Frequency and patterns of protease gene resistance mutations in HIV-infected patients treated with lopinavir/ritonavir as their first protease inhibitor

BACKGROUND: Selection of protease mutations on antiretroviral therapy (ART) including a ritonavir-boosted protease inhibitor (PI) has been reported infrequently. Scarce data exist from long-term cohorts on resistance incidence or mutational patterns emerging to different PIs. METHODS: We studied UK patients receiving lopinavir/ritonavir as their first PI, either while naive to ART or having previously received non-PI-based ART. Virological failure was defined as viral load ≥ 400 copies/mL after previous suppression <400 copies/mL, or failure to achieve <400 copies/mL during the first 6 months. pol sequences whilst failing lopinavir or within 30 days after stopping were analysed. Major and minor mutations (IAS-USA 2008-after exclusion of polymorphisms) were considered. Predicted susceptibility was determined using the Stanford HIVdb algorithm. RESULTS: Three thousand and fifty-six patients were followed for a median (IQR) of 14 (6-30) months, of whom 811 (27%) experienced virological failure. Of these, resistance test results were available on 291 (36%). One or more protease mutations were detected in 32 (11%) patients; the most frequent were I54V (n = 12), M46I (n = 11), V82A (n = 7) and L76V (n = 3). No association with viral subtype was evident. Many patients retained virus predicted to be susceptible to lopinavir (14, 44%), tipranavir (26, 81%) and darunavir (27, 84%). CONCLUSIONS: This study reflects the experience of patients in routine care. Selection of protease gene mutations by lopinavir/ritonavir occurred at a much higher rate than in clinical trials. The mutations observed showed only partial overlap with those previously identified by structural chemistry models, serial cell culture passage and genotype-phenotype analyses. There remained a low degree of predicted cross-resistance to other widely used PIs.