16.15: Transdermal Drug Delivery Systems

Transdermal drug delivery systems (TDDS) enable the controlled release of drugs across the skin into systemic circulation. They are particularly advantageous for drugs with short half-lives or narrow therapeutic indices, as they maintain consistent plasma concentrations and reduce the risk of subtherapeutic or toxic levels.

TDDS are categorized into monolithic, reservoir, and mixed systems. Monolithic systems embed the drug in a polymer matrix, where diffusion governs release. Reservoir systems contain the drug in a liquid or gel compartment, separated from the skin by a rate-controlling membrane. Mixed systems combine both, offering an initial rapid release followed by sustained diffusion through the matrix, which allows for greater control over release profiles.

The suitability of a drug for TDDS is determined by two main rate-limiting steps: diffusion from the patch and skin permeation. Ideal candidates are drugs with low molecular weight, moderate lipophilicity, and high potency. TDDS are unsuitable for compounds that require high doses, are extremely lipophilic or hydrophilic, cause skin irritation, or undergo significant skin metabolism. Large molecules, such as proteins or peptides, typically do not permeate the skin effectively without the use of enhancement techniques.

TDDS offer several advantages: they are noninvasive, bypass gastrointestinal degradation and hepatic first-pass metabolism, and improve patient compliance due to ease of use and reduced dosing frequency. These features make them ideal for chronic treatments where maintaining stable drug levels is essential. Common clinical examples include hormone replacement drugs (Climara, Androderm), pain management drugs (Duragesic, Lidoderm), and nicotine replacement drugs (Nicoderm CQ, Habitrol), contraceptives (Xulane, Twirla), cardiovascular drugs (Minitran, Catapres-TTS), neurological/psychiatric drugs (Exelon Patch, Daytrana), and drugs for motion sickness (Transderm Scop).

These applications demonstrate the system’s capability for precise and sustained drug delivery, enhancing therapeutic outcomes while minimizing systemic side effects.

Transdermal drug delivery systems or TDDS use patches to release drugs through the skin into systemic circulation at a controlled rate.

These systems benefit drugs with short half-lives or narrow therapeutic indices by maintaining stable plasma levels.

They include monolithic systems—in which drugs are embedded in a polymer matrix—and reservoir systems—in which drugs are stored in a compartment with a rate-controlling membrane.

Mixed systems combine both types, offering an initial controlled release followed by diffusion through the matrix.

Two steps limit the choice of TDDS: diffusion from the patch and skin permeation.

Advantages include noninvasive application, bypassing GI degradation and first-pass metabolism, and good patient compliance.

However, they are unsuitable for high-dose drugs, large molecules, extremely lipophilic or hydrophilic compounds, skin irritants, or drugs heavily metabolized in the skin.

Common examples of transdermal delivery systems include scopolamine and contraceptives like Ortho Evra.