The Correlation between Plasma Concentrations of Protease Inhibitors, Medication Adherence and Virological Outcome in HIV-Infected Patients

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Abstract
Background: Although adherence clearly influences response to antiretroviral therapy (ART), accurate assessment of adherence is problematic. The objective of this analysis was to assess the independent predictive value of protease inhibitor (PI) concentrations as a supplement to self-report as markers of medication adherence. Methods: This retrospective analysis was conducted from a prospective clinical trial designed to compare the outcomes of frequent versus infrequent HIV RNA measurement used to manage antiretroviral therapy. For 131 patients, self-reported medication adherence, HIV RNA levels, CD4 counts and PI concentrations (unannounced, random samples) were measured at baseline (when patients changed to a new regimen) and every 2 months thereafter. The change in HIV RNA from baseline to month 6 (area-based measure) was used to evaluate overall response. The proportion of measured PI concentrations below the detection limit was used as an alternative marker of adherence. An undetectable concentration would be expected after missing a single dose. Results: The mean baseline CD4 count was 125 cells/mm3 and the mean HIV RNA level was 4.7 log10 copies/ml. The mean change in log10 HIV RNA was -0.73 copies/ml. The mean percentage of self-reported adherence was 91% (range: 15–100%) and the mean proportion of undetectable PI concentrations was 27% (range: 0–100%, mean 2.5 samples/patient). The correlation between the two measures was -0.23 ( P=0.009). In a multivariate model, percentage of visits with undetectable PI concentrations ( P=0.02), percentage of medication adherence ( P=0.02), baseline HIV RNA level ( P=0.005), prior PI use ( P=0.0004), prior lamivudine (3TC) use ( P=0.0009) and randomization to the frequent HIV RNA measurement group ( P10 additional HIV RNA reduction compared with those who had no detectable concentrations. Conclusions: Repeated, random PI concentration values are independently predictive of virological response and may add to self-report of adherence in understanding the response to ART.

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