- The aim of this study was to investigate the percutaneous penetration and dermal metabolism of a new potential anti-acne prodrug—TU-2100 [bis( o -carboxyphenyl ethyl ester)nonanedioate] in guinea pigs. The fluxes of this agent through excised skin after applications of TU-2100 gels at 3 and 10% concentrations were similar. However, after 24 h from the time of drug application, the total amounts of permeated TU-2100 into the skin compartment and through the skin into the receiver were 271.7 (±30.7 S.E.) μg/cm 2 from the 3% gel and 779.4.0 (±98.5 S.E.) μg/cm 2 from the 10% gel, demonstrating a relatively high skin accumulation. Higher degradation of TU-2100 to ethylsalicylate occurred after application of drug at 10% concentration than after the application of 3% gel. In contrast, the fraction of permeated drug metabolized was twofold higher after the 3% gel application than after the 10% gel ( F m =20 vs. 10.5 mole %). Since F m is reversibly related to the total permeating drug, the obtained values actually reflect the significant difference in TU-2100 permeation from the 3% (271.7 μg) and the 10% (779.4 μg) gels. An in vivo–in vitro comparison revealed similar drug accumulations in the skin after application of both 3 and 10% gels, however, skin metabolism was found to be significantly higher in vivo than in vitro.