Objective: To assess the patterns of HIV phenotypic cross-resistance to protease inhibitors (PI) in patients experiencing viral load rebound on combination therapy including a PI.
Methods: Phenotypic analysis of sensitivity to indinavir, nelfinavir, saquinavir, ritonavir and amprenavir was carried out using a single-cycle recombinant virus assay. Viral protease was sequenced by automated dideoxynucleotide chain termination.
Results: Of the 108 patients studied, 68 had received indinavir, 50 ritonavir, 25 saquinavir and eight nelfinavir. The majority (71%) had received only one PI. The incidence of cross-resistance between indinavir, nelfinavir, ritonavir and saquinavir was high (60-90%). Cross-resistance to amprenavir was less frequent (37-40%). However there was some correlation between levels of sensitivity to amprenavir and indinavir (r2 = 0.34; P < 0.01). Conversely, the correlation between levels of sensitivity to indinavir and saquinavir was poor (r2 = 0.25), particularly for patients who had not received saquinavir. The degree of cross-resistance correlated with the level of resistance and with the total number of mutations in the protease gene (P < 0.05, chi square test) but could not be significantly correlated to any one particular mutation or combination of mutations. Mutation 184V was significantly associated with cross-resistance to amprenavir, with no mutations at codon 50 observed, while mutations associated with cross-resistance to saquinavir differed according to the treatment received.
Conclusions: These results suggest that, although the total number of protease mutations correlates with the degree of cross-resistance, the specific mechanisms accounting for primary resistance and for cross-resistance may be different.