CYP2B6 (c.516G→T) and CYP2A6 (*9B and/or *17) polymorphisms are independent predictors of efavirenz plasma concentrations in HIV‐infected patients
Authors: Kwara Awewura , Lartey Margaret , Sagoe Kwamena W. , Rzek Naser L. , Court Michael H. 
Published: January 2009

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

• Cytochrome P450 (CYP) 2B6 polymorphisms, particularly c.516G→T, are strongly associated with plasma efavirenz concentrations, but do not entirely explain interindividual variability in efavirenz exposure.

• In vitro data suggest that CYP2A6 is involved in the metabolism of efavirenz.

• Rifampicin can induce the function and activity of the main metabolizing for efavirenz and causes small (22–26%) reductions in efavirenz area under the curve during co‐administration, although with wide interindividual variability.

WHAT THIS STUDY ADDS

• Identifies CYP2B6 516G→T polymorphism and carriers of CYP2A6*9B and/or *17 variants as independent predictors of efavirenz mid‐dose concentration in human immunodeficiency virus‐infected patients.

• Factors such as concurrent therapy with rifampicin‐containing tuberculosis regimen, gender and body mass index had no a significant influence on efavirenz mid‐dose concentration.

• Provides in vivo evidence that CYP2A6 is likely to be involved in the metabolism of efavirenz.

AIMS Interindividual variability in efavirenz pharmacokinetics is not entirely explained by the well‐recognized CYP2B6 516G→T single nucleotide polymorphism. The aim of this study was to determine whether polymorphisms in the CYP2A6 gene can be used to enhance the predictability of efavirenz concentrations in human immunodeficiency virus (HIV)‐infected native African patients.

METHODS Mid‐dose efavirenz plasma concentrations were determined at 4 and 8 weeks following initiation of antiretroviral therapy in 65 HIV‐infected Ghanaian patients. Selected CYP2B6 and CYP2A6 genotypes were determined by commercial 5′‐nuclease assays. Relationships between averaged 4‐ and 8‐week mid‐dose efavirenz concentrations, demographic variables and genotypes were evaluated by univariate and multivariate statistical approaches including gene–gene interactions.

RESULTS CYP2B6 c.516G→T, CYP2B6 c.983T→C, CYP2A6*9B and CYP2A6*17 allele frequencies were 45, 4, 5 and 12%, respectively. Rifampicin therapy, gender, age and body mass index had no significant influence on efavirenz mid‐dose concentrations. Median efavirenz concentrations were more than five times higher (P < 0.001) in patients with CYP2B6 c.516TT genotype compared with GG and GT genotypes. Although none of the CYP2A6 genotypes was associated with altered efavirenz concentrations individually, CYP2A6*9B and/or CYP2A6*17 carriers showed a 1.8 times higher median efavirenz concentration (P= 0.017) compared with noncarriers. Multiple linear regression analysis indicated that the CYP2B6 c.516G→T polymorphism and CYP2A6 slow‐metabolizing variants accounted for as much as 36 and 12% of the total variance in efavirenz concentrations, respectively.

CONCLUSIONS Our findings support previous work showing efavirenz oxidation by CYP2A6, and suggest that both CYP2A6 and CYP2B6 genotyping may be useful for predicting efavirenz plasma concentrations.