High frequency of spontaneous helicase-primase inhibitor (BAY 57-1293) drug-resistant variants in certain laboratory isolates of HSV-1.

Herpes simplex virus (HSV) helicase-primase is the target for a new group of potent antivirals that show great promise in vivo. A claimed advantage of this class of compounds is the low rate of drug resistance, which is reported to occur at a lesser rate than acyclovir (ACV)-resistance in cell culture. We confirmed that BAY 57-1293 is highly active against HSV-1 and superior to ACV when tested in Vero cells. Notably, drug resistance was detected in laboratory working stocks in two different strains of HSV at 10(-4) to 10(-5) and there was evidence that the resistant variants were present in the virus population before the selection was applied. Plaque-purified clones obtained from the parental viruses showed a lower level of resistance selection in the presence of drug (10-6) and this value is similar to published reports. In the case of HSV-1 SC16, no difference was observed between a working stock and a plaque-pure clone in the rate of resistance to the nucleoside analogue ACV. The working stocks were found to contain variants with resistance to BAY 57-1293 ranging from approximately 15-fold to 4,000-fold suggesting that these viruses have the potential to subvert effective therapy. Sequence analysis of HSV-1 helicase protein showed that most of the amino acid substitutions in the variants described in this study tallied with published results, with some interesting exceptions in the case of HSV-1 strain PDK. Resistant variants did not readily revert to a sensitive phenotype in the absence of the inhibitor and representative BAY 57-1293-resistant variants were cross-resistant to an alternative helicase-primase inhibitor, BILS 22 BS. Variants resistant to BAY 57-1293 retained sensitivity to the nucleoside analogue, ACV.