Pharmacokinetic analysis of percutaneous absorption: evidence of parallel penetration pathways for methotrexate.

Compartmental models were developed to describe the penetration of a drug from a topically applied vehicle through the skin. Data for in vitro penetration of methotrexate through hairless mouse skin from vehicles varying in pH from 3.5 to 6.5 were computer-fitted to estimate model parameters. Comparison of lag time and the exponential coefficient suggested that parallel penetration pathways exist. The fraction of drug penetrating through the shunt pathway increased as vehicle pH and ionization increased. Penetration curves were quantitatively partitioned into bulk tissue and shunt contributions. At pH 6.5, flux through the shunt pathway predominated.