56. In vitro cytotoxic activities of hydroxamate-β-carboline based hybrid compounds 71(a–d).

Compounds No.	R	In Vitro Antiproliferative Activity (IC50 µM)
		HCT116	SUMM-7721	HepG2	Mcf-7	Huh-7
71a	3,4,5 (MeO)3-Ph	0.82	1.06	0.65	2.25	1.52
71b	3-MeO-Ph	0.89	1.22	1.02	2.18	1.52
71c	4-Me-Ph	0.78	0.84	0.53	1.56	0.96
71d	4-NO2-Ph	1.41	2.61	1.51	4.03	2.78
SAHA	-	5.53	5.61	6.26	4.48	4.95

[137].

M.F.A. Mohamed et al. (2017) synthesized hybrids of hydroxamic acid and chalcone derivatives and evaluate their antiproliferative activity against different cell lines. Most of the synthesized compounds displayed potent anticancer activity. Among them, 72(a) exhibited potent activity against cancer cell lines including HEPG2, MCF-7 and HcF-116 (human colon cancer cell), having IC₅₀ values of 0.62, 2.05 and 2.92 µM, respectively and control, SAHA having IC₅₀ values of 3.33, 2.18 and 1.23, respectively. The structure of hydroxamic acid based chalcone derivatives is given in Figure 69 and the in vitro cytotoxic activities of hybrid compounds 72(a–d) are listed in

Table 57. In vitro cytotoxic activities of hydroxamic acid based chalcone hybrid compounds 72(a–d).

Compound No.	R	R1	Antiproliferative Activity (IC50 µM)
			HeptG2	MCF-7	HCF-116
72a	4-OH3	OH	0.620.04	2.050.48	2.920.52
72b	H	OH	3.550.77	2.210.44	2.180.33
72c	4-F	OH	8.721.65	3.200.82	6.741.59
72d	4-OCH3		7.172.01	12.992.99	3.020.81
SAHA	-	-	3.330.74	2.180.35	1.230.08

[138].

Chao ding et al. (2017) synthesized and investigated novel 6-(1,2,3-triazol-4-yl) 4-aminoquinazolin derivatives possessing a hydroxamic acid moiety for antiproliferative activity against two cell lines. Most of the synthesized compounds displayed potent anticancer activity. Among them, 73(a) exhibited potent activity against cell lines including A549 and BT-474 (human breast cancer cells) with IC₅₀ values of 0.51 and 3.63 µM, respectively. Controls were lapatinib (IC₅₀: 1.740.28 and 0.100.02) and SAHA (IC₅₀: 2.57 and 2.67). The structure of hydroxamic acid based 4-aminoquinazolin derivatives is given in Figure 70 and the in vitro cytotoxic activities of hybrid compounds 73(a–d) (IC₅₀, µM) are listed in

Table 58. In vitro cytotoxic activities of hydroxamic acid based 4-aminoquinazolin hybrid compounds 73(a–d).

Compound No.	R	n	Antiproliferative Activity (IC50 µM)
			A549	BT-474
73a		1	0.51	3.63
73b		2	0.63	3.88
73c		2	3.68	2.24
73d		1	8.46	14.65
Lapatinib	-	-	1.74	0.10
SAHA	-	-	2.57	2.67

[139].

D.T.M. Dung et al. (2017) synthesized hybrids of indoline-based N-hydroxy propenamides and evaluated their antiproliferative activity against different cell lines. Most of the synthesized compounds displayed potent anticancer activity. Among them, compound 74(a) exhibited potent actively against three different cell lines including SW620 (colon cancer), Aspe-1(prostate cancer) and PC-3 at with IC₅₀ values 3.05, 6.83 and 7.30 (µM), respectively with control SAHA having IC₅₀ values of 1.44, 7.04, 5.30 (µM), respectively. The structure of hydroxamic acid based indoline derivatives is given in Figure 71 and the in vitro cytotoxic activities of hybrid compounds 74(a–d) are listed in

Table 59. In vitro cytotoxic activities of hydroxamic acid based indoline hybrid compounds 74(a–d).

Compound No.	R	Cytotoxicity Cell Line (IC50 µM)
		SE620	AsPc-1	PC3
74a	5-Br	3.05	6.83	7.30
74b	5-F	3.30	7.28	11.50
74c	5-Cl	3.42	22.35	17.44
74d	5-CH3	3.44	7.47	12.25
SAHA	-	1.44	7.04	5.30

[140].

Hydroxamic Acid Based Hybrids That Are FDA Approved or under Clinical Trial

Hydroxamic acid based hybrids that are FDA approved or under clinical trial, and, their chemical structure and current status are listed in

Table 60. Hydroxamic acid-based hybrid compounds approved or under clinical trials.

Company Name	Compound Name	Drug Structure	Drug Target	Type of Cancer	Status	References
Merck and Co., Inc	Vorinostat (SAHA)		histone deacetylase inhibitor	cutaneous T cell lymphoma	Approved	[141]
Novartis	Panobinostat		non-selective histone deacetylase inhibitor	multiple myeloma	Approved	[142]
4SC AG	Resminostat		histone deacetylase inhibitor	hepatocellular carcinoma	Approved	[143]
Helsinn and MEI Pharma	Pracinostat		histone deacetylase inhibitor	Acute Myeloid Leukemia	Clinical Trial	[144]
Italfarmaco Group’s	Givinostat		histone deacetylase inhibitor	chronic lymphocytic leukemia	Clinical Trial	[144]
Xynomic Pharmaceuticals, Inc.	Abexinostat		histone deacetylase inhibitor	Hodgkin’s lymphoma	Clinical Trial	[144]

.

3.12. Ferrocene Hybrids

Quirante et al. (2011) synthesized ferrocene-indole based hybrids (Figure 72) and evaluated their cytotoxic activity against A549 cells using 5-fluorouracil (5-FU) as a positive control. They synthesized 14 molecules, and 12 showed cytotoxic activity at IC₅₀ values below 100 µM, where 5-FU had an IC₅₀ value of <5 µM. Among these molecules, ferrocene-indole hybrid 75(a) showed the strongest cytotoxic activity with an IC₅₀ value of 5 µM. Cytotoxic activities of hybrid compounds 75(a–d) are listed in

Table 61. In vitro cytotoxic activities of ferrocene-indole hybrid compounds 75(a–d).

Compounds No.	R1	R2	X	A549 (µM)
75a	OCH3	H	H	5
75b	H	Cl	H	33
75c	Cl	H	N	10
75d	NO2	H	H	14
5-Fluorouracil				<5

[145].

Xian-Feng Huang et al. (2014) synthesized novel hybrids of ferrocene containing pyrazolyl moieties and evaluated their anti-proliferative activity against different cell lines. Most of the synthesized compounds displayed potent anticancer activity. Among them, compound 76(c) exhibited potent activity against three different cell lines; A549 (human lung cancer cell), Hep G2 (human liver cancer cell) and MDA-MB-45 (human breast cancer cell) with IC₅₀ values of 4.44, 20.82, and 4.89 (µM), respectively. Control 5-Fluro Uracil had IC₅₀ values of 16.2, 17.6, 2.80 and cisplatin 0.87, 0.74 and 1.14, respectively. The structure of ferrocene derivatives containing pyrazolyl moiety is given in Figure 73 and in vitro cytotoxic activities of hybrid compounds 76(a–d) are listed in

Table 62. In vitro cytotoxic activities of ferrocene containing pyrazolyl hybrid compounds 76(a–d).

Compound No.	R	Antitumor Activity (µM)
		A549	HepG2	MDA-MB-45
76a		4.47	26.70	4.50
76b		5.31	10.89	5.39
76c		4.44	20.82	4.89
76d		7.93	8.43	6.98
5-FU	-	16.8	17.6	2.80
Cisplatin	-	0.87	0.74	1.14

[146].

Frans J. Smit et al. (2016) synthesized hybrids of ferrocenyl-chalcone amide, and evaluate their antitumor activity against different cell lines. Most of the synthesized compounds displayed potent anticancer activity. Among them, compound 77(a) exhibited potent activity against three different cell lines, including Tk-10 (renal) UACC-62 (melanoma), and, MCF-7 at IC₅₀ 2.4, 3.0 and 2.5 μM, respectively. Control parthenolide (PTD)-had IC₅₀ values of 6.4, 15.0 and 5.8, respectively. The structure of ferrocenyl-chalcone amide derivatives are shown in Figure 74 and the in vitro cytotoxic activities of hybrid compounds 77(a–d) are listed in

Table 63. In vitro cytotoxic activities of ferrocenyl-chalcone amide hybrid compounds 77(a–d).

Compound No.	R	Anticancer Activity IC50 (µM)
		TK-10	UACC-62	MCF-7
77a		2.4	3.0	2.5
77b		2.3	6.0	2.8
77c		2.5	3.0	2.8
77d		2.5	3.5	3.0
PTD	-	6.4	15.0	5.8

[147].

J.N. Wei et al. (2019) synthesized novel hybrids of ferrocene-coumarin moiety and evaluate their antiproliferative activity against six different human cancer cell lines. Most of the synthesized compounds displayed potent anticancer activity. Among them, compound 78(a) exhibited potent activity against six different cell lines: BIU-87 (Human Bladder Cancer cell), SGC-790 (human cancer gastric cells), EC9706 and ECa1090 (human esophageal cancer cells), MCF-7 (human breast adenocarcinoma cells) and Jurkat (human leukemia cell) with IC₅₀ values 1.09, 10.61, 25.89, 36.38, 12.10 and 53.01 (µM), respectively. Control adriamycin had IC₅₀ values of 6.09, 5.44, 8.56, 6.52, 7.95, 4.50 µM, respectively. The structure of ferrocene-coumarin derivatives is given in Figure 75 and the in vitro cytotoxic activities of hybrid compounds 78(a–d) are listed in

Table 64. In vitro cytotoxic activities of ferrocene-coumarin hybrid compounds 78(a–d).

Compound No.	Name	Anticancer Activity IC50 (µM)
		BIU-87	SGC-7901	EC-9706	ECa-109	MCF-7	Jurkat
78a	4-Methyl-7-hydroxy-8-nitrocoumarin-ferrocene butyrate conjugate	1.09	1.061	25.89	36.38	12.10	53.01
78b	4-Methyl-7-hydroxycoumarin-ferrocene butyrate conjugate	4.48	16.61	41.26	27.97	15.34	54.90
78c	7-Hydroxycoumarin-ferrocene butyrate conjugate	5.24	7.46	82.83	42.47	28.10	55.77
78d	7-Hydroxy-8-nitrocoumarin-ferrocene butyrate conjugate	15.27	19.78	N	40.00	23.91	38.81
Adriamycin	-	6.09	5.44	8.56	6.52	7.90	4.50

[148].

S. Panaka et al. (2016) synthesized hybrids of ferrocenyl chalcogeno (sugar) triazole conjugates and evaluated their antitumor activity against different cell lines. Most of the synthesized compounds displayed potent anticancer activity. Among them, compound 79(a) exhibited potent activity against five different cell lines including A549 (human lung cancer cell), MDA-MB-231 (human breast cancer cell), MCF-7 (human breast adenocarcinoma cell), HeLa (immortal human cell) and HEK-293T (normal non-tumorigenic human embryonic kidney) at IC₅₀ values 2.9, 3.35, 5.58 and 11.6 (μM), respectively. A control, doxorubicin, had IC₅₀ values of 0.36, 0.47, 0.98 and 0.89, respectively. The structure of ferrocenyl-chalcogeno (sugar) triazole conjugates is given in Figure 76 and the in vitro cytotoxic activities of hybrid compounds 79(a–d) (IC₅₀, µM) are listed in

Table 65. In vitro cytotoxic activities of ferrocene-chalcogeno (sugar) triazole conjugate compounds 79(a–d).

Compound No.	E	R	Anticancer Activity IC50 (µM)
			A549	MDA-MB-231	MCF-7	HeLa	HEK-293T
79a	Se (Selenium)		2.9	3.35	5.85	11.6	-
79b	Se (Selenium)		3.71	>200	>200	18.3	--
79c	S (Sulphur)		11.6	>200	>200	14.8	-
79d	S (Sulphur)		>200	9.7	>200	>200	-
Doxorubicin			0.39	0.47	0.98	0.89	-

[149].

3.13. Curcumin-Based Hybrids

Saiharish Raghavan et al. (2015) synthesized hybrids of curcumin and quinolone and evaluated their anticancer activity against different cell lines. Most of the synthesized compounds displayed potent anticancer activity. Among them, compound 80(a) exhibited potent activity against four different cell lines including A549 (human lung cancer cell), MCF-7 (human breast adenocarcinoma cell), SKOV3 (ovarian cancer cell line) and H460 (human lung cancer cell) at IC₅₀ values 23.9, 36.2,12.8, 21.75, respectively. The structure of curcumin-quinolone derivatives is given in Figure 77 and in vitro cytotoxic activities of hybrid compounds 80(a–d) are listed in

Table 66. In vitro cytotoxic activities of curcumin-quinolone hybrid compounds 80(a–d).

Compound No.	R	R1	Anticancer Activity IC50 (µM)
			A549	MCF-9	SKOV3	H460
80a	-CH3		23.9	36.2	12.8	21.75
80b	-H		21.3	151.56	19	25.4
80c	-OCH3		20.6	>100	14.04	38.14
80d	-F		40.45	25.0	>100	44.3

[150].

G. Banuppriya et al. (2018) synthesized hybrids of curcumin and sulfonamides and evaluated their anticancer activity against two cell lines. Most of the synthesized compounds displayed potent anticancer activity. Among them, compound 81(a) exhibited potent activity against two different cell lines including A549 (human lung cancer cell) and AGS (human gastric adenocarcinoma cell) at IC₅₀ values 1.29 and 10.16 (µM), respectively with control Curcumin at IC₅₀ values 25.33 and 20.76, respectively. The structure of curcumin-sulfonamide derivatives is given in Figure 78 and in vitro cytotoxic activities of hybrid compounds 81(a–d) are listed in

Table 67. In vitro cytotoxic activities of curcumin-sulfonamide hybrid compounds 81(a–d).

Compound No.	R1	R2	R3	R4	R5	Anticancer Activity IC50 (µM)
						A549	AGS
81a	H	OMe	OH	H	H	1.29	10.16
81b	H	H	Me	H	H	6.25	11.94
81c	H	H	Cl	H	H	12.56	22.31
Curcumin	-	-	-	-	-	25.33	20.76

[151].

H.R. Puneeth et al. (2016) synthesized hybrids of curcumin-pyrazole and evaluated their anticancer activity against different cell lines. Most of the synthesized compounds displayed potent anticancer activity. Among them, compound 82(a) exhibited potent activity against four different cell lines including HeLa (human cervical cell), MCF-7 (human breast adenocarcinoma cell), K562 (human immortalized myelogenous leukemia cell) and HEK293T (normal non-tumorigenic human embryonic kidney) at IC₅₀ values 45.54, 34.99, 25.55 and >1000 (µM), respectively with control Paclitaxel at IC₅₀ values 11.61, 9.12, 8.43 and 1.43 (µM), respectively. The structure of curcumin-pyrazole derivatives is given in Figure 79 and in vitro cytotoxic activities of hybrid compounds 82(a–c) (IC₅₀, µM) are listed in

Table 68. In vitro cytotoxic activities of curcumin-pyrazole hybrid compounds 82(a–c).

Compound No.	R	Anticancer Activity IC50 (µM)
		HeLa	MCF-7	K562	HEK293K
82a	o-Cl	45.56	34.99	25.55	>1000
82b	H	42.56	52.88	36.99	NT
82c	o, p-diNO2	56.45	43.60	42.35	NT
Paclitaxel	-	0.0061	0.0053	0.0049	NT

[152].

Peiju Qui et al. (2013) synthesized hybrids of curcumin, pyrimidine and urea and evaluated their anticancer activity against different cell lines. Most of the synthesized compounds displayed potent anticancer activity. Among them, compound 83(a) exhibited potent activity against two different cell lines including HT29 (human colon adenocarcinoma cell) and HCT116 (human colon cancer cell) at IC₅₀ values 7.10.4 and 6.21.2 (µM), respectively with control 5-fluorouracil at IC₅₀ 11.61, 9.12, 8.43 and 1.43 (µM), respectively. The structure of curcumin pyrimidine derivatives is given in Figure 80 and in vitro cytotoxic activities of hybrid compounds 83(a–c) (IC₅₀, µM) are listed in

Table 69. In vitro cytotoxic activities of curcumin-pyrimidine hybrid compounds 83(a–c).

Compound. No.	R1	R2	R3	R4	Anticancer Activity IC50 (µM)
					HT29	HCT116
83a	OCH3	OCH3	H	CH2CH2OH	7.10.4	6.21.2
83b	OCH3	OCH3	H	CH2(CH2)2OH	12.81.9	9.96.9
83c	OCH3	OCH3	H	CH2CH3	38.525.4	46.423.6

[153].

Sahil Sharma et al. (2015) synthesized hybrids of curcumin and isatin and evaluated their anticancer activity against different cell lines. Most of the synthesized compounds displayed potent anticancer activity. Among them, compound 84(a) exhibited potent activity against four different cell lines including THP-1 (Leukemia), COLO-205 (Colon), HCT-(116) and PC-3 (Prostate) at IC₅₀ values of 2.87, 4.15, 1.12 and 5.67 (µM), respectively. The structure of curcumin-isatin derivatives is given in Figure 81 and in vitro cytotoxic activities of hybrid compounds 84(a–c) are listed in

Table 70. In vitro cytotoxic activities of curcumin-isatin hybrid compounds 84(a–c).

Compound No.	X1	X2	X3	X4	X5	R	Anticancer Activity IC50 (µM)
							THP-1	CoLo-205	HCT-116	PC-3
84a	H	OCH3	OCH3	OCH3	H	H	2.87	4.15	1.12	5.67
84b	H	OCH3	OCH3	H	H	H	4.31	5.78	2.92	6.44
84c	H	H	OCH3	H	H	H	4.96	6.72	3.45	8.95

[154].

3.14. Triazole-Based Hybrids

Li Ying et al. (2015) synthesized hybrids of triazole, pyrimidine and urea and evaluated their anticancer activity against different cell lines. Most of the synthesized compounds displayed potent anticancer activity. Among them, compound 85(a) exhibited potent activity against four different cell lines including EC-109 (squamous cell carcinoma cells), MCF-7 (human breast adenocarcinoma cells), MGC-803 (immortal human cells) and B16-F10 (murine melanoma cells) at IC₅₀ 2.96, 3.11, 3.60 and 4.55 (µM), respectively and with control 5-fluorouracil an IC₅₀ of 11.61, 9.12, 8.43 and 1.43 (µM), respectively. The structure of triazole-pyrimidine derivatives is given in Figure 82 and in vitro cytotoxic activities of hybrid compounds 85(a–d) are listed in

Table 71. In vitro cytotoxic activities of triazole-pyrimidine hybrid compounds 85(a–d).

Compound No.	R1	R2	Anticancer Activity IC50 (µM)
			EC-109	MCF-7	MGC-803	B16-F10
85a	m-CF3	p-Br	2.96	3.11	3.60	4.55
85b	m-CF3	7-CH3	32.29	6.12	7.58	9.58
85c	m-CF3	m,p,m-Tri-OCH3	>64	8.14	8.21	24.43
85d	m-CF3	p-CH(CH3)2	24.22	8.62	>64	1.70
5-FU	-	-	11.61	9.12	8.43	1.43

[155].

Madasu Chandrashekhar et al. (2016) synthesized hybrids of triazole–myrrhanore Cand evaluated their anticancer activity against different cell lines. Most of synthesized compounds displayed potent anticancer activity. Among them, compound 86(a) exhibited good anticancer activity against different cell lines including A549 (human lung cancer cell), Hela (human cervical cell), MCF-7 (human breast adenocarcinoma cell), DU-I45 (human prostate cancer cell) and HepG2 (human hepatocellular carcinoma cell) at IC₅₀ 06.16, 07.76, 09.59, 08.83 and 09.52 (µM), respectively, with control doxorubicin 2.81, 2.57, 1.13, 1.41 and 3.01 (µM), respectively. The structure of triazole–myrrhanore C derivatives is given in Figure 83 and in vitro cytotoxic activities of hybrid compounds 86(a–c) (IC₅₀, µM) are listed in

Table 72. In vitro cytotoxic activities of triazole –myrrhanore C hybrid compounds 86(a–c).

Compound No.	R	Anticancer Activity IC50 (µM)
		A549	Hela	MCF-7	DU-I45	HepG2
86a		06.16	07.76	09.59	08.83	09.52
86b		08.12	12.83	14.66	11.14	14.08
86c		11.22	22.96	16.59	16.37	34.87
Doxorubicin	-	2.81	2.57	1.13	1.41	3.01

[156].

Zahra Najafi et al. (2015) synthesized hybrids of triazole and isoxazole and evaluated their anticancer activity against different cell lines. Most of the synthesized compounds displayed potent anticancer activity. Among them, compound 87(a) exhibited potent activity against two different cell lines including MCF-7 and T47D (breast cancer cell line) at IC₅₀ > 100 and 27.7 µM, respectively with control etoposide at IC₅₀ 7.5 and 7.9, respectively (µM). The structure of triazole-isoxazole derivatives is given in Figure 84 and in vitro cytotoxic activities of hybrid compounds 87(a-b) (IC₅₀, µM) are listed in

Table 73. In vitro cytotoxic activities of triazole-isoxazole hybrid compounds 87(a–c).

Compound No.	Ar1	Ar2	Cytotoxic Activity IC50 (µm)
			MCF-7	T47D
87a	Ph		>100	27.7 ± 0.1
87b	Ph		85.7 ± 4.5	>100
Etoposide	-	-	7.5 ± 0.32	7.9 ± 0.45

[157].

Y.C. Duan et al. (2013) synthesized hybrids of 1,2,3-triazole and dithiocarbamate and evaluated their anticancer activity against different cell lines. Most of the synthesized compounds displayed potent anticancer activity. Among them, compound 88(a) exhibited potent activity against four different cell lines including MGC-803 (immortal human cell), MCF-7 (human breast adenocarcinoma cells), PC-3 (human pancreatic cancer cell) and EC-109 (squamous cell carcinoma cell) at IC₅₀ 0.73, 5.67, 11.61 and 2.44 µM, respectively with control 5-fluorouracil at IC₅₀ 7.01, 7.54, 27.07 and 3.34 µM, respectively. The structure of triazole-dithiocarbamate derivatives is shown in Figure 85 and in vitro cytotoxic activities of hybrid compounds 88(a–c) (IC₅₀, µM) are listed in

Table 74. In vitro cytotoxic activities of triazole-dithiocarbamate hybrid compounds 88(a–c).

Compound No.	R	Anticancer Activity IC50 (µM)
		MGC-803	MCF-7	PC-3	EC-109
88a	o-F	0.73	5.67	11.61	2.44
88b	o-Cl	0.49	6.09	12.45	11.93
5-FU	-	7.01	7.54	27.07	3.3

[158].

R.M. Kumbhare et al. (2015) synthesized hybrids of triazole thiazole and evaluated their anticancer activity against different cell lines. Most of the synthesized compounds displayed potent anticancer activity. Among them, compound 89(a) exhibited potent activity against four different cell lines including MCF-7 (human breast cancer), A549 (human lung cancer), A375 (human melanoma cancer), and MCF-10A (normal breast epithelial cells) with IC₅₀ values of 2.12, 5.48, 4.7 and 29.33 µM, respectively. Control doxorubicin had comparative IC₅₀ 0.12, 3.13, 7.2 and 24.0 µM and paclitaxel at IC₅₀ 2.58, 4.9, 8.0, 38.0 µM, respectively. The structure of triazole based thiazole derivatives is given in Figure 86 and in vitro cytotoxic activities of hybrid compounds 89(a–c) are listed in

Table 75. In vitro cytotoxic activities of triazole-thiazole hybrid compounds 89(a–c).

Compound No.	R	Anticancer Activity IC50 (µM)
		MCF-7	A549	A375	MCF-10A
89a	5-(CF3)-1,2,4-thiadiazole	2.12	5.48	4.7	29.33
89b	4,5-CF3C6H4	2.52	4.97	4.67	2.5
89c	2,4-FC6H3	3.0	4.45	6.02	2.2
Doxorubicin	-	0.12	3.13	7.2	24.0
Paclitaxel	-	2.58	4.9	8.0	38.0

[159].

3.15. Benzimidazole-Based Hybrids

R. Sivaramakarthikeyan et al. (2020) synthesized hybrids of benzimidazole and pyrazole and evaluated their anticancer activity against different cell lines. Most of the synthesized compounds displayed potent anticancer activity. Among them, compound 90(a) exhibited potent activity against three different cell lines including SW1990 (human pancreatic adenocarcinoma cell), AsPC1 (human pancreatic tumor cell), MRCS (marginal reticular cells) with IC₅₀ values of 30.9, 32.8, 80.0 µM, respectively and control gemicitabine at IC₅₀ values of 35.09, 39.27 and 54.17 µM, respectively. The structure of benzimidazole-pyrazole derivatives is given in Figure 87 and in vitro cytotoxic activities of hybrid compounds 90(a–c) (IC₅₀, µM) are listed in

Table 76. In vitro cytotoxic activities of benzimidazole-pyrazole hybrid compounds 90(a-b).

Compound No.	R1	R2	R3	Anticancer Activity IC50 (µM)
				SW1990	AsPC1	MRCS
90a	F	H	H	30.9	32.8	80.0
90b	o-Me	H	H	57.6	62.4	>100
Gemicitabine	-	-	-	35.09	39.27	54.17

[160].

Kun Pena Shao et al. (2014) synthesized hybrids of benzimidazole–pyrimidine and evaluated their anticancer activity against different cell lines. Most of the synthesized compounds displayed potent anticancer activity. Among them, compound 91a exhibited potent activity against three different cell lines including MCF-7 (human breast adenocarcinoma cells), MGC-803 (immortal human cells), EC-9706 (esophagus squamous cell carcinoma) and SMMC-7721 (human hepatocellular carcinoma cells) with IC₅₀ values of 1.43, 1.33, 3.33, 20.50 (µM), respectively with control 5-Fluorouracil had IC₅₀ values 7.12, 3.45, 8.07 and 15.08 (µM), respectively. The structure of benzimidazole-pyrimidine derivatives is given in Figure 88 and in vitro cytotoxic activities of hybrid compounds 91(a–c) (IC₅₀, µM) are listed in

Table 77. In vitro cytotoxic activities of benzimidazole-pyrimidine hybrid compounds 91(a–c).

Compound No.	R1	R2	Anticancer Activity IC50 (µM)
			MCF-7	MGC-803	EC-9706	SMMC-7721
91a	H	4-CH3-C6H5-NH-	1.43	1.33	3.33	20.50
91b	H	4-CH3O-C6H5-NH-	2.90	2.03	5.83	10.55
91c	H	4-F-C6H5-NH-	4.24	2.30	8.55	22.58
5-FU	-	-	7.12	3.45	8.07	15.08

[161].

Pankaj Sharma et al. (2016) synthesized hybrids of benzimidazole and thiazolidinedione and evaluated their anticancer activity against different cell lines. Most of the synthesized compounds displayed potent anticancer activity. Among them, compound 92a exhibited potent activity against five different cell lines including PC-3, DU-145 (prostate cancer), MDA MB-231 (breast cancer), A549 (lung cancer) and MCF10A (normal breast epithelial cells) with IC₅₀ values of 39.87, 31.41, 29.18, 11.46 and >100 (µM), respectively with control 5-Fluorouracil had IC₅₀ values of 45.32, 40.58, 35.98, 30.47 (µM), respectively. The structure of benzimidazole-pyrimidine derivatives is given in Figure 89 and in vitro cytotoxic activities of hybrid compounds 92(a–c) are listed in

Table 78. In vitro cytotoxic activities of benzimidazole–thiazolidinedione hybrid compounds 92(a–c).

Compound No.	NR1R2	R3	Anticancer Activity IC50 (µM)
			PC-3	DU-145	MDA MB-231	A549	MCF10A
92a	R1 = H, R2 = 4-phenylthiazol	Methyl	39.87	31.41	29.18	11.46	>100
92b	6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline	Ethyl	33.54	>50	>50	29.40	----
92c	6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline	Methyl	41.10	37.32	36.03	13.35	>100
5-FU	-	-	45.32	40.58	35.98	30.47	-

[162].

Lei Shi et al. (2014) synthesized benzimidazole-quinazoline hybrids and evaluated their anticancer activity against different cell lines. Most of the synthesized compounds displayed potent anticancer activity. Among them, compound 93a exhibited potent activity against two different cell lines including Hep-G2 (human liver carcinoma cells) and MCF-7 (human breast adenocarcinoma cell) at IC₅₀ 8.7 and 1.5 (µM), respectively. Control golvatinib had IC₅₀ values of 65.5 and 49.6 µM, respectively. The structure of benzimidazole-quinazoline derivatives is given in Figure 90 and in vitro cytotoxic activities of hybrid compounds 93(a–c) are listed in

Table 79. In vitro cytotoxic activities of benzimidazole-quinazoline hybrid compounds 93(a–c).

Compound No.	R	Proliferative Inhibition IC50 (µM)
		Hep-G2	MCF-7
93a	4-F	8.7	12.6
93b	4-Cl	15.8	3.6
93c	4-Br	24.7	8.3
Galvatinib	-	65.5	49.6

[163].

Reddymasu Sireesha et al. (2021) synthesized hybrids of benzimidazole and β-carboline and evaluated their anticancer activity against different cell lines. Most of the synthesized compounds displayed potent anticancer activity. Among them, compound 94a exhibited potent activity against four different cell lines, including MCF-7 (human breast cancer cell line), A549 (a human lung cancer cell line), Colo-205 (a human colon cancer cell line) and A2780 (a human ovarian cancer cell line) with IC₅₀ values of 0.22, 1.55, 1.68 and 1.16 (µM), respectively, with the control etoposide having IC₅₀ values of 2.11, 3.08, 0.13 and 1.31 (µM), respectively. The structure of benzimidazole–β-carboline derivatives is given in Figure 91 and in vitro cytotoxic activities of hybrid compounds 94(a–c) are listed in

Table 80. In vitro cytotoxic activities of benzimidazole-β-Carboline hybrid compounds 94(a–c).

Compound No.	R	Cytotoxicity of Compound IC50 (µM)
		MCF-7	A549	Colo-205	A2780
94a	5,6-dicyano	0.220	1.550	1.680	1.160
94b	5,6-dimethoxy	0.0920	0.720	0.340	1.230
94c	5,6-dimethyl	0.810	1.900	0.410	1.800
Etoposide	-	2.110	3.080	0.130	1.310

[164].

Benzimidazole Based Hybrids That Are FDA Approved or under Clinical Trials

Benzimidazole based hybrids that are FDA approved/under clinical trials and their current status are given in

Table 81. Current status of benzimidazole based hybrids that are approved or under clinical trials.

Company Name	Compound Name	Drug Structure	Drug Target	Type of Cancer	Status	Reference
Eli Lilly and Company	Abemaciclib		cyclin dependent kinase-4 (CDK4) and CDK6 inhibitor	negative metastatic breast cancer	Approved	[165]
AbbVie	Veliparib		PARP inhibitor	ovarian cancer	Clinical Trial	[166]
Allarity Therapeutics	Dovitinib		pan tyrosine kinase inhibitor	prostate cancer	Clinical Trial	[167]
Novartis	Nazartinib		EGFR kinase inhibitor	non-small cell lung cancer	Clinical Trial	[168]

.

3.16. Isatin Containing Hybrids

In humans and other mammals, isatin is an endogenous compound that has a variety of pharmacological properties, including anticancer activity [169]. Human health is facing several difficulties in the modern medical period, especially with regard to human cancers. As a result, new therapies that selectively target tumor cells will unavoidably be added to the therapeutic arsenal for these cancers [170].

AZIZ et al. (2021) synthesized different sets of isatin-based benzoazine hybrids, i.e., isatin quinoxaline quinazoline and phthalazines hybrids. All the synthesized hybrids were evaluated in vitro for their antiproliferative activity against three human cancer cell lines, namely breast cancer (ZR-75), human colon cancer (HT-29) and lung cancer (A-549). The structure of isatin-based benzoazine derivatives is given in Figure 92 and in vitro cytotoxic activities of hybrid compounds 95(a–c) are listed in

Table 82. In vitro cytotoxic activities of isatin-based benzoazine hybrid compounds 95(a–c).

Compound No.	R	ZR-75(µM)	HT-29 (µM)	A-549 (µM)	Average IC50(µM)
95a	H	13.25	6.69	7.19	9.04
95b	F	5.9	5.31	5.39	5.53
95c	Cl	7.77	6.23	7.02	7.01
Sunitinib		8.31	10.14	5.87	8.11

[171].

Meleddu et al. (2017) synthesized isatin-dihydropyrazole derivatives and then evaluated their capability to inhibit tumor cell growth against nine human cancer cell lines, namely A549 (lung carcinoma), IGR39 (melanoma), U87 (glioblastoma), MDA-MB-231 (breast cancer), MCF-7 (breast adenocarcinoma), and BT474 (invasive ductal carcinoma), H1299 (non-small cell lung carcinoma), BxPC-3 (pancreatic adenocarcinoma), SKOV-3 (ovarian cancer) and human foreskin fibroblasts. Sunitinib was the standard drug taken for evaluation of anticancer activity. The structure of dihydropyrazole isatin based hybrids is given in Figure 93 and the in vitro cytotoxicity of hybrid compounds 96(a–d) against human cancer cell lines is shown in

Table 83. In vitro cytotoxic activities of isatin-dihydropyrazole hybrid compounds 96(a–d).

Compound No.	Ar	R	EC50 Values (µM)
			IGR39	A549	U87	Fibroblasts	MCF-7	BT474	H1299	BxPC-3	SKOV-3
96a		5-Cl	0.33	0.34	0.38	-	0.07	0.09	0.0.1	0.06	0.06
96b		5-OCH3	0.50	0.73	0.67	0.27	0.27	0.24	0.15	0.10	-
96c		5-CH3	0.14	0.18	0.23	0.15	0.31	-	-	0.10	-
96d		5-Cl	2.97	-	5.76	-	-	-	-	-	-
Sunitinib	-		-	-	-	0.30	0.96	0.90	1.54	2.5	1.36

[172].

Wagdy et al. (2015) synthesized isatin-pyridine derivatives and tested their anti-proliferative activity against three human tumor cancer cell lines, i.e., hepatocellular carcinoma (HEPG2), lung cancer (A549), and breast cancer (MCF-7) using a sulforhodamine B (SRB) colorimetric assay. Doxorubicin has been used as a reference cytotoxic compound. The structure of pyridine isatin based hybrids is given in Figure 94 and in vitro cytotoxicity of compounds 97(a–c), or 98, 99(a–c) against human cancer cell lines is shown in

Table 84. In vitro cytotoxic activities of isatin-pyridine hybrid compounds [97(a–c), 98, 99 (a–c)].

Compound No.	X	Ar	IC50 (µM)
			A549	HepG2	MCF-7
97a	H		>200	>200	>200
97b	Cl		>200	>200	>200
97c	Br		>200	>200	>200
98	-	-	19.3	2.5	11.6
99a	H	-	16.8	>200	14.7
99b	F	-	19.7	11.5	10.4
99c	Cl	-	10.8	8.7	6.3
Doxorubicin	-	-	7.6	6.9	6.1

[173].

Singh et al. (2015) synthesized isatin-coumarin hybrids which contain triazole as a linker, and investigated their in vitro cytotoxicity activity against four human cancer cell lines (COLO-205, THP-1, HCT-116 and PC-3) using sulforhodamine B19. The cells were given 48 h to multiply in the presence of a test substance. COLO-205, THP-1 and HCT-116 (three of the four cancer cell lines tested) were sensitive to the synthesized hybrids, and the THP-1 cancer cell line was the most susceptible to these hybrids, whereas PC-3 was found to be resistant. The structure of isatin-based coumarin hybrids is given in Figure 95 and in vitro cytotoxicity of compounds 100(a–d) against human cancer cell lines is shown in

Table 85. In vitro cytotoxic activities of isatin-based coumarin hybrid compounds 100(a–d).

Compound No.	R	n	IC50 (µM)
			THP-1	COLO-205	HCT-116
100a	H	1	0.73	3.45	3.04
100b	F	1	1.99	6.67	5.41
100c	Cl	1	5.47	8.87	5.77
100d	Br	1	6.43	10.53	8.09

[174].

Wabli et al. (2017) synthesized isatin-indole derivatives and evaluated their antiproliferative activity on three cell lines: ZR-75 (human breast), HT-29 (colon) and A-549 (lung). Compound 101(d) had an average IC₅₀ value of 1.17 μM against the tested human cancer cell lines, making it the most active compound, with a potency approximately seven times greater than that of sunitinib. The structure of isatin-based indole hybrids is given in Figure 96 and in vitro cytotoxicity IC₅₀ (µM) of compounds 101(a–d) against human cancer cell lines is shown in

Table 86. In vitro cytotoxic activities of isatin-indole hybrid compounds 101(a–d).

Compound No.	X	R	IC50 (µM)			Average IC50 (µM)
			ZR-75	HT-29	A-549
101a	H	H	22.73	19.74	12.98	18.48
101b	Br	H	21.92	21.93	16.01	19.95
101c	OCH3	CH3	1.48	5.73	1.93	3.05
101d	Cl		0.74	2.02	0.76	1.17
Sunitinib	-	-	8.31	10.14	5.87	8.11

[175].

Panga et al. (2020) synthesized isatin-benzoic acid conjugates and tested their in vitro cytotoxic activity against MCF-7 and HeLa cell lines. All the synthesized isatin-benzoic acid conjugates showed moderate to strong cytotoxicity against both HeLa and MCF-7 cell lines with IC₅₀ values ranging from 4.02 to 17.83 μM and 17.14 to 24.6 μM, respectively. Among all synthesized compounds, 102(b) showed maximum activity on both cell lines. The structure of isatin-based benzoic acid hybrids is given in Figure 97 and in vitro cytotoxicity of compounds 102(a–d) against human cancer cell lines is shown in

Table 87. In vitro cytotoxic activities of isatin-benzoic hybrid compounds 102(a–d).

Compound No.	R1	R2	R3	R4	R5	R6	IC50 (µM)
							HeLa	MCF-7
102a	I	H	Cl	H	OC2H	H	9.24	6.57
102b	Br	Br	Cl	H	OC2H	H	8.34	5.88
102c	Br	H	H	OCH3	H		16.68	11.32
102d	Br	Br	H	OCH3	H		10.44	9.46
Vinblastine	-	-	-	-	-		7.14	4.02

[176].

Eldehna et al. (2016) synthesized isatin-thiazolo benzimidazole hybrids and evaluated their anti-proliferative efficacy against MCF-7 and MDA-MB-231 breast cancer cell lines by using sulforhodamine B colorimetric (SRB) assay. Staurosporine was utilized as a positive control, and the results were shown as IC₅₀ values. Compounds 103(a) and 104(a) had maximum anticancer activity towards the tested cell lines. The structure of isatin-based thiazolo-benzimidazole hybrids is shown in Figure 98, and the in vitro cytotoxicity IC₅₀ (µM) of compounds 103(a–c), 104(a–c) against human cancer cell lines is shown in

Table 88. In vitro cytotoxic activities of isatin-thiazolo benzimidazole hybrid compounds 103(a–c) and, 104(a–c).

Compound No.	R	R1	IC50 Values
			MCF	MDA-MB-231
103a	H	-	2.02	6.50
103b	F	-	16.83	42.64
103c	Br	-	9.39	8.62
104a	H	CH3	3.01	2.60
104b	H		9.80	18.06
104c	H		1.27	13.73
Staurosporine	-	-	3.81	4.29

[170].

Isatin Containing FDA-Approved Hybrids

Isatin containing FDA-approved hybrids are depicted in Figure 99 [177].

3.17. Pyrrolo-Benzodiazepines Based Hybrids

Pyrrolo-benzodiazepines (PBD) are naturally found in many actinomycetes species. PBD block transcription factors and promote DNA replication by binding covalently to DNA, and thus inhibits cell growth [178].

Bose et al. (2012) synthesized hybrids of pyrrole and benzodiazepine and tested cytotoxic activities of the synthesized compounds was in vitro against five tumor cell lines: THP-1 (human acute monocytic leukemia), U-937 (human histiocytic lymphoma), HL-60 (human promyelocytic leukemia), Jurkat (human T-cell leukemia) and A-549 (lung carcinoma). Among the synthesized compounds, 105(a) was the most potent. The structure of pyrrolo-benzodiazepine hybrids is given in Figure 100 and the in vitro cytotoxicity IC₅₀ of compounds 105(a–c) against human cancer cell lines is shown in

Table 89. In vitro cytotoxic activities of pyrrolo-benzodiazepine hybrid compounds 105(a–c).

Compound No.	R	IC50 Values (µM)
		THP-1	U-937	HL-60	Jurkat	A-549
105a	F	0.49	3.44	4.13	6.58	nd
105b	OMe	2.14	6.03	6.39	11.27	nd
105c	H	3.09	5.41	7.27	9.43	nd
Etoposide		2.16	17.94	1.83	5.35	17.94
Camptothecin		0.07	1.98	0.60	0.026	0.008

[179].

Kamal et al. (2012) synthesized pyrrolo-benzodiazepine conjugated with benzo indolone derivatives. The synthesized derivatives were assessed for their anticancer activity in human cancer cell lines of the lung, skin, colon and prostrate by using the MTT assay. These new conjugates exhibited encouraging anticancer activity, with IC₅₀ values ranging from 1.05 to 36.49 µM. Doxorubicin and DC81, the positive controls, displayed IC₅₀ values in the range of 0.03–2.51 µM and 0.86–1.65 µM, respectively. Compound 106(d) had the maximum anticancer activity. The structure of pyrrolo-benzodiazepine-based benzoindolone hybrids is given in Figure 101 and in vitro cytotoxicity of compounds 106(a–d) against human cancer cell lines is shown in

Table 90. In vitro cytotoxic activities of pyrrolo-benzodiazepine-based benzoindolone compounds 106(a–d).

Compound No.	X	n	IC50 (µM)
			A431	A549	Colo-205	PC-3
106a	CO	1	3.04	13.85	8.65	7.67
106b	CO	2	1.34	9.80	3.31	3.23
106c	CO	3	3.71	21.94	9.76	15.45
106d	SO2	4	1.72	1.05	1.21	1.52
DC-81	-	-	1.65	1.11	0.86	1.19
Dox	-	-	0.03	1.02	1.69	2.51

[180].

Li et al. (2021) synthesized new pyrrolo [2,1-c] [1,4] benzodiazepine-3,11-dione (PBD) derivatives and evaluated their HDAC6 inhibitory effect. When R = H, the activity of the linker with the benzene ring was moderate. However, when the linker was extended by one methylene, the compound’s activity drastically dropped (107a vs. 107b). The structure of pyrrolo-benzodiazepine-dione hybrids is given in Figure 102 and the in vitro IC₅₀ of compounds [107(a–c),108(a–c)] against HDAC6 enzyme is shown in

Table 91. In vitro cytotoxic activities of pyrrolo-benzodiazepine-dione hybrid compounds [107(a–c),108(a–c)].

Compound No.	R	m/n	IC50 Value (nM)
			HDAC6 Enzyme
107a	H	0	38.80
107b	H	1	147
107c	H	3	>300
108a	Cl	5	23.77
108b	Cl	6	27.80
108c	Cl	7	137.77
Tubacin	-	-	5.36
Tubastatin A	-	-	22.36

[181].

Chen et al. (2013) synthesized a new series of PBD-triazole hybrids and their cytotoxicity was investigated on various mouse and human cells, taking tubastatin A as a positive control. Cis-hybrids (109a–c) were much more cytotoxic than trans isomers (110a–c) in sensitive melanoma (A375). Compound 109a exhibited a higher inhibitory activity compared to that of other agents on A375 cells. The structure of triazole pyrrolo-benzodiazepine hybrids is given in Figure 103 and the in vitro cytotoxicity of compounds [109(a–c), 110(a–c)] against cancer cell lines is shown in

Table 92. In vitro cytotoxic activities of triazole-pyrrolo-benzodiazepines hybrid compounds [109(a–c),110(a–c)].

Compound No.	R	IC50 Value
		A375
109a	H	2.2
109b	2-OCH3	4.2
109c	3-OCH3	3.6
110a	H	4.5
110b	2-OCH3	5.2
110c	3-OCH3	5.3

[182].

FDA Approved Drugs Containing Pyrrolo-Benzodiazepines

FDA approved drugs containing pyrrolo-benzodiazepines include tomaymycin, sibiromycin and neothramycin, which are depicted in Figure 104 [178].

3.18. Chalcone-Based Hybrids

Chalcone compounds are one of the most important fundamental categories of natural products as they are abundant within tea leaves, fruits and vegetables, fruits, and are of great interest because of their pharmacological effectiveness in treating many diseases. Some of the naturally occurring chalcones are depicted in Figure 105 [183].

One of the most significant bioactive substances with a chalcone structure (isoliquiritigenin) was isolated from liquorice roots. Butein, a physiologically active flavonoid found in Rhus verniciflua (Stokes’ bark), has been shown to have strong anticancer effects on a variety of cancer types.

Chalcone-based hybrid compounds have the potential to improve selectivity and anticancer activity while also overcoming drug resistance. Therefore, combining the chalcone moiety with additional anticancer pharmacophores is a promising strategy for creating new anticancer drugs. In recent times, a number of chalcone hybrids have been prepared and evaluated for their cytotoxic activity; some of them are found to have remarkable activity both in vitro and in vivo [184].

Zahrani et al. (2020) synthesized chalcone-based phenothiazine derivatives and evaluated their cytotoxic activity against two carcinoma cell lines (human breast cancer cell line MCF-7 and human hepatocellular carcinoma HEPG-2 cells) compared with anticancer standard drugs cisplatin and doxorubicin under the similar conditions following the MTT (methylthiazol-tetrazolium) colorimetric assay. 111(a) and 111(b) were the most effective compounds with IC₅₀ values of 7.14 µg/mL and 7.6 g/mL, respectively. The structure of chalcone based phenothiazine hybrids is shown in Figure 106 and the in vitro cytotoxicity of compounds 111(a–d) against cancer cell lines is shown in

Table 93. In vitro cytotoxic activities of chalcone-based phenothiazine hybrid compounds [111(a–d),111(a)].

Compound No.	Ar	IC50 (µM)
		MCF-7	HepG-2
111a		12	7.6
111b		13.8	7.14
111c		15.4	11.6
111d		19.2	14.7
Cisplatin	-	5.71	3.67
Doxorubicin	-	0.35	0.36

[183].

Shivapriya et al. (2021) produced chalcone benzoxadiazole hybrids and evaluated their cytotoxic activity against the human epidermal carcinoma (KB) cell line using MTT assay. Chalcone-benzoxadiazole hybrids were prepared through the Claisen–Schmidt condensation reaction. The structure of chalcone-based benzoxadiazole hybrids is given in Figure 107 and the in vitro cytotoxicity of compounds 112(a–d) against cancer cell lines is shown in

Table 94. In vitro cytotoxic activities of chalcone-based benzoxadiazole hybrid compounds [112(a–d),112(a)].

Compound No.	R	IC50 (µM)
		KB
112a	4-OCH3	15
112b	4-F	15
112c	4-Br	15
112d	4-Cl	31

[185].

Alswah (2017) et al., designed and synthesized a series of triazolo-quinoxaline-chalcone derivatives 113a–d, and evaluated their cytotoxic activity against three target cell lines: human breast adenocarcinoma (MCF-7), human colon carcinoma (HCT-116) and human hepatocellular carcinoma (HEPG-2) using the MTT assay method with doxorubicin as a reference drug. The initial results showed that some of the chalcones exhibited significant antiproliferative effects against most of the cell lines, with selective or non-selective behavior, with IC₅₀ values found to be in the 1.65 to 34.28 µM range. Compound 113(a) showed maximum cytotoxic activity towards given cell lines. The structure of chalcone-based triazolo-quinoxaline hybrids is given in Figure 108 and the in vitro cytotoxicity of synthesized hybrids against cancer cell lines are shown in

Table 95. In vitro cytotoxic activities of chalcone-based triazolo-quinoxaline hybrid compounds [113(a–d),113(a)].

Compound No.	R1	R2	R3	R4	R5	IC50 (µM)
						MCF-7	HCT-116	HepG-2
113a	H	H	OCH3	H	H	8.23	9.57	34.28
113b	H	H	H	H	H	58.92	57.49	105.21
113c	OCH3	H	H	H	H	24.04	25.60	30.72
113d	OH	H	H	H	H	22.17	19.76	17.32
Doxorubicin	-	-	-	-	-	0.27	1.55	0.22

[186].

Yepes et al. (2021) synthesized a new series of hybrid molecules by inclusion of two scaffolds: chalcone and melatonin. To achieve this goal, biologically active chalcone was attached via a non-hydrolizable thioalkyloxy linker to the corresponding melatonin bioisostere scaffold. The anticancer activity of the synthesized hybrids was evaluated against an in vitro model of colorectal cancer. In this study, two cell lines were used, i.e., the non-malignant (CHO-K1) and human colon adenocarcinoma cells (SW480). The reference drug used was 5-FU. Compound 114(a) maximum activity. The structure of chalcone based melatonin hybrids is given in Figure 109 and in vitro cytotoxicity of compounds 114(a–d) against cancer cell lines is shown in

Table 96. In vitro cytotoxic activities of chalcone based melatonin hybrid compounds [114(a–d),114(a)].

Compound No.	R	24 h		48 h
		IC50 (µM) SW480 Cells	IC50 (µM) CHO-K1 Cells	IC50 (µM) SW480 Cells	IC50 (µM) CHOK1 Cells
114a		1.26	6.40	0.24	0.16
114b		29.49	20.48	4.14	4.95
114c		30.46	10.12	2.43	1.13
114d		49.82	>100	1.77	10.61
5-fluorouracil	-	-	543.5	174.3	173.2

[187].

Ma et al. (2021) synthesized chalcone-based quinoxalin as anti-cancer hybrids. All of the synthesized compounds (115a–e) were tested in vitro for their anticancer activities against, PC12, BPH-1 and MCF-7 cells using the MTT assay. With IC₅₀ values in the micro molar range (9.1–98.7 mM) against all tested cancer cells, all synthesized compounds demonstrated moderate to good antiproliferative activity and compound 115(e) showed maximum cytotoxic activity. The structure of chalcone-based quinoxalin hybrids is given in Figure 110 and the in vitro cytotoxicity of compounds 115(a–e) against cancer cell lines is shown in

Table 97. In vitro cytotoxic activities of chalcone-based quinoxaline hybrid compounds [115(a–e),115(e)].

Compound No.	Ar	IC50 Value (mM)
		BPH-1	PC12	MCF-7
115a		48.5	42.1	72.4
115b		54.8	48.5	54.8
115c		64.2	73.1	80.9
115d		76.9	81.1	77.3
115e		10.4	16.7	9.1
Doxorubicin		14.1	22.5	9.2

[188].

3.19. Coumarin-Based Hybrids

The coumarin scaffold (2H-1-benzopyran-2-one) is extensive in nature, and it’s derivatives exhibit various antibacterial, antifungal, antimalarial, and anticancer pharmacological properties.

Kamaldeep Paul et al. (2013) synthesized coumarin-benzimidazole hybrids. The synthesized compounds showed potent activity against leukemia cancer cells (CCRF-CEM, HL-60(TB), K-562, RPMI-8226), colon cancer cells (HCT-116, HCT-15), melanoma cancer cells (LOX IMVI, UACC-257) and breast cancer cells (MCF7, T-47D). The compounds containing substitution (NR₁R₂) were the most potent with inhibition of most of the cell lines. The compound with ethanolamine as a substituent (NR₁R₂) at position 7 of the coumarin-benzimidazole scaffold, was the most potent synthesized compound. The structure of coumarin-benzimidazole hybrids is given in Figure 111 and the in vitro % growth inhibition of compounds 116(a–e) against cancer cell lines is shown in

Table 98. In vitro cytotoxic activities of coumarin-benzimidazole hybrid compounds 116(a–e).

Compound No.	Substitution		% Growth Inhibition	Maximal Inhibition
116a	NR1R2 = Ethylenediamine		20.5	Colon Cancer
			33.98	Breast Cancer
116b	NR1R2 = Ethanolamine		80.51	Leukemia
116c	NR1R2 = Morpholine		62.12	Breast Cancer
116d	NR1R2 = Methylpiperazine		56.39	Leukemia
116e	NR1R2 = 2-Aminoethylmorpholine		23.26	Small cell lung cancer
			22.91	CNS Cancer
5-FU			47.9	Leukemia

[189].

R. An et al. synthesized amide containing 1,2,3-triazole hybrids, which showed moderate to excellent activity against MDB-MB 231 cell lines under both normoxic and hypoxic conditions. The structure of coumarin-triazole hybrids is given in Figure 112 and the in vitro IC₅₀ of compounds 117 (a–e) against cancer cell lines are shown in

Table 99. In vitro cytotoxic activities of coumarin containing 1,2,3-triazole hybrid compounds 117(a–e).

Compound No.	Substitution	IC50(mM)		IC50 normoxia/IC50 hypoxia
		Hypoxia	Normoxic
117a		23.47	24.41	1.04
117b		75.21	73.77	0.98
117c		6.72	6.78	1.01
117d		0.03	1.34	46.31
117e		73.82	91.61	1.24
Dox		0.60	1.07	1.79

[190].

H.A. Elshemy et al., (2017) synthesized coumarin-chalcone hybrids having anticancer activity against HEPG2, leukemia, and WI-38 cell lines. All coumarin-chalcone hybrids had potent activity against HEPG2 and K562, and weak activity against WI-38 cell. Among the coumarin-chalcone hybrids, 4-methoxyphenyl chalcone 118(a) was more potent than 3,4-dimethoxyphenyl chalcone 118(b), which showed higher activity than chalcone with 3,4,5-trimethoxyphenyl moiety 118(c).

In the coumarin-acrylohydrazide series, compound 119(c) showed the highest activity against a leukemia cell line (k562) while compound (119a and 119b) showed potent activity against the HEPG2 cell line and moderate activity against the WI-38 cell line. The structure of coumarin containing chalcone hybrids is shown in Figure 113 and the in vitro IC₅₀ (µM) of compounds [118(a–c),119(a–c)] against cancer cell lines is shown in

Table 100. In vitro cytotoxic activities of coumarin containing chalcone hybrid compounds [118(a–c),119(a–c)].

Compound No.	Substitution			IC50 (µm)
	R1	R2	R3	HEPG2	Leukemia K562	WI-38
118a	H	OCH3	H	0.65	1.09	292.7
118b	OCH3	OCH3	H	0.82	O.93	33.09
118c	OCH3	OCH3	OCH3	2.02	1.36	9.55
119a	H	OCH3	H	0.77	3.96	48.4
119b	OCH3	OCH3	H	0.93	8.71	3.3
119c	OCH3	OCH3	OCH3	1.57	0.49	11.1
Cisplatin				2.56	2.02	NA

[191].

Mohit Sanduja et al. (2020) synthesized uracil-coumarin-based compounds as potent anticancer hybrid compounds. The uracil-coumarin hybrid compounds inhibited the MCF-7 cancer cell proliferation more effectively compared to standard 5-FU. The most potent compound 120b (GI₅₀ = 1.55µM) contained A with fluorine atom as R with two carbon chain lengths between triazole and coumarin moieties. The structure of coumarin containing uracil hybrids is given in Figure 114, and in vitro GI₅₀ (µM) of compounds 120(a–e) against cancer cell lines is shown in

Table 101. In vitro cytotoxic activities of coumarin-based uracil hybrid compounds 120(a–e).

Compound No.	Substitution (R)	n	MCF-7(GI50-µM)
120a	H	1	10.99
120b	F	1	1.55
120c	Cl	1	2.57
120d	Br	1	3.34
120e	I	1	3.96
5-FU			7.28

[192].

Zhuo zing et al., (2018) synthesized novel coumarin-based furoxin hybrids which had potent activity against Hela cell proliferation. The compound 121(e) had the highest antiproliferative activity and was more potent than standard, doxorubicin. The structure of coumarin containing furoxin hybrids is given in Figure 115 and the in vitro IC₅₀ of compounds 121(a–e) against cancer cell lines is shown in

Table 102. In vitro cytotoxic activities of coumarin-based furoxin hybrid compounds 121(a–e).

Compound No.	Substitution®	IC50 (µM)
121a		32.32
121b		4.85
121c		5.16
121d		5.95
121e		4.72
Dox		10.21

[193].

3.20. Nitrogen Mustard Based Hybrids

Shengtao Xu et al. (2014) synthesized natural oridonin bearing nitrogen mustard hybrid as potential anticancer compound. The hybrid showed activity against the multidrug resistant cell lines SW620/AD300 and NCL-H460/MX20. Compound 122(b) induced apoptosis and affected cell cycle progression in human hepatoma (Bel-7402) cells. The structure of nitrogen mustard contain oridonin hybrids is given in Figure 116 and the in vitro IC₅₀ of compounds 122(a–d) against the cancer cell line (Bel-7402) is shown in

Table 103. In vitro cytotoxic activities of nitrogen mustard-containing oridonin hybrid compounds 122(a–d).

Compound No.	Substitution (R1)	IC50 (µM) against Bel-7502
122a		8.35
122b		0.50
122c		1.43
122d		7.91
Chlorambucil		49.31

[194].

Laczkowski et al. (2016) synthesized nitrogen mustard-based thiazole hybrid drugs that showed antiproliferative activity against human cancer cell lines MV4-11, A549, MCF-7 and HCT-116. Among the all derivatives, 123(a–e) was most potent and exhibited activity against human leukemia (MV4-11) cells. The compounds 123(b) and 123(e) had potent activity against MCF-7 and HCT116. The structure of nitrogen mustard contain thiazole hybrids is given in Figure 117 and the in vitro IC₅₀ of compounds 123(a–e) against cancer cell lines is shown in

Table 104. In vitro cytotoxic activities of nitrogen mustard-containing thiazole hybrid compounds 123(a–e).

Compound No.	Substitution (Ar)	IC50 (µg/mL)	Log P
123a		3.63	6.15
123b		3.95	5.74
123c		3.29	6.80
123d		4.26	6.35
123e		2.17	6.53
Cisplatin		0.31	-

[195].

Kolesinska et al. (2012) synthesized hybrids containing a nitrogen mustard-triazine scaffold as potent anticancer compounds. These were prepared by rearrangement of mono, bis and tris-(1,3,5-triazin-2-yl)-1,4-diazacyclo [2.2.2] octanium chlorides leading to the formation of 2-chloroethylamino fragments attached to 1,3,5-triazine via one, two or three piperazine, respectively. Their cytotoxicity and alkylating activity depended on substituents on the triazine ring and nitrogen mustard. Among the above mentioned compounds, 124(b) and 124(d) were the most potent. The structure of nitrogen mustard-containing triazine hybrids is shown in Figure 118 and the in vitro IC₅₀ of compounds 124(a–d) against cancer cell lines is shown in

Table 105. In vitro cytotoxic activities of nitrogen mustard-containing triazine hybrid compounds 124(a–d).

Compound No.	Substitution (R)	IC50 (µg/mL)
		Breast Cancer (T47D)	Prostate Cancer (LNCaP)	Colorectal Cancer (SW707)
124a	CH3	27.86	15.78	32.98
124b	C6H5CH2	1.40	0.99	3.45
124c	CF3CH2	96.84	18.27	96.84
124d	C6H5	2.60	1.47	2.93

[196].

Acharya et al. (2017) synthesized androstene oxime-nitrogen mustard hybrids and nitrogen mustard conjugates of various steroidal oximes. The conjugation was achieved by oxime-ester linkage. The 17-E-steroidal oxime benzoic acid mustard ester, 3β acetoxy-17E-[p-(N,N-bis(2-chloroethyl)amino]benzoyloxiamino-androst-5ene 125(a) showed the highest growth inhibition on IGROV (ovarian cancer cells) with a GI₅₀ of 0.937 µg/mL. The structure of androstene oxime-nitrogen mustard hybrids is shown in Figure 119 and the in vitro GI₅₀ (µg/mL) of compounds (125(a-b),126(a-b)) against cancer cell lines is shown in

Table 106. In vitro cytotoxic activities of androstane oxime-nitrogen mustard hybrid compounds [125(a-b),126(a-b)].

Compound No.	Substitution (n)	GI50 (µM)
		K-562	HL-60
125a	0	3.90	7.01
125b	1	13.6	36.0
126a	0	12.4	32.2
126b	1	40.1	61.2

[197].

3.21. Pyrazole-Based Hybrids

Hassan et al. (2021) synthesized indole-pyrazole hybrids as anticancer agents. The newly synthesized hybrids were screened for their cytotoxicity activities in vitro against four human cancer types, i.e., colorectal (HCT-116), breast carcinoma (MCF-7), liver carcinoma (HepG2) and lung carcinoma (A549). Among all indole-pyrazole hybrids, 126a and 126b showed excellent anticancer activity against the HepG2 cancer cell line with an IC₅₀ of 6.1 and 7.9 µM, respectively. The structure of the pyrazole-based indole hybrid is given in Figure 120 and the in vitro IC₅₀ of compounds 127(a–e) against cancer cell lines are shown in

Table 107. In vitro cytotoxic activities of pyrazole-based indole hybrid compounds 127(a–e).

Compound No.	Substitution		IC50(µM)
	Ar	Ar1	HCT-116	MCF-7	HepG2	A549
127a	C6H5	C6H5	17.4	10.6	6.1	23.7
127b	4-CH3-C6H4	C6H5	19.6	14.5	7.9	14.1
127c	H	4-CH3-O-C6H4	31.9	22.2	35.8	43.4
127d	C6H5	4-CH3-O-C6H4	25.3	17.4	27.2	58.7
127e	4-CH3-C6H4	4-CH3-O-C6H4	37.4	16.2	25.8	40.8
Dox	-	-	40.0	64.8	24.7	58.1

[198].

Somaia et al. (2015) synthesized benzofuran-pyrazole-based hybrids as anticancer drugs. The newly synthesized compounds showed remarkable growth inhibitory activity against leukemia (CCRF-CEM), A549 (lung carcinoma), and HCT-116 (colorectal carcinoma) cells. Compound 128c showed good src inhibition activity at 10 µM and was found to be most potent compound. The structure of benzofuran-pyrazole hybrids is given in Figure 121 and in vitro IC₅₀ of compounds 128(a–d) against cancer cell lines is shown in

Table 108. In vitro cytotoxic activities of benzofuran-pyrazole-based hybrid compounds 128(a–d).

Compound No.	Substitution (R)	Cell Growth Promotion Percentage (At 10−5 M Concentration)
		CCRM-CEM	A549/ATCC	HCT-116
128a	4-Cl-C6H4-	-	-	-
128b	Cyclohexyl	-	-	-
128c	2-pyroryl	90.65	62.56	27.86
128d	2-furyl	78.58	93.57	74.8

[199].

Abdelaal et al. (2021) synthesized quinazoline-pyrazole hybrids as potential antiproliferative agents. The newly synthesized compounds showed activity against hepatocellular carcinoma (liver) HEPG2, mammary gland (breast) MCF-7 and colon cancer HCT-116 cells. The structure of quinazoline-pyrazole hybrids is given in Figure 122 and the in vitro IC₅₀ of compounds (129–133) against cancer cell lines is shown in

Table 109. In vitro cytotoxic activities of quinazoline-pyrazole-based compounds (129–131) and pyrazole derivatives (132–133).

Compound No.	IC50 (µM)
	HePG2	HCT-116	MCF-7
129	65.14	45.95	79.88
130	4.99	6.83	4.63
131	16.18	11.78	18.72
132	6.85	3.46	9.07
133	73.11	88.60	69.16

[200].

Washim Akhtar et al. (2021) synthesized 15 novel pyrazoline-pyrazole hybrids, all of which showed activity in the MTT growth inhibition assay against five cancer cells lines: MCF-7, A549, SiHa, COLO205 and HePG2 cells. Compound 134 (b) was found to active against A549, SiHa, COLO205 and HePG2 cell lines with IC₅₀ values of 4.94, 4.54, 4.98 and 2.09 µM, respectively, and was non-toxic against normal cells. The structure of pyrazoline-pyrazole hybrids is given in Figure 123 and in vitro IC₅₀ of compounds 134(a–e) against cancer cell lines is shown in

Table 110. In vitro cytotoxic activities of pyrazoline-pyrazole-based hybrid compounds 134(a–e).

Compound No.	Substitution		IC50 (µM)
	R	R1	MCF-7	A549	SiHa	COLO205	HepG2
134a	4-Cl	4-OCH3	31.38	15.30	4.54	12.95	>50
134b	H	4-OCH3	27.37	4.94	4.58	4.86	2.09
134c	H	4-F	29.31	10.27	5.67	6.69	8.56
134d	H	4-Cl	30.05	11.50	6.40	7.16	>50
134e	H	H	>50	26.25	>50	31.27	>50
5-FU			2.08	4.35	5.78	4.00	19.01

[201].

Garima Verma et al. (2018) synthesized pyrazole acrylic acid-based oxadiazole and amide hybrids as anticancer and antimalarial agents. The anticancer activity of newly synthesized compounds was measured using the sulforhodamine B assay. Compounds 135(a–c) demonstrated promising results against all tested cell lines. The non-cyclized compounds (amide derivatives) favored anticancer activity. The structure of pyrazole acrylic acid based oxadiazole hybrids is shown in Figure 124 and the in vitro IC₅₀ of compounds 135(a–c) against cancer cell lines is shown in

Table 111. In vitro cytotoxic activities of pyrazole acrylic acid-based oxadiazole hybrid compounds 135(a–c).

Compound No.	Substitution		IC50 (µM)
	R	X	HCT-116	SW-620	MCF-7	HT-29
135a	4-CH3		1.8	3.6	2.0	4.4
135b	4-CH3		10	>10	10.9	8.5
135c	H		1.9	5.0	6.4	4.6
Paclitaxel			0.004	0.006	0.006	0.005

[202].

3.22. Pyridine-Based Hybrids

Chetan B. Sangani et al. (2014) synthesized biquinoline-pyridine hybrids as potential EGFR and HER-2 kinase inhibitors by base catalyzed cyclocondensation through one potent multicomponent reaction. All the synthesized compounds were tested against A549 (adenocarcinomic human alveolar basal epithelial) and Hep G2 (liver cancer) cell lines. Enzyme inhibitory activity was measured against HER-2. Compound 136(d) was found to be most potent compound. The structure of biquinoline-pyridine hybrids is given in Figure 125 and the in vitro IC₅₀ of compounds 136(a–e) against cancer cell lines is shown in

Table 112. In vitro cytotoxic activities of biquinoline-pyridine hybrid compounds 136(a–e).

Compound No.	Substitution		IC50 (µM)
	R1	R2
136a	H	CN	4.8
136b	OCH3	CN	5.4
136c	H	COOMe	1.1
136d	CH3	COOEt	0.09
136e	OCH3	COOEt	0.2
Erlotinib			0.03

[203].

W.M. Eldehna et al. (2015) synthesized isatin-pyridine hybrids as potential antiproliferative agents. They showed antiproliferative activity against hepatocellular carcinoma (HEPG2), lung cancer (A549) and breast cancer (MCF-7) cell lines. Compound 137 showed the most potent activity against HEPG2 with an IC₅₀ of 2.5 µM, while compound 138(c) was the most potent compound against A549 and MCF-7 cell line with IC₅₀ values of 10.8 and 6.3 µM, respectively. The structure of isatin-pyridine hybrids is shown in Figure 126 and the in vitro IC₅₀ of compounds [137,138(a–d)] against cancer cell lines is shown in

Table 113. In vitro cytotoxic activities of isatin-pyridine hybrid compounds [137,138(a–d)].

Compounds No.	Substitution (X)	IC50 (µM)
		HepG2	A549	MCF-7
138a	H	NA	16.8	14.7
138b	F	11.5	19.7	10.4
138c	Cl	8.7	10.8	6.3
138d	Br	59.1	85	14.9
Dox		6.9	7.6	6.1

[173].

E.K. Hamza et al. (2020) synthesized pyrazolo [3,4] pyridine hybrids as potential anticancer agents. The activity of the newly synthesized compounds was evaluated in vitro against two human cancer cell lines (HCT116 and MCF-7). Compounds 139(a–d) showed activity against HCT116 and compounds 140(a–d) showed activity against MCF-7 cancer cell lines. The structure of pyrazolo [3,4] pyridine hybrids is shown in Figure 127 and the in vitro IC₅₀ (µM) of compounds [139(a–d),140(a–d)] against cancer cell lines is shown in

Table 114. In vitro cytotoxic activities of pyrazolo [3,4] pyridine hybrid compounds [139(a–d),140(a–d)].

Compound No.	Substitution (Ar)	IC50 (µM)
		HCT116 *	MCF-7 **
139a, 140a		6.4	70.9
139a, 140b		19.8	55
139c, 140c		8.1	48.7
139d, 140d		9.8	32.6
Dox		9.5	65.6

* 139(a–d), ** 140(a–d).

[204].

4. Conclusions

For a very long time, medicinal chemists from all over the world have been trying to develop new and effective cancer treatments. Combination therapy and hybrid chemotherapeutics haves become more common, because a complex disease such as cancer cannot be properly treated with a single drug. This study presents rational approaches behind the design of anticancer agents employing molecular hybridization. This method has potential because it combines two moieties to create new molecular scaffolds. Molecular hybridization offers a wide range of applications since it can produce compounds with distinct and/or multiple modes of action and minimal side effects. The few examples included in this article are not intended to be an exhaustive collection of anticancer hybrids, but to provide a quick explanation of the idea and its potential uses for researchers working in this field.