To our knowledge, this is the first report on the association of polymorphisms in CYP2B6
516 with long-term concentrations of efavirenz using hair samples. Since lifelong daily therapy is required for the treatment of HIV infection, studies that identify SNPs that can influence antiretroviral exposure over prolonged periods may guide dose-optimization strategies. Given the extent of treatment discontinuation, toxicities [29
], and adherence difficulties in HIV-infected individuals, use of pharmacogenetics to personalize care for these individuals should improve treatment selection, dosing, and outcomes [30
]. Indeed, HIV medicine is one of the few fields that routinely uses pharmacogenetic screening to reduce rates of a medication-associated reaction already.
With the availability of new genotyping technology, the paradigm in pharmacogenetics has shifted from examining a small number of genes for a limited number of allelic variants to systematically assessing SNPs in a range of genes relevant for ADME [20
]. In a large heterogeneous population of HIV-infected patients, we performed a comprehensive search of 182 SNPs and 45 haplotypes in 9 genes implicated in efavirenz ADME and modeled them with nongenetic traits previously found to influence efavirenz AUC. Host traits were examined in combination with other factors that influence drug metabolism, to identify both genetic and nongenetic contributors to exposure.
Orange juice consumption and 2 additional SNPs (CYP2B6
983C and an ABCB1
haplotype, both of which have been found to influence plasma levels in the literature [15
]) were associated with modest elevations in AUC, but less so with hair levels. Estimated effects on hair levels were generally smaller than those on AUCs, possibly reflecting imprecision in our self-reported adherence measure, as well as the need to add 1 before logarithmic transformation [28
]. Inhibition of intestinal p-glycoprotein transport or downregulation of enteric CYP3A4 by citrus components in oranges or orange juice could lead to enhanced bioavailability and increased short-term exposure to efavirenz. We identified 1 additional genetic association (rs2002042
in the ABCC2 transporter) that influenced hair levels but not AUCs. The unusual pattern of heterozygotes having lower levels in hair than either type of homozygote must be regarded as preliminary. The most important finding of this study was the consistent effect of the CYP2B6
516T rare allele on tripling efavirenz exposure as measured via AUCs and hair levels.
Efavirenz is prescribed commonly for HIV infection, and a number of studies have examined the relationship between host traits and plasma efavirenz levels [15
]. Certain CYP2B6
SNPs have consistently been linked to higher plasma efavirenz levels and higher rates of early discontinuation [45
]. Individuals homozygous for the CYP2B6
516T rare allele (TT), referred to as “slow metabolizers,” consistently demonstrate higher plasma levels than those in individuals carrying the TG/GG genotypes. However, dose modification of efavirenz on the basis of the presence of SNPs that might transiently increase plasma levels is not routinely performed. Although authors have argued for the synthesis of pharmacogenetic testing with therapeutic drug monitoring to individualize dosing [47
], the lack of a “gold standard” to monitor exposure to antiretroviral therapy and the uncertain significance of a SNP's influence on a plasma antiretroviral level remain significant barriers to dose-optimization protocols.
Plasma levels as a measure of exposure have a number of limitations, including insensitivity to day-to-day variation [48
], dependence on the accurate reporting of last dose taken, intraindividual variations in diet, medications, and illicit substance use. Plasma levels may not reflect typical patterns of medication use and are subject to “white-coat” effects, in which adherence improves prior to medical visits [49
]. Previous studies examining the relationship between genetic traits and efavirenz exposure have used single plasma levels or nonlinear mixed effects modeling of plasma concentrations to calculate population pharmacokinetic parameters as estimates of exposure. Short-term exposure is more robustly estimated by AUCs from intensive PK sampling, which was the measure used in this analysis. Although AUC measurement after witnessed dosing more accurately estimates exposure than single plasma levels, intensive PK sampling is cumbersome and expensive for the routine clinical setting.
We measured long-term drug exposure by monitoring efavirenz concentrations in small samples of hair. Because the concentrations of medications in hair reflect drug uptake from the systemic circulation over weeks to months, they are not subject to bias from the “white-coat” effect or inaccurate recall of the time at which the last dose was taken. Drug levels in hair therefore provide an advantage over plasma concentrations in estimating an average level of exposure over time [50
] and provide a more feasible exposure measure in the clinical setting than AUCs. Genetic traits that influence both AUCs and hair concentrations of medications are likely to be durable in their effects.
Given the high rates of efavirenz discontinuation in clinical settings, identifying traits that lead to durable effects on exposure is important. This comprehensive search for SNPs in genes associated with efavirenz ADME demonstrated that individuals homozygous for the CYP2B6 516T (“TT” genotype) had >3-fold increases in short-term and long-term efavirenz exposure. This SNP's effect on exposure over the prolonged duration represented by hair levels is reported for the first time. Hair testing for exposure has feasibility and cost advantages over intensive PK sampling. Polymorphisms that affect exposure may be more important to assess in individuals whose capacity to clear a medication is already limited by nongenetic factors. Therefore, genetic testing coupled with hair measurement may be helpful in optimizing efavirenz dosing in the clinical setting, particularly when other risk factors for high exposure are present.