Effect of rifampicin-based antitubercular therapy and the cytochrome P450 2B6 516G>T polymorphism on efavirenz concentrations in adults in South Africa

Background Rifampicin induces expression of the cytochrome P450 isoenzyme 2B6 (CYP2B6), which metabolizes efavirenz. The CYP2B6 516G>T polymorphism impairs efavirenz metabolism and occurs more commonly in Africans than in Caucasians. We explored the effect of rifampicin-based antitubercular therapy and the 516G>T polymorphism on efavirenz concentrations in HIV- infected patients in South Africa. Methods Between-patient and within-patient comparisons were made of mid-dosing interval efavirenz plasma concentrations in adults on antiretroviral therapy including efavirenz 600 mg daily, with and without antitubercular therapy. Results There were 142 participants (40 were on antitubercular therapy and 102 were controls), the mean weight was 66 kg. Median efavirenz concentration was 2.4 mg/l (interquartile range IQR 1.3–3.1) and 1.8 mg/l (IQR 1.4–4.4) in participants on antitubercular therapy and controls, respectively ( P=0.734). Paired efavirenz concentrations during and after antitubercular therapy in 17 participants were also similar ( P=0.113). Genotyping results were 60 (49%) G/G homozygotes, 46 (38%) G/T heterozygotes and 16 (13%) T/T homozygotes. In a multivariate logistic regression model adjusted for sex, weight and concomitant antitubercular therapy, the 516G>T polymorphism was strongly associated with high (>4 mg/l) efavirenz concentrations: odds ratio (OR) 4.4 (95% confidence interval CI 1.3–14.9) for G/T versus G/G and 31.1 (95% CI 6.6–146.6) for T/T versus G/G. High efavirenz concentrations were associated with severe sleep disturbance ( P=0.048). Low (T polymorphism occurred commonly and was associated with high efavirenz concentrations.