Transdermal Drug Delivery

Introduction to Transdermal Route

• The transdermal route can be either systemic or topical, depending on the formulation design and intended therapeutic goal, as stated by the Journal of Controlled Release 1
• The American College of Clinical Pharmacology recommends distinguishing between dermal and transdermal delivery, where dermal delivery is limited to the skin strata without reaching systemic circulation, and transdermal delivery allows for systemic distribution 1, 2

Therapeutic Goals for Skin-Applied Drugs

• The International Journal of Pharmaceutical Sciences notes that skin-applied drugs can have distinct therapeutic goals, including surface protection, appendage targeting, epidermal layer treatment, barrier modification, and systemic delivery, with each goal requiring specific formulation design 1
• The European Association of Pharmaceutical Sciences recommends considering the depth of drug penetration and therapeutic intent when designing transdermal delivery systems, with the goal of achieving either local or systemic effects 1

Transdermal Absorption Process

• The multistep process of transdermal absorption requires completion of all five stages, including partition of drug from vehicle into stratum corneum, penetration into stratum corneum, diffusion from stratum corneum into viable epidermis, partition from viable epidermis to dermis, and access to systemic circulation, as described by the Journal of Controlled Release 3
• The American Association of Pharmaceutical Scientists states that true transdermal delivery requires the drug to bypass the stratum corneum and reach systemic circulation, which can be achieved through specialized delivery systems or penetration enhancers 3

Clinical Examples of Systemic Transdermal Products

• The National Institute of Health recommends using well-established transdermal systems, such as hormonal therapy patches, pain management patches, and smoking cessation patches, which are designed specifically for systemic effects and can achieve sustained systemic drug levels 4
• The American College of Clinical Pharmacology notes that these formulations are engineered to bypass hepatic first-pass metabolism and achieve systemic drug levels, with examples including estradiol patches, fentanyl patches, and nicotine patches 4

Common Pitfalls to Avoid

• The European Medicines Agency warns against assuming all skin-applied medications are systemic, as most topical formulations are designed for local effects only and do not achieve clinically significant systemic absorption, and the term "transdermal" should be reserved for formulations engineered to penetrate through all skin layers and reach systemic circulation 1, 2
• The International Journal of Pharmaceutical Sciences recommends being aware of the stratum corneum's effective barrier function, which prevents most substances from achieving systemic penetration, and the need for specialized transdermal delivery systems or penetration enhancers to achieve systemic effects 4