Effects of clopidogrel and itraconazole on the disposition of efavirenz and its hydroxyl metabolites: Exploration of a novel CYP2B6 phenotyping index

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT •Cytochrome P450 (CYP) 2B6 is the enzyme primarily responsible for the metabolism of many clinically important drugs, including efavirenz, which it converts to 8-hydroxyefavirenz and then to 8,14-hydroxyefavirenz. •The CYP2B6*6 polymorphism influences efavirenz pharmacokinetics, but a validated phenotyping method for predicting CYP2B6 activity in human subjects is not yet available. •The disposition of 8,14-dihydroxyefavirenz in humans in vivo is unknown. WHAT THIS STUDY ADDS •This study is the first quantitative examination of 8,14-dihydroxyefavirenz pharmacokinetics in human subjects. •The 8,14-dihydroxyefavirenz:efavirenz AUC(0,120h) ratio correlates with efavirenz oral clearance and is sensitive and specific to CYP2B6 activity alterations. •The 8,14-dihydroxyefavirenz:efavirenz AUC(0,120h) ratio may be a useful phenotyping index for CYP2B6 activity in vivo. AIMS To evaluate the effects of clopidogrel and itraconazole on the disposition of efavirenz and its hydroxyl metabolites in relation to the CYP2B6*6 genotype and explore potential phenotyping indices for CYP2B6 activity in vivo using a low dose of oral efavirenz. METHODS We conducted a randomized three phase crossover study in 17 healthy Korean subjects pre-genotyped for the CYP2B6*6 allele (CYP2B6*1/*1, n= 6; *1/*6, n= 6; *6/*6, n= 5). Subjects were pretreated with clopidogrel (75mg day^-1 for 4 days), itraconazole (200mg day^-1 for 6 days), or placebo and then given a single dose of efavirenz (200mg). The plasma (0-120h) and urine (0-24h) concentrations of efavirenz and its metabolites (7- and 8-hydroxyefavirenz and 8,14-dihydroxyefavirenz) were determined by LC/MS/MS. RESULTS This study is the first to delineate quantitatively the full (phase I and II) metabolic profile of efavirenz and its three hydroxyl metabolites in humans. Clopidogrel pretreatment markedly decreased AUC(0,48h), C_max and Ae(0,24h) for 8,14-dihydroxyefavirenz, compared with placebo; 95% CI of the ratios were 0.55, 0.73, 0.30, 0.45 and 0.25, 0.47, respectively. The 8,14-dihydroxyefavirenz:efavirenz AUC(0,120h) ratio was significantly correlated with the weight-adjusted CL/F of efavirenz (r²≈ 0.4, P < 0.05), differed with CYP2B6*6 genotype and was affected by clopidogrel pretreatment (P < 0.05) but not by itraconazole pretreatment. CONCLUSIONS The disposition of 8,14-dihydroxy-EFV appears to be sensitive to CYP2B6 activity alterations in human subjects. The 8,14-dihydroxyefaviremz:efavirenz AUC(0,120h) ratio is attractive as a candidate phenotyping index for CYP2B6 activity in vivo.