What is the ideal molecular weight range for an API candidate for a transdermal drug?

The ideal molecular weight range for an Active Pharmaceutical Ingredient (API) candidate intended for transdermal delivery is generally considered to be between 100 and 500 Daltons (g/mol). This range is based on the permeability properties that influence how effectively a drug can penetrate the skin barrier.

1. Molecular Size Considerations: Molecules below 100 Daltons may not have sufficient binding affinity for skin receptors, while those above 500 Daltons can struggle to penetrate the skin effectively due to increased size and potential polarity.

2. Lipophilicity: Transdermal drugs should ideally possess a balance of lipophilicity and hydrophilicity to enhance diffusion through lipid membranes while maintaining adequate solubility in the delivery formulation.

3. Permeability: Molecules within this molecular weight range are generally small enough to navigate through the stratum corneum, the outermost layer of skin, which acts as the primary barrier to drug absorption.

In conclusion, while it may vary depending on specific formulations and delivery systems, a molecular weight of 100 to 500 Daltons is a widely accepted guideline for the ideal characteristics of an API targeting transdermal delivery.