Functional characterization of cytochrome P450 2B6 allelic variants

Bupropion and 4-OH-bupropion pharmacokinetics in relation to genetic polymorphisms in CYP2B6

Bupropion is applied in depression and smoking cessation. Genetic polymorphisms in cytochrome P450 2B6 (CYP2B6) may cause variability in bupropion pharmacokinetics since hydroxylation is known to be mediated by CYP2B6. Bupropion may be a probe drug for CYP2B6 activity in humans. Bupropion pharmacokinetics were studied after a single oral dose of 150 mg in 121 healthy male volunteers. The amino acid polymorphisms R22C, Q172H, S259R, K262R and R487C were analysed by polymerase chain reaction and restriction fragment length polymorphism and plasma concentrations were measured by high-performance liquid chromatography. Pharmacokinetic analysis was performed by non-parametric methods and by population pharmacokinetic modelling. A unimodal distribution of bupropion and hydroxybupropion kinetic parameters was detected with a mean (range) area under the curve (AUC) of 3.64 (0.89-8.14) micromol.h/l for bupropion and 25.5 (6.72-75.3) micromol.h/l for hydroxybupropion. Population kinetic analysis revealed that bupropion total clearance via CYP2B6 alleles *1, *2, *5 and *6 did not differ, but clearance via allele *4 was 1.66-fold higher compared to wild-type allele *1 (P=0.001). Corresponding to the high clearance of bupropion, carriers of the CYP2B6 genotype *1/*4 had significantly higher Cmax of hydroxybupropion compared to all other genotypes (P=0.03). Only a minor fraction of the variability in bupropion and hydroxybupropion kinetics could be explained by the known CYP2B6 amino acid variants, in particular by the CYP2B6*4 allele. The role of this allele should also be studied in other CYP2B6 substrates, including cyclophosphamide, halothane, mianserin, promethazine and propofol.

Multiple Novel Nonsynonymous CYP2B6 Gene Polymorphisms in Caucasians: Demonstration of Phenotypic Null Alleles

The human microsomal cytochrome P450, CYP2B6, is involved in the biotransformation of several clinically important drugs. By complete sequence analysis of the human CYP2B6 gene coding regions in selected Caucasian DNA samples, we identified the five novel missense mutations 62A>T (Q21L in exon 1), 136A>G (M46V in exon 1), 12820G>A (G99E in exon 2), 13076G>A (R140Q in exon 3), and 21388T>A (I391N in exon 8). The recently described but functionally uncharacterized variant 13072A>G (K139E) was also observed. Haplotype analysis indicated the presence of at least six novel alleles that code for the protein variants CYP2B6.10 (Q21L, R22C), CYP2B6.11 (M46V), CYP2B6.12 (G99E), CYP2B6.13 (K139E, Q172H, K262R), CYP2B6.14 (R140Q), and CYP2B6.15 (I391N). Heterologous expression in COS-1 cells revealed comparable levels of CYP2B6 apoprotein and bupropion hydroxylase activity for CYP2B6.1 (wild type) and CYP2B6.10, whereas all other variants exhibited reduced expression and/or function. The three amino acid changes M46V, G99E, and I391N resulted in almost unmeasurable (M46V) or undetectable (G99E and I391N) enzyme activity, despite the presence of residual protein. The K139E change led to completely abolished protein expression; as a consequence, no function was detected. Expression in insect cells by recombinant baculoviruses confirmed these results and demonstrated the virtual absence of incorporated heme in these protein variants. The collective allele frequency of the four very low or null activity variants M46V, G99E, K139E, and I391N was 2.6% in a Caucasian study population. These data provide further insight into the genetic variability of CYP2B6 and demonstrate the existence of phenotypic null alleles in this gene.

Identification of CYP2B6 sequence variants by use of multiplex PCR with allele-specific genotyping

BACKGROUND

Cytochrome P450 2B6 (CYP2B6) has a role in the metabolism of many clinically important substances, but the variation within the CYP2B6 gene has not been fully characterized. The aim of the present study was to develop a reliable and robust assay for determining genotypic variants.

METHODS

We used a two-stage procedure. An initial multiplex PCR reaction amplified the relevant gene fragments in exonic and regulatory regions to ensure isolation of CYP2B6 from its similar pseudogene (CYP2B7). This product was then genotyped by allele-specific PCR.

RESULTS

The assay detected the following published single-nucleotide polymorphisms: C64T (Arg22Cys), C78T, G216C, G516T (Gln172His), C777A (Ser259Arg), A785G (Lys262Arg), and C1459T (Arg487Cys), as well as additional loci found within the single-nucleotide polymorphism (SNP) databases: A1190G, C1268A, C1330T, A1382G, A1402T, and an A/T SNP in intron 2 (A12917T). This approach detected all common, previously reported alleles and identified a new allele (CYP2B6*4C) present in 2.2% of a Caucasian population. Genotypic frequencies obtained were consistent with previously published results.

CONCLUSIONS

This method is simple, reliable, rapid, and amenable to automation and could facilitate the large-scale genotypic analysis of CYP2B6.

The Unique Regulation of Brain Cytochrome P450 2 (CYP2) Family Enzymes by Drugs and Genetics

Cytochrome P450 (CYP) enzymes in the brain may have a role in the activation or inactivation of centrally acting drugs, in the metabolism of endogenous compounds, and in the generation of damaging toxic metabolites and/or oxygen stress. CYPs are distributed unevenly among brain regions, and are found in neurons, glial cells and at the blood-brain interface. They have been observed in mitochondrial membranes, in neuronal processes and in the plasma membrane, as well as in endoplastic reticulum. Brain CYPs are inducible by many common hepatic inducers, however many compounds affect liver and brain CYP expression differently, and some CYPs which are constitutively expressed in liver are inducible in brain. CYP induction is isozyme-, brain region-, cell type- and inducer-specific. While it is unlikely that brain CYPs contribute to overall clearance of xenobiotics, their punctate, region- and cell-specific expression suggests that CNS CYPs may create micro-environments in the brain with differing drug and metabolite levels (not detected or predicted by plasma drug monitoring). Coupled with the sensitivity of CNS CYPs to induction, this may in part account for inter-individual variation in response to centrally acting drugs and neurotoxins, and may have implications for individual variation in receptor adaptation and cross-tolerance to different drugs. In addition, genetic variation in brain CYPs, depending on the type of polymorphism (structural versus regulatory), will alter enzyme activity. These aspects of brain CYP expression regulation and genetic influences are illustrated in this review using mRNA, protein, and enzyme activity data for CYP2D1/6, CYP2E1 and CYP2B1/6 in rat and human brain. The role of CYP-mediated metabolism in the brain, a highly heterogeneous and complex organ, is a new and relatively unexplored field of scientific enquiry. It holds promise for furthering our undestanding of inter-individual variability in response to centrally acting drugs as well as risk for neurological diseases and pathogies.

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

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.

Functional Characterization of Single Nucleotide Polymorphisms with Amino Acid Substitution in CYP1A2, CYP2A6, and CYP2B6 Found in the Japanese Population

As a part of the studies conducted by the Pharma SNPs Consortium (PSC), the enzyme activities of CYP1A2, CYP2A6 and CYP2B6 variants with altered amino acids as a result of single nucleotide polymorphisms (SNPs) found among the Japanese population were analyzed under a unified protocol using the same lots of reagents by the laboratories participating in the PSC. Mutations in CYP1A2, CYP2A6 and CYP2B6 were introduced by site-directed mutagenesis and the wild type and mutated CYP molecules were expressed in Escherichia coli. The expressed cytochrome P450s were purified and the enzyme activities were measured in reconstitution systems. CYP1A2 and CYP1A2Gln478His did not show any differences in 7-ethoxyresorufin O-deethylase activity. CYP2A6 and CYP2A6Glu419Asp metabolized coumarin to form 7-hydroxycoumarin in a similar manner, whereas CYP2A6Ile471Thr showed low activity compared to the wild-type CYP2A6. CYP2B6, CYP2B6Pro167Ala and CYP2B6Arg487Cys showed the same activity for 7-ethoxy-4-triflouromethyl-coumarin O-deethylation. However, CYP2B6Gln172His was roughly twice as active as CYP2B6 and the other CYP2B6 variants for 7-ethoxy-4-triflouromethylcoumarin O-deethylation activity. Although higher inter- and intra-laboratory variations were observed for the calculated Km and V(max) values because the studies were conducted in several different laboratories, the degree of variations was reduced by the increased number of analyses and the adoption of a simple analysis system.

Hepatic CYP2B6 expression: gender and ethnic differences and relationship to CYP2B6 genotype and CAR (constitutive androstane receptor) expression

CYP2B6 metabolizes many drugs, and its expression varies greatly. CYP2B6 genotype-phenotype associations were determined using human livers that were biochemically phenotyped for CYP2B6 (mRNA, protein, and CYP2B6 activity), and genotyped for CYP2B6 coding and 5'-flanking regions. CYP2B6 expression differed significantly between sexes. Females had higher amounts of CYP2B6 mRNA (3.9-fold, P < 0.001), protein (1.7-fold, P < 0.009), and activity (1.6-fold, P < 0.05) than did male subjects. Furthermore, 7.1% of females and 20% of males were poor CYP2B6 metabolizers. Striking differences among different ethnic groups were observed: CYP2B6 activity was 3.6- and 5.0-fold higher in Hispanic females than in Caucasian (P < 0.022) or African-American females (P < 0.038). Ten single nucleotide polymorphisms (SNPs) in the CYP2B6 promoter and seven in the coding region were found, including a newly identified 13072A>G substitution that resulted in an Lys139Glu change. Many CYP2B6 splice variants (SV) were observed, and the most common variant lacked exons 4 to 6. A nonsynonymous SNP in exon 4 (15631G>T), which disrupted an exonic splicing enhancer, and a SNP 15582C>T in an intron-3 branch site were correlated with this SV. The extent to which CYP2B6 variation was a predictor of CYP2B6 activity varied according to sex and ethnicity. The 1459C>T SNP, which resulted in the Arg487Cys substitution, was associated with the lowest level of CYP2B6 activity in livers of females. The intron-3 15582C>T SNP (in significant linkage disequilibrium with a SNP in a putative hepatic nuclear factor 4 (HNF4) binding site) was correlated with lower CYP2B6 expression in females. In conclusion, we found several common SNPs that are associated with polymorphic CYP2B6 expression.

Human drug metabolising cytochrome P450 enzymes: properties and polymorphisms

The cytochrome P450s are responsible for about 75% of phase I dependent drug metabolism and for the metabolism of a huge amount of dietary constituents and endogenous chemicals. The human has 59 active genes, and 6 of those encode important drug metabolising enzymes. About 40% of cytochrome P450 dependent drug metabolism is catalysed by polymorphic enzymes and such drug P450 interactions are frequently seen in adverse drug reaction reports. In this contribution an update of human cytochrome P450 enzymology and pharmacogenetics is given with particular emphasis on CYP1B1, CYP2B6, CYP2E1 and CYP3As.

High-throughput screening assays for CYP2B6 metabolism and inhibition using fluorogenic vivid substrates

CYP2B6 is a highly polymorphic P450 isozyme involved in the metabolism of endo- and xenobiotics with known implications for the activation of many procarcinogens resulting in carcinogenesis. However, lack of validated high-throughput screening (HTS) CYP2B6 assays has limited the current understanding and full characterization of this isozyme's involvement in human drug metabolism. Here, we have developed and characterized a fluorescence-based HTS assay employing recombinant human CYP2B6 and 2 novel fluorogenic substrates (the Vivid CYP2B6 Blue and Cyan Substrates). Assay validation included testing the inhibitory potency of a panel of drugs and compounds known to be metabolized by this isozyme, including CYP2B6 substrates, inhibitors, and known inducers. Compound rankings based on inhibitory potency in the Vivid CYP2B6 Blue and Cyan Assays matched compound rankings based on relative affinity measurements from previously published data (K(i), K(d), or K(m) values) for the CYP2B6 isozyme. In conclusion, these assays are proven to be robust and sensitive, with broad dynamic ranges and kinetic parameters allowing screening in HTS mode of a large panel of compounds for CYP2B6 metabolism and inhibition, and are a valuable new tool for CYP2B6 studies.