Finite dose percutaneous drug absorption: a BASIC program for the solution of the diffusion equation.

A BASIC program to obtain the solution for the diffusion equation describing the finite dose percutaneous absorption pharmacokinetics is shown. Drug amount in the solution matrix, Av, that in the skin which is regarded as a simple diffusion membrane. As, and the drug flux from the skin which is provided by the local circulation per unit area, J, and the blood drug concentration. C, expressed as functions of time are obtained using Fourier series. Usually, a small number (less than 10) of series is enough to obtain the accurate values for Av, As, J, and C. It is also shown that the solution of the diffusion equation reported previously for the specific case where both the diffusion coefficient of a drug through the vehicle and skin-capillary clearance are very large is expressed as a limit.