Assessment of dermal bioavailability: predicting the input function for topical glucocorticoids using stratum corneum sampling

Predicting the dermal bioavailability of topically delivered drugs is challenging. In this work, minimally invasive stratum corneum (SC) sampling was used to quantify the delivery of betamethasone valerate (BMV) into the viable skin. Betnovate® cream (0.1% w/w BMV) was applied at three doses (2, 5 and 10 mg cm-2) to the ventral forearms of 12 healthy volunteers. The mass of drug in the SC was measured using a validated tape-stripping method (a) after a 4-hour “uptake” period, and (b) following a 6-hour “clearance” period subsequent to cream removal. Concomitantly, the skin blanching responses to the same doses were assessed with a chromameter over 22 hours post-application. BMV uptake into the SC was significantly higher for the 5 mg cm-2 dose compared to those of 2 and 10 mg cm-2. In all cases, ~30% of the drug in the SC at the end of uptake period was cleared in the subsequent 6 hours. From the SC sampling data, the average drug flux into the viable epidermis, and its first-order elimination rate constant from the SC, were estimated as 4 ng cm-2 hr-1 and 0.07 hr-1, respectively. In contrast, skin blanching results were highly variable and insensitive to the dose of cream applied. The SC sampling method was able to detect a 50% difference between two applied doses with 80% power; detection of a 20% difference would require a larger sample size. SC sampling enabled quantitative metrics describing corticosteroid delivery to the viable epidermis to be determined.