Factors affecting the drug absorption through skin

The absorption of drugs through the skin, or transdermal absorption, is influenced by several physicochemical and formulation factors

Physicochemical Factors:

Molecular Size: Smaller molecules generally penetrate the skin more easily than larger ones. Molecules typically need to be less than 500 Daltons to efficiently pass through the skin barrier.

Lipophilicity: Drugs that are more lipophilic (fat-loving) tend to penetrate the skin better. This is because the skin's outer layer, the stratum corneum, is rich in lipids.

Hydrophilicity: While lipophilic drugs penetrate more easily, extremely hydrophilic (water-loving) drugs often have trouble crossing the lipid-rich stratum corneum.

Partition Coefficient: The partition coefficient between the skin and the vehicle affects drug absorption. A higher partition coefficient indicates better penetration through the skin.

pH: The pH of the drug formulation can affect the drug's ionization state, influencing its ability to penetrate the skin. The skin's pH is slightly acidic, which can affect the drug's absorption depending on its pKa.

Solubility: The solubility of the drug in the vehicle (the substance in which the drug is delivered) is crucial. Drugs need to be sufficiently soluble in the vehicle to be effectively delivered through the skin.

Formulation Factors:

Vehicle or Base: The type of vehicle (e.g., cream, gel, patch) can impact drug absorption. Vehicles that enhance the solubility and stability of the drug and facilitate its penetration through the skin are preferred.

Penetration Enhancers: Certain substances, known as penetration enhancers, can temporarily disrupt the skin barrier to increase drug absorption. Examples include alcohols, surfactants, and fatty acids.

Occlusive Properties: Occlusive dressings or formulations that create a barrier over the skin can increase moisture and enhance drug absorption by softening the stratum corneum.

Release Rate: The rate at which the drug is released from the formulation affects absorption. Controlled-release formulations can provide a steady drug delivery rate, improving therapeutic efficacy.

Particle Size: For formulations that include particles, such as microemulsions or nanoparticles, the size of these particles can influence the rate and extent of absorption.

Additives: Stabilizers, preservatives, and other additives in the formulation can affect the drug's stability and, consequently, its absorption.

Overall, optimizing these factors can significantly enhance the efficiency and effectiveness of transdermal drug delivery systems.