ISOLATION OF HUMAN ADENOVIRUS TYPE-5 VARIANTS RESISTANT TO THE ANTIVIRAL CIDOFOVIR

Purpose. Cidofovir (S-HPMPC) is a potent broad-spectrum antiviral drug with potential clinical application against infections caused by huma n cytomegalovirus, herpes simplex virus, and adenovirus (AD). This stu dy sought to determine whether variants of AD5 could be isolated in vi tro that demonstrated increased resistance to this new antiviral drug. Methods. Homogenous stocks of wild-type AD5 (ATCC strain VR-5) were g enerated from isolated plaques grown in A549 cells. The stocks subsequ ently were serially passaged in cells containing increasing levels (fr om 5 to 75 mu g/ml) of cidofovir. The recovered virus either was passa ged, titrated, or assayed for 50% inhibitory concentration (IC50) of c idofovir. Results, Three independently isolated variants were obtained that demonstrated increased resistance to cidofovir. Viral resistance to the drug increased on stepwise passage in higher concentrations. C ompared to the ATCC AD5 reference (IC50 = 6.2 mu g/ml), stable cidofov ir-resistant variants showed fivefold to eightfold resistance (AD5 R1 IC50 = 36.5 mu g/ml; AD5 R2 IC50 = 36.7 mu g/ml; and AD5 R3 IC50 = 32. 6 mu g/ml; analysis of variance, P = 0.000001). However, a variable nu mber of passages (1 to 13) at each concentration of cidofovir was perf ormed to obtain robust infectious virus suitable for testing at the ne xt higher concentration. All resistant virus isolates grew to levels o f virus titer comparable to the parental virus and showed no apparent phenotypic changes in growth rates, plaque size, or efficiency of plaq ue formation. Conclusions. The successful isolation of AD5 variants in tissue culture resistant to cidofovir has important clinical implicat ions with respect to the anticipated use of this antiviral drug in tre ating adenoviral ocular infections.