Opposite effects of cytomegalovirus UL54 exonuclease domain mutations on acyclovir and cidofovir susceptibility

Acyclovir has weak activity against human cytomegalovirus (CMV). Despite some efficacy as prophylaxis, more potent anti-CMV drugs are preferred. Acyclovir resistance of CMV has been little studied. The viral UL97 kinase phosphorylates acyclovir, and cross-resistance of ganciclovir-resistant mutants is documented. However, UL54 exonuclease domain mutants may confer ganciclovir and cidofovir resistance by a mechanism that does not apply to acyclovir as an obligate chain terminator. To test for differential susceptibilities, 11 exonuclease domain mutants were tested for their 50% inhibitory concentrations (EC50s) of acyclovir in comparison with cidofovir. The 5 mutants with the highest cidofovir EC50s (>10-fold increased over wild type) all had acyclovir EC50s less than 20% of wild type. The relatively common N408K mutant had an acyclovir EC50 of 6 μM, comparable to that reported for wild type varicella-zoster virus. Several foscarnet-resistant UL54 mutants outside the exonuclease domains, some with low-grade ganciclovir/cidofovir cross-resistance, showed various degrees of acyclovir resistance. Based on these in vitro data, acyclovir may become a therapeutic option when a highly cidofovir-resistant exonuclease mutation is present without a simultaneous mutation in UL97.