Transdermal Delivery: Challenges and Opportunities

Dear Colleagues,

The skin, being the largest organ in humans, which covers the whole body, has become recognized as an important drug delivery route. Unlike most other organs in the body, the skin can be reached directly. Therefore, there is a great interest in the skin as a site of drug application, both for dermal and transdermal drug delivery. Transdermal delivery involves the application of a drug to the skin in order to achieve systemically active drug levels to treat systemic diseases. Transdermal drug delivery systems (TDDS) offer a number of advantages compared to conventional drug delivery systems, leading to systemic drug effects. TDDS enable the drugs to be directly delivered to the systemic circulation, circumventing hepatic first-pass effects and avoiding the interaction of the drug with the gastrointestinal tract. Furthermore, TDDS act as a “depot”, controlling the rate of drug input over a prolonged period of time and ensuring constant plasma levels. Regarding drugs with a narrow therapeutic margin, their undesirable side effects, particularly those associated with pulsed peak plasma levels, may be reduced via the use of a TDDS. In addition, the use of a TDDS enables the dose frequency to be reduced. It is worth noting that TDDS are non-invasive, i.e., they avoid needle delivery, which is associated with pain and patient phobia (involved with intravenous therapy), and TDDS drugs can be self-administered. The aforementioned advantages of TDDS lead to enhanced patient compliance.

However, the effectiveness of transdermal drug delivery is often hindered by challenges such as the low permeability of the most apical layer of the skin, the stratum corneum, leading to limitations in the skin permeation of drugs and thus low therapeutic effectiveness due to the insufficient plasma levels of drugs. In addition, unfavorable drug properties such as inadequate solubility and partition coefficient (log P), as well as their high molecular weight values, limit the efficacy of transdermal drug delivery. Only few drug molecules yield skin permeability coefficients sufficiently high to develop clinically active plasma levels. Thus, the market for transdermal patches comprises patches with only few low-molecular-weight drugs.

Therefore, significant efforts have been devoted to developing strategies to overcome the impermeability of intact human skin, which provides the main barrier for drug penetration. These strategies involve chemical (formulation-based) and physical penetration enhancement techniques. Chemical penetration enhancement methods involve the manipulation of the drug or vehicle in order to enhance the drug diffusion through the skin; this also includes the use of different nanocarriers (e.g., vesicles, nanoparticles, etc.). Among physical methods, iontophoresis, electroporation, and ultrasound are the most studied enhancement methods.

The primary focus of this Special Issue is to address the opportunities and challenges associated with transdermal drug delivery and to demonstrate how various percutaneous penetration enhancement strategies can overcome these challenges, thereby improving the therapeutic efficacy of TDDS.

Dr. Nina Dragićević
Guest Editor

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• transdermal drug delivery
• nanocarrier
• iontophoresis
• electroporation
• ultrasound
• liposome
• nanoparticle