The distribution of esterases in the skin of the minipig.

Skin esterases serve an important pharmacological function as they can be utilised for activation of topically applied ester prodrugs. Understanding the nature of these enzymes, with respect to their role and local activity, is essential to defining the efficacy of ester prodrugs. Minipigs are used as models to study the kinetics of absorption of topically applied drugs. Their skin has structural properties very similar to human skin. However, regional distribution differences in esterase activity from site-to-site could influence cross-species extrapolation. Investigation of the regional site variation of minipig skin esterase activity will facilitate standardization of topically applied drug studies. Furthermore, the characterization of regional skin variation, will aid in translation of minipig results to better predictions of human esterase activity. Here we report the variation in rates of hydrolysis by minipig skin taken from different regional sites, using the esterase-selective substrates: phenyl valerate (carboxylesterase), phenyl acetate (arylesterase) and p-nitrophenyl acetate (general esterase). Skin from ears and back of male minipig showed higher activity than female. Skin from minipig ears and the back showed the highest level of esterase activity and was similar to human breast skin used in vitro absorption studies. These results suggest that skin from the minipig back is an appropriate model for preclinical human skin studies, particularly breast skin. This study supports the use of the minipig, with topical application to the back, as a model for the investigation of pharmacokinetics and metabolism of ester prodrugs.