Extended acyclovir therapy for herpes genitalis: changes in virus sensitivity and strain variation.

The in-vitro sensitivity of herpes simplex virus isolated from patients enrolled in clinical trials of orally administered chronic suppressive acyclovir therapy was determined. Approximately 500 isolates from 250 patients were examined. There was no increased incidence of recovery of virus less sensitive to acyclovir in vitro following chronic administration of acyclovir or intermittent therapy for recurrences during treatment periods of four months to four years. Changes in in-vitro sensitivity of virus isolated from two patients who shed virus less sensitive to acyclovir were studied to determine the possible role of continued antiviral therapy in selecting for progressively more resistant phenotypes. Increasing resistance was not documented. Both patients who shed less sensitive virus derived clinical benefit from chronic orally administered acyclovir treatment.