Feasibility of measuring the bioavailability of topical betamethasone dipropionate in commercial formulations using drug content in skin and a skin blanching bioassay.

An in vivo technique has been developed which simultaneously compares a skin blanching bioassay with drug content in human stratum corneum following topical application of four 0.05% betamethasone dipropionate formulations. Bioavailability of drug from commercial cream and ointment formulations was assessed by quantification of drug content in tape-stripped stratum corneum and skin blanching in the treated skin site under occluded conditions. Tape-stripping removed stratum corneum to a varying degree between individuals but was consistent (35%) within an individual with all formulations, day to day. A correlation (r = 0.9935) between the amount of drug in the treated stratum corneum normalized for surface area and the corresponding skin blanching score was observed with four 0.05% betamethasone dipropionate formulations. Increasing the amount of drug in the tape-stripped stratum corneum correlated with an increased skin blanching score. Ointment formulations delivered more drug to the skin and produced greater blanching scores than the cream formulations. Topical corticosteroid content in the treated skin site can therefore be quantified and correlates well with the resulting pharmacodynamic activity.