Pharmacogenetic determinants of interindividual variability in bupropion hydroxylation by cytochrome P450 2B6 in human liver microsomes

Abstract

Bupropion is primarily metabolized in human liver by cytochrome P450 (CYP) 2B6, an isoform that shows high interindividual variability in expression and catalysis. The aim of this study was to identify mechanisms underlying this variability through comprehensive phenotype-genotype analysis of a well-characterized human liver bank (n = 54). There was substantial variability in microsomal bupropion hydroxylation activities (over 45-fold) and CYP2B6 protein content (over 288-fold), with excellent correlation between protein and activity values (rs = 0.88). CYP2B6 mRNA levels showed less variability (13-fold) and poorer correlation (rs = 0.44) to CYP2B6 protein resulting from 20-30% of livers that contained substantial CYP2B6 mRNA, but low CYP2B6 protein. Livers were genotyped for the common coding polymorphisms (Q172H, K262R and R487C) and 14 additional variations identified by sequencing of the gene promoter to -3000 bp. Of 14 haplotypes that were inferred, *1A (reference), *1H (-2320t>c; -750t>c) and *6B (-1456t>c; -750t>c; Q172H; K262R) were most common with frequencies of 0.28, 0.20 and 0.26, respectively. Alcohol use history (P = 0.011) and *6B haplotype (P = 0.011) were identified as significant predictors of bupropion hydroxylation. A consideration of the effects of these variables on CYP2B6 mRNA and protein levels suggests that alcohol use is associated with enhanced CYP2B6 gene transcription, but the presence of at least one *6B allele reduces this effect on bupropion hydroxylation at the post-transcriptional level. In conclusion, the results of this study indicate that interindividual variability in bupropion hydroxylation is a consequence of interactions between environmental and genetic influences on CYP2B6 gene function.

Differential induction of ethanol-metabolizing CYP2E1 and nicotine-metabolizing CYP2B1/2 in rat liver by chronic nicotine treatment and voluntary ethanol intake

“…Exposure to ethanol alone, and in combination with nicotine, could increase the hepatic metabolism of CYP2B1/2 substrates thereby affecting their efficacy and/or toxicity. This may be of clinical importance for a number of drugs metabolized by CYP2B such as bupropion (Hesse et al, 2000), efavirenz (Ward et al, 2003), and cyclophosphamide (Chang et al, 1993), where alterations in plasma levels can result in altered therapeutic outcomes and toxicities (Hesse et al, 2004;Rotger et al, 2005).…”

“…Chronic ethanol induces rat liver CYP2B1/2 levels, via a transcriptional mechanism, resulting in increased in vitro metabolism of nicotine in rat liver microsomes (Adir et al, 1980;Schoedel et al, 2001). Chronic nicotine treatment does not alter rat or monkey hepatic CYP2B levels (Lee et al, 2006;Miksys et al, 2000a) nor are the levels higher in human smokers (Hesse et al, 2004).…”

Brain Drug-Metabolizing Cytochrome P450 Enzymes are Active In Vivo, Demonstrated by Mechanism-Based Enzyme Inhibition

“…Alterations in brain CYP levels through genetic variation or through induction by commonly used drugs such as nicotine or ethanol would result in alterations in local CYP metabolism, which may contribute to the variation seen in efficacy, interactions, side effects, and toxicities of drugs that enter and act on the CNS (Gervasini et al, 2004). For example, brain CYP2B6 protein levels are higher in smokers compared to nonsmokers (Miksys et al, 2003), whereas hepatic levels are unchanged (Hesse et al, 2004), suggesting that smokers may have different therapeutic responses or side effects to CYP2B6 substrates. The anesthetic propofol is inactivated by CYP2B6, and there is evidence that smokers require a larger dose of propofol (Lysakowski et al, 2006) and report fewer postoperative side effects (Chimbira and Sweeney, 2000).…”

Cytochrome P450 pharmacogenetics and cancer

“…The role of the different CYP2B6 alleles for the in vivo metabolism of drugs is still largely unknown. CYP2B6*5 (R487C) and CYP2B6*7 (Q172H, K262R and R487C) variants have been suggested to cause significantly reduced protein expression levels in human liver (Lang et al, 2001), but other studies have not confirmed this (Xie et al, 2003;Hesse et al, 2004). The CYP2B6*6 allele (Q172H and K262R) has been associated with a decreased protein expression, but higher activity using cyclophosphamide as substrate (Xie et al, 2003).…”