In Vivo Evaluation of Acyclovir Prodrug Penetration and Metabolism Through Rat Skin Using a Diffusion/Bioconversion Model

Purpose. In order to evaluate the in vivo penetration of prodrugs which undergo metabolism in skin, we analyzed thein vivo penetration profiles of acyclovir prodrugs based on a two-layer skin diffusion model in consideration of metabolic process.Methods. Acyclovir prodrugs (e.g., valerate, isovalerate and pivarate) were used as model prodrugs and the amounts excreted in urine were measured after percutaneous application. In vivo penetration profiles were then estimated by employing a deconvolution method and the penetration of acyclovir prodrugs was analyzed using a diffusion model. Subsequently, diffusion, partitioning and metabolic parameters were compared under in vitro and in vivo conditions.Results. Although total penetration amounts at the end of the experiment were similar for the three prodrugs, the ratio of intact prodrug to total penetration amount differed significantly. Moreover, the excretion and absorption profiles were also very different for each prodrug. Enzymatic hydrolysis rate constants calculated under in vivo conditions were considerably larger than those obtained in the skin homogenate and in vitro penetration experiments.Conclusions. The present skin diffusion/bioconversion model combined with computer analysis enables us to comprehensively account for diffusion, partitioning and metabolism during in vivo percutaneous absorption. Nevertheless, different enzymatic hydrolysis rate constants obtained under bothin vivo and in vitro conditions demonstrate the difficulty of obtaining accurate values for in vivo enzymatic activity from related in vitro experiments.