Clinical and virologic characterization of acyclovir-resistant varicella-zoster viruses isolated from 11 patients with acquired immunodeficiency syndrome

We studied the clinical resistance to acyclovir of infections with varicell a-zoster viruses (VZV) in patients with acquired immunodeficiency syndrome, and we correlated it to virologic analyses. Eleven patients with VZV infec tions (treated with acyclovir, 30 mg/kg/day, given intravenously, or 4 g/da y, given orally) were included in the study because of the failure of 10 da ys of acyclovir therapy. Susceptibility of VZV isolates to acyclovir was te sted using a plaque reduction assay to determine the 50% inhibitory concent ration (IC50) of acyclovir and the SI50 (IC50 of the patient isolate/IC50 o f the reference strain) to acyclovir. The thymidine kinase (TK) gene, which supports the resistance, was sequenced on amplified products. Only 3 patie nts had a significant increase in the IC50, as compared with the IC50 of th e reference strain (SI50 of greater than or equal to4), and a mutation in t he TK gene. For the other 8 patients, the clinical resistance was not confi rmed by the virologic results: the SI50 was <4, and no mutation was detecte d in the TK gene. Because no acyclovir-resistant strain appeared during a s horter period of time, we suggest an increase in the duration of the treatm ent to 21 days before acyclovir resistance is suspected.