Modest decreases in NNRTI susceptibility do not influence virological outcome in patients receiving initial...

To assess the prevalence of modest (< 10-fold) decreases in baseline non-nucleoside reverse transcriptase inhibitor (NNRTI) susceptibility and their impact on virological response to NNRTI-containing triple therapy in drug-naive individuals.

Baseline HIV resistance phenotype, genotype and response to therapy were examined retrospectively for all antiretroviral-naive individuals initiating therapy with two nucleoside analogues and an NNRTI in British Columbia, Canada, between 05/1997 and 08/1999 (n = 279), followed until July 31 2001. Time to viral suppression (first of at least two consecutive plasma viral loads < 400 copies HIV RNA copies/ml) and viral rebound (to > or = 400 copies/ml after first pVL < 400 copies HIV RNA copies/ml), were estimated by Kaplan-Meier methods. Multivariate analyses were performed using Cox proportional hazards regression.

Nevirapine was the most commonly prescribed NNRTI (96%). Four- to 10-fold decreased susceptibility to NNRTIs was observed in > 30% of untreated individuals at baseline, an observation strongly driven by decreased susceptibility to delavirdine (22.4%). A > 10-fold decrease in susceptibility to any NNRTI was observed only rarely (< 2%). There was no association between four- and 10-fold decreased baseline susceptibility to NNRTIs and virological outcome (P > 0.05). In multivariate analyses, the strongest predictors of poor virological response to NNRTI-based therapy were baseline plasma viral load and the proportion of time on therapy in the first year of follow-up. There was no relationship between the presence of previously reported mutations associated with decreased NNRTI susceptibility (at codons 135 and 283 in HIV reverse transcriptase) and virological response.

These data suggest that the clinically significant level of resistance to NNRTIs, particularly nevirapine, in drug-naive individuals is likely greater than four- to 10-fold.