Estimation of intradermal disposition kinetics of drugs: I. Analysis by compartment model with contralateral tissues

Purpose. The objectives of dermal application of drugs are not only systemic therapeutics, but also local ones. We would expect its intradermal kinetics to be dependent on its therapeutic purpose. To develop more efficient drugs for local or systemic therapeutics, it will be important to estimate quantitatively the intradermal disposition of drugs applied topically. We tried, therefore, to develop the compartment model to describe the intradermal disposition kinetics after topical application of drugs. Methods. In vivo percutaneous absorption study for antipyrine, a model compound, was performed using rats with tape-stripped skin, using the assumption that the stratum corneum permeability to drugs would be improved enough not to be a rate-limiting process. Results. To analyze the results obtained, a 4-compartment model, composed of donor cell, viable skin, muscle, and plasma compartments, was applied. Although the fitting lines obtained could describe the concentration-time profiles of antipyrine in each compartment very well, the concentration profiles in the contralateral tissues were extensively overestimated. Therefore, we developed a 6- compartment model which included the viable skin and muscle in the contralateral site, and analyzed the concentration-time curve of each compartment. The fitting curves were in good agreement with the experimental data for all the compartments including the contralateral viable skin and muscle, and thus, this model was recognized to be adequate for the estimation of intradermal kinetics after topical application. Judging from the obtained values of clearance from viable skin to plasma and from viable skin to muscle, about 80% of antipyrine penetrated into viable skin, which suggested it was absorbed into circulating blood and 20% was transported to muscle under viable skin. Conclusions. Pharmacokinetic analysis using the 6- compartment model would be very useful for the estimation of local and systemic availability after topical application of drugs.