Novel animal model for evaluating topical efficacy of antiviral agents: flux versus efficacy correlations in the acyclovir treatment of cutaneous herpes simplex virus type 1 (HSV-1) infections in hairless mice.

This report describes the study of a novel animal model for the topical treatment of cutaneous herpes virus infections, with a focus upon the relationship between the dermal flux of the antiviral agent and the effectiveness of the topical therapy. A recently developed (trans)dermal delivery system (TDS) for controlling acyclovir (ACV) fluxes was employed in the treatment of cutaneous herpes simplex virus type 1 (HSV-1) infections in hairless mice. The TDS's were fabricated with rate-controlling membranes to provide nearly constant fluxes of ACV for up to 3 to 4 days. At the end of each experiment an extraction procedure was used to determine the residual ACV, validating the drug delivery performance of the TDS. Virus was inoculated into the skin of the mice at a site distant from the TDS area, and the induced lesion development was evaluated to distinguish between topical and systemic effectiveness of the therapy. In the main protocol, ACV therapy was initiated 0, 1, 2, and 3 days after virus inoculation and the lesion development "scored" on Day 5. The topical efficacies of 1- and 2-day-delayed treatments were essentially the same as that of a 0-day-delayed treatment, while the topical efficacy of a 3-day-delayed treatment was much poorer. Also, in the cases of 0-, 1-, and 2-day-delayed treatments, topical efficacy increased with increasing flux in the range of 10 to 100 micrograms/cm2-day. When the ACV flux was 100 micrograms/cm2-day or greater, a maximum 100% topical efficacy was obtained. The results for systemic efficacy were shifted to higher fluxes: approximately 10-fold greater ACV fluxes were necessary to provide efficacy equal to the topical efficacy results. The animals treated with a high ACV flux (350-500 micrograms/cm2-day) lived significantly longer than those treated with a low ACV flux (10-125 micrograms/cm2-day) and those of untreated (placebo) animals. Further, their mean survival time decreased with an increase in the time delay for ACV treatment. In contrast, the mean survival time for the animals which received a low ACV flux was similar to that of the control animals and remained unaltered with an increase in the time delay for ACV treatment. The approach developed in this study should be valuable in (a) the screening of new antiviral agents for the topical treatment of cutaneous herpes virus infections and (b) in the optimization of drug delivery systems (i.e., topical formulations).