Chemical Release from Topical Formulations across Synthetic Membranes: Infinite Dose

Drug release rates from topical preparations are sometimes measured by monitoring the cumulative mass of drug appearing in a receptor solution (MR). If the topical formulation and receptor solution are in direct contact, then MR increases linearly with √t. When a synthetic membrane is placed between the topical formulation and receptor solution, drug appearance in the receptor solution is delayed and MR is not immediately linear in √t. As a result, linear regressions of MR with √t produce positive values for the √t-intercept. Here, we mathematically model chemical release from an infinite-dose, topical formulation across synthetic membrane to quanitiatively determine the physical meaning of the √t-intercept. To correctly determine drug diffusivity in the topical formulation, the experiment must be conducted long enough that MR is linear in √t. Theoretically based procedures are presented for testing which data should not be used in linear regression of MR with √t. Theoretical predictions are compared with previously published experimental results for ethyl salicylate across a poly(dimethylsiloxane) (Silastic) membrane and for hydrocortisone across several different synthetic membranes.