Functional Characterization of Human CYP2C9 Allelic Variants in COS-7 Cells

The Impact of CYP2C9*11 Allelic Variant on the Pharmacokinetics of Phenytoin and (S)-Warfarin

Cytochrome P450 2C9 (CYP2C9) is responsible for the oxidative metabolism of about 15% of commonly used drugs, some of which are characterized by a narrow therapeutic window. CYP2C9 is highly polymorphic, and over 60 alleles have been described. CYP2C9*2 and CYP2C9*3 are the most common polymorphisms among White patients and both are associated with decreased activity. The evidence concerning the functional importance of less frequent variant alleles is scarce. The objective of the current study was to characterize the in vivo activity of CYP2C9 among carriers of CYP2C9*11, one of the "African" alleles and the fourth most common CYP2C9 variant allele among White patients by using two prototype substrates, phenytoin and (S)-warfarin. Single 300-mg phenytoin and 20-mg warfarin doses were given to 150 healthy Ethiopian Jewish participants who were nonsmokers, at least one week apart. (S)-warfarin oral clearance and phenytoin metabolic ratio (PMR) derived from the ratio of 5-(4-hydroxyphenyl)-5-phenylhydantoin in 24-hour urine collection to plasma phenytoin 12 hours (PMR 24/12) or 24 hours (PMR 24/24) post dosing, were used as markers of CYP2C9 activity. PMR 24/12 and PMR 24/24 were reduced by 50% and 62.2%, respectively, among carriers of CYP2C9*1/*11 (n = 13) as compared with carriers of CYP2C9*1/*1 (n = 127) (false discovery rate (FDR) q < 0.001). The respective decrease in (S)-warfarin oral clearance was 52.6% (FDR q < 0.001). In conclusion, the enzyme encoded by CYP2C9*11 is characterized by a more than 50% decrease in the enzymatic activity, resembling the extent of decrease associated with CYP2C9*3 ("no-function allele"). Among patients of African ancestry, CYP2C9*11 genetic analysis should be considered prior to prescribing of narrow therapeutic window drugs such as phenytoin, warfarin, nonsteroidal anti-inflammatory drugs, or siponimod.

Impact of genetic and non-genetic factors on hepatic CYP2C9 expression and activity in Hungarian subjects

CYP2C9, one of the most abundant hepatic cytochrome P450 enzymes, is involved in metabolism of 15–20% of clinically important drugs (warfarin, sulfonylureas, phenytoin, non-steroid anti-inflammatory drugs). To avoid adverse events and/or impaired drug-response, CYP2C9 pharmacogenetic testing is recommended. The impact of CYP2C9 polymorphic alleles (CYP2C9*2, CYP2C9*3) and phenoconverting non-genetic factors on CYP2C9 function and expression was investigated in liver tissues from Caucasian subjects (N = 164). The presence of CYP2C9*3 allele was associated with CYP2C9 functional impairment, and CYP2C9*2 influenced tolbutamide 4′-hydroxylase activity only in subjects with two polymorphic alleles, whereas the contribution of CYP2C8*3 was not confirmed. In addition to CYP2C9 genetic polymorphisms, non-genetic factors (co-medication with CYP2C9-specific inhibitors/inducers and non-specific factors including amoxicillin + clavulanic acid therapy or chronic alcohol consumption) contributed to the prediction of hepatic CYP2C9 activity; however, a CYP2C9 genotype–phenotype mismatch still existed in 32.6% of the subjects. Substantial variability in CYP2C9 mRNA levels, irrespective of CYP2C9 genotype, was demonstrated; however, CYP2C9 induction and non-specific non-genetic factors potentially resulting in liver injury appeared to modify CYP2C9 expression. In conclusion, complex implementation of CYP2C9 genotype and non-genetic factors for the most accurate estimation of hepatic CYP2C9 activity may improve efficiency and safety of medication with CYP2C9 substrate drugs in clinical practice.

Effect of CYP2C9 *11/*11 genotype on initial and long-term warfarin dose requirement and therapeutic response

The warfarin dose requirement and therapeutic response of a 42-year-old African-American male with genotype CYP2C9 *11/*11, VKORC1 -1639GG and CYP4F2 433Val/Val anticoagulated for ischemic stroke is described herein. Warfarin was dosed according to the institution's personalized medicine program recommendations of a 10 mg mini-load dose, followed by dose decreases to 4-6 mg/day through discharge. Stable international normalized ratio was achieved after eight doses, with good overall long-term maintenance of therapeutic international normalized ratio over several years with warfarin doses of 3.1-4.3 mg/day. This case report sheds further light on the clinical impact of CYP2C9 *11/*11 on warfarin dose requirements, short- and long-term treatment response and practical considerations for warfarin management in suspected carriers of rare variant CYP2C9 alleles.

Functional characterization of the chlorzoxazone 6-hydroxylation activity of human cytochrome P450 2E1 allelic variants in Han Chinese

BACKGROUNDS

Cytochrome P450 (P450) 2E1 is one of the primary enzymes responsible for the metabolism of xenobiotics, such as drugs and environmental carcinogens. The genetic polymorphisms of the CYP2E1 gene in promoter and coding regions have been identified previously in the Han Chinese population from four different geographic areas of Mainland China.

METHODS

To investigate whether genetic variants identified in the CYP2E1 coding region affect enzyme function, the enzymes of four single nucleotide polymorphism (SNP) variants in the coding region (novel c.1009C>T, causing p.Arg337X, where X represents the translational stop codon; c.227G>A, causing p.Arg76His; c.517G>A, yielding p.Gly173Ser; and c.1263C>T, presenting the highest allele frequency), two novel alleles (c.[227G>A;1263C>T] and c.[517G>A;1263C>T]), and the wild-type CYP2E1 were heterologously expressed in COS-7 cells and functionally characterized in terms of expression level and chlorzoxazone 6-hydroxylation activity. The impact of the CYP2E1 variant sequence on enzyme activity was predicted with three programs: Polyphen 2, PROVEAN and SIFT.

RESULTS

The prematurely terminated p.Arg337X variant enzyme was undetectable by western blotting and inactive toward chlorzoxazone 6-hydroxylation. The c.1263C>T and c.[517G>A;1263C>T] variant enzymes exhibited properties similar to those of the wild-type CYP2E1. The CYP2E1 variants c.227G>A and c.[227G>A;1263C>T] displayed significantly reduced enzyme activity relative to that of the wild-type enzyme (decreased by 42.8% and 32.8%, respectively; P < 0.01). The chlorzoxazone 6-hydroxylation activity of the c.517G>A transfectant was increased by 31% compared with the wild-type CYP2E1 enzyme (P < 0.01). Positive correlations were observed between the protein content and enzyme activity for CYP2E1 (P = 0.0005, r 2 = 0.8833). The characterization of enzyme function allelic variants in vitro was consistent with the potentially deleterious effect of the amino acid changes as determined by prediction tools.

CONCLUSIONS

These findings indicate that the genetic polymorphisms of CYP2E1, i.e., c.1009C>T (p.Arg337X), c.227G>A (p.Arg76His), and c.517G>A (p.Gly173Ser), could influence the metabolism of CYP2E1 substrates, such as chlorzoxazone.

Cytochrome P450 1A1, 2C9, 2C19, and 3A4 Polymorphisms Account for Interindividual Variability of Toxicological Drug Metabolism in Cynomolgus Macaques

Cytochromes P450 (P450s) and their genetic variants in humans are important drug-metabolizing enzymes partly accounting for interindividual variations in drug metabolism and toxicity. However, these genetic variants in P450s have not been fully investigated in cynomolgus macaques, a nonhuman primate species widely used in toxicological studies. In this study, genetic variants found in cynomolgus CYP1A1, CYP2C9 (formerly CYP2C43), CYP2C19 (CYP2C75), and CYP3A4 (CYP3A8) were assessed on functional importance. Resequencing of CYP1A1 in cynomolgus macaques found 18 nonsynonymous variants, of which M121I and V382I were located in SRSs, domains potentially important for P450 function. By further analyzing these two variants, V382I was significantly associated with lower drug-metabolizing activities in the liver for the heterozygotes than the wild types. Similarly, the heterozygotes or homozygotes of CYP2C9 variants (A82V and H344R) and CYP2C19 variant (A490V) showed significantly lower drug-metabolizing activities in the liver than the wild types. Moreover, the homozygotes of CYP3A4 variant (S437N) showed significantly higher activities than the wild type in the liver. Kinetic analyses using recombinant proteins revealed that CYP2C9 variants (A82V and H344R) showed substantially lower  K_s values than the wild type, although CYP1A1 variant (V382I) showed kinetic parameters similar to the wild type. Likewise, CYP2C19 variant (A490V) showed substantially a lower  V_max/ K_m value than the wild type, whereas CYP3A4 variant (S437N) showed a higher V_max/ K_m value than the wild type. These results suggest the toxicologically functional importance of CYP2C9 variants (A82V and H344R), CYP2C19 variant (A490V), and CYP3A4 variant (S437N) for hepatic drug metabolism in cynomolgus macaques.

Genetic Polymorphisms and in Vitro Functional Characterization of CYP2C8, CYP2C9, and CYP2C19 Allelic Variants

Genetic variations in CYP 2C (CYP2C) subfamily, CYP2C8, CYP2C9, and CYP2C19 contribute to interindividual variability in the metabolism of clinically used drugs. Changes in the drug metabolizing activity of CYP2C members may cause unexpected and serious adverse drug reactions and inadequate therapeutic effects. Therefore, CYP2C gene polymorphism is used as a genome biomarker for predicting responsiveness to administered drugs. The most direct method for understanding the extent of the effects of CYP2C gene polymorphism on drug pharmacokinetics is by evaluating the blood and urine concentrations of the drug in subjects. However, in vivo tests are highly invasive, and considering the risk of adverse drug reactions, the burden on the patient may be significant. In addition, examining the functions of rare variant enzymes with an allele frequency of ≤1% requires at least several hundred subjects. Furthermore, it is extremely difficult to evaluate the functions of all variant enzymes in an in vivo test. On the other hand, in vitro enzyme activity can be evaluated using a heterologous expression system to avoid the aforementioned problems. In vitro tests are extremely important as they complement in vivo information. This review focuses on recent findings of in vitro studies on 3 highly polymorphic CYP2C members: CYP2C8, CYP2C9, and CYP2C19.

In Vitro Functional Characterisation of Cytochrome P450 (CYP) 2C19 Allelic Variants CYP2C19*23 and CYP2C19*24

Cytochrome P450 (CYP) 2C19 is essential for the metabolism of clinically used drugs including omeprazole, proguanil, and S-mephenytoin. This hepatic enzyme exhibits genetic polymorphism with inter-individual variability in catalytic activity. This study aimed to characterise the functional consequences of CYP2C19*23 (271 G>C, 991 A>G) and CYP2C19*24 (991 A>G, 1004 G>A) in vitro. Mutations in CYP2C19 cDNA were introduced by site-directed mutagenesis, and the CYP2C19 wild type (WT) as well as variants proteins were subsequently expressed using Escherichia coli cells. Catalytic activities of CYP2C19 WT and those of variants were determined by high performance liquid chromatography-based essay employing S-mephenytoin and omeprazole as probe substrates. Results showed that the level of S-mephenytoin 4'-hydroxylation activity of CYP2C19*23 (V _max 111.5 ± 16.0 pmol/min/mg, K _m 158.3 ± 88.0 μM) protein relative to CYP2C19 WT (V _max 101.6 + 12.4 pmol/min/mg, K _m 123.0 ± 19.2 μM) protein had no significant difference. In contrast, the K _m of CYP2C19*24 (270.1 ± 57.2 μM) increased significantly as compared to CYP2C19 WT (123.0 ± 19.2 μM) and V _max of CYP2C19*24 (23.6 ± 2.6 pmol/min/mg) protein was significantly lower than that of the WT protein (101.6 ± 12.4 pmol/min/mg). In vitro intrinsic clearance (CL_int = V _max/K _m) for CYP2C19*23 protein was 85.4 % of that of CYP2C19 WT protein. The corresponding CL_int value for CYP2C19*24 protein reduced to 11.0 % of that of WT protein. These findings suggested that catalytic activity of CYP2C19 was not affected by the corresponding amino acid substitutions in CYP2C19*23 protein; and the reverse was true for CYP2C19*24 protein. When omeprazole was employed as the substrate, K _m of CYP2C19*23 (1911 ± 244.73 μM) was at least 100 times higher than that of CYP2C19 WT (18.37 ± 1.64 μM) and V _max of CYP2C19*23 (3.87 ± 0.74 pmol/min/mg) dropped to 13.4 % of the CYP2C19 WT (28.84 ± 0.61 pmol/min/mg) level. Derived from V _max/K _m, the CL_int value of CYP2C19 WT was 785 folds of CYP2C19*23. K _m and V _max values could not be determined for CYP2C19*24 due to its low catalytic activity towards omeprazole 5'-hydroxylation. Therefore, both CYP2C19*23 and CYP2C19*24 showed marked reduced activities of metabolising omeprazole to 5-hydroxyomeprazole. Hence, carriers of CYP2C19*23 and CYP2C19*24 allele are potentially poor metabolisers of CYP2C19-mediated substrates.