Biochemical Characterization of a Virus Isolate, Recovered from a Patient with Herpes Keratitis, That Was Clinically Resistant to Acyclovir

In vitro susceptibility assays of herpes simplex virus (HSV) do not necessarily correlate with treatment outcome. An HSV type 1 (HSV-1) isolate, N4, recovered from a patient who presented with herpes keratitis with localized immunosuppression, was characterized for susceptibility. Although the 50% inhibitory concentration (IC50) for this isolate was less than the accepted breakpoint for defining resistance to acyclovir (>2.0 µg/mL), the following lines of evidence suggest that the isolate was acyclovir resistant: (1) the clinical history confirmed that the infection was nonresponsive to acyclovir; (2) the in vitro susceptibility was similar to that of a thymidine kinase (TK)-negative, acyclovir-resistant virus SLU360; (3) the IC50 of acyclovir was more than 10 times the IC50 for an acyclovir-susceptible control strain; (4) plaque-purified clonal isolates were resistant to acyclovir (IC50s, >2.0 µg/mL); and (5) biochemical studies indicated that the HSV-1 N4 TK was partially impaired for acyclovir phosphorylation. Although residue changes were found in both the viral tk and pol coding regions of HSV-1 N4, characterization of a recombinant virus expressing the HSV-1 N4 polymerase suggested that the TK and Pol together conferred the acyclovir-resistance phenotype.