What property is particularly important for drugs formulated in transdermal patches?

• A. High molecular weight
• B. Volatility
• C. Lipophilicity
• D. Hydration level

Answer: (C)

Transdermal delivery relies on passive diffusion through the skin’s lipid-rich outer layer. The drug must partition from the patch into these lipids, so its ability to partition into the skin—reflected by lipophilicity—is the key factor that governs permeation. If a compound is too hydrophilic, it won’t enter the lipid barrier effectively; if it’s too volatile, it may evaporate from the patch; if it has very high molecular weight, diffusion through the barrier becomes slow. Hydration alters the barrier but is not the intrinsic property driving permeation. Therefore, lipophilicity is the most critical property for drugs in transdermal patches.