Factors affecting human percutaneous penetration of methotrexate and its analogues in vitro.

Factors affecting the percutaneous penetration of methotrexate (MTX) and its analogues through excised human skin were studied. With application of 0.05% MTX solution to the epidermis, 0.07% of the applied dose was recovered in the epidermis and less than 0.005% penetrated completely through the skin. The percent of drug which penetrated remained constant from 0.05% to 2.0% MTX and was also unchanged by increasing incubation temperature from 28 degrees C to 37 degrees C. Cellophane tape stripping of stratum corneum following drug incubation removed essentially all the MTX that had been measured in whole epidermis. Binding of MTX was localized to the outermost layers of the stratum corneum. With removal of the stratum corneum prior to drug incubation, 25% of the applied MTX penetrated through the skin. Various vehicles were tested to determine their effect on MTX percutaneous absorption. Dimethylsulfoxide (80%), dimethylacetamide (25%), or retinoic acid (0.1%) had no effect on penetration. Maximum enhancement of penetration by 25-fold and 143-fold was obtained with retinoic acid (saturated solution in aqueous ethanol) and C-10-methylsulfoxide (2.5%), respectively. The penetration of lipid-soluble derivatives of MTX was also tested. The compound that penetrated best was the dimethyl ester of dichloro-MTX with a 3-fold increase in penetration over MTX.