DRUG AND VEHICLE DEPOSITION FROM TOPICAL APPLICATIONS - LOCALIZATION OF MINOXIDIL WITHIN SKIN STRATA OF THE HAIRLESS MOUSE

The cutaneous bioavailability of topical 2% minoxidil solution was ver ified in live hairless mice. Minoxidil and propylene glycol deposition on the skin surface, epidermis and dermis from the single-dose in viv o study were compared with the results from previous in vitro studies. A distinct difference is apparent in the epidermis where the in vitro values are 11-22 times higher than the in vivo values for minoxidil a nd 8-16 times higher for propylene glycol. The differences were not as great in the dermis. Percutaneous absorption of the drug appeared to be a very small fraction of the applied dose. Similarly shaped stratum corneum and plasma concentration profiles and the relatively constant dermal profiles of minoxidil and propylene glycol open the possibilit y of transappendageal routes being involved in percutaneous absorption . The greater amount of drug and vehicle found in the dermis from in v itro studies can be explained by the absence of dermal clearance. The overestimation in the amount of drug found in the epidermis in vitro m ay also be attributable to poor dermal clearance. On the whole, the st udy raises questions about the use of in vitro tissue dispositions for bioavailability assessment and bioequivalence demonstration.