A case report of a patient carrying CYP2C9*3/4 genotype with extremely low warfarin dose requirement

Identification and characterization of a novel CYP2C9 allelic variant in a warfarin-sensitive patient

Aim: To determine the genetic basis of the low warfarin dose requirement in a Chinese patient. Materials & methods: Bi-directional sequencing of CYP2C9, VKORC1 and CYP4F2 genes was performed. CYP2C9 variants were highly expressed in yeast and insect-cell microsomes. Three typical CYP2C9 probe drugs were used to evaluate the catalytic activity. Results: A novel missense mutation (1400T>C) was identified in CYP2C9 and had been named as new allele *60. When expressed in yeast and insect cells, compared with wild-type enzyme, variant CYP2C9.60 exhibited lower protein expression capacity and showed significantly decreased metabolic activities for the hydroxylation of S-warfarin, tolbutamide and diclofenac. Conclusion: The novel mutation can greatly decrease the enzymatic activity of the CYP2C9 enzyme both in vitro and in vivo.

Low dose requirement for warfarin treatment in a patient with CYP2C9*3/*13 genotype

BACKGROUND

Cytochrome P450 2C9 (CYP2C9) is the major isoform of the CYP2C subfamily, and is involved in the metabolism of many clinically important therapeutic agents. Here we describe a patient who was intolerant of warfarin treatment because of impaired drug metabolism related to the CYP2C9*3/*13 genotype and high warfarin sensitivity associated with the VKORC1 1173TT genotype.

CASE

A 64-year-old Korean man had taken warfarin for treatment of an occluded left subclavian artery and atrial fibrillation. Although the warfarin doses prescribed were not high (14 mg/week) during the induction of anticoagulation, the prothrombin time (PT) was elevated to over 120 s after two weeks of warfarin therapy. For this patient, a very low dose of warfarin (4 mg/week) was required in order to achieve target INR values.

CONCLUSIONS

To the best of our knowledge, this is the first report of a patient with CYP2C9*3/*13 and VKORC1 1173TT genotypes who had slower than normal warfarin metabolism, resulting in the need to administer an extremely low dose of warfarin in order to achieve the target INR value. Our report reinforces the relevance of pharmacogenetic testing to explain warfarin dose variability and to enable individualized dosage adjustments for improved warfarin treatment outcomes.

Role of pharmacogenomics in drug discovery and development

Pharmacogenetics and pharmacogenomics are two major emerging trends in medical sciences, which influence the success of drug development and therapeutics. In current times, though pharmacogenetic studies are being done extensively for research, its application for drug development needs to get started on a large scale. The major determinants of success of a new drug compound, viz safety and efficacy, have become more predictable, with the advent of pharmacogenetic studies. There is a need felt for pharmacogenomic studies, where the effects of multiple genes are assessed with the study of entire genome.Pharmacogenetic studies can be used at various stages of drug development. The effect of drug target polymorphisms on drug response can be assessed and identified. In clinical studies, pharmacogenetic tests can be used for stratification of patients based on their genotype, which corresponds to their metabolizing capacity. This prevents the occurrence of severe adverse drug reactions and helps in better outcome of clinical trials. This can also reduce attrition of drug compounds. Further, the variations in drug response can be better studied with the wider application of pharmacogenomic methods like genome wide scans, haplotype analysis and candidate gene approaches. The cost of pharmacogenetic testing has become very low, with the advent of newer high throughput genotyping systems. However, the cost of pharmacogenomic methods continues to be very high. As the treatment with several drugs is being more and more pharmacogeneticaly guided (e.g. warfarin and irinotecan), the FDA has laid down guidelines for pharmaceutical firms regarding submission of pharmacogenetic data for their drug products in labelling.

Pharmacogenetic testing of CYP2C9 and VKORC1 alleles for warfarin

American College of Medical Genetics statements and guidelines are designed primarily as an educational resource for medical geneticists and other health care professionals to help them provide quality medical genetic services. Adherence to these standards and guidelines does not necessarily ensure a successful medical outcome. These statements and guidelines should not be considered inclusive of all proper procedures and tests or exclusive of other procedures and tests that are reasonably directed to obtaining the same results. In determining the propriety of any specific procedure or test, the health care professional should apply his or her own professional judgment to the specific clinical circumstances presented by the individual patient or specimen. It may be prudent, however, to document in the patient's record the rationale for any significant deviation from these standards and guidelines. Warfarin (Coumadin) is a potent drug that when used judiciously and monitored closely, leads to substantial reductions in morbidity and mortality from thromboembolic events. However, even with careful monitoring, initiation of warfarin dosing is associated with highly variable responses between individuals and challenges achieving and maintaining levels within the narrow therapeutic range that can lead to adverse drug events. Variants of two genes, CYP2C9 and VKORC1, account for 30-50% of the variability in dosing of warfarin; thus, many believe that testing of these genes will aid in warfarin dosing recommendations. Evidence about this test is evolving rapidly, as is its translation into clinical practice. In an effort to address this situation, a multidisciplinary expert group was organized in November 2006 to evaluate the role of CYP2C9 and VKORC1 testing in altering warfarin-related therapeutic goals and reduction of adverse drug events. A recently completed Rapid-ACCE (Analytical, Clinical Validity, Clinical Utility, and Ethical, Legal, and Social Implications) Review, commissioned to inform this work group, was the foundation for this analysis. From this effort, specific recommendations for the appropriate use of CYP2C9 and VKORC1 testing were developed and are presented here. The group determined that the analytical validity of these tests has been met, and there is strong evidence to support association between these genetic variants and therapeutic dose of warfarin. However, there is insufficient evidence, at this time, to recommend for or against routine CYP2C9 and VKORC1 testing in warfarin-naive patients. Prospective clinical trials are needed that provide direct evidence of the benefits, disadvantages, and costs associated with this testing in the setting of initial warfarin dosing. Although the routine use of warfarin genotyping is not endorsed by this work group at this time, in certain situations, CYP2C9 and VKORC1 testing may be useful, and warranted, in determining the cause of unusual therapeutic responses to warfarin therapy.