Identification of I50L as the signature atazanavir (ATV)-resistance mutation in treatment-naive HIV-1-infected patients receiving ATV-containing regimens.

Atazanavir (ATV) is a once-daily human immunodeficiency virus (HIV) protease inhibitor (PI) shown to be effective and well tolerated. ATV has a distinct resistance profile relative to other PIs, with susceptibility maintained against 86% of isolates resistant to 1-2 PIs. Clinical isolates obtained from PI-naive patients designated as experiencing virologic failure while receiving ATV-containing regimens contained a unique isoleucine-to-leucine substitution at amino acid residue 50 (I50L) of the HIV-1 protease. The I50L substitution, observed in all isolates exhibiting phenotypic resistance to ATV, emerged in a variety of different backgrounds and was most frequently accompanied by A71V, K45R, and/or G73S. Viruses containing an I50L substitution were growth impaired, displayed ATV-specific resistance, and had increased susceptibilities (</=0.4 of reference strain) to other PIs. Comparison of viruses bearing I50L with those bearing I50V revealed specific resistance to ATV and amprenavir, respectively, with no evidence of cross-resistance. The unique I50L substitution is the signature mutation for resistance to ATV.