Foreword

Transdermal drug delivery offers several advantages, including sustained effects, avoidance of hepatic first-pass metabolism, and ease of administration. The stratum corneum, the outermost layer of the skin, serves as the primary barrier to drug permeation, functioning as a bioprotective mechanism. Effectively controlling this barrier function could enhance drug efficacy while ensuring patient friendly formulation. Additionally, the skin is a crucial application site not only for topical and transdermal medications but also for cosmetics, which are widely used by the general populations.

The development of transdermal drug delivery systems, which began in the 1960s, may have already reached a turning point. Many products, numerous papers and conference presentations have been launched. The time required for drug absorption is a major drawback of transdermal formulation. This was overcome by the development of a tulobuterol transdermal formulation, which effectively suppresses asthma attacks before waking up by ensuring that blood concentration reach therapeutic levels before wake up, this proved that the drug could be delivered to the body. Furthermore, the limitation of delivering only a small amount of drug to the body is mitigated in the transdermal formulation of rivastigmine, as it helps prevent side effects such as tremors, which frequently occur with the rapid increase in blood concentration seen in oral administration. Thus, what was once considered a drawback of transdermal absorption is now a beneficial feature of this formulation.

This “Current Topics” section includes one review article and two regular articles. The first review, conducted by Prof. Takagi and Dr. Kage, explores the application of hyaluronic acid. Hyaluronic acid is a linear chain of N-acetylglucosamine and D-glucuronic acid with molecular weights ranging from 800000 to 1200000. Hyaluronic acid is a naturally occurring polysaccharide with a high water retention ability. It can hold up to 6 L of water per gram. It is found in the skin, joints, and eyes, and plays a crucial role in hydration and elasticity. It is used in cosmetics (lotions, sera, and creams), aesthetic medicine (hyaluronic acid fillers for wrinkle reduction and volume enhancement), and healthcare (joint lubrication and ophthalmic treatments). Furthermore, its levels naturally decline with age, decreasing by nearly 50% in patients in their 40s. External supplementation through skincare, dietary supplements, and injections helps maintain youthful skin. In summary, hyaluronic acid is vital for skin hydration and elasticity, but its levels decline with age.

The second study by our group focused on the effect of alcohols on the improvement of the skin permeability of drugs. Some alcohols are commonly used in drug formulations, and are known to facilitate skin permeability of drugs. This study compared the effects of short chain alcohols on skin permeation of drugs with varying properties to identify specific skin permeation pathways. In drug skin permeation experiments, it is necessary not only to interpret the phenomena that form the basis of formulation development but also to focus on and understand the permeation pathways of the drug itself.

The third study, conducted by Prof. Fukami’s research group, focused on enhancing the absorption of patch-type anti-inflammatory drug formulation. This study investigated the interactions between hydrophilic drugs and acrylic polymers in matrix-type adhesive patches to improve transdermal drug permeability. Loxoprofen sodium hydrate (LP-Na), a poorly permeable non-steroidal anti-inflammatory drug (NSAID), was incorporated into patches made of acrylic polymers with varying functional groups and molecular weights. Lactic acid (LA) was added as a permeability enhancer. Skin permeation tests revealed that patches using carboxyl-functional acrylic polymers (AO) increased permeability by 2.5 times, with an additional 1.5-fold increase when LA was added. These findings suggest that interactions between LP-Na, acrylic polymers, and LA enhance drug diffusion from adhesive patches, improving transdermal drug delivery efficiency.

I would hope that this special feature is recognized as both a culmination of accumulated research findings and a foundation for future advancements. Transdermal absorption is not just a conventional drug delivery method but also a field where new perspectives continue to emerge in this evolving era.