Drug combinations and effect parameters of zidovudine, stavudine, and nevirapine in standardized drug-sensitive and resistant HIV type 1 strains.

Reference strains of HIV-1 from the NIH AIDS Research and Reference Reagent Program, including wild-type IIIB, G762-3, and AZT resistant with RT 215T-->Y (G910-11/AZT); 67D-->N, 70K-->R, 215T-->F, 219K-->Q (G691-2/AZT); as well as nevirapine (NEV) resistant with 181Y-->C (N119/NEV); and 103K-->N, 181Y-->C (A17/NEV), were subjected to quantitative parametric efficacy analysis using AZT, stavudine (D4T), and nevirapine (NEV) singly or in combinations in MT4 or MT2 cells. The median-effect principle and combination index (CI) method of Chou-Talalay (see Ref. 26) have been used, which take into account both the potency (Dm value or EC50) and the shape of the dose-effect curve (m value). Under standardized assay conditions, G910-11 and G691-2 strains showed 600- and 7800-fold resistance to AZT, and N119 and A17 strains showed 3600- and 1000-fold resistance to NEV at the EC50 level, respectively. AZT-resistant strains exhibited slight cross-resistance to D4T. Computerized analysis indicates that IIIB gave sigmoidal dose-effect curves (m = 2.8, 3.4, and 3.1 for AZT, D4T, and NEV, respectively) whereas drug-resistant strains showed negative sigmoidicity toward the corresponding AZT or NEV, with m = 0.27-0.73. Therefore, the degrees of drug resistance are drastically different at classic EC50 and at therapeutically more relevant EC95 levels (ranging from severalfold to several log orders). Combinations of AZT+NEV and AZT+NEV+D4T showed synergism against IIIB, G762-3 (wild type) and A17/NEV, G910-11/AZT strains. D4T+NEV and AZT+D4T showed nearly additive or moderate antagonism. Synergism or additive effect leads to a favorable dose-reduction index (DRI). The present study on RT inhibitors provides quantitative assessment of the combinations of AZT, NEV, and D4T against HIV infections involving drug-sensitive and drug-resistant HIVs.