TY - JOUR T1 - Effects of the CYP2B6*6 Allele on Catalytic Properties and Inhibition of CYP2B6 in vitro: Implication for the Mechanism of Reduced Efavirenz Metabolism and Other CYP2B6 Substrates in vivo JF - Drug Metabolism and Disposition JO - Drug Metab Dispos DO - 10.1124/dmd.111.042416 SP - dmd.111.042416 AU - Cong Xu AU - Evan T. Ogburn AU - Yingying Guo AU - Zeruesenay Desta Y1 - 2012/01/01 UR - http://dmd.aspetjournals.org/content/early/2012/01/09/dmd.111.042416.abstract N2 - The mechanism by which CYP2B6*6 allele alters drug metabolism in vitro and in vivo is not fully understood. To test the hypothesis that altered substrate binding and/or catalytic properties contribute to its functional consequences, efavirenz 8-hydroxylation and bupropion 4-hydroxylation were determined in CYP2B6.1 and CYP2B6.6 proteins expressed without and with cytochrome b5 (Cyt b5) and in human liver microsomes (HLMs) obtained from liver tissues genotyped for the CYP2B6*6 allele. The susceptibility of the variant protein to inhibition was also tested in HLMs. Significantly higher Vmax and Km values for 8-hydroxyefavirenz formation and 2-fold lower intrinsic clearance (Clint) were noted in expressed CYP2B6.6 protein (-b5) compared to that of CYP2B6.1 protein; this effect was abolished by Cyt b5. The Vmax and Clint values for 4-hydroxybupropion formation were significantly higher in CYP2B6.6 than in CYP2B6.1 protein, with no difference in Km, while co-expression with Cyt b5 reversed the genetic effect on these kinetic parameters. In HLMs, CYP2B6*6/*6 genotype was associated with markedly lower Vmax (and moderate increase in Km) and thus lower Clint values for efavirenz and bupropion metabolism, but no difference in catalytic properties was noted between CYP2B6*1/*1 and *1/*6 genotypes. Inhibition of efavirenz 8-hydroxylation by voriconazole was significantly greater in HLMs with the CYP2B6*6 allele (Ki=1.6 ± 0.8 μM) than HLMs with CYP2B6*1/*1 genotype (Ki= 3.0 ± 1.1 μM). In conclusion, our data suggest the CYP2B6*6 allele influences metabolic activity by altering substrate binding and catalytic activity in substrate- and Cyt b5-dependent manner. It may also confer susceptibility to inhibition. ER -