Mid-dosing interval concentration of atazanavir and virological outcome in patients treated for HIV-1 infection.

OBJECTIVES: We investigated the clinical significance of monitoring the mid-dosing interval atazanavir (ATV) concentration (measured 12 +/- 2 h after intake; C(12 h)) in patients taking this drug once daily in the evening. METHODS: We retrospectively selected HIV-infected patients harbouring ATV-susceptible virus who underwent therapeutic drug monitoring (TDM) of ATV C(12 h) during routine out-patient visits, and we correlated C(12 h) to the 24-week virological response and toxicity. RESULTS: A total of 115 plasma samples from 86 patients (76.7% with baseline HIV RNA<50 HIV-1 RNA copies/mL) were analysed. ATV plasma concentrations showed high inter-individual variability. ATV plasma levels were higher in samples obtained from patients taking boosted regimens (P<0.001) and not concomitantly receiving acid-reducing agents (P=0.007). In a multivariate model, ritonavir boosting, use of acid-reducing agents and liver cirrhosis showed an independent association with ATV level. Virological response at 24 weeks was observed for 94 of the 115 samples (81.7%). We identified a concentration cut-off of 0.23 mg/L which predicted virological response at 24 weeks: samples with a C(12 h)< or =0.23 mg/L showed virological failure in 41.2% of cases, whereas samples with a C(12 h)>0.23 mg/L showed virological failure in 14.3% of cases (P=0.021). In multivariate analysis, C(12 h)>0.23 mg/L was an independent predictor of virological response [odds ratio (OR) 4.23, P=0.031]. ATV levels correlated with concomitant unconjugated bilirubin levels (r=0.223, P=0.037), but a concentration cut-off predictive of moderate/severe hyperbilirubinaemia could not be identified. CONCLUSIONS: We identified a C(12 h) efficacy threshold that predicted virological response; this could be useful for morning TDM in selected subjects receiving ATV in the evening. Results must be interpreted with caution given the retrospective design of the study.