Mechanisms and quantitative prediction of Efavirenz metabolism, pharmacogenetics and drug interactions

Abstract

The antiretroviral drug efavirenz remains a cornerstone for treatment-naïve HIV patients. Subsequent to the demonstration that efavirenz is a substrate of cytochrome P450 (CYP) 2B6, a number of clinical studies found that the CYP2B6*6 allele is significantly associated with higher efavirenz exposure and/or adverse reactions. However, the mechanism of reduced efavirenz metabolism by this genetic variant is not fully understood and whether this variant exhibits differential susceptibility to metabolic inhibition is also unknown. Ths use of efavirenz is further complicated by the drug interactions associated with it. Therefore, I hypothezised that 1) the CYP2B6*6 allele reduces efavirenz metabolism by altering catalytic properties of CYP2B6; 2) efavirenz alters the pharmacokinetics of co-administered drugs by inhibiting drug metabolizing enzymes. A series of studies was carried out in hepatic microsomal preparations to determine the functional consequences of the CYP2B6*6 allele and to assess inhibition potency of efavirenz on 8 CYPs. The major findings for these studies include: 1) the CYP2B6*6 allele reduces efavirenz metabolism by decreasing substrate binding and catalytic efficiency; 2) functional consequences of the CYP2B6*6 allele appear to be substrate- and cytochrome b5-dependent; 3) the CYP2B6*6 allele confers increased susceptibility to metabolic inhibition; and 4) efavirenz inhibits the activities of CYP2B6, 2C8, 2C9 and 2C19 at therapeutically relevant concentrations. In addition, I explored the hypothesis that the incorporation of in vitro mechanism by which the CYP2B6*6 allele reduced efavirenz metabolism predicts the genetic effect of this allele on efavirenz clearance after a single oral dose by modeling approach. A pharmacogenetics-based in vitro-in vivo extrapolation (IVIVE) model was developed to predict human efavirenz clearance. Taken together, results from this dissertation provide new mechanistic information on how the CYP2B6*6 allale alters substrate metabolism and drug interactions; demonstrate new mechanisms of efavirenz-mediated inhibition interactions; and demonstrate the utility of a pharmacogenetics-based predictive model that can serve as a basis for future studies with efavirenz and other CYP2B6 substrates. Overall these data provide improved understanding of genetic and non-genetic determinant of efavirenz disposition and drug interactions associated with it.