by Karthikeshwar Kasirajan, MD

Despite numerous advances in the invasive management of venous thromboembolism (VTE), oral anticoagulants remain the recommended treatment once discharged for a minimum of three months. Despite the availability of newer anticoagulants, warfarin remains the most commonly prescribed drug for VTE, with a >90% efficacy in preventing recurrent VTE. Interestingly, warfarin is the only drug with a genome-based dosing guideline in the package insert. More than 120 drugs now contain pharmacogenetic information in their Food and Drug Administration (FDA) approved labeling in the form of a black box warning. A growing body of literature suggests that they are frequently related to Cytochrome P450 (CYP450) gene polymorphism. For many drugs with a narrow therapeutic window (e.g., warfarin), these individual genetic variations can result in significant adverse drug reaction (ADR) related to under or over dosing. These ADRs are a major impact to the health care system, due to the staggering costs and resource utilization related to ER visits and hospital admissions required to manage these complications. In a study published in 2001, the overall cost to the healthcare system to manage ADR was $177.4 billion.(1)

Anticoagulants account for close to 50% of these emergency room (ER) visits for ADRs.(2) Unfortunately, warfarin accounts for 33% of all ER visits for ADRs, making this the commonest drugs for ADR-related ER visits and subsequent hospital admissions.(2) Additionally, warfarin demonstrates significant variation in the individual patient’s response based on their unique genetic makeup. This fact has prompted the FDA to insert a black box warning in the prescribing information, recommending use of genetic testing before drug prescription, and a genome-based guide to drug dosage.(3,4)

The CYP450 System

The liver is the major site of drug biotransformation, and the cytochrome P450 (CYP450) enzymes located in the membranes of the smooth endoplasmic reticulum are responsible for most drug metabolism. Genetic differences in humans affect the function of several of the CYP450 enzymes. Current knowledge of the genetic polymorphism affecting drug metabolism has increased exponentially in the last few years leading to a whole new branch of science referred to as pharmacogenomics.

To a large extent, the plasma concentration of drugs is determined by the individual patient’s metabolic status. The consequences of CYP450 polymorphisms range from serious toxicity to ineffective drug therapy. Genetically determined reductions in CYP450 enzyme activity (poor metabolizers) for active drugs have important implications for narrowtherapeutic index drugs such as warfarin, where increased plasma concentrations contribute to bleeding complications.

Among the 50 different CYP450 enzyme pathways, eight of these are responsible for the metabolism of more than 80% of all prescribed drugs. The CYP450 2C9 is responsible for warfarin metabolism and will be discussed further.

Warfarin and CYP450 2C9

The cytochrome 2C9 pathway is responsible for metabolism of about 15% of prescribed drugs in the US. (5) Among drugs metabolized via this pathway, s-warfarin is the most commonly prescribed oral anticoagulant with the highest incidence of ADR resulting in an ER visit among all prescribed drugs in the US.(2,6) Chemically, warfarin is composed of two enantiomorphic isomers referred to as a racemic mixture. The s-warfarin is three to five times as potent as R-warfarin as an anticoagulant, has a shorter halflife, and is metabolized predominantly by the 2C9 pathway. The two common 2C9 polymorphisms have only a fraction of the level of enzyme activity of the wild-type *1(Normal metabolizer: NM): 12% for 2C9*2 and 5% for 2C9*3.(7) Hence, lower doses of warfarin will be required by those with the *2 and *3 allelle (Poor Metabolizer: PM). In one study the patients with 2C9*2 or *3 allele required between 61% and 86% of the dose needed by *1 allele homozygotes (NM).(8) The discovery of the target for warfarin in addition to its metabolic pathway changed the approach to warfarin dosing. The vitamin K epoxide reductase complex subunit 1 (VKORC1) was identified as target enzyme for warfarin, and single-nucleotide polymorphisms in VKORC1 were shown to be associated with the dose of warfarin required to achieve a target INR value.(9)

In a study involving almost 900 patients for whom genetic information on CYP2C9 and VKORC1 was made available to prescribing physicians with a matched historical control group of patients who were started on warfarin therapy without genetic information, the results demonstrated dramatic improvements in patient outcomes in the group with genetic profiling. Six months after study initiation, the all-cause hospitalization rate was 18.5% in the patients whose physicians received genotype reports and 25.5% in the control patients, a 28% relative reduction that was statistically significant. Hospitalizations for bleeding or thromboembolic events occurred in 6% of the genotyped patients and in slightly more than 8% of the controls, a 27% relative reduction that was again statistically significant. Warfarin genotyping was linked with a relative drop in all-cause hospitalization of 31%, and a relative drop in hospitalizations for bleeding or thromboembolism of 28%, both statistically significant effects, after the researchers controlled for baseline differences in patients’ age, comorbid conditions, other drugs used, warfarin indication, prior gastrointestinal bleeding, venous thromboembolism, history of hospitalization, and propensity score. (10) The FDA revised the label on warfarin in February 2010, providing genotype-specific ranges of doses and suggesting that genotypes be taken into consideration (Figure 1). The wide availability of CYP2C9 and VKORC1 genotyping and decision tools should have facilitated the wide spread clinical use of this information. Nevertheless, the clinical adoption of genotype-guided administration of warfarin has been slow, probably due to contradictory findings from two small prospective trials of Caucasians showing no benefit with genotype-guided warfarin. (11,12) A multicenter randomized- controlled clinical trial in the US is currently underway to further assess the clinical utility of warfarin pharmacogenetics and results should be available by year end.(13)

These genetic variations account for approximately 40% of the effect on the final dose; age, gender and body weight account for another 30% of the variations. A variety of dosing algorithms are currently available using genetic information in combination with age, gender, and body weight for determining the best final dose for a stable INR. Environmental factors such as other drugs and type of food consumed account for the other 30% of variations noted in the drug dosage. Interestingly, warfarin is the only drug to date that includes a black box warning wherein a dosing chart is included. (14)

Newer Oral Anticoagulants

Factor Xa Inhibitor

Rivaroxaban, a direct factor Xa inhibitor, became the
first newer alternate drug in the US for management of patients with VTE. In the EINSTEIN (15) study, rivaroxaban demonstrated a 85% reduction in relative risk compared to placebo. However, the incidence of major or clinically relevant hemorrhage was higher with rivaroxaban (6.0%) compared to placebo (1.2%). Rivaroxaban is metabolized by CYP450(3A4) pathway. As vast majority of patients are normal metabolizers (99%), the therapeutic efficacy is very predictable. Unfortunately for drugs metabolized by the CYP4503A4 pathway, a variety of drugs can work as inducers or inhibitors of this pathway (Table II). Hence, a significant source of variability is due to the potential interactions with some strong inhibitors or inducers of the CYP450 3A4 enzyme. For example, patients on commonly prescribed antibiotics such as ciprofloxacin or azithromycin have the potential to have much higher serum levels of rivaroxaban due to inhibition of the 3A4 pathway necessary for the metabolic elimination of rivaroxaban.

Apixaban, a direct factor Xa inhibitor, is the latest drug to receive approval with superior results demonstrated in the AMPLIFY-EXT(16) study. Recurrent VTE was much lower (1.7%) compared to placebo (8.8%), with bleed rates that were comparable to placebo at 2.5 mg bid dose (3.2% vs. 2.7%). This drug is also metabolized by the CYP4503A4 pathway.

Direct Thrombin Inhibitor

Dabigatran, a direct thrombin inhibitor, was compared in the RE-MEDY(17) trial with warfarin. Dabigatran was non-inferior in the prevention of recurrent VTE (1.8%) compared to warfarin (1.3%) with a significant reduction
in the incidence of incidence of clinically relevant bleeding (5.5% vs. 10.2%). The RE-SONATE trial(17) compared dabigatran with a placebo, demonstrating a better efficacy for prevention of recurrent VTE (0.4% vs. 5.6%) but with a higher bleed rate compared to placebo (5.3% vs. 1.8%). The major disadvantage of the newer anticoagulants is the lack of a specific reversing agent. A summary of the three newer anticoagulants is given in table III.

Despite the promising results of the newer anticoagulants with the need for less intense monitoring, they have been only effectively studied in low-risk patients, as was the entry criteria for clinical trials. Typical patients in clinical practice have multiple coexisting risk factors, are older (study patients average age was 56 years), and have more risk factors for bleeding. Other confounding factors, such as a higher cardiac event rate seen in patients receiving dabigatran when compared to warfarin, has not been fully evaluated. Real world experience involving use in high risk patients who benefit the most from prolonged anticoagulation and a direct head-to-head comparison between the three newer drugs may help answer the best drug for short-term and prolonged anticoagulation. Until such studies are available, it may be best to restrict use of the newer anticoagulants to patients who have difficulty in achieving and maintaining therapeutic INR. We are currently in the early adoption state of pharmacogenomics testing in the US. The greatest benefit will be seen in patients using multiple drugs that are metabolized by the CYP450 pathway, as genetic testing may help minimize significant drug on drug interaction. Fortunately, pharmacogenomics testing can help predict many of these drug complications, as most prescribed drugs are metabolized by the liver. With the increasing appreciation of the benefits of testing, it is widely anticipated that pharmacogenomics testing will become the standard of clinical practice in the US within the next five years.