Polymorphisms in warfarin drug target and metabolizing enzyme genes, in addition to nongenetic factors, were important determinants of warfarin dose requirements.

Background Initiation of warfarin therapy using trial-and-error dosing can cause bleeding. Clinical factors explain only 20%–30% of the variability in the therapeutic dose of warfarin. Single nucleotide polymorphisms (SNPs) in the cytochrome P450 2C9 (CYP2C9) gene correlate with the clearance of S-warfarin and SNPs in the vitamin K epoxide reductase (VKORC1) gene predict warfarin sensitivity. We test the hypothesis that the combination of clinical and pharmacogenetic information can predict the therapeutic warfarin dose. Methods We collected DNA, demographic variables, laboratory values, and medication histories from patients taking warfarin. Subjects either attended an outpatient anticoagulation clinic or participated in the PREVENT (prevention of venous thromboembolism) study. After PCR amplification, we used Pyrosequencing® to genotype DNA regions for 2 coding CYP2C9 SNPs, *2 (C430T) and *3 (A1075C), and for 4 noncoding VKORC1 SNPs: C861A, A5808C, G6853C, and G9041A. Using multiple regression, we quantified the association between therapeutic warfarin dose and clinical and genetic factors in a derivation cohort of 900 participants and a validation cohort of 100 participants. Results The VKORC1 G6853C SNP was the first variable to enter the stepwise regression equation and was associated with a 27% decrease in the warfarin dose per allele in Caucasian patients. The VKORC1 A5808C SNP was associated with a 33% decrease per allele in warfarin dose in African-American patients. Other significant (p < 0.05) predictors of the therapeutic warfarin dose, in order of entry into the regression equation and their effect on warfarin dose were: body surface area (+12% per SD increase), CYP2C9*3 (−33% per allele), CYP2C9*2 (−20% per allele), age (−7% per decade), target INR (+8% per 0.5 unit increase), amiodarone use (−24%), African-American race (+12%), smoker (+9%), and simvastatin or fluvastatin use (−5%). A dosing equation that included these pharmacogenetic and clinical factors explained 52% of the dose variability in derivation cohort and 55% of the variability in the validation cohort. Conclusions The therapeutic warfarin dose can be estimated from clinical and pharmacogenetic factors that can be obtained when warfarin is started. Use of this dosing equation has potential to aid in the prediction of an optimal warfarin dose, which may decrease the risk of bleeding during the initiation of warfarin therapy.

Cytochrome P450 (CYP) 2C8 is responsible for the oxidative metabolism of many clinically available drugs from a diverse number of drug classes (e.g., thiazolidinediones, meglitinides, NSAIDs, antimalarials and chemotherapeutic taxanes). The CYP2C8 enzyme is encoded by the CYP2C8 gene, and several common nonsynonymous polymorphisms (e.g., CYP2C8*2 and CYP2C8*3) exist in this gene. The CYP2C8*2 and *3 alleles have been associated in vitro with decreased metabolism of paclitaxel and arachidonic acid. Recently, the influence of CYP2C8 polymorphisms on substrate disposition in humans has been investigated in a number of clinical pharmacogenetic studies. Contrary to in vitro data, clinical data suggest that the CYP2C8*3 allele is associated with increased metabolism of the CYP2C8 substrates, rosiglitazone, pioglitazone and repaglinide. However, the CYP2C8*3 allele has not been associated with paclitaxel pharmacokinetics in most clinical studies. Furthermore, clinical data regarding the impact of the CYP2C8*3 allele on the disposition of NSAIDs are conflicting and no definitive conclusions can be made at this time. The purpose of this review is to highlight these clinical studies that have investigated the association between CYP2C8 polymorphisms and CYP2C8 substrate pharmacokinetics and/or pharmacodynamics in humans. In this review, CYP2C8 clinical pharmacogenetic data are provided by drug class, followed by a discussion of the future of CYP2C8 clinical pharmacogenetic research. Keywordsamodiaquine; CYP2C8; cytochrome P450 2C8; human; ibuprofen; NSAID; paclitaxel; repaglinide; thiazolidinedione The field of pharmacogenetics is aimed at understanding how genetic variation contributes to interindividual variability in drug disposition (i.e., pharmacokinetics) and drug response (i.e., pharmacodynamics). In terms of drug disposition, the cytochrome P450 (CYP) metabolizing enzymes are a major area of study, as they catalyze the oxidative metabolism of numerous drugs and endogenous compounds. Enzymes in the CYP2C family (e.g., CYP2C8, CYP2C9, CYP2C18 and CYP2C19) are significant contributors to drug disposition and metabolize 20% of clinically available drugs [1,2]. Recently, the role of the CYP2C8 isoenzyme has garnered considerable interest in the fields of drug metabolism and pharmacogenetics. This is largely a †Author for correspondence: University of Colorado Denver, School of Pharmacy, Department of Pharmaceutical Sciences, 12700 East 19th Avenue, Box C238-P15, Aurora, CO 80045, USA, Tel.: +1 303 724 6126; Fax: +1 303 724 7266; christina.aquilante@ucdenver.edu. For reprint orders, please contact: reprints@futuremedicine.com Financial & competing interests disclosureDr Aquilante currently holds investigator-initiated research grants from the NIH (K23 DK073197), American College of Clinical Pharmacy and Tibotec Therapeutics. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materia...