Analysis of skin penetration enhancement based on a two-layer skin diffusion model with polar and nonpolar routes in the stratum corneum: dose-dependent effect of 1-geranylazacycloheptan-2-one on drugs with different lipophilicities.

The effects of 1-geranylazacycloheptan-2-one (GACH) on the in vitro skin penetration of seven drugs with various lipophilicities were studied. The penetration of drugs from aqueous vehicle through the guinea pig skin was increased depending on the pretreatment dose of GACH. The largest enhancement was observed for 5-fluorouracil (5-FU) and a bell-shaped relationship was obtained between enhancement ratio and the octanol/water partition coefficients of drugs. Further, Laplace transformed equations for percutaneous absorption of drug were derived from Fick's second law based on a two-layer skin diffusion model with polar and nonpolar routes in the stratum corneum. Through curve-fitting of their equations to penetration profiles using a nonlinear regression program MULTI(FILT) combined with a fast inverse Laplace transform (FILT) algorithm, the action mechanism of GACH was discussed in terms of its effect on the partitioning and diffusivity of drugs in each domain. With an increase in pretreatment dose of GACH, the estimated partition parameters of drugs into the nonpolar route increased but their diffusivities were little affected. The analysis based on a linear free-energy relationship suggested that the increase in partitioning of drugs into the nonpolar route was explained by its increasing polarity with GACH pretreatment.