Long-term foscarnet therapy remodels thymidine analogue mutations and reverses resistance to zidovudine.

Objective: To study the evolution of multi-drug-resistant HIV-1 in treatment-experienced patients receiving foscarnet (PFA) as part of salvage therapy and to investigate the virological consequences of emerging mutations. Methods: Genotypic and phenotypic resistance tests were performed on plasma viruses from seven patients at baseline and during treatment with PFA. The phenotypic effects of mutations suspected to be associated with PFA resistance were evaluated by site-directed mutagenesis of wild-type or thymidine analogue mutations (TAM)carrying pNL4-3. Reversion of single mutations was performed in a patient-derived recombinant clone. Results: Baseline multi-drug-resistant isolates exhibited hypersusceptibility to PFA. In two patients who received >12 months of PFA treatment, a novel mutation pattern including K706, V75T, K219R and L228R emerged. These viruses had 3-6-fold resistance to PFA, compared to baseline, and 14-39-fold resistance to lamivudine, in the absence of M184V. In wild-type clones mutations K706 and V75T induced moderate PFA resistance. In the case of TAMs, combinations of >= 3 mutations (K706+K219R+L228R +/- V75T) induced PFA resistance and decreased zidovudine resistance 3-13fold. These mutants exhibited high-level lamivudine resistance (>20-fold) without mutation M1184V. Reversion of K706 -> R and K219R -> E in a patientderived clone confirmed the contribution of individual mutations and the negative association between PFA resistance and zidovudine resistance. Conclusions: In the context of multiple TAMs, hypersusceptibility to PFA was observed and a novel pattern of resistance, including alternative amino acid substitutions at TAM loci, emerged. This mutational pattern was associated with decreases in zidovudine resistance and a 2-20-fold decrease in resistance to zidovudine surprisingly high-level lamivudine resistance.