Single Amino Acid Substitution Within the Helicase of Varicella Zoster Virus Makes It Resistant to Amenamevir

A helicase-primase inhibitor, amenamevir (ASP2151), is the active pharmaceutical ingredient of a drug for the herpes zoster that is caused by reactivation of varicella-zoster virus (VZV). Here we report a new amenamevir-resistant VZV isolated under the selection pressure of amenamevir. The resistant virus has a nonsynonymous mutation K350N in the helicase gene ORF55. A recombinant virus artificially constructed harboring the ORF55 K350N also acquired amenamevir resistance, and thus the single amino-acid substitution in helicase is revealed to be responsible for the resistance. We observed that the drug-resistant virus and the ORF55 K350N recombinant virus have high resistance to amenamevir, as the EC₅₀ values in a plaque reduction assay were > 100 μM, while the two viruses remained susceptible to the nucleoside analog drug acyclovir. No defect in viral growth was observed for these resistant viruses in a plaque size assay in human malignant melanoma cells. However, defect in plaque formation was observed from resistant virus in human fetal lung fibroblast cells, showing that the growth of the resistant virus is dependent on the cell type. We observed that the single amino-acid substitution in the helicase induces amenamevir resistance, confirming the importance of the helicase in amenamevir's inhibition of virus growth. Our findings highlight the importance of regulating the clinical use of amenamevir to minimize the risk of the emergence of helicase K350N mutation, especially in the long-term use of amenamevir by immunosuppressed patients.