Topical drug delivery from thin applications: theoretical predictions and experimental results

Stainless-steel templates of various thicknesses (75, 200, 800, and 1600 microns) were used to apply propylene glycol/water gels containing methyl or propyl p-aminobenzoates to silicone rubber membranes, and drug delivery was studied with the use of the Bronaugh diffusion cell under conditions in which the drug was initially in thermodynamic equilibrium with respect to the application and membrane. Theoretical diffusion profiles were generated with the use of a model which assumes that diffusional gradients exist within the application. To use the model equation, previously derived for the initial condition in which the drug is in thermodynamic equilibrium with respect to the application and membrane, drug diffusivity in both the application and the membrane and the drug's membrane/vehicle partition coefficient were independently determined. In general, agreement between experimental and theoretical results was within 25%.