Background:  The availability of HAART for patients who have previously failed protease inhibitor- (PI)-containing regimens is limited by the substantial amount of cross-resistance among the currently approved PI. AG-001859 is a novel PI that demonstrates potent in vitro antiviral activity against strains of HIV resistant to the currently approved PI.

Methods:  The ability of HIV to develop in vitro resistance to AG-001859 was evaluated by serial passage of wild type HIV-1 NL4-3 in the presence of increasing concentrations of compound. As a control, HIV-1 was passaged in parallel in the presence of escalating concentrations of lamivudine (3TC), a nucleoside reverse transcriptase inhibitor that rapidly selects for resistant HIV-1 variants in vitro.

Results:  An HIV-1 variant exhibiting high-level resistance to 3TC (> 100-fold) was selected after only 20 days in culture (passage 4), and contained the single characteristic 3TC-associated resistance substitution M184I. In contrast, resistance to AG-001859 was slow to emerge. A gradual accumulation of mutations in the protease gene was observed, with the first mutation (I84V) appearing after 132 days in culture (passage 28) followed by V82I (163 days; passage 35), and then M46L (193 days; passage 43). In addition to changes in protease, a mutation in the P1-P6 gag proteolytic processing site (NFLQ®NFFQ) appeared on day 163 (passage 35) and remained present throughout the duration of the experiment. Phenotypic analyses of virus containing the I84V (passage 33), V82I/I84V (passage 40), and M46L/V82I/I84V (passage 46) substitutions demonstrated 1-, 3-, and 12-fold reduced susceptibility to AG‑001859, respectively. AG-001859-resistant virus containing the V82I/I84V (passage 40) and M46L/V82I/I84V (passage 46) substitutions demonstrated < 10-fold reduced susceptibility to the approved PI evaluated (nelfinavir, atazanavir, lopinavir, tipranavir, and indinavir), with the exception of amprenavir, which exhibited a 25-fold reduction in activity against the M46L/V82I/I84V (passage 46) virus.

Conclusions:  Under experimental conditions conducive to the rapid emergence of high-level resistance to 3TC, strains of HIV-1 resistant to AG-001859 were slow to emerge, with a gradual increase in AG-001859 resistance observed as mutations in the protease and gag cleavage sites accumulated.

Keywords: Protease Inhibitor; AG-001859; Resistance