Characterization of herpes simplex viruses selected in culture for resistance to penciclovir or acyclovir

Penciclovir (PCV), an antiherpesvirus agent in the same class as acyclovir (ACV), is phosphorylated in herpes simplex virus (HSV)-infected cells by th e viral thymidine kinase (TK). Resistance to ACV has been mapped to mutatio ns within either the TK or the DNA polymerase gene. An identical activation pathway, the similarity in mode of action, and the invariant cross-resista nce of TK-negative mutants argue that the mechanisms of resistance to PCV a nd ACV are likely to be analogous. A total of 48 HSV type 1 (HSV-1) and HSV -2 isolates were selected after passage in the presence of increasing conce ntrations of PCV or ACV in MRC-9 cells. Phenotypic analysis suggested these isolates were deficient in TK activity. Moreover, sequencing of the TK gen es from ACV-selected mutants identified two homopolymeric G-C nucleotide st retches as putative hot spots, thereby confirming previous reports examinin g Acv(r) clinical isolates. Surprisingly, mutations identified in PCV-selec ted mutants were generally not in these regions but distributed throughout the TK gene and at similar frequencies of occurrence within A-T or G-C nucl eotides, regardless of virus type. Furthermore, HSV-1 isolates selected in the presence of ACV commonly included frameshift mutations, while PCV-selec ted HSV-1 mutants contained mostly nonconservative amino acid changes. Data from this panel of laboratory isolates show that Pcv(r) mutants share cros s-resistance and only limited sequence similarity with HSV mutants identifi ed following ACV selection. Subtle differences between PCV and ACV in the i nteraction with viral TK or polymerase may account for the different spectr a of genotypes observed for the two sets of mutants.