Mechanism of Action

Atazanavir (ATV) is an azapeptide HIV-1 protease inhibitor (PI). The compound selectively inhibits the virus-specific processing of viral Gag and Gag-Pol polyproteins in HIV-1 infected cells, thus preventing formation of mature virions.

Antiviral Activity in Cell Culture

Atazanavir exhibits anti-HIV-1 activity with a mean 50% effective concentration (EC50) in the absence of human serum of 2 to 5 nM against a variety of laboratory and clinical HIV-1 isolates grown in peripheral blood mononuclear cells, macrophages, CEM-SS cells, and MT-2 cells.

Atazanavir has activity against HIV-1 Group M subtype viruses A, B, C, D, AE, AG, F, G, and J isolates in cell culture. Atazanavir has variable activity against HIV-2 isolates (1.9 to 32 nM), with EC50 values above the EC50 values of failure isolates. Two-drug combination antiviral activity studies with atazanavir showed no antagonism in cell culture with PIs (amprenavir, indinavir, lopinavir, nelfinavir, ritonavir, and saquinavir), NNRTIs (delavirdine, efavirenz, and nevirapine), NRTIs (abacavir, didanosine, emtricitabine, lamivudine, stavudine, tenofovir DF, and zidovudine), the HIV-1 fusion inhibitor enfuvirtide, and two compounds used in the treatment of viral hepatitis, adefovir and ribavirin, without enhanced cytotoxicity.

Resistance

In Cell Culture: HIV-1 isolates with a decreased susceptibility to atazanavir have been selected in cell culture and obtained from patients treated with atazanavir or atazanavir with ritonavir. HIV-1 isolates with 93- to 183-fold reduced susceptibility to atazanavir from three different viral strains were selected in cell culture by 5 months. The substitutions in these HIV-1 viruses that contributed to atazanavir resistance include I50L, N88S, I84V, A71V, and M46I. Changes were also observed at the protease cleavage sites following drug selection. Recombinant viruses containing the I50L substitution without other major PI substitutions were growth impaired and displayed increased susceptibility in cell culture to other PIs (amprenavir, indinavir, lopinavir, nelfinavir, ritonavir, and saquinavir). The I50L and I50V substitutions yielded selective resistance to atazanavir and amprenavir, respectively, and did not appear to be cross-resistant.

Clinical Studies of Treatment-Naive Subjects: Comparison of Ritonavir-Boosted Atazanavir vs Unboosted Atazanavir: Study AI424-089 compared atazanavir 300 mg once daily with ritonavir 100 mg vs atazanavir 400 mg once daily when administered with lamivudine and extended-release stavudine in treatment-naive subjects with HIV-1 infection. A summary of the number of virologic failures and virologic failure isolates with atazanavir resistance in each arm is shown in Table 23.

Clinical Studies of Treatment-Naive Subjects Receiving Atazanavir 300 mg with Ritonavir 100 mg: In Phase 3 Study AI424-138, an as-treated genotypic and phenotypic analysis was conducted on samples from subjects who experienced virologic failure (HIV-1 RNA ≥400 copies/mL) or discontinued before achieving suppression on atazanavir with ritonavir (n=39; 9%) and lopinavir/ritonavir (n=39; 9%) through 96 weeks of treatment. In the atazanavir with ritonavir arm, one of the virologic failure isolates had a 56-fold decrease in atazanavir susceptibility emerge on therapy with the development of PI resistance-associated substitutions L10F, V32I, K43T, M46I, A71I, G73S, I85I/V, and L90M. The NRTI resistance-associated substitution M184V also emerged on treatment in this isolate conferring emtricitabine resistance. Two atazanavir with ritonavir-virologic failure isolates had baseline phenotypic atazanavir resistance and IAS-defined major PI resistance-associated substitutions at baseline. The I50L substitution emerged on study in one of these failure isolates and was associated with a 17-fold decrease in atazanavir susceptibility from baseline and the other failure isolate with baseline atazanavir resistance and PI substitutions (M46M/I and I84I/V) had additional IAS-defined major PI substitutions (V32I, M46I, and I84V) emerge on atazanavir treatment associated with a 3-fold decrease in atazanavir susceptibility from baseline. Five of the treatment failure isolates in the atazanavir with ritonavir arm developed phenotypic emtricitabine resistance with the emergence of either the M184I (n=1) or the M184V (n=4) substitution on therapy and none developed phenotypic tenofovir disoproxil resistance. In the lopinavir/ritonavir arm, one of the virologic failure subject isolates had a 69-fold decrease in lopinavir susceptibility emerge on therapy with the development of PI substitutions L10V, V11I, I54V, G73S, and V82A in addition to baseline PI substitutions L10L/I, V32I, I54I/V, A71I, G73G/S, V82V/A, L89V, and L90M. Six lopinavir/ritonavir virologic failure isolates developed the M184V substitution and phenotypic emtricitabine resistance and two developed phenotypic tenofovir disoproxil resistance.

Clinical Studies of Treatment-Naive Subjects Receiving Atazanavir 400 mg without Ritonavir: atazanavir-resistant clinical isolates from treatment-naive subjects who experienced virologic failure on atazanavir 400 mg treatment without ritonavir often developed an I50L substitution (after an average of 50 weeks of atazanavir therapy), often in combination with an A71V substitution, but also developed one or more other PI substitutions (eg, V32I, L33F, G73S, V82A, I85V, or N88S) with or without the I50L substitution. In treatment-naive subjects, viral isolates that developed the I50L substitution, without other major PI substitutions, showed phenotypic resistance to atazanavir but retained in cell culture susceptibility to other PIs (amprenavir, indinavir, lopinavir, nelfinavir, ritonavir, and saquinavir); however, there are no clinical data available to demonstrate the effect of the I50L substitution on the efficacy of subsequently administered PIs.

Clinical Studies of Treatment-Experienced Subjects: In studies of treatment-experienced subjects treated with atazanavir or atazanavir with ritonavir, most atazanavir-resistant isolates from subjects who experienced virologic failure developed substitutions that were associated with resistance to multiple PIs and displayed decreased susceptibility to multiple PIs. The most common protease substitutions to develop in the viral isolates of subjects who failed treatment with atazanavir 300 mg once daily and ritonavir 100 mg once daily (together with tenofovir DF and an NRTI) included V32I, L33F/V/I, E35D/G, M46I/L, I50L, F53L/V, I54V, A71V/T/I, G73S/T/C, V82A/T/L, I85V, and L89V/Q/M/T. Other substitutions that developed on atazanavir with ritonavir treatment including E34K/A/Q, G48V, I84V, N88S/D/T, and L90M occurred in less than 10% of subject isolates. Generally, if multiple PI resistance substitutions were present in the HIV-1 virus of the subject at baseline, atazanavir resistance developed through substitutions associated with resistance to other PIs and could include the development of the I50L substitution. The I50L substitution has been detected in treatment-experienced subjects experiencing virologic failure after long-term treatment. Protease cleavage site changes also emerged on atazanavir treatment, but their presence did not correlate with the level of atazanavir resistance.

Cross-Resistance

Cross-resistance among PIs has been observed. Baseline phenotypic and genotypic analyses of clinical isolates from atazanavir clinical trials of PI-experienced subjects showed that isolates cross-resistant to multiple PIs were cross-resistant to atazanavir. Greater than 90% of the isolates with substitutions that included I84V or G48V were resistant to atazanavir. Greater than 60% of isolates containing L90M, G73S/T/C, A71V/T, I54V, M46I/L, or a change at V82 were resistant to atazanavir, and 38% of isolates containing a D30N substitution in addition to other changes were resistant to atazanavir. Isolates resistant to atazanavir were also cross-resistant to other PIs with >90% of the isolates resistant to indinavir, lopinavir, nelfinavir, ritonavir, and saquinavir, and 80% resistant to amprenavir. In treatment-experienced subjects, PI-resistant viral isolates that developed the I50L substitution in addition to other PI resistance-associated substitution were also cross-resistant to other PIs.

Baseline Genotype/Phenotype and Virologic Outcome Analyses

Genotypic and/or phenotypic analysis of baseline virus may aid in determining atazanavir susceptibility before initiation of atazanavir with ritonavir therapy. An association between virologic response at 48 weeks and the number and type of primary PI resistance-associated substitutions detected in baseline HIV-1 isolates from antiretroviral-experienced subjects receiving atazanavir with ritonavir once daily or lopinavir/ritonavir (fixed-dose product) twice daily in Study AI424-045 is shown in Table 24.

Overall, both the number and type of baseline PI substitutions affected response rates in treatment-experienced subjects. In the atazanavir with ritonavir group, subjects had lower response rates when 3 or more baseline PI substitutions, including a substitution at position 36, 71, 77, 82, or 90, were present compared to subjects with 1–2 PI substitutions, including one of these substitutions.

The response rates of antiretroviral-experienced subjects in Study AI424-045 were analyzed by baseline phenotype (shift in susceptibility in cell culture relative to reference, Table 25). The analyses are based on a select population with 62% of subjects receiving an NNRTI-based regimen before study entry compared to 35% receiving a PI-based regimen. Additional data are needed to determine clinically relevant break points for atazanavir.