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Background and Purpose: The ATP-dependent biliary efflux transporter ABCC2, also known as multidrug resistance protein 2 (MRP2), is essential for the cellular disposition and detoxification of various xenobiotics including drugs as well as endogenous metabolites. Common functionally relevant ABCC2 genetic variants significantly alter drug responses and contribute to side effects. The aim of this study was to determine functional consequences of rare variants identified in subjects with European ancestry using in silico tools and in vitro analyses., Experimental Approach: Targeted next-generation sequencing of the ABCC2 gene was used to identify novel variants in European subjects (n = 143). Twenty-six in silico tools were used to predict functional consequences. For biological validation, transport assays were carried out with membrane vesicles prepared from cell lines overexpressing the newly identified ABCC2 variants and estradiol β-glucuronide and carboxydichlorofluorescein as the substrates., Key Results: Three novel rare ABCC2 missense variants were identified (W227R, K402T, V489F). Twenty-five in silico tools predicted W227R as damaging and one as potentially damaging. Prediction of functional consequences was not possible for K402T and V489F and for the common linked variants V1188E/C1515Y. Characterisation in vitro showed increased function of W227R, V489F and V1188E/C1515Y for both substrates, whereas K402T function was only increased for carboxydichlorofluorescein., Conclusion and Implications: In silico tools were unable to accurately predict the substrate-dependent increase in function of ABCC2 missense variants. In vitro biological studies are required to accurately determine functional activity to avoid misleading consequences for drug therapy., (© 2024 British Pharmacological Society.)

CYP2D6 metabolizes 20-25% of all clinically used drugs and its complex genetic polymorphism is a major determinant of drug safety and efficacy. We investigated the basis for the functional difference between the two common alleles *2 (g.2851C>T + g.4181G>C, normal function) and *41 (additional intronic g.2989G>A, reduced function). A recently reported far-distant enhancer polymorphism rs5758550A/G linked to *2 has been suggested to play a decisive role. Genotyping of two white cohorts confirmed strong linkage of rs5758550G to *2, whereas no influence was found on metabolic ratio of sparteine or hepatic expression. Genomic plasmid constructs carrying individual variants or combinations thereof were expressed in COS1 and Huh7 cells. Both g.2851C>T(R296C) and g.2989G>A reduced enzyme activity and protein levels similarly by ~ 50-65% compared to reference (*1), whereas the double variant had only ~ 20% activity. Although the unexpected loss of function caused by g.2851C>T was compensated by g.4181G>C (mimicking the EM-phenotype of *2), the additional loss of function due to intronic g.2989G>A in the triple variant was not compensated (mimicking the IM-phenotype of *41). We also confirmed increased erroneous splicing in carriers of g.2989G>A but not of g.2851C>T as a likely explanation for the impaired function of *41. In conclusion, our data demonstrate g.2989G>A as causal variant of impaired allele CYP2D6*41 whereas triple-haplotypes have to be considered to explain the functional difference between *2 and *41. These data are important for genotyping strategies and clinical implementation of CYP2D6 pharmacogenetics., (© 2020 The Authors. Clinical Pharmacology & Therapeutics published by Wiley Periodicals LLC on behalf of American Society for Clinical Pharmacology and Therapeutics.)

Interindividual variability in cytochrome P450 3A4 (CYP3A4) is believed to be largely heritable; however, predictive genetic factors have remained scarce. Using a candidate-gene approach in a human liver bank, we identified single-nucleotide polymorphisms (SNPs) in the Ah-receptor nuclear translocator (ARNT), glucocorticoid receptor (GR), progesterone receptor membrane component 2 (PGRMC2), and peroxisome proliferator-activated receptor-α (PPARA) that are associated with CYP3A4 phenotype. Validation in atorvastatin-treated volunteers confirmed a decrease in atorvastatin-2-hydroxylation in carriers of PPARA SNP rs4253728. Homozygous carriers expressed significantly less PPAR-α protein in the liver. Moreover, shRNA-mediated PPARA gene knockdown in primary human hepatocytes decreased expression levels of the PPAR-α target ACOX1 and of CYP3A4 by more than 50%. In conclusion, this study identified novel genetic determinants of CYP3A4 that, together with nongenetic factors, explained 52, 55, and 33% of hepatic CYP3A4 mRNA, protein, and atorvastatin-2-hydroxylase activity, respectively. These findings have implications for variability in response to drug substrates of CYP3A4.

Background: The antiepileptic drug carbamazepine (CBZ) is a potent inducer of human drug metabolism, resulting in serious interactions with many commonly prescribed drugs. The molecular mechanisms underlying this response are not well understood, however, and the spectrum of CBZ-inducible genes in human liver has not been thoroughly investigated., Methods: The availability of liver ribonucleic acid from 2 epileptic patients treated with CBZ and from 7 control subjects enabled us to study the global induction response of drug-metabolizing enzymes, drug transporters, and nuclear receptors in vivo., Results: Using expression profiling, we identified 64 significantly up-regulated transcripts but only 1 significantly down-regulated transcript (SLC22A5). We confirmed the induction of several genes that previously have been shown to be inducible by drugs in vitro, including multiple cytochrome P450 (CYP) genes in the CYP1A, CYP2A, CYP2B, CYP2C, and CYP3A subfamilies, as well as glutathione S-transferase A1, uridine diphosphate-glucuronosyltransferase 1As, the drug transporter ABCC2, and the nuclear receptors CAR (constitutive androstane receptor) and PXR (pregnane X receptor). Moreover, we identified a number of additional genes not previously known to be induced by CBZ, including CYP39A1, sulfotransferase 1A1, glutathione S-transferase Z1, and the drug transporters SLCO1A2, ABCG2, and ABCB7, as well as the glucocorticoid and aldosterone receptors. In transactivation studies in CV-1 cells, we demonstrated that both CBZ and its major metabolite, CBZ-10,11-epoxide, activate the nuclear receptor PXR in a concentration-dependent fashion and at therapeutic concentrations with 50% inhibitory concentration values of approximately 50 micromol/L., Conclusions: CBZ is a potent inducer of a broad spectrum of drug-metabolizing enzymes and drug transporters in the human liver, and these effects are mediated at least in part by activation of PXR.

Aim: The aim of this study was to investigate the activity of the drug-metabolizing enzyme cytochrome P450 (CYP) 2B6 before and after in vivo induction by rifampin (INN, rifampicin) in white subjects and Chinese subjects by use of the probe drug bupropion (INN, amfebutamone)., Methods: Healthy male white subjects (n = 9) and Chinese subjects (n = 9) (age range, 19-34 years) of known CYP2B6 genotype received orally administered bupropion (Zyban SR, 150 mg) alone and during daily treatment with rifampin (600 mg). Blood samples were taken for up to 72 hours after each bupropion dose, and plasma concentrations of bupropion and its active metabolites, hydroxybupropion, threohydrobupropion, and erythrohydrobupropion, were measured by HPLC. The subjects' CYP2B6 genotype was determined by use of a matrix-assisted laser desorption /ionization-time of flight (MALDI-TOF) mass spectrometry assay., Results: Rifampin treatment increased the apparent clearance of bupropion in Chinese subjects and white subjects combined (n = 16) from 2.6 L x h(-1) x kg(-1) (95% confidence interval [CI], 2.3-3.0 L x h(-1) x kg(-1)) after bupropion alone to 7.9 L x h(-1) x kg(-1) (95% CI, 6.8-10.1 L x h(-1) x kg(-1)) during rifampin treatment. Rifampin treatment decreased the half-life of bupropion from 15.9 hours (95% CI, 13.5-20.4 hours) to 8.2 hours (95% CI, 6.7-12.4 hours). Rifampin treatment increased the hydroxybupropion maximum concentration from 395 ng/mL (95% CI, 341-497 ng/mL) to 548 ng/mL (95% CI, 490-638 ng/mL), decreased the area under the concentration-time curve extrapolated to infinity of hydroxybupropion from 14.7 microg x h/mL (95% CI, 12.7-18.4 microg x h/mL) to 8.4 microg x h/mL (95% CI, 7.4-10.2 microg x h/mL), and reduced the elimination half-life of hydroxybupropion from 21.9 hours (95% CI, 20.3-24.0 hours) to 10.7 hours (95% CI, 8.6-14.5 hours). There was no significant difference in the pharmacokinetics of bupropion or hydroxybupropion between white subjects and Chinese subjects before and after treatment with rifampin, once corrected for body weight., Conclusions: Rifampin significantly induces CYP2B6 activity in vivo, and the clinical consequences of potential interactions between rifampin and CYP2B6 substrates deserve further investigation. Rifampin appears to induce the elimination of hydroxybupropion. Differences in bupropion pharmacokinetics that were observed between white subjects and Chinese subjects can be attributed to differences in body weight, suggesting that, for a given subject weight, CYP2B6 activity is similar in white subjects and Chinese subjects.

Background: Individuals with the cytochrome P450 (CYP) 2D6 intermediate metabolizer (IM) phenotype have low residual enzyme activity and compose about 10% to 15% of white populations. Their identification is clinically relevant but remains unsatisfactory because of incomplete characterization of the major allele involved, termed CYP2D6*41 (-1584C, R296C, S486T)., Methods: To search for novel mutations, resequencing of the entire CYP2D6 gene was performed in selected individuals. Genotype-phenotype correlation analysis was done in a population sample of 308 white subjects phenotyped with sparteine and previously genotyped for all major alleles., Results: A total of 16 novel polymorphic positions were identified, of which 7 were located within 2.4 kilobases of previously uncharacterized 2D7-2D6 intergenic sequence and 9 were located within intronic regions. The novel mutation 2988G>A in intron 6 appeared to be specifically associated with the IM phenotype. Further analysis in the population sample demonstrated that 2988G>A was strongly linked to allele *41 but not to any other alleles including *1, *2, *2xN, *4, *6, *7, *8, *9, *10, and *35. The overall frequency of the novel polymorphism was 8.4% in the normal white population. Compared with conventionally determined *41, 2988G>A was shown to have improved predictivity for the IM phenotype. With 2988G>A being taken into account, alleles *1, *2, and *35 (-1584G, V11M, R296C, S486T) were found to be phenotypically equivalent., Conclusions: CYP2D6 genotyping can be considerably simplified by using 2988G>A as a marker for *41 and by omitting genotyping for the functionally equivalent alleles *2 and *35.