Warfarin is a widely used anticoagulant in the treatment and prevention of thrombosis. It was initially marketed as a pesticide against rats and mice and is still used for this purpose. It was approved for use as a medication in the early 1950s and is widely prescribed. Despite its common use, warfarin therapy can be associated with significant bleeding complications. Achieving a safe therapeutic response can be difficult because of warfarin’s narrow therapeutic index and great individual variability in the dose required, which is mostly a consequence of individual genetic variants. This fact is well known among clinicians and a wide range, from 1 mg/day to 20 mg/day, of warfarin maintenance doses are observed across the population. To maintain a therapeutic level of anti-thrombosis and to minimize the risk of bleeding complications, warfarin therapy requires intensive monitoring via the INR to guide its dosing. The INR is used to monitor the effectiveness of warfarin and measures of the pathway of blood coagulation. The INR is used to standardize the results for a prothrombin time. The INR is the ratio of a patient’s prothrombin time to a control sample, raised to the power of the index value for the analytical system used.

Several factors increase the risk of over-anticoagulation: genetic polymorphisms affecting the metabolizing enzymes, impaired liver function, drug interactions, congestive heart failure, diarrhea, fever, and diets rich in vitamin K [1]. Nevertheless, genetic factors and drug interactions mostly account for the risk of over-anticoagulation. Warfarin metabolism involves primarily the cytochrome P450 (CYP) enzymes. Some loss-of-function CYP2C9 and VKORC1 polymorphisms are known to be associated with decreased enzymatic activity and as a result, with increased risk of hemorrhage. These are CYP2C9*2 (Cysl44/Ile359), CYP2C9*3 (Argl44/Leu359) [2] and VKORC1 (−1639G > A) [3].

Warfarin-induced hemorrhage is an important complication of anticoagulation therapy. A review of many studies shows average yearly rates of warfarin-related bleeding as high as 0.8%, 4.9%, and 15%, for fatal, major and minor bleeding complications, respectively [4].

Vitamin K is required by proteins C and S, together with clotting factors II, VII, IX, and X, to allow assembly of the procoagulant enzyme complexes necessary to generate fibrin. Warfarin as an anticoagulant agent has the ability to interfere with the recycling of vitamin K in the liver. The pharmacologic effect of warfarin is mediated by the inhibition of vitamin K epoxide reductase complex subunit 1 (EC 1.1.4.1) [5,6].

Warfarin consist of (R)- and (S)-warfarin enantiomers. (R)- and (S)-warfarins differ in their relative plasma concentrations, in their antithrombotic potency and in the specific isoenzymes responsible for their metabolism. (S)-warfarin has a three-to-five times greater anticoagulant effect than the (R)-enantiomer and accounts for 60% to 70% of warfarin’s overall anticoagulant activity. (S)-warfarin is metabolized almost exclusively by CYP2C9 [7,8,9], see Figure 1.

The activity of the CYP2C9 enzyme has a significant impact on the clearance of (S)-warfarin and as a consequences on anticoagulant effect. In the presence of genetic variations where the activity of CYP2C9 is reduced, clearance of (S)-warfarin is also reduced. Activity of CYP2C9 between individuals can vary by more than 20-fold. (R)-warfarin is metabolized by multiple different CYP enzymes [10].

While several single-nucleotide polymorphisms of CYP2C9 have been reported, the CYP2C9*2 (Cysl44/Ile359) and CYP2C9*3 (Argl44/Leu359) polymorphisms have been identified as clinically relevant [11]. Both of these variants are associated with decreased enzymatic activity [12,13,14,15,16,17,18].

Homozygous CYP2C9*3 variant genotypes have only 5% to 10% metabolic efficiency compared to the wild-type genotype. As a result, compared to wild-type CYP2C9*1*1 controls, enzyme activity and the median maintenance warfarin dose for CYP2C9*3*1 heterozygotes was reduced by 40%, and by approximately 90% for CYP2C9*3*3 homozygotes [11,12,13].

Furuya [19] and Steward [14] showed that the CYP2C9*2 variant is also associated with reduced warfarin elimination. Heterozygotes demonstrate 40% and homozygotes 15% of the wild-type enzyme activity, causing dose adjustment for heterozygote CYP2C9*2 individuals down to 20% less than the standard dose.

Margaglione [15] has also demonstrated bleeding rates as high as 27.9 per 100 patient-years in carriers of CYP variants. In this study, findings were adjusted for other common variables associated with increased bleeding risk, such as increased age, drug interactions and abnormal liver function.

Several studies of the *2 and *3 CYP2C9 polymorphisms consistently show that patients with at least one CYP2C9 allele polymorphism have reduced warfarin requirements [15,20,21,22,23,24].

Freeman [25] reported reduced warfarin weekly dosages for carriers of CYP2C9*2 or CYP2C9*3 alleles compared with patients who were homozygous for the wild-type allele (0.307 mg/kg/wk and 0.397 mg/kg/wk, respectively).

Taube [23] compared warfarin maintenance dosages in 683 patients carrying different CYP2C9 genotypes. Mean warfarin maintenance dosages were 86% in patients with CYP2C9*1/CYP2C9*2, 79% in patients with CYP2C9*1/CYP2C9*3, 82% in compound heterozygotes CYP2C9*2/CYP2C9*3, and 61% in patients homozygous for CYP2C9*2.

Furthermore, Aithal [20] warns that even when warfarin dosages are decreased, carriers of CYP2C9 poor metabolizer alleles experience a rate of major bleeding that is 3.68-fold higher than the rate seen in patients with the wild type genotype.

The frequency of CYP2C9 alleles is ethnically related [22,26]. Approximately 20% of the white population carries one of the loss-of-function CYP2C9 alleles, and it is estimated that 1% of whites carry two such alleles [10]. The frequency of the CYP2C9*2 allele reportedly ranges from 8–13% in different Caucasian populations. CYP2C9*2 is present in 4% of African-Americans and rare among Japanese individuals [27,28]. The frequency of CYP2C9*3 is 6–10% among Caucasian populations and 3.8% in Japanese populations [28,29]. This data suggests that a substantial fraction of the Caucasian patient population may carry at least one defective CYP2C9 allele. In this group, the usual prescription dosage of warfarin may lead to major or even life-threatening hemorrhage.

Drug-drug interaction is a main concern in adverse drug reactions. It has been shown that metronidazole and cimetadine increase the prothrombin time in patients on warfarin therapy. Chloramphenicol enhances warfarin’s effect by inhibiting the action of the hepatic P450 system [10].

Where depression coexists with cardiovascular disease, warfarin is commonly prescribed in combination with selective serotonin reuptake inhibitors (SSRIs). Case reports suggest that some SSRIs can interact with warfarin to increase the likelihood of bleeding [30]. SSRIs cause adverse effects in isolation [31,32] and can interact with other medications by inhibiting various isoenzymes of the CYP450 enzyme group [33,34].

The primary complication occurring with warfarin treatment is bleeding. SSRIs may increase the risk of bleeding during warfarin therapy by hindering platelet aggregation through depletion of platelet serotonin levels [35,36,37]. Some SSRIs may also inhibit the oxidative metabolism of warfarin by CYP2C9 [38].

Duncan [38] and Sayal [39] have warned that antidepressants with a known or predictable interaction with warfarin, such as fluoxetine and fluvoxamine, should be avoided in patients receiving warfarin because of the risk of adverse outcomes.