The therapeutic range for efavirenz plasma concentrations is unclear and some studies found no correlation with viral non-suppression. Efavirenz concentrations are variable, driven in part by polymorphisms in CYP2B6. We hypothesised that efavirenz mid-dosing concentrations, together with CYP2B6 metaboliser genotype, could predict viral non-suppression. Participants starting first-line efavirenz-based antiretroviral therapy were monitored for 48 weeks. HIV-RNA and efavirenz mid-dose interval concentrations were determined at Weeks 16 and 48. CYP2B6 metaboliser genotype status was determined by 516G→T and 983T→C polymorphisms. Cox proportional hazards modelling was used to predict viral non-suppression and to determine the most predictive efavirenz mid-dosing concentration threshold. In total, 180 participants were included. Median efavirenz concentrations were 2.3 mg/L (IQR 1.6-4.6 mg/L) and 2.2 mg/L (IQR 1.5-3.9 mg/L) at Weeks 16 and 48, respectively. Moreover, 49 (27.2%), 84 (46.7%) and 39 (21.7%) participants had extensive, intermediate or slow CYP2B6 metaboliser genotype, respectively. Log2 efavirenz concentrations [adjusted hazard ratio (aHR) = 0.77, 95% CI 0.67-0.89] and baseline CD4 cell count (aHR = 0.994, 95% CI 0.989-0.998), but not CYP2B6 genotype, were predictive of viral non-suppression. For every doubling of efavirenz concentration there was a 23% decrease in the hazard of non-suppression. A threshold of 0.7 mg/L was found to be the efavirenz mid-dosing concentration that was most predictive of non-suppression. Mid-dosing efavirenz concentrations are predictive of viral non-suppression, but the currently recommended lower therapeutic limit (1 mg/L) is higher than our finding. Knowledge of CYP2B6 metaboliser genotype is not required for prediction of virological outcomes.
Keywords: CYP2B6; Efavirenz; Pharmacogenetic; Pharmacokinetic; Therapeutic drug monitoring; Virological failure.
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