Abstract

Helicase–primase inhibitors (HPIs), e.g. BAY 57-1293, are extremely active against HSV in cell culture and animal infection models. They target the helicase–primase (HP) complex which is involved in virus DNA replication. Using BAY 57-1293 at inhibitory concentrations (e.g. 10–100 times the IC50) it was possible to detect HPI-resistant viruses in two different laboratory working stocks of HSV-1 following a single passage with the inhibitor in Vero cells. Furthermore, resistance selection occurred when the inhibitor was continuously present from prior to virus inoculation suggesting that certain resistance mutations may pre-exist in virus populations at relatively high frequency. PCR data will be presented to confirm these observations. It was shown subsequently that 2 out of 10 recent clinical isolates of HSV-1 also contained BAY 57-1293-resistant variants at 10−4 to 10−5 p.f.u. This is similar to the laboratory isolates and 10–100 times the previously reported spontaneous rate for HPI-resistance mutations (10−6) in plaque-purified HSV-1 strains. The most common resistance mutations involved three amino acid residues just down-stream from the predicted helicase motif IV in HSV-1 UL5 and one residue near the C-terminus of the primase (UL52). We also showed that certain HPI-resistance mutations in UL5 are associated with increased or decreased virus growth in tissue culture with concomitant effects on pathogenicity.