Amprenavir-resistant HIV-1 exhibits lopinavir cross-resistance and reduced replication capacity

Objectives: To evaluate protease inhibitor (PI) cross-resistance and reductions in replication capacity conferred by amprenavir-selective mutations. Methods: HIV-1(IIIB) variants derived from passage in increasing concentrations of amprenavir were studied, as well as 3'Gag/protease recombinants derived from them. These strains progressively accumulated mutations at codons 10, 46, 47, 50 and 84 in the protease as well as a p1/p6 cleavage site mutation at codon 449 in Gag. Their susceptibility (IC50) to various PI and their corresponding replication capacities were evaluated by a single-cycle growth assay and compared with measures using competitive cultures and p24 antigen production. Results: Amprenavir susceptibility decreased with increasing numbers of protease mutations. Changes in lopinavir susceptibility paralleled changes in amprenavir susceptibility. Certain amprenavir-selected mutants conferred greater than 10-fold cross-resistance to lopinavir, including PrL10F/M461/150V-GagL449F (19-fold) and PrL10F/M46I/147V/150V-GagL449F (31-fold). Moreover, one isolate with only two mutations in the protease (L10F/84V) and GagL449F displayed a 7.7-fold increase in lopinavir IC50. Low-level cross-resistance to ritonavir and nelfinavir was also observed. The replication capacity of viruses containing either 184V or 150V was at least 90% lower than the reference virus in the single-cycle assay. The order of relative replication capacity was wild-type > L10F > L10F/184V > L10F/M461/150V > L10F/M461/147V/150V. Conclusion: These results indicate that until more comprehensive genotype-phenotype correlations between amprenavir and lopinavir susceptibility are established, phenotypic testing may be preferable to genotyping to detect cross-resistance, and should be considered when switching patients from a failing amprenavir-containing regimen. This study also provides data on the concordance of replication capacity measurements generated using rapid single-cycle growth and competition assays. (C) 2002 Lippincott Williams Wilkins.