Efficacy and Safety of Dabigatran vs. Warfarin for Stroke
After more than five decades as the preferred anticoagulant worldwide, warfarin is being challenged by a new rival that doesn't require careful dosage monitoring. Pradaxa (dabigatran) was unanimously approved by the FDA on October 19, 2010 for treating atrial fibrillation (AF) patients, who are at an increased risk for suffering stroke and systemic embolisms (U.S. Food and Drug Administration). Dabigatran acts by binding directly to thrombin. Warfarin (Coumadin) is an anticoagulant that functions by inhibiting the synthesis of vitamin K-dependent clotting factors (Lemos et al., 770-771). Variations in the levels of vitamin K in the diet can influence how effective a given dose of warfarin is for a patient, so appropriate therapeutic dosages are determined on an individual basis periodically through a standardized clotting test (international normalized ratio [INR]). Even though warfarin can reduce the risk of stroke in AF patients by as much as 68%, patient compliance with treatment (time in therapeutic range [TTR]) is a significant problem because of the requisite periodic monitoring of dose effectiveness and the risk of major bleeding events (Ezekowitz, 2007). Because dabigatran doesn't require the labor and time intensive monitoring that warfarin does, the expectation is that patient compliance should increase.
Atrial fibrillation increases the risk of stroke and systemic embolisms because the upper chambers of the heart begin to beat irregularly, thus impairing blood flow efficiency. Slowed blood flow can result in blood pooling in the heart chambers and clots can form. If these clots enter the brain they could cause a stroke. For this reason AF patients are often prescribed anticoagulants to help prevent clot formation. The prevalence of AF in the United States increases from less than 1% for persons under the age of 60, to over 10% for persons over 80 (Centers for Disease Control and Prevention [CDC], 2003). Deaths linked to AF affect about 1 in 4000 people each year, but 84% occur in people over the age of 75.
Atrial fibrillation patients often suffer from coronary artery disease, so the focus of a dabigatran phase 2 clinical trial was to determine the safety of combining this anticoagulant with aspirin (Ezekowitz et al., 2007). Three dabigatran doses were administered (50, 150, and 300 mg twice daily) to patients for 12 weeks, alone or in combination with 81 or 325 mg aspirin, thus creating nine experimental groups. The patients treated with warfarin (INR = 2.0-3.0) were used as a control. The primary outcome of concern was bleeding events and the results indicated major (6% increase, p < 0.02) or all types of bleeding events (26% increase, p = 0.0003) occurred more often in patients treated with 300 mg dabigatran twice a day plus aspirin. On the other end of the dosage spectrum, 50 mg dabigatran was less likely to cause bleeding than all other conditions (15% increase vs. 300 mg p = 0.0002; 11% increase vs. 150 mg p = 0.01; 11% increase vs. warfarin, p = 0.044), but the two embolisms that occurred during the study happened in patients taking 50 mg dabigatran twice daily, alone or with 81 mg of aspirin. The last result indicates 50 mg dabigatran, with or without 81 mg of aspirin, doesn't provide effective protection against stroke or systemic embolisms. This possibility was supported by finding plasma D-dimer levels were inversely correlated with dabigatran dose (13% increase at 50 mg, p = 0.0008; 3% increase at 150 mg, p = 0.027; 300 mg p =0.267) when compared to warfarin. The results of this study suggest 150 mg dabigatran twice daily is a relatively safe and effective dose for coronary artery patients taking aspirin. This conclusion is limited by the small sample size (502), consisting mostly of men (411), which precludes making stronger statements about safety and efficacy for the general population.
A direct head-to-head comparison between 150 mg dabigatran twice daily and warfarin (INR = 2.0-3.0) was conducted using patients beginning anticoagulation therapy at 228 clinics in 29 countries (Schulman et al., 2009). The efficacy outcomes of concern were embolism and associated deaths. The safety outcomes were all types of bleeding, acute coronary symptoms, and several other adverse events. During the 6-month treatment period no difference in the rate of embolism or related deaths were observed between the two anticoagulants (noninferiority, p < 0.001). There was no difference in the number of major bleeding events between the two drugs, but when both major and clinically-relevant bleeding events were combined, patients...
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