COMPARISON OF A DNA-PROBE ASSAY WITH THE PLAQUE REDUCTION ASSAY FOR MEASURING THE SENSITIVITY OF HERPES-SIMPLEX VIRUS AND VARICELLA-ZOSTER VIRUS TO PENCICLOVIR AND ACYCLOVIR

A DNA probe assay was compared with the plaque reduction assay to dete rmine the sensitivity of clinical isolates of herpes simplex virus (HS V) and varicella-zoster virus (VZV) to penciclovir and acyclovir in MR C-5 cells. In both assays, penciclovir and acyclovir shared comparable activity against cell-free virus (CFV) preparations of VZV and herpes simplex virus type 1 (HSV-1) isolates, whilst acyclovir was significa ntly more active than penciclovir against herpes simplex virus type 2 (HSV-2) isolates in both the DNA probe assay (P less than or equal to 0.01) and the plaque reduction assay (P less than or equal to 0.01). H owever, the 50% effective concentrations (EC(50)s) were generally lowe r in the DNA probe assay and the correlation between the plaque reduct ion and DNA probe assays was poor for either compound. Six acyclovir-r esistant strains of HSV-1 derived in cell culture were also tested for susceptibility to penciclovir and acyclovir, in the DNA probe and pla que reduction assays. The relative susceptibilities of these strains w ere comparable, for example, one ACV-resistant strain was susceptible to penciclovir in both assays. Further comparisons of the assay method s were made using cell-associated VZV (CAV). As with CFV the EC(50)s w ere significantly lower in the DNA probe assay than the plaque reducti on assay for penciclovir (P less than or equal to 0.01) and acyclovir (P less than or equal to 0.01). In the DNA probe assay there was no si gnificant difference in the EC(50)s for either penciclovir or acyclovi r when comparing CAV with CFV. However, in the plaque reduction assay the EC(50)s for CAV were significantly higher than those for CFV for b oth penciclovir (P less than or equal to 0.01) and acyclovir (P less t han or equal to 0.01). Overall the DNA probe assay is objective, does not require prior titration of isolates and provides opportunities for automation. It is more suitable for sensitivity testing of large numb ers of clinical isolates than the well-established plaque reduction as say.