In Vivo Resistance to a Human Immunodeficiency Virus Type 1 Proteinase Inhibitor: Mutations, Kinetics, and Frequencies

Resistance to saquinavir (Ro 31-8959), an inhibitor of human immunodeficiency virus type 1 proteinase, was studied in peripheral blood mononuclear cell-derived proviral DNA from patients undergoing prolonged treatment. A Leu90→Met exchange was the predominant resistance mutation in vivo; Gly48→Val or doubly mutant virus was rarely observed. After 8–12 months of treatment with saquinavir alone (600 mg, 3 times/day) or in combination with zidovudine (200 mg, 3 times/day), ∼45% of all patients carried provirus with mutant proteinase; the incidence was lower (22%) in patients treated with a combination of saquinavir, zidovudine, and dideoxycytidine. There was a good relationship between genotypic analysis of saquinavir resistance and data from virus assays, confirming that Leu90→Met and Gly48→Val are the essential exchanges in the proteinase that determine loss of sensitivity to this inhibitor. Absence of genotypic resistance correlated with a sustained decrease in plasma viral RNA. There was a positive correlation between a Met90 mutation and some residues at natural polymorphic sites (positions 10, 36, 63, and 71).