CYP2C9
Cytochrome P450 family 2 subfamily C member 9 (abbreviated CYP2C9) is an enzyme protein. The enzyme is involved in metabolism, by oxidation, of both xenobiotics, including drugs, and endogenous compounds, including fatty acids. In humans, the protein is encoded by the "CYP2C9" gene. The gene is highly polymorphic, which affects the efficiency of the metabolism by the enzyme. Function. CYP2C9 is a crucial cytochrome P450 enzyme, which plays a significant role in the metabolism, by oxidation, of both xenobiotic and endogenous compounds. CYP2C9 makes up about 18% of the cytochrome P450 protein in liver microsomes. The protein is mainly  expressed in liver, duodenum and small intestine. About 100 therapeutic drugs are metabolized by CYP2C9, including drugs with a narrow therapeutic index such as warfarin and phenytoin, and other routinely prescribed drugs such as acenocoumarol, tolbutamide, losartan, glipizide, and some nonsteroidal anti-inflammatory drugs. By contrast, the known extrahepatic CYP2C9 often metabolizes important endogenous compounds such as serotonin and, owing to its epoxygenase activity, various polyunsaturated fatty acids, converting these fatty acids to a wide range of biological active products. In particular, CYP2C9 metabolizes arachidonic acid to the following eicosatrienoic acid epoxide (EETs) stereoisomer sets: 5"R",6"S"-epoxy-8Z,11Z,14Z-eicosatetrienoic and 5"S",6"R"-epoxy-8Z,11Z,14Z-eicosatetrienoic  acids; 11"R",12"S"-epoxy-8Z,11Z,14Z-eicosatetrienoic and 11"S",12"R"-epoxy-5Z,8Z,14Z-eicosatetrienoic acids; and 14"R",15"S"-epoxy-5Z,8Z,11Z-eicosatetrainoic and 14"S",15"R"-epoxy-5Z,8Z,11Z-eicosatetrainoic acids. It likewise metabolizes docosahexaenoic acid to epoxydocosapentaenoic acids (EDPs; primarily 19,20-epoxy-eicosapentaenoic acid isomers [i.e. 10,11-EDPs]) and eicosapentaenoic acid to epoxyeicosatetraenoic acids (EEQs, primarily 17,18-EEQ and 14,15-EEQ isomers). Animal models and a limited number of human studies implicate these epoxides in reducing hypertension; protecting against myocardial infarction and other insults to the heart; promoting the growth and metastasis of certain cancers; inhibiting inflammation; stimulating blood vessel formation; and possessing a variety of actions on neural tissues including modulating neurohormone release and blocking  pain perception (see epoxyeicosatrienoic acid and epoxygenase). In vitro studies on human and animal cells and tissues and in vivo animal model studies indicate that certain EDPs and EEQs (16,17-EDPs, 19,20-EDPs, 17,18-EEQs have been most often examined) have actions which often oppose those of another product of CYP450 enzymes (e.g. CYP4A1, CYP4A11, CYP4F2, CYP4F3A, and CYP4F3B) viz., 20-Hydroxyeicosatetraenoic acid (20-HETE), principally in the areas of blood pressure regulation, blood vessel thrombosis, and cancer growth (see 20-Hydroxyeicosatetraenoic acid, epoxyeicosatetraenoic acid, and epoxydocosapentaenoic acid sections on activities and clinical significance). Such studies also indicate that the  eicosapentaenoic acids and EEQs are: 1) more potent than EETs in decreasing hypertension and pain perception; 2) more potent than or equal in potency to the EETs in suppressing inflammation; and 3) act oppositely from the EETs in that they inhibit angiogenesis, endothelial cell migration, endothelial cell proliferation, and the growth and metastasis of human breast and prostate cancer cell lines whereas EETs have stimulatory effects in each of these systems. Consumption of omega-3 fatty acid-rich diets dramatically raises the serum and tissue levels of EDPs and EEQs in animals as well as humans, and in humans is by  far the most prominent change in the profile of polyunsaturated fatty acids metabolites caused by dietary omega-3 fatty acids. CYP2C9 may also metabolize linoleic acid to the potentially very toxic products, vernolic acid (also termed leukotoxin) and coronaric acid (also termed isoleukotoxin); these linoleic acid epoxides cause multiple organ failure and acute respiratory distress in animal models and may contribute to these syndromes in humans. Pharmacogenomics. The "CYP2C9" gene is highly polymorphic. At least 20 single nucleotide polymorphisms (SNPs) have been reported to have functional evidence of altered enzyme activity. In fact, adverse drug reactions (ADRs) often result from unanticipated  changes in CYP2C9 enzyme activity secondary to genetic polymorphisms. Especially for CYP2C9 substrates such as warfarin and phenytoin, diminished metabolic capacity because of genetic polymorphisms or drug-drug interactions can lead to toxicity at normal therapeutic doses. The label CYP2C9*1 is assigned by the Pharmacogene Variation Consortium (PharmVar) to the most commonly observed human gene variant. Other relevant variants are cataloged by PharmVar under consecutive numbers, which are written after an asterisk (star) character to form an allele label. The two most well-characterized variant alleles are CYP2C9*2 (NM_000771.3:c.430C>T, p.Arg144Cys, rs1799853) and CYP2C9*3  (NM_000771.3:c.1075A>C, p. Ile359Leu, rs1057910), causing reductions in enzyme activity of 30% and 80%, respectively. Metabolizer phenotypes. On the basis of their ability to metabolize CYP2C9 substrates, individuals can be categorized by groups. The carriers of homozygous CYP2C9*1 variant, i.e. of the *1/*1 genotype, are designated extensive metabolizers (EM), or normal metabolizers. The carriers of the CYP2C9*2 or CYP2C9*3 alleles in a heterozygous state, i.e. just one of these alleles (*1/*2, *1/*3) are designated intermediate metabolizers (IM), and those carrying two of these alleles, i.e. homozygous (*2/*3, *2/*2 or *3/*3) — poor metabolizers (PM). As a  result, the metabolic ratio - the ratio of unchanged drug to metabolite - is higher in PMs. A study of the ability to metabolize warfarin among the carriers of the most well-characterized CYP2C9 genotypes (*1, *2 and *3), expressed as percentage of the mean dose in patients with wild-type alleles (*1/*1), concluded that the mean warfarin maintenance dose was 92% in *1/*2, 74% in *1/*3, 63% in *2/*3, 61% in *2/*2 and 34% in 3/*3. CYP2C9*3 reflects an Ile359-Leu (I359L) change in the amino acid sequence, and also has reduced catalytic activity compared with the wild type (CYP2C9 *1) for substrates other than warfarin. Its prevalence varies with race as: Test panels of variant alleles. The Association for Molecular Pathology Pharmacogenomics (PGx) Working Group in 2019 has recommended a minimum panel of variant alleles (Tier 1) and an extended panel of variant alleles (Tier 2) to be included in assays for CYP2C9 testing. CYP2C9 variant alleles recommended as Tier 1 by the PGx Working Group include CYP2C9 *2, *3, *5, *6, *8, and *11. This recommendation was based on their well-established functional effects on CYP2C9 activity and drug response availability of reference materials, and their appreciable allele  frequencies in major ethnic groups. The following CYP2C9 alleles are recommended for inclusion in tier 2: CYP2C9*12, *13, and *15. CYP2C9*13 is defined by a missense variant in exon 2 (NM_000771.3:c.269T>C, p. Leu90Pro, rs72558187). CYP2C9*13 prevalence is approximately 1% in the Asian population, but in Caucasians this variant prevalence is almost zero. This variant is caused by a T269C mutation in the CYP2C9 gene which in turn results in the substitution of leucine at position-90 with proline (L90P) at the product enzyme protein. This residue is near the access point for substrates and the  L90P mutation causes lower affinity and hence slower metabolism of several drugs that are metabolized CYP2C9 by such as diclofenac and flurbiprofen. However, this variant is not included in the tier 1 recommendations of the PGx Working Group because of its very low multiethnic minor allele frequency and a lack of currently available reference materials. As of 2020, the evidence level for CYP2C9*13 in the PharmVar database is limited, comparing to the tier 1 alleles, for which the evidence level is definitive. Additional variants. Not all clinically significant genetic variant alleles have been registered by PharmVar. For example, in  a 2017 study, the variant rs2860905 showed stronger association with warfarin sensitivity (<4 mg/day) than common variants CYP2C9*2 and CYP2C9*3. Allele A (23% global frequency) is associated with decreased dose of warfarin as compared to the allele G (77% global frequency). Another variant, rs4917639, according to a 2009 study, has strong effect on warfarin sensitivity, almost the same as if CYP2C9*2 and CYP2C9*3 were combined into a single allele. The C allele at rs4917639 has 19% global frequency. Patients with the CC or CA genotype may require decreased dose of warfarin as compared to  patients with the wild-type AA genotype. Another variant, rs7089580 with T allele having 14% global frequency, is associated with increased CYP2C9 gene expression. Carriers of AT and TT genotypes at rs7089580 had increased CYP2C9 expression levels comparing to wild-type AA genotype. Increased gene expression due to rs7089580 T allele leads to increased rate of warfarin metabolism and increased warfarin dose requirements. In a study published in 2014, the AT genotype showed slightly higher expression than TT, but both much higher than AA. Another variant, rs1934969 (in studies of 2012 and 2014) have been shown to affect the ability  to metabolize losartan: carriers of TT genotype have increased CYP2C9 hydroxylation capacity for losartan comparing to AA genotype, and, as a result, lower metabolic ratio of losartan, i.e. faster losartan metabolism. Ligands. Most inhibitors of CYP2C9 are competitive inhibitors. Noncompetitive inhibitors of CYP2C9 include nifedipine, phenethyl isothiocyanate, medroxyprogesterone acetate and 6-hydroxyflavone. It was indicated that the noncompetitive binding site of 6-hydroxyflavone is the reported allosteric binding site of the CYP2C9 enzyme. Following is a table of selected substrates, inducers and inhibitors of CYP2C9. Where classes of agents are listed, there may be exceptions within the class. Inhibitors of  CYP2C9 can be classified by their potency, such as: Epoxygenase activity. CYP2C9 attacks various long-chain polyunsaturated fatty acids at their double (i.e. alkene) bonds to form epoxide products that act as signaling molecules. It along with CYP2C8, CYP2C19, CYP2J2, and possibly CYP2S1 are the principle enzymes which metabolizes 1) arachidonic acid to various epoxyeicosatrienoic acids (also termed EETs); 2) linoleic acid to 9,10-epoxy octadecaenoic acids (also termed vernolic acid, linoleic acid 9:10-oxide, or leukotoxin) and 12,13-epoxy-octadecaenoic (also termed coronaric acid, linoleic acid 12,13-oxide, or isoleukotoxin); 3) docosahexaenoic acid to various epoxydocosapentaenoic acids (also termed  EDPs); and 4) eicosapentaenoic acid to various epoxyeicosatetraenoic acids (also termed EEQs). Animal model studies implicate these epoxides in regulating: hypertension, Myocardial infarction and other insults to the heart, the growth of various cancers, inflammation, blood vessel formation, and pain perception; limited studies suggest but have not proven that these epoxides may function similarly in humans (see epoxyeicosatrienoic acid and epoxygenase pages). Since the consumption of omega-3 fatty acid-rich diets dramatically raises the serum and tissue levels of the EDP and EEQ metabolites of the omega-3 fatty acid, i.e. docosahexaenoic and eicosapentaenoic acids, in animals and humans and in  humans is the most prominent change in the profile of polyunsaturated fatty acids metabolites caused by dietary omega-3 fatty acids, eicosapentaenoic acids and EEQs may be responsible for at least some of the beneficial effects ascribed to dietary omega-3 fatty acids.