Efavirenz-based antiretroviral regimen is preferred during rifampin-containing antituberculous therapy. The adult fixed dose of 600 mg per day is associated with wide interindividual variability in plasma concentrations, as well as clinical outcome [1–3]. The variability in efavirenz concentrations is even greater during coadministration with rifampin or rifampin-containing antituberculous therapy [4–6], suggesting a variable degree of drug–drug interaction. Among healthy volunteers, rifampin caused a significant reduction in efavirenz area under the curve (AUC) in 10 volunteers, but two volunteers had higher AUC on than off rifampin . In HIV/tuberculosis (TB)-coinfected patients, the change in efavirenz AUC with concomitant rifampin administration ranged from −65% to +37% in one study . Efavirenz is primarily metabolized by hepatic CYP2B6, with some contributions from CYP3A4/5 , CYP2A6 , and UGT2B7 enzymes . The lack of induction of efavirenz metabolism by rifampin in some individuals is contrary to the expected effect of rifampin on CYP2B6 activity .
A bimodal effect of rifampin-containing therapy on efavirenz plasma concentrations has also been reported, with paradoxically higher concentrations with antituberculous therapy in the patients who appeared to be slow efavirenz metabolizers (based on phenotype only, as enzyme genotyping was not done) [5,12]. Also, some HIV/TB-coinfected patients have required decreased efavirenz doses and/or discontinuation during rifampin-containing therapy because of severe toxicities associated with elevated efavirenz concentrations [13,14].
Elevations of efavirenz plasma concentration with rifampin-containing therapy that occur predominantly in persons with slow metabolizing phenotype suggest the possibility of a genotype-dependent drug–drug interaction. In this study, we investigated the potential for such an interaction between rifampin-containing antituberculous therapy and CYP2B6 516G→T polymorphism using data from a pharmacokinetic study conducted previously. The characteristics of the patients and results of the primary analysis are reported elsewhere . All patients received efavirenz 600 mg per day with didanosine and lamivudine, and pharmacokinetic sampling was performed on day 28 of therapy. Antiretroviral therapy was initiated within 2–8 weeks of antituberculous therapy in coinfected patients and only those on concurrent therapy at the time of pharmacokinetic sampling were included in this analysis. The study was approved by the Institutional Review Board of the Nogouchi Memorial Institute for Medical Research, Ghana. Informed written consent was obtained from all patients.
Pharmacokinetic samples were drawn approximately 12 h after the efavirenz dose was taken. Efavirenz plasma concentrations were measured using a validated high-performance liquid chromatography/ultraviolet method . Volunteers were genotyped for CYP2B6 c.516G→T (Q172H and rs3745274) using the fluorometric 5′-nuclease genotyping assay (Applied Biosystems, Foster City, California, USA). Two-way analysis of variance (ANOVA), which incorporates an interaction model, was used to investigate the effects of CYP2B6 516G→T polymorphism and rifampin-containing TB therapy. As in previous studies [15,17], concentration data were not normally distributed and so they were log-transformed prior to analysis. In instances where the ANOVA indicated a significant effect (P < 0.05), post-hoc multiple pairwise comparison testing was performed using the Student–Newman–Keuls method to identify groups that were significantly different from each other.
The analysis included 38 HIV-infected patients on efavirenz-based therapy and 18 HIV/TB-coinfected patients on efavirenz-based and rifampin-containing antituberculous therapies. Concentration data from individual volunteers categorized by CYP2B6 516G→T genotype and type of therapy are shown in Fig. 1. The two-way ANOVA revealed a statistically significant effect of CYP2B6 516 genotype on efavirenz plasma concentrations, as well as an interaction between CYP2B6 516G→T genotype and the antituberculous therapy (P < 0.001 for CYP2B6 516G→T polymorphism; P = 0.089 for rifampin-containing therapy; and P = 0.022 for interaction). Post-hoc pairwise analysis showed that patients with CYP2B6 516TT genotype who were receiving concurrent therapy had a significantly higher geometric mean efavirenz concentration than those on only antiretroviral therapy (14689 vs. 6012 ng/ml, respectively; P = 0.006) (Fig. 1). Patients with CYP2B6 516GT genotype on concurrent antituberculous therapy had a slightly higher geometric mean efavirenz concentration than those on HAART only (1901 vs. 1778 ng/ml) and the patients with the CYP2B6 516GG genotype on antituberculous therapy had a lower geometric mean efavirenz concentrations than those with the CYP2B6 516GG genotype on HAART only (1291 vs. 1552 ng/ml), but these latter differences between therapeutic groups did not achieve statistical significance (P > 0.05).
As a CYP2B6 enzyme inducer, rifampin coadministration would be expected to enhance efavirenz clearance and reduce plasma levels. However, in this study, we found that the patients with CYP2B6 516TT genotype and receiving concomitant antituberculous therapy had higher efavirenz plasma concentrations than those volunteers with the same genotype, but not receiving antituberculous therapy. These findings are consistent with previous reports of elevated efavirenz concentration with rifampin-containing antituberculous therapy in HIV/TB-coinfected patients who appeared to have slow metabolizing phenotype, as genotyping was not done [5,12,14]. The paradoxical effect of antituberculous therapy in this study may be due to increased susceptibility of the CYP2B6 (172-histidine) variant allozyme (resulting from the c.516G→T polymorphism) to inhibition by one (or more) of the antituberculous drugs as compared with the reference CYP2B6 (172-glutamine) enzyme. Alternately, efavirenz metabolism may be inhibited by effects on the non-CYP2B6 accessory pathways leading to higher efavirenz concentrations. CYP2A6-mediated 7-hydroxylation is identified as an important alternate pathway in the metabolism of efavirenz  and may be the major pathway for efavirenz clearance in CYP2B6 slow metabolizers . Thus, inhibition of CYP2A6 by one (or more) of the antituberculous drugs could explain the higher concentrations. Studies are needed to determine the mechanism by which addition of the standard four-drug antituberculous therapy causes higher efavirenz concentrations in CYP2B6 slow metabolizers.
Our study has some limitations. First, the sample size is small and the lack of significant effect of antituberculous therapy on efavirenz concentrations in the CYP2B6 516GG and the CYP2B6 516GT genotype groups should be interpreted with caution. Second, the study compares two different patient populations and the differences seen in the CYP2B6 516TT genotype group could be due to population differences. However, our observation is supported by previous studies that showed paradoxically higher efavirenz concentrations during antituberculous therapy in the patients who appeared to be slow efavirenz metabolizers [5,12]. If our results are confirmed in larger studies, CYP2B6 genotyping could inform rational decisions about efavirenz dosing or alternate therapy in HIV/TB-coinfected patients.
This research was supported in part by a K23 developmental award (NIH K23 AI071760) to A.K. from NIAID and ACRiA grant from Doris Duke Foundation to M.L. and M.H.C. was supported by grant R01GM061834 from the National Institute of General Medical Sciences (NIGMS), National Institutes of Health (Bethesda, Maryland, USA).
We thank the study participants, the Medical Officers at the Fever's Unit, Korle-Bu Teaching Hospital, staff of Clinical Virology, University of Ghana Medical School, and the Study Coordinators, as well as the Study Nurse for all their valuable assistance in recruitment and evaluation of patients, as well as obtaining and handling the samples.
The University of North Carolina at Chapel Hill, Center for AIDS research #9P30 AI50410, Clinical Pharmacology and Analytical Chemistry Laboratory (CPACL) performed the efavirenz concentrations.
A.K. has previously received a research grant not related to this study from Bristol Myer-Squibb. M.L., M.H.C., and K.W.S. report no conflict of interest.
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