Both baseline HIV-1 drug resistance and antiretroviral drug levels are associated with short-term virologic responses to salvage therapy.

Background: To determine the impact of HIV-1 drug resistance at baseline and antiretroviral drug levels (DL) during follow-up on virologic response to the next antiretroviral regimen.
Methods: Baseline genotypic and phenotypic susceptibility was obtained for plasma virus from patients failing a protease inhibitor-containing regimen. Untimed plasma antiretroviral DL were performed and the distribution of DL after 12 weeks of follow-up was classified as above (DLHigh) or below (DLLow) the median. Inhibitory quotients [IQ = (DL at week 12)/(fold change in IC50 to wild-type)] were determined for each drug in the regimen. Primary outcome was change in log10 plasma HIV-1 RNA viral load (DeltaVL) from baseline to 12 weeks.
Results: There were 137 patients who had baseline resistance data available for the antiretroviral drugs used in the salvage regimen, and DL at week 12. Each drug with DLHigh was associated with DeltaVL = -0.40 (P = 0.0002) while each drug with DLLow had DeltaVL = -0.16 (P = 0.11). In multivariate models DeltaVL associated with each active drug (defined by genotype) with DLHigh was -0.48 log10 (P < 0.0001), and with each active drug with DLLow was -0.22 (P = 0.03). The DeltaVL was -0.18 if no drugs in the regimen had an IQ > median, compared to -0.58 for one drug, -1.06 for two drugs, -0.86 for three drugs, and -1.44 for four or five drugs with IQ > median (P < 0.0001 for trend).
Conclusions: In salvage therapy, both the number of active drugs and the DL for each drug in the new regimen determine the antiviral response.