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Impact of the Enhanced Permeability and Retention (EPR) Effect and Cathepsins Levels on the Activity of Polymer-Drug Conjugates

Reading School of Pharmacy, University of Reading, Whiteknights, Reading RG6 6AD, UK
Author to whom correspondence should be addressed.
These authors contributed equally to this work.
Polymers 2014, 6(8), 2186-2220;
Original submission received: 19 February 2014 / Revised: 28 June 2014 / Accepted: 14 July 2014 / Published: 20 August 2014
(This article belongs to the Special Issue Polymers for Drug Delivery)


Polymer-drug conjugates have demonstrated clinical potential in the context of anticancer therapy. However, such promising results have, to date, failed to translate into a marketed product. Polymer-drug conjugates rely on two factors for activity: (i) the presence of a defective vasculature, for passive accumulation of this technology into the tumour tissue (enhanced permeability and retention (EPR) effect) and (ii) the presence of a specific trigger at the tumour site, for selective drug release (e.g., the enzyme cathepsin B). Here, we retrospectively analyse literature data to investigate which tumour types have proved more responsive to polymer-drug conjugates and to determine correlations between the magnitude of the EPR effect and/or expression of cathepsin B. Lung, breast and ovarian cancers showed the highest response rate (30%, 47% and 41%, respectively for cathepsin-activated conjugates and 31%, 43%, 40%, across all conjugates). An analysis of literature data on cathepsin content in various tumour types showed that these tumour types had high cathepsin content (up to 3835 ng/mg for lung cancer), although marked heterogeneity was observed across different studies. In addition, these tumour types were also reported as having a high EPR effect. Our results suggest that a pre-screening of patient population could bring a more marked clinical benefit.

1. Introduction

Polymer-drug conjugates (PDCs) are nano-sized drug delivery systems, in which one or more chemotherapeutic agent is covalently linked to a water-soluble polymer (Figure 1a) [1,2]. The main rationale for PDCs stems from their ability to passively accumulate into the tumour tissue by means of the enhanced permeability and retention (EPR) effect (Figure 1b) [3]. This is a unique feature of tumour vasculature, which makes the tumour vasculature hyper-permeable to macromolecules compared to normal vasculature. As a result, PDCs have been shown to: (a) selectively target tumour tissues; (b) be less toxic and display an extended half-life compared to the parent free drug (e.g., maximum tolerated dose (MTD) for N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-doxorubicin (320 mg/m2) is four-fold higher than for the parent compound doxorubicin (60 mg/m2)) [4].
Figure 1. (a) Schematic representation of a polymer-drug conjugate (PDC); (b) EPR effect facilitating passive tumour targeting of PDCs; and (c) Lysosomotropic delivery of PDC.
Figure 1. (a) Schematic representation of a polymer-drug conjugate (PDC); (b) EPR effect facilitating passive tumour targeting of PDCs; and (c) Lysosomotropic delivery of PDC.
Polymers 06 02186 g001
Nineteen PDCs have undergone/are undergoing clinical evaluation (Table 1 and Figure 2). Clinical trials have shown evidence of tumour responses to PDC to various degrees, and some conjugates (e.g., poly-l-glutamic acid (PGA)-paclitaxel) have progressed to Phase III trials [5,6]. In spite of several advantages of PDCs over the relevant parent drugs, such as a more favourable toxicity profile, superior quality of life and a significant survival rate [7], to date, no such conjugate has entered the market place, which suggests that system design and/or patient selection are still suboptimal.
When looking at possible reasons for the slow progression of the development of this technology, various factors need to be considered. First, PDCs are intrinsically more complex systems than low molecular weight drugs, for instance they have an inherent degree of variability due to the polydispersity of the carrier. In addition, and of greater significance, the anti-tumour activity of PDCs relies on two factors: (a) passive tumour accumulation via the EPR effect; (b) drug release following a biological stimulus (e.g., enzyme or pH) (Figure 1c). Failure to identify the correct patient/tumour type population, which displays a sufficient level of EPR effect and a sufficient level of enzymes, would negatively bias the results obtained with this technology.
Many studies have reported evidence for the EPR effect in a variety of solid tumours [8,9,10,11] but the factors that affect the magnitude of such an effect are still unclear. Recent studies have considered different in vivo tumour models and these have reported that both the size and tumour type can affect the magnitude of the EPR effect [12,13]. A discussion group constituted by world experts of the EPR effect has looked at the heterogeneity of such a phenomenon and has identified its key influencing factors, mainly: (a) the nature of the vascular bed and stroma, including the presence or absence of lymphatics; (b) tumour size, type and location; and (c) patient characteristics such as age, gender, body composition, and treatment [10,13,14]. With regards to expression of the various activating enzymes, again the situation is heterogeneous and unclear. It is well known that enzymes that have been exploited for activation of PDCs, such as cathepsin B, are expressed in tumour tissues [2,13,15,16,17]. However, it has been recently noted that various factors, such as hormone levels, can affect the abundance of such enzymes. For instance, cathepsin B levels are higher in pre-menopausal women than in post-menopausal women as the former have higher oestrogen levels than the latter [5,18,19].
Table 1. Current clinical status of polymer-drug conjugates (PDCs) and types of release triggers.
Table 1. Current clinical status of polymer-drug conjugates (PDCs) and types of release triggers.
Release TriggerCode/Product NameCompositionLinker/SpacerStatusReference
CathepsinsFCE28068/PK1HPMA copolymer-doxorubicinAmide/Peptide dPhase II[4,20]
CathepsinsFCE28069/PK2HPMA copolymer-doxorubicin-galctosamineAmide/Peptide dPhase I[21]
CathepsinsDE-310Carboxymethyldextran-exatecanAmide/Peptide dPhase I[22]
CathepsinsDelimotecan (MEN 4901/T-0128)Carboxymethyldextran-T2513TriglycinePhase I[23]
Esterases/Acid hydrolysis a Cathepins bCT-2106/PGA-CPTPGA-camptothecinEsterPhase I[24]
Esterases/Acid hydrolysis a Cathepins bCT-2103/PGA-PTX XYOTAX™/OPAXIO®PGA-paclitaxelEsterPhase III[5,6,19,25,26,27,28,29,30,31,32,33,34]
Acid hydrolysis/Cathepsins cAP5280HPMA copolymer-carboplatinateAminomalonate/Peptide dPhase I/II[35]
Acid hydrolysis/Cathepsins cAP5346/ProLindac®HPMA copolymer-DACH oxiplatinateAminomalonate/Peptide ePhase I[36]
Hydrolysis/EsterasesPNU166945/HPMA-PTXHPMA copolymer-paclitaxelEsterPhase I discontinued[37]
Hydrolysis/EsterasesPNU166148//HPMA-CPT/MAG-CPTHPMA copolymer-camptothecinEsterPhase I discontinued[38,39,40]
Hydrolysis/EsterasesEZN246/PEG-CPT/Pegamotecan/Prothecan™PEG-camptothecinEsterPhase II discontinued[41,42,43]
Hydrolysis/EsterasesPEG-PTXPEG-paclitaxelEsterPhase I discontinued[44]
Hydrolysis/EsterasesEZN-2208/PEG-SN-38PEG-SN-38GlycinamidoesterPhase II[45,46]
Hydrolysis/EsterasesNKTR-102PEG-IrinotecanGlycinamidoesterPhase II/III[47,48,49,50]
Hydrolysis/EsterasesNKTR-105PEG-Docetaxel-Phase I[51]
Hydrolysis/EsterasesXMT-1001PHF-camptothecinSuccinamidoesterPhase I[52]
EsterasesCRLX101/IT-101Cyclodextrin-camptothecinGlycinamidoesterPhase II[53]
pH-sensitiveONCOFID-PTMHA-paclitaxelHydrazonePhase I/II[54]
AD-70, DOX-OXDOxidised dextran-doxorubicinSchiff’s basePhase I discontinued[55]
Notes: a Mediates the cleavage of the active drug from the polymeric backbone; b Mediates the cleavage of the biodegradable polymeric backbone; c Mediates the cleavage of the peptide linker/spacer; d Spacer-Gly-Phe-Leu-Gly; e Spacer-Gly-Gly-Gly. HPMA, N-(2-Hydroxypropyl) methacrylamide; PGA, Poly-l-glutamic acid; PEG, Polyethylene glycol; PHF, Poly(1-hydroxyl-methylethylene hydroxyl-methyl-formal); HA, Hyaluronic acid.
In this paper, we retrospectively analyse data obtained from the literature concerning clinical trials carried out on PDCs in order to determine whether there is a connection between the clinical responses of various tumour types and the levels of enzyme expression/magnitude of the EPR effect in such tumour types. First, we analysed clinical data within the literature to identify the tumour types for which marked tumour responses were observed. Then, we documented the content of cathepsin B that has been reported for the various tumour types. Finally, we determined the extent of the EPR effect, which has been, reported for these various tumour types.
Figure 2. Chemical structures of the PDCs discussed in this article. (a) PDCs for which the release of the drug is triggered via an enzymatic mechanism; and (b) PDCs for which the release of the drug is triggered by a non-enzymatic mechanism. Note: The chemical structure of NKTR-105 is not available.
Figure 2. Chemical structures of the PDCs discussed in this article. (a) PDCs for which the release of the drug is triggered via an enzymatic mechanism; and (b) PDCs for which the release of the drug is triggered by a non-enzymatic mechanism. Note: The chemical structure of NKTR-105 is not available.
Polymers 06 02186 g002

2. Experimental Section

The data presented in this paper were collected via systematic literature searches using search engines such as PubMed, Science Direct and Web of Science. No date restrictions were applied to the searches.

2.1. Terminology

This paper reports data collected from various sources that are derived from both clinical and pre-clinical studies. Different studies used different terminologies when referring to the various cancer types, with the terminologies ranging from technical/very specific (e.g., glioblastoma) to more general (e.g., brain cancer). In addition, in some cases, the effects of the tumours on adjacent organs are reported (e.g., gastroesophageal cancer). To ensure that a consistent terminology is used within this paper, clinical tumour types and preclinical models were grouped under general umbrella terms summarised in Table 2.
Table 2. Summary of the terminology used in this article to identify various tumour types/tumour models.
Table 2. Summary of the terminology used in this article to identify various tumour types/tumour models.
Tumour Type
(Terminology Used in This Article)
Tumour Type
(Terminology Used in the Original Reference)
BreastBreast, mammary gland, MCF-7, MDA-MB-231, BT 20 and DU4476-cell lines, Walker 256-murine model, MX-1, MAXF 449-human xenograft.
Colon and/or rectumColon, rectum, colorectal, anus, C26 NL-17, HT-29, LS174T-human xenograft.
Head/Neck and brainBrain, glioblastoma, head/neck, thyroid, salivary gland, follicular, papillary, tongue, maxillary sinus, parotid gland, IMR-32, SK-N-SH, SK-N-DZ- human xenograft.
LungNon-small cell lung cancer, small cell lung cancer, bronchial, Meta-7-murine model, H522, COR L23-human xenograft.
Oesophagus, stomach and intestineOesophageal, cardioesophageal, stomach, gastroesophageal, gastrointestinal, intestine, small bowel, peritoneal carcinosis, OCUM-2MLN-human xenograft.
OvaryOvarian, Oca-1-murine model, A2780 cell line/human xenograft, SK-OV-3, OVCAR-3-human xenograft.
PancreasPancreatic, PAXF 546-human xenograft.
SkinMelanoma, basal cell, histosarcoma, B16F10, A431-murine model, MEXF 276-human xenograft.
UrinaryBladder, urinary tract, urethal, urothelial, urachus.
The term indicates the tumour types which were either studied in a very low sample size (n < 3) and/or those for which low responses were observed.
Adrenal, adenoid cystic, adenocarcinoma (unknown primary), bone (ewing sarcoma, osteosarcoma), cervix (uterine, leiomysarcoma uteri, ME180-human xenograft), fibrosarcoma (S-180, Meth A-murine model), gall bladder, kidney, leiomysarcoma, liver (cholangiocarcinoma, ampullary, bile duct, L1210-murine model, VX-2 carcinoma), lymphoma, mesothelioma, neuroendocrine, prostrate, sarcoma (unknown), soft tissue sarcoma, squamous cell sarcoma (unknown origin), solid tumours (unknown), unknown primary tumours, MAC 15A-murine model, MAC 26-murine model, RXF 486, RXF 1220, ME 180-human xenograft.

2.2. Clinical Status of the PDCs

The data on the current clinical status of the PDCs, reported in Table 1, were gathered from articles obtained using the key word “polymer-drug conjugate*”. The clinical data related to each individual conjugate, which are reported in the supplementary information, were gathered using the name of the individual conjugate.
The percentage tumour response reported for each conjugate, per tumour type, was determined considering all of the clinical responses observed (i.e., number of (partial response (PR) + complete response (CR) + stable disease (SD) + minor responses (MR)), as well as the total number of patients evaluated per tumour type using the following formula:
Response to conjugate A in tumour X (%) = (Total clinical responses observed for conjugate A in tumour X/Number of patients evaluated for conjugate A in tumour X) × 100
The results obtained for each conjugate for each tumour type (reported in supplementary information, Tables S1 to S19) were then further processed in order to obtain an overall response to all PDCs per tumour type.
The overall percentage tumour response for each tumour type was determined using the following formula:
Overall response in tumour X (%) = Σ[(Response to conjugate A in tumour X (%) × number of patients with tumour X in which conjugate A was tested) + (Response to conjugate B in tumour X (%) × number of patients with tumour X in which conjugate B was tested) + (Response to conjugate n in tumour X (%) × number of patients with tumour X in which conjugate n was tested)]/Total number of patients with tumour X

2.3. Cathepsin Level

The data on cathepsin levels in various tumour types were collected via literature searches using the following key words and their combinations: “cathepsin level*”, “cathepsin content*”, “tumour/tumor types”.

2.4. EPR Effect

The data related to the EPR effect in different tumour types were retrieved via literature searches using the following key words (and their combinations): “enhanced permeability and retention effect”, “EPR effect”, “tumour/tumor*”, “drug accumulation”, “biodistribution” and the names of the individual conjugates. In addition, original research articles quoted in reviews related to the EPR effect, which were not identified from the general search, were also considered.
Priority was given to articles related to PDCs, however, as the EPR effect is a phenomenon that applies to any macromolecular system, clinical and preclinical data relating to other such systems (e.g., liposomes and micelles) were also included.

3. Results and Discussion

Many clinical studies have been carried out on PDCs (e.g., OPAXIO) [26]. The main clinical outcomes have been summarized in extensive reviews [55,56,57], but, as yet, a detailed analysis of which tumour types are proving more responsive to this type of drug delivery technology is still missing. A number of studies have already attempted to identify correlations between tumour types and enzyme content or the magnitude of the EPR effect, but this approach has been largely carried out in preclinical models and systematic assessments of these factors and clinical activities are missing, which is surprising since the latter are two key pre-requisites for the activity of PDCs.
In an attempt to provide a more comprehensive study that links clinical outcomes, enzyme content, and the EPR effect, we initially documented all the PDCs that have been explored in clinical trials, alongside the triggers that have been used to promote drug release in each (Table 1). Of the 19 conjugates that have been tested in patients, the vast majority (17) rely, at least partially, on the presence of enzymes for drug release (Table 1). Of the various enzymes exploited as triggers, cathepsins, a family of lysosomal proteases, are the most widely targeted (eight of the conjugates reported in Table 1 rely on cathepsins action for drug release). This is not surprising, as cathepsins have been reported to be linked with cancer progression. In particular, cathepsin B has been connected with tumour invasion [58].

3.1. Effect of Tumour Type on Clinical Response

To assess the clinical responses observed in different tumour types, we analysed clinical data for PDCs retrieved from our literature searches. Although the purpose of phase I, II and III clinical trials and their patient selection criterias are different, the objective responses (i.e., partial response (PR), complete response (CR), stable disease (SD) and minor responses (MR)) are considered as the end point in all such studies [59]. Therefore for each conjugate, evidence of a clinical response (PR, CR SD and MR) in the various tumour types has been summarised in detail in Tables S1–S19 in the supplementary information. A representative data set with clinical responses for the top three highest responsive tumour types for each conjugate has been provided in Table 3. For example, in the case of HPMA copolymer-doxorubicin (PK1; FCE28068), the top three highest responsive tumour types are lung, breast, colon and/or rectum, with tumour responsive rate of 57%, 53%, and 4%, respectively calculated from both phase II clinical trials and I. Likewise, the top three highest responsive tumour types based on the calculation from all the available clinical phase trials have been summarised for the other eighteen PDCs.
Table 3. Clinical data for the top three tumour types for the nineteen PDCs that are released by both cathepsins and change in pH.
Table 3. Clinical data for the top three tumour types for the nineteen PDCs that are released by both cathepsins and change in pH.
Tumour TypeNo. of Patients per Tumour Total a [Ph I/Ph II/Ph III]Clinical Responses b Total (Ph I/Ph II/Ph III)Tumour Response Rate c (%) Total [Ph I/Ph II/Ph III]
No. of SDNo. of PRNo. of MRNo. of CRNo. of OSNo. of NR
HPMA copolymer-doxorubicin (PK1; FCE28068) [4,20]
Lung31[(2/29(21) */-]8[-/8/-]5[2/3/-]---10[0/10/-]57[100/52/-]
Breast20[3/17(14) */-]5[-/5/-]3[-/3/-]1[1/-/-]--8[2/6]53[33/57/-]
Colon and/or rectum24[8/16/-]--1[1/-/-]--23[7/16/-]4[4/-/-]
HPMA copolymer-doxorubicin-galactosamine (PK2; FCE28069) [21]
Colon and/or rectum6[6/-/-]-----6[6/-/-]0
Carboxymethyldextran-exatecan; DE-310 [22]
Adenocarcinoma (Unknown primary)2[2/-/-]1[1/-/-]--1[1/-/-]-0100[100/-/-]
Delimotecan; MEN 4901/T-0128 [23]
Head/Neck and brain2[2/-/-]-1[1/-/-]---1[1/-/-]50[50/-/-]
Colon and/or rectum7[7/-/-]-1[1/-/-]---6[6/-/-]14[14/-/-]
Poly-l-glutamic acid-camptothecin; PGA-CPT; CT-2106 [24]
Poly-l-glutamic acid-paclitaxel; PGA-PTX; CT2103; XYOTAX; OPAXIO® [5,6,19,24,25,26,27,28,29,30,31,32,33]
HPMA copolymer-carboplatin; HPMA-carboplatin; AP5280 [35]
Colon and/or rectum12[12/-/-]2[2/-/-]----10[10/-/-]17[17/-/-]
HPMA copolymer-platinate; HPMA-Pt; AP5346 [36]
Oesophagus, stomach and intestine1[1/-/-]1[1/-/-]----0100[100/-/-]
Polyethylene-camptothecin; PEG-CPT; EZN246; Pegmaotecan; Prothecan™ [41,42,43]
Oesophagus, stomach and intestine42[7/35/-]14[-/14/-]5[-/5/-]3[2/1/-]--20[5/15/-]52[29/57/-]
Unknown primary5[5/-/-]--1[1/-/-]--4[4/-/-]20[20/-/-]
Multiarm-polyethylene-SN38; EZN-2208 [45,46]
Oesophagus, stomach and intestine3[3/-/-]2[2/-/-]----1[1/-/-]67[67/-/-]
PHF-CPT; MER-1001; XMT-1001 [52]
Solid tumours (Unspecified)8[8/-/-]3[3/-/-]----5[5/-/-]38[38/-/-]
Cyclodextrin-camptothecin; CRLX101; IT-101 [53]
Solid tumours *35[35/-/-]28[28/-/-]----7[7/-/-]80[80/-/-]
HA-paclitaxel (ONCOFID-P™) [54]
Oxidized dextran-Dox; OXD-DOX (AD-70) [55]
Colon and/or rectum6[6/-/-]1[1/-/-]----5[5/-/-]17[17/-/-]
Oesophagus, stomach and intestine2[2/-/-]-----2[2/-/-]0
HPMA copolymer-paclitaxel; HPMA-PTX; PNU166945 [37]
Solid tumours 112[12/-/-]2[2/-/-]1[1/-/-]---9[9/-/-]25[25/-/-]
HPMA copolymer-camptothecin; HPMA-CPT; PNU166148 [38,39,40]
Solid tumours 240[40/-/-]5[5/-/-]-1[1/-/-]--34[34/-/-]15[15/-/-]
Polyethylene glycol-paclitaxel; PEG-PTX [44]
Solid tumours 313 NA NCNC
Multi-arm-polyethylene glycol-pacliaxtel; PEG-PTX; NKTR-102 [47,48,49]
Solid tumours 4125[32/68/-]28[-/28/-]24[7/17/-]6[6/-/-]2[2/-/-]-65[8/23/-]48[47/66/-]
Multi-arm-polyethylene glycol-docetaxel; NKTR-105 [51]
Notes: a Indicates total number of patients considered for the clinical evaluation; b Clinical responses includes the total of SD, Stable disease; PR, Partial response; MR, Minor response; CR, Complete response; OS, Overall survival; NR, No response; c Tumour response rate (%) is the added responses (SD + PR + MR + CR + OS) per tumour type. NA, Not available, NC, Not calculated. * Indicates number of patients evaluated for the tumour responses. The term “Solid tumours” has been mentioned wherever the clinical responses per tumour type have not been reported in the respective study. 1 Solid tumours includes (number of patients is denoted in the brackets for those that are available in the study)-Ovary (4); Breast (2); Colon and/or rectum (2); Lung (1) and others (3); 2 Solid tumours includes-Colon and/or rectum (18); Ovary (2); Oesophagus, stomach and intestine (4); Unknown primary (4); Head/Neck and brain (2); Lung (6); Kidney (3); Adrenal (1); Cervix (1); Bone (1); Mesothelioma (1); Prostate (1) and Sarcoma (3); 3 Solid tumours includes-Colon and/or rectum (3); Breast (2); Neuroendocrine (2); Lung (1); Prostate (1) and Others (4); 4 Solid tumours includes-Ovary; Breast; Adrenal; Oesophagus, stomach and intestine; Lymphoma; Lung; Cervix; Head/Neck and brain; Urinary and Breast.
Finally, summary graphics assembling the percentage responses per tumour type for all conjugates were compiled (Figure 3 and Table 4). Figure 3 and Table 4 shows the percentage responses observed for cathepsin-activated conjugates. Breast, ovary and lung cancer were the cancer types in which the highest response rates were observed (>30%). However, the responses observed for the various conjugates in each tumour type were variable, for example, the HPMA copolymer-oxaliplatin conjugate (AP5346/ProLindac) showed no responses in breast cancer while the HPMA copolymer-doxorubicin conjugate (PK1) showed 53% response in that tumour. This variability might be due to the fact that the latter relies solely on cathepsins for drug release, and hence has a more specific activation than the former, for which both cathepsins and acid hydrolysis play a role in the release of oxaliplatin. This finding however can also be affected to some extent by the nature of the therapeutic agent.
Figure 3. Graphical representation of the tumuor response rates for PDCs (release mediated only by cathepsin).
Figure 3. Graphical representation of the tumuor response rates for PDCs (release mediated only by cathepsin).
Polymers 06 02186 g003
Table 4. Tumour response rates for PDCs (release mediated only by cathepsin).
Table 4. Tumour response rates for PDCs (release mediated only by cathepsin).
Tumour TypeTumour response rate (%)
PDC BreastOvaryLungPancreasSkinOesophagus, Stomach and IntestineUrinaryHead/Neck and BrainColon and/or Rectum
Normalised Average *474130272118171312
Notes: * Averages were normalised to take into account of the number of patients evaluated for each conjugated, see experimental section; “-” indicates that the conjugate was not evaluated against the particular tumour type.
Figure 4 and Table 5 show the result for all conjugates, independent of their activation mechanism. The results are similar to that observed for cathepsin-activated PDCs, with breast, ovary and lung cancer still achieving >30% responses. However, urinary cancer and cancers of the oesophagus, stomach and intestines showed a high response as well (48% and 34%, respectively).
Figure 4. Graphical representation of the tumuor response rates for PDCs (release mediated both by cathepsin and pH).
Figure 4. Graphical representation of the tumuor response rates for PDCs (release mediated both by cathepsin and pH).
Polymers 06 02186 g004

3.2. Cathepsin Levels in Different Tumour Types

Having assessed which tumour types showed the highest responses to PDCs, we investigated which tumour types had been reported to express the highest levels of cathepsins.
Cathepsins are lysosomal cysteine proteases involved in the bulk degradation of intracellular and endocytosed proteins. Cathepsin B is a large lysosmal protease that is generally over expressed in tumour tissues and plays a very important role in the release and activation of PDCs. Several studies have indicated that its co-existing isoenzymes such as cathepsin D, L, H, S and X are also overexpressed in tumour tissues and are thought to play a role in proteolysis [60,61,62,63]. Some studies have been carried out to elucidate precisely which isoenzyme contributes to drug release. For example, extensive studies carried out in the 80s have identified cathepsin B as the primary enzyme responsible for doxorubicin release from the HPMA copolymer-Dox conjugate, when a Gly-Phe-Leu-Gly (GFLG) linker was used. However, the release mechanism for other conjugates has not been investigated so extensively. Also, it is known that various isoenzymes can contribute to drug release from peptidyl linkers, for example, both cathepsin L and cathepsin B were found to contribute to the release of daunorubicin when GPLG and GPPL linkers were used [64]. Similarly, both cathepsin B and cathepsin H contributed to the release of 5-FU, albeit to a different extent [65]. The level of expression of these cathepsins in tumour tissues (especially cathepsin B) is very crucial for the selective release of the drug from the PDC within the tumour environment (Table 1). Also, several studies have indicated that cathepsin expression in tumours varies greatly [63] which is affected by various parameters including gender, age and hormone levels [14,18]. Therefore, to determine if there is a correlation between the level of cathepsin expression in a tumour type, and the clinical response to cathepsin-activated PDCs, data on cathepsin levels is presented for clinical as well as for in vivo and in vitro samples of different tumour types to obtain reliable results (Table 6). The information on tumour grades and other parameters that are related to the cathepsin’s expression were limited in the original studies, hence these details have not been considered in our present study.
Table 5. Tumour response rates for PDCs (release mediated both by cathepsin and pH).
Table 5. Tumour response rates for PDCs (release mediated both by cathepsin and pH).
Tumour typeTumour response rate (%)
PDC UrinaryBreastOvaryOesophagus, Stomach and IntestineLungSkinPancreasColon and/or RectumHead/Neck and Brain
PNU166945 a
PNU166148 a
Normalised average *48434034312118169
Notes: * Averages were normalised to take into account of the number of patients evaluated for each conjugated, see experimental section; “-” indicates that the conjugate was not evaluated against the particular tumour type; a The clinical responses for this conjugate are considered under the tumour type “others” as the responses per specific tumour type has not been mentioned in the original references [36,37,38,39]. NC: Not considered.
Table 6. Cathepsin levels found in clinical and preclinical sample of different tumour types (clinical studies highlighted in grey).
Table 6. Cathepsin levels found in clinical and preclinical sample of different tumour types (clinical studies highlighted in grey).
Tumour TypeType of Cathepsin (CAT)Clinical (C)/Pre-Clinical (PC)/In vitro (IV)Sample Size (n)Cathepsin ContentReference
LungCAT BC10510.65 ng/mL[66]
CAT BC17448 ng/mg of protein[67]
CAT BPC159High *[68]
CAT DC171304 ng/mg of protein[67]
CAT SC604.2 ± 0.22 ng/mg of protein[69]
CAT HC123172 ± 86 ng/mg of protein[70]
CAT LC10526.16 ng/mL[66]
CAT LC173835 ng/mg of protein[68]
Head/Neck and brainCAT BC84High *[71]
CAT BC47High *[72]
CAT BC32High *[73]
CAT BPCNAHigh *[74]
CAT BPCNAHigh *[75]
CAT BPC11High *[76]
CAT BPCNAHigh *[77]
CAT DPC71300 ng/mg of protein[78]
CAT SPC11Low *[76]
CAT HPC71500 ng/mg of protein[79]
CAT LPC11Low *[76]
Oesophagus, stomach and intestineCAT BC25325.9 ng/mg of protein[80]
CAT BC17510.83 ± 1.8 ng/mL[81]
CAT BPCNALow *[67]
CAT LC2543.6 ng/mg of protein[80]
Colon and/or rectumCAT BC7213.38 ng/mL[81]
CAT BC108168 ± 86 ng/mg of protein[82]
CAT BC60253.5 ng/mg of protein[82]
Colon and/or rectumCAT B C7455 ± 5 hg/mg of protein[83]
CAT BPC40High *[84]
CAT XC7717.4 ng/mL[85]
CAT HC747 ± 1 ng/mg of protein[83]
CAT LC7450 ± 10 ng/mg of protein[83]
CAT LC60274 ng/mg of protein[82]
BreastCAT BC3074 ng/mg of protein[86]
CAT BPC (DU4475)4High *[86]
CAT BIV (BT20)NALow *[87]
CAT DC57High *[88]
CAT XIV (MCF-7)NA2.5 ng/mL[89]
CAT XIV (MDA-MB-231)NA37 ng/mL[89]
OvaryCAT LC31816.1 ± 5.1 ng/mL[90]
PancreasCAT BPCNAHigh *[91]
UrinaryCAT BPC7High *[67]
LiverCAT BC2813.46 ng/mL[81]
Notes: * The cathepsin levels are expressed as high and low (rather than a specific value) for those entries in which definite quantitative values were not available in the original study; NA, Not applicable; “n” = number of human/animal subjects.
Two tumours, lung and breast, which have shown the highest percentage response clinically to PDCs activated by cathepsins, also showed a generally high content of this class of enzyme (in some studies, as high as 3835 ng/mg for lung cancer). This correlation is as expected, as the presence of a trigger is essential for the activation of a prodrug, but has not been widely reported from analysis of experimental data. It should also be noted that the content of cathepsins reported in various studies concerning the same tumour type were highly variable. This finding suggests that heterogeneity might be also present in the tumour samples of the patients that have undergone treatment with PDCs and this could potentially explain why only a fraction of the patients enrolled in the clinical studies responded to the treatment [92,93]. Differences in the content of cathepsins are particularly marked for breast cancer. For example, the content of cathepsin B was reported to be “high” in the study from Bremer et al. [86] but “low” from that of Hulkower et al. [87]. It is also key to highlight that, as mentioned previously, cathepsins level have been found to be affected by oestrogen levels and therefore an additional element of variability is added depending on whether the sample is taken from a post-menopausal or pre-menopausal woman [5,18].

3.3. Magnitude of the EPR Effect in Different Tumour Types

A second, but at least equally important, factor for activity of PDCs is the extent of the EPR effect in a tumour. To establish if there was a correlation between the magnitude of the EPR effect and the tumour’s response to PDCs, data on drug accumulation of different drug delivery technologies such as PDCs and liposomes in clinical tumour samples as well as in preclinical samples were collected as summarised in Table 7. Although this comparison holds a limitation with respect to the heterogeneity of systems (different nanoconstructs and different phases of clinical research) and heterogeneous tumour types, pooling the results from such heterogeneous data was necessary due to the limited clinical studies on EPR effect with PDCs. Different methodologies have been used in different studies to evaluate the extent of passive accumulation in the tumour both qualitatively and quantitatively. For example, in some studies analogues of conjugates containing radioisotopes were used with imaging techniques where the results were expressed qualitatively (i.e., expressed as whether drug accumulation was or was not observed) and quantitatively (expressed as % ID/Kg of drug uptake in tumour). Similarly with the staining technique using Evan’s blue dye the results were expressed both qualitatively and quantitatively (expressed as % dose/g/tumour). A few studies involved the comparison between the free drug accumulation and the nano particle accumulation in which case the results were expressed in terms of degree of their accumulation. Some studies have indicated that the extent of the EPR effect is also dependent on the tumour sizes [3,13,94] but due to the lack of information on the tumour sizes in most of the studies, this factor was not considered in our evaluation.
EPR-mediated accumulation was generally observed in all the tumour types where the highest percentage responses were reported (namely breast, lung and ovary). Studies on liposomal drug accumulation have shown poor EPR effects in liver and pancreatic tumours [95,96]. Colorectal cancer was another tumour type where poor EPR effect has been reported in some studies ([11,20] and Table 7). This observation is probably mainly due to the fact that large tumours in liver, pancreas and prostate possess hypovascular properties with low drug accumulation potential. However, it should be remembered that the smaller tumours possess high vascular density and can exhibit profound EPR effect [97].
This observation also correlates with a recent study on validation of the tumour models for EPR activity, where high drug accumulation has been reported for both large and small lung tumours, and a low drug accumulation has been reported for the large breast tumours [13]. Disparities observed across the various studies are might be due to the heterogeneity within and between the tumour types (size and histological differences) and the diversity of the nanopharmaceutical characteristics [14].
Table 7. Extent of passive accumulation of macromolecular system in different tumour types in both clinical and pre-clinical samples, clinical studies highlighted in grey.
Table 7. Extent of passive accumulation of macromolecular system in different tumour types in both clinical and pre-clinical samples, clinical studies highlighted in grey.
Tumour TypeClinical (C)/Pre-Clinical (PC)Sample Size (n)Macromolecule System UsedRemarksReference
BreastC2PDC1.8%–5.9% dose of PDC uptake in tumour[20]
C5Liposome5.3% ± 2.6% ID/Kg of drug uptake in tumour #[97]
C6Liposome4–16 fold higher drug accumulation in tumour than the free drug[98]
PC (MX-1, Human xenograft)10Protein-conjugate33% higher drug accumulation in tumour than the free drug[99]
PC (MAXF 449, Human xenograft)NAPDC1.0%–0.1% dose/g/tumour drug accumulation[13]
PC (Mouse)NAPDC5.29% dose/g for HMW
3.18% dose/g for LMW
PC (MX-1)NAPDC207 fold higher tumour exposure than the free drug (SN-38)[101]
PC (Walker 256, Rat)NAProtein-conjugate7 fold higher drug accumulation in tumour than the free drug[102]
PC (Mouse)NANanoparticleDrug accumulation observed in tumour *[103]
PancreasCNALiposomesLow drug accumulation observed in tumour *[95]
PC (Mouse)40Micelle3 fold higher drug accumulation in tumour than the free drug[104]
LungC6PDCNo drug accumulation observed in tumour *[20]
C4Liposome18.3% ± 5.7% ID/Kg of drug uptake in tumour #[97]
C3Liposome4–16 fold higher drug accumulation in tumour than the free drug[98]
C15LiposomeHigher drug accumulation observed in tumour *[105]
PC (Meta-7, Mouse)NAPDC3.5%–4.7% dose/g/tumour drug accumulation[13]
PC (COR L23, Human xenograft)NAPDC4.7%–12.2% dose/g/tumour drug accumulation[13]
PC (B16, Mouse)NALiposomes10.6% ± 0.2% ID/g of tumour[106]
PC (B16F10, Mouse)NAPDC8.82% dose/g for HMW 3.23% dose/g for LMW[100]
PC (B16, Mouse)NAMicelle2–3 fold higher drug accumulation in tumour than the free drug[107]
PC (B16, Mouse)NAPDC6–12 fold higher drug accumulation in tumour than the free drug[28]
PC (B16, Mouse)3PDCHigher drug accumulation observed in tumour *[108]
LungPC (A431, Mouse)NAProtein-conjugate24 fold higher drug accumulation in tumour than the free drug[109]
PC (B16, Mouse)NAPDC30–63 fold higher drug accumulation in tumour than the free drug[110]
PC (B16, Mouse)NAPDC16.3 fold higher drug accumulation in tumour than the free drug[111]
OvaryC3Liposome4–16 fold higher drug accumulation in tumour than the free drug[98]
PC (Mouse)NAPDCDrug accumulation observed in tumour *[28]
PC (Oca-1, Mouse)NAPDC5 fold higher drug accumulation in tumour than the free drug[112]
PC (Oca-1, Mouse)NAPDC28–38 times higher drug accumulation in tumour than the free drug[113]
PC (A2780, Human xenograft)NAPDCDrug accumulation observed in tumour *[114]
PC (OVCAR-3, Human xenograft)NAPDC45 fold higher drug accumulation in tumour than the free drug[115]
Oesophageal, stomach and intestinePC (Mouse)NAPDCDrug accumulation observed in tumour *[116]
PC (OCUM-2MLN, Human xenograft)NAMicelleDrug accumulation observed in tumour *[117]
Colon and/or rectumC5PDCNo drug accumulation observed in tumour *[20]
C10PDC64 fold higher drug accumulation in tumour than the free drug[118]
PC (HT29, Human xenograft)4Liposome1.7 fold higher drug accumulation for 0.6 mol % PEG-conjugate in tumour than the free drug[119]
PC (C26 NL-17, Mouse)NALiposomeHigher drug accumulation observed in tumour *[120]
PC (Mouse)NAMicelleDrug accumulation observed in tumour *[121]
Colon and/or rectumPC (LS174T, Human xenograft)NAPDC160 fold higher drug accumulation in tumour than the free drug [122]
PC (LS174T, Human xenograft)NALiposomes6.3% ± 2.9% ID/g of tumour[106]
PC (Mouse)NAPDCDrug accumulation observed in tumour *[123]
PC (LS174T, Human xenograft)NAPDCDrug accumulation observed in tumour *[124]
PC (HT29, Human xenograft)NAPDCDrug accumulation observed in tumour *[125]
PC (HT29, Human xenograft)111Nano crystal (3H-PTX)Low drug accumulation observed in tumour *[126]
Head/Neck and brainC6PDC2.2% ± 2.1% dose at 2–3 h
1.3% ± 0.4% dose at 24 h
0.5% ± 0.3% dose at 8 days uptake in tumour
C10Liposome13–19 times higher accumulation in tumour as compared to the normal brain tissue[127]
C5Liposome7–13 times higher accumulation in tumour as compared to the normal brain tissue[127]
C7Liposome33.0% ± 15.8% ID/Kg of drug uptake in tumour #[97]
Adenocarcinoma (Unknown)PC (MAC 26, Mouse)NAPDC6.9%–10.8% dose/g/tumour drug accumulation[13]
Adenocarcinoma (Unknown)PC (MAC 15A, Mouse)NAPDC8.2%–12.6% dose/g/tumour drug accumulation[13]
CervixPC (ME180, Human xenograft)NALiposomeDrug accumulation observed in tumour *[128]
FibrosarcomaPC (S-180, Mouse)NAPolymer conjugatesDrug accumulation observed in tumour *[129]
PC (S-180, Mouse)NAMicelle13 fold higher drug accumulation in tumour than the free drug[130]
PC (S-180, Mouse)NAPDCDrug accumulation observed in tumour *[131]
PC (S-180, Mouse)NAProtein-conjugateDrug accumulation observed in tumour*[10]
FibrosarcomaPC (Meth A, Mouse)NA PDC Drug accumulation observed in tumour *[132]
PC (S-180, Mouse)NAProtein-conjugate4 fold higher drug accumulation in tumour than the free drug[8]
PC (S-180, Mouse)NAPDCDrug accumulation observed in tumour *[133]
LiverC31LiposomesLow drug accumulation observed in tumour *[95]
C3LiposomesLow drug accumulation observed in tumour *[96]
PC (Mouse)NAMicelle4 fold higher drug accumulation in tumour than the free drug[134]
PC (VX-2, Rabbit)NAPolymer-protein conjugate (SMANCS-Lipidol)Drug accumulation observed in tumour *[135]
PC (VX-2, Rabbit)NAPolymer-protein conjugate (Lipidol)Drug accumulation observed in tumour *[136]
PCNAPDCDrug accumulation observed in tumour *[137]
ProstatePC (Human xenograft)NA89Zr-DFO-mAlb (Polymer-protein conjugate)Drug accumulation observed in tumour *[138]
PC (Human xenograft)NAPDCDrug accumulation observed in tumour *[139]
PC (Rat)NAPDCDrug accumulation observed in tumour *[140]
PC (Rat)NAPDCDrug accumulation observed in tumour *[141]
Notes: * The quantitative data on accumulation in tumour is not available in the original research paper; # related free drug uptake is not stated in the original study; “n” = number of human/animal subjects.
Two decades since the first PDC entered the clinical evaluation [2], one might wonder about the margin of advantage of PDCs over their corresponding parent drugs in terms of overall survival and the patient’s quality of life. Such an advantage of a drug is evaluated only at phase III of the clinical trial, thus since only 2 PDCs have undergone phase III clinical trials (Opaxio and NKTR-102) and the clinical data is available to-date only for Opaxio, any conclusion with regards to the overall survival and patient’s quality of life is derived from Opaxio’s performance. Opaxio, in comparison with its conventional paclitaxel-based treatment, showed some safety-related advantages such as alopecia was rare, nausea and vomiting were uncommon and hypersensitivity reactions were rarely observed. A significant survival benefit has also been observed for women receiving Opaxio vs. paclitaxel [7]. These promising results have provided the confidence that PDCs can give therapeutic benefits in the field of oncology. The use of PDCs in combination with the application of diagnostic tools to measure protease expression and EPR effect would be a rational approach for the further development and clinical use of PDCs.

4. Conclusions

In this study, we have reported analysis of data from the literature whereby we have documented and examined the expression of enzymes (cathepsins) within various tumour types, the magnitude of the EPR effect in the various tumour types, and the clinical responses observed for PDCs. We have determined that the highest percentages of clinical responses to PDCs were observed for lung cancer, which was also found to express high levels of the cathepsin enzymes. Breast and ovarian cancer also showed high clinical responses to PDCs, which correlated well with high levels of cathepsins observed in these tumour types, and with reports indicating the presence of the EPR effect. This finding is in line with other studies [13,14] and in agreement with that concluded by others (e.g., [13]). Moreover, our study also suggests that careful patient selection, in the form of pre-screening for enzyme content and the EPR effect, would be a rational approach for the further development and clinical application of PDCs. This could ultimately result in a more consistent efficacy of this drug delivery system in the clinical setting.


Financial support from the Indian Government (A.K.R.) and the Felix Trust (D.R.) is gratefully acknowledged.

Author Contributions

A.K.R. and D.R. jointly carried out the research and wrote the manuscript; H.M.I.O. co-led the research and writing of the manuscript; F.G. conceived the original idea, led the research and writing of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.


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MDPI and ACS Style

Rajora, A.K.; Ravishankar, D.; Osborn, H.M.I.; Greco, F. Impact of the Enhanced Permeability and Retention (EPR) Effect and Cathepsins Levels on the Activity of Polymer-Drug Conjugates. Polymers 2014, 6, 2186-2220.

AMA Style

Rajora AK, Ravishankar D, Osborn HMI, Greco F. Impact of the Enhanced Permeability and Retention (EPR) Effect and Cathepsins Levels on the Activity of Polymer-Drug Conjugates. Polymers. 2014; 6(8):2186-2220.

Chicago/Turabian Style

Rajora, Amit K., Divyashree Ravishankar, Helen M. I. Osborn, and Francesca Greco. 2014. "Impact of the Enhanced Permeability and Retention (EPR) Effect and Cathepsins Levels on the Activity of Polymer-Drug Conjugates" Polymers 6, no. 8: 2186-2220.

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