CYP2B6 genotype, but not rifampicin-based anti-TB cotreatments, explains variability in long-term efavirenz plasma exposure.

We investigated the effects of rifampicin-based anti-TB treatment on plasma efavirenz exposure and the implications of CYP2B6 genotype. Antiretroviral therapy-naive Ugandan HIV patients without (n = 157) or with TB coinfection (n = 106) were enrolled and treated with efavirenz-based highly active antiretroviral therapy alone or with rifampicin-based anti-TB therapy, respectively. Efavirenz plasma concentration was determined on day 3 and weeks 1, 2, 8, 12, 16, 20, 24, 28 and 32. Rifampicin-based anti-TB cotreatment reduced plasma efavirenz exposure during the first 2 weeks (p < 0.05), but no significant effect was observed afterwards. Although not significant, rifampicin-based anti-TB cotreatment inconsistently increased efavirenz exposure over time, which was reduced immediately after completing anti-TB therapy. CYP2B6*6, *11 and ABCB1 c.4036A>G genotypes were significant predictors of efavirenz plasma exposure. Plasma efavirenz exposure is mainly influenced by CYP2B6 genotype, but not by rifampicin cotreatment. Therefore, no efavirenz dosage adjustment during rifampicin cotreatment is required in Ugandans.