Cytochrome P450 2C19 Gene Research Articles

Two Single-Tube Tetra-Primer Assays to Detect the CYP2C19*2 and *3 Alleles of S-Mephenytoin Hydroxylase

Cytochrome P450 2C19 (CYP2C19) metabolizes several therapeutic agents, such as S -mephenytoin, omeprazole, propranolol, and imipramine. Interindividual differences in CYP2C19 activity divide the population into extensive metabolizers and poor metabolizers (PMs). PMs may suffer adverse effects when treated with a routine clinical dose of a drug inactivated by CYP2C19 or may not gain therapeutic benefit from prodrugs activated by CYP2C19. For example, the antimalarial drug proguanil is administered as the prodrug and requires activation by CYP2C19. PMs were found to totally lack the active metabolite cycloguanil in their plasma and are at risk for failed protection from plasmodium infection by proguanil (1)(2). The PM phenotype occurs in 2–5% of Caucasians and Africans and 10–23% of Orientals (3). Seven alleles have been described in the CYP2C19 gene that produce an inactive CYP2C19 enzyme; however, two alleles account for the majority of the PM phenotypes. The CYP2C192 and the CYP2C193 alleles are found in ∼87% of PMs in Caucasians and ∼98% of PMs in Orientals (4). CYP2C192 is the most prevalent PM allele, with a G-to-A nucleotide substitution in exon 5, which produces an aberrant splice site (5). CYP2C193 is found mainly in Orientals, with a G-to-A nucleotide substitution at position 636 in exon 4, which produces a premature stop codon (6). Several methods based on PCR amplification of the CYP2C19 locus are used to genotype the PM alleles. The most widely used methods are restriction fragment length polymorphism (RFLP) analysis (7)(8) and allele-specific amplification (ASA) (9). However, ASA and RFLP include transfer steps that increase hands-on time and the chance of contamination or confusion of samples. To decrease hands-on time and to facilitate genotyping of the CYP2C19 alleles that predict the majority of …

Genetic association between sensitivity to warfarin and expression of CYP2C9*3.

Cytochrome P4502C9 (CYP2C9) is largely responsible for terminating the anticoagulant effect of racemic warfarin via hydroxylation of the pharmacologically more potent S-enantiomer to inactive metabolites. Mutations in the CYP2C9 gene result in the expression of three allelic variants, CYP2C9*1, CYP2C9*2 and CYP2C9*3. Both CYP2C9*2 and CYP2C9*3 exhibit altered catalytic properties in vitro relative to the wild-type enzyme. In the present study, a patient was genotyped who had proven unusually sensitive to warfarin therapy and could tolerate no more than 0.5 mg of the racemic drug/day. PCR-amplification of exons 3 and 7 of the CYP2C9 gene, followed by restriction digest or sequence analysis, showed that this individual was homozygous for CYP2C9*3. In addition, patient plasma warfarin enantiomer ratios and urinary 7-hydroxywarfarin enantiomer ratios were determined by chiral-phase high performance liquid chromotography in order to investigate whether either parameter might be of diagnostic value in place of a genotypic test. Control patients receiving 4-8 mg warfarin/day exhibited plasma S:R ratios of 0.50 +/- 0.25:1, whereas the patient on very low-dose warfarin exhibited an S:R ratio of 3.9:1. In contrast, the urinary 7-hydroxywarfarin S:R ratio of 4:1 showed the same stereoselectivity as that reported for control patients. Therefore, expression of CYP2C9*3 is associated with diminished clearance of S-warfarin and a dangerously exacerbated therapeutic response to normal doses of the racemic drug. Analysis of the plasma S:R warfarin ratio may serve as a useful alternative test to genotyping for this genetic defect.