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Relationship of skin target site free drug concentration (C*) to the in vivo efficacy: an extensive evaluation of the predictive value of the C* concept using acyclovir as a model drug.

Author(s): Mehta SC, Afouna MI, Ghanem AH, Higuchi WI, Kern ER

Affiliation(s): Department of Pharmaceutics and Pharmaceutical Chemistry, University of Utah, Salt Lake City 84112, USA.

Publication date & source: 1997-07, J Pharm Sci., 86(7):797-801.

For the past few years, our laboratory has been involved in the development of a novel approach for predicting topical in vivo efficacy based on the estimation of skin target site free drug concentration (C*) from in vitro flux data. We have used acyclovir (ACV) as a model drug in the treatment of cutaneous herpes simplex virus type 1 infections in hairless mice. The goal of this study was to rigorously evaluate the applicability of this approach over the entire range of topical efficacy (i.e., from 0 to 100%). We employed a variety of ACV formulations differing in solvent compositions, enhancers, and excipients (and therefore in their efficacies) to achieve this goal. The C* values were estimated from the in vitro flux data obtained in an in vivo-in vitro experimental design that closely approximated the in vivo treatment protocol. For the in vivo antiviral efficacy studies, a finite dose of ACV formulation was applied twice a day, beginning the day after virus inoculation, for 4 days. The lesions were scored on the fifth day, and the efficacies were calculated as described earlier. Our results indicate that, for a variety of formulations over a wide range of efficacies, the predictions based on C* are in good agreement with the observed in vivo efficacies. These findings strongly demonstrate the predictive value of C* over the entire range of topical efficacy, thereby further strengthening its potential for future studies. The findings also indicate that although the excipients in a formulation may alter the rate and extent of available drug at the target site, in these cases, they do not seem to have any effect on the in vivo potency of the drug.

Page last updated: 2006-01-31

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