First report of warfarin dose requirements in patients possessing the CYP2C9*12 allele Travis J. O'Brien a , Robert S. Kidd b , Craig A.H. Richard b , Ngoc-Han Ha c , Preston Witcher b , Linda V. Tran a , April Barbour d , Matthew Tuck d,e,f , Samantha D. McIntosh e,f , Jacqueline N. Douglas d , Arthur F. Harralson a,b, ⁎ a The George Washington University, Department of Pharmacology and Physiology, Washington, DC, United States b Shenandoah University, Bernard J. Dunn School of Pharmacy, Winchester, VA, United States c Laboratory of Cancer Biology and Genetics, National Cancer Institute, National Institutes of Health, Bethesda, MD, United States d The George Washington University, Department of Medicine, Washington, DC, United States e Uniformed Services University of the Health Sciences, Bethesda, MD, United States f Veterans Affairs Medical Center, Washington, DC, United States abstract article info Article history: Received 19 March 2013 Received in revised form 8 May 2013 Accepted 8 May 2013 Available online 17 May 2013 Keywords: Warfarin CYP2C9 CYP2C9*12 rs9332239 Background: Warfarin is the most frequently prescribed anticoagulant in North America and Europe. It is ad- ministered as a racemate, but S-warfarin is principally responsible for its anticoagulant activity. Cytochrome P450 (CYP) 2C9 is the enzyme primarily responsible for the metabolism of S-warfarin. Numerous variant alleles of CYP2C9 have been identified. The CYP2C9*12 (rs9332239) allele harbors a P489S substitution in CYP2C9 which has been shown to result in a 40% decline in catalytic activity in vitro. Cases: Four Caucasian patients with a low mean weekly warfarin dose (MWWD) were genotyped for CYP2C9, VKORC1 and APOE variant alleles. None of the four patients carried the common CYP2C9 variant alleles (*2, *3, *5, *6, *7, *8, *9, *11, *13) despite a relatively low MWWD (23.4 ± 7.94 mg) compared to 208 patients car- rying the CYP29C9*1 genotype (32.2 ± 12.65 mg). Given that CYP2C9*12 confers decreased in vitro activity to the enzyme, we investigated whether these patients carried this allele. All four patients were CYP2C9*12 CT heterozygotes. Individual comparisons with patients possessing the same VKORC1 and APOE genotypes also demonstrated lower dose requirements in the patients that possessed CYP2C9*12 allele. Conclusions: There are no reports of the clinical impact of rs9332239 on CYP2C9 substrates. This is the first report of patients with the rare CYP2C9*12 genotype and lower warfarin dose requirements. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Warfarin is a commonly prescribed oral anticoagulant for the pre- vention of thromboembolic events. However, warfarin has a narrow therapeutic index and a large interindividual variability exists with regard to warfarin dose requirements for appropriate anticoagulation. Factors such as age, weight, height, medications and diet have been shown to contribute to the variability in warfarin dosing [1–4]. Addi- tionally, a significant proportion of this variability in warfarin re- quirements appears to be genetically based [5]. Warfarin produces its therapeutic effect through the inhibition of Vitamin K epoxide reductase (VKORC1), a product of the VKORC1 gene [6]. A properly functioning VKORC1 enzyme is required for the activation, of vitamin K-dependent clotting factors (II, III, IX and X). Some VKORC1 genetic variants result in decreased levels of activated clotting factors, a higher tendency towards clinically significant bleeding, and a lower warfarin dose to attain a therapeutic INR [7]. Warfarin is primarily metabolized by CYP2C9-dependent hydroxyl- ation [8]. A significant proportion of the population possesses variant alleles of CYP2C9 that result in decreased metabolic activity leading to decreased warfarin clearance and a higher risk for serious bleeding associated with higher plasma levels of S-warfarin [9]. Patients with CYP2C9 gene variants that confer decreased enzymatic activity require a lower warfarin dose to attain a therapeutic international normalized ratio (INR) [9–11]. The increasing importance of phar- macogenetics on warfarin dosing is evidenced by the inclusion of information regarding warfarin for gene variants in CYP2C9 and VKORC1 on the product label by the United States Food and Drug Administration (USFDA) [12]. In addition to CYP2C9 and VKORC1, several other gene variants have been implicated in impacting therapeutic warfarin dose including apo- lipoprotein E (APOE) which facilitates the uptake of vitamin K-rich lipo- proteins into cells [13–16]. Three common ApoE variants have been identified (E2, E3, E4) with ApoE2 and ApoE4 alleles conferring the highest and lowest levels of vitamin K, respectively [24,25]. Consistent with this, ApoE4 carriers have a lower warfarin dose requirement that carriers of the other ApoE common alleles [17–21]. Clinica Chimica Acta 424 (2013) 73–75 ⁎ Corresponding author at: Bernard J. Dunn School of Pharmacy, at The George Washington University, Virginia Science & Technology Campus, 45085 University Drive, 202S, Ashburn, VA 20147-2766, United States. Tel.: +1 703 726 3718. E-mail address: Aharrals@su.edu (A.F. Harralson). Contents lists available at ScienceDirect Clinica Chimica Acta journal homepage: www.elsevier.com/locate/clinchim 0009-8981/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cca.2013.05.008