"Aug. 10, 2011 -- The newly approved drug Xarelto appears to prevent strokes at least as well as the standard treatment warfarin in people who have a heart condition that puts them at high risk for blood clots, a study shows.
Mechanism of Action
Dabigatran and its acyl glucuronides are competitive, direct thrombin inhibitors. Because thrombin (serine protease) enables the conversion of fibrinogen into fibrin during the coagulation cascade, its inhibition prevents the development of a thrombus. Both free and clot-bound thrombin, and thrombin-induced platelet aggregation are inhibited by the active moieties.
At recommended therapeutic doses, dabigatran etexilate prolongs the coagulation markers such as aPTT, ECT, and TT. INR is relatively insensitive to the exposure to dabigatran and cannot be interpreted the same way as used for warfarin monitoring.
The aPTT test provides an approximation of PRADAXA's anticoagulant effect. The average time course for effects on aPTT, following approved dosing regimens in patients with various degrees of renal impairment is shown in Figure 1. The curves represent mean levels without confidence intervals; variations should be expected when measuring aPTT. While advice cannot be provided on the level of recovery of aPTT needed in any particular clinical setting, the curves can be used to estimate the time to get to a particular level of recovery, even when the time since the last dose of PRADAXA is not precisely known. In the RE-LY trial, the median (10th to 90th percentile) trough aPTT in patients receiving the 150 mg dose was 52 (40 to 76) seconds.
Figure 1 : Average Time
Course for Effects of Dabigatran on aPTT, Following Approved PRADAXA Dosing
Regimens in Patients with Various Degrees of Renal Impairment*
*Simulations based on PK data from a study in subjects with renal impairment and PK/aPTT relationships derived from the RE-LY study; aPTT prolongation in RE-LY was measured centrally in citrate plasma using PTT Reagent Roche Diagnostics GmbH, Mannheim, Germany. There may be quantitative differences between various established methods for aPTT assessment.
The degree of anticoagulant activity can also be assessed by the ecarin clotting time (ECT). This test is a more specific measure of the effect of dabigatran than activated partial thromboplastin time (aPTT). In the RE-LY trial, the median (10th to 90th percentile) trough ECT in patients receiving the 150 mg dose was 63 (44 to 103) seconds.
No prolongation of the QTc interval was observed with dabigatran etexilate at doses up to 600 mg.
Dabigatran etexilate mesylate is absorbed as the dabigatran etexilate ester. The ester is then hydrolyzed, forming dabigatran, the active moiety. Dabigatran is metabolized to four different acyl glucuronides and both the glucuronides and dabigatran have similar pharmacological activity. Pharmacokinetics described here refer to the sum of dabigatran and its glucuronides. Dabigatran displays dose-proportional pharmacokinetics in healthy subjects and patients in the range of doses from 10 to 400 mg.
The absolute bioavailability of dabigatran following oral administration of dabigatran etexilate is approximately 3 to 7%. Dabigatran etexilate is a substrate of the efflux transporter P-gp. After oral administration of dabigatran etexilate in healthy volunteers, Cmax occurs at 1 hour post-administration in the fasted state. Coadministration of PRADAXA with a high-fat meal delays the time to Cmax by approximately 2 hours but has no effect on the bioavailability of dabigatran; PRADAXA may be administered with or without food.
The oral bioavailability of dabigatran etexilate increases by 75% when the pellets are taken without the capsule shell compared to the intact capsule formulation. PRADAXA capsules should therefore not be broken, chewed, or opened before administration.
Dabigatran is approximately 35% bound to human plasma proteins. The red blood cell to plasma partitioning of dabigatran measured as total radioactivity is less than 0.3. The volume of distribution of dabigatran is 50 to 70 L. Dabigatran pharmacokinetics are dose proportional after single doses of 10 to 400 mg. Given twice daily, dabigatran's accumulation factor is approximately two.
Dabigatran is eliminated primarily in the urine. Renal clearance of dabigatran is 80% of total clearance after intravenous administration. After oral administration of radiolabeled dabigatran, 7% of radioactivity is recovered in urine and 86% in feces. The half-life of dabigatran in healthy subjects is 12 to 17 hours.
After oral administration, dabigatran etexilate is converted to dabigatran. The cleavage of the dabigatran etexilate by esterase-catalyzed hydrolysis to the active principal dabigatran is the predominant metabolic reaction. Dabigatran is not a substrate, inhibitor, or inducer of CYP450 enzymes. Dabigatran is subject to conjugation forming pharmacologically active acyl glucuronides. Four positional isomers, 1-O, 2-O, 3-O, and 4-O-acylglucuronide exist, and each accounts for less than 10% of total dabigatran in plasma.
An open, parallel-group single-center study compared dabigatran pharmacokinetics in healthy subjects and patients with mild to moderate renal impairment receiving a single dose of PRADAXA 150 mg. Exposure to dabigatran increases with severity of renal function impairment (Table 3). Similar findings were observed in the RE-LY trial.
Table 3 : Impact of Renal Impairment on Dabigatran
|Renal Function||CrCl (mL/min)||Increase in AUC||Increase in Cmax||t½ (h)|
|+Patients with severe renal impairment were not studied in RE-LY. Dosing recommendations in subjects with severe renal impairment are based on pharmacokinetic modeling [see DOSAGE AND ADMINISTRATION and Use in Specific Populations].|
Administration of PRADAXA in patients with moderate hepatic impairment (Child-Pugh B) showed a large inter-subject variability, but no evidence of a consistent change in exposure or pharmacodynamics.
Impact of Other Drugs on Dabigatran
Rifampin: Rifampin 600 mg once daily for 7 days followed by a single dose of dabigatran decreased its AUC and Cmax by 66% and 67%, respectively. By Day 7 after cessation of rifampin treatment, dabigatran exposure was close to normal [see WARNINGS AND PRECAUTIONS and DRUG INTERACTIONS].
In studies with the P-gp inhibitors ketoconazole, amiodarone, verapamil, and quinidine, the time to peak, terminal half-life, and mean residence time of dabigatran were not affected. Any observed changes in Cmax and AUC are described below.
Dronedarone: Simultaneous administration of dabigatran etexilate and dronedarone (administered once or twice daily) increases exposure to dabigatran by 70 to 140% compared to dabigatran alone. The increase in exposure is only 30 to 60% higher compared to dabigatran alone when dronedarone is administered 2 hours after dabigatran etexilate.
Ketoconazole: Systemic ketoconazole increased dabigatran AUC and Cmax values by 138% and 135%, respectively, after a single dose of 400 mg, and 153%, and 149%, respectively, after multiple daily doses of 400 mg.
Verapamil: When dabigatran etexilate was coadministered with oral verapamil, the Cmax and AUC of dabigatran were increased. The extent of increase depends on the formulation of verapamil and timing of administration. If verapamil is present in the gut when dabigatran is taken, it will increase exposure to dabigatran with the greatest increase observed when a single dose of immediate-release verapamil is given one hour prior to dabigatran (AUC increased by a factor of 2.4). If verapamil is given 2 hours after dabigatran, the increase in AUC is negligible. In the population pharmacokinetics study from RE-LY, no important changes in dabigatran trough levels were observed in patients who received verapamil.
Amiodarone: When dabigatran etexilate was coadministered with a single 600 mg oral dose of amiodarone, the dabigatran AUC and Cmax increased by 58% and 50%, respectively. The increase in exposure was mitigated by a 65% increase in the renal clearance of dabigatran in the presence of amiodarone. The increase in renal clearance may persist after amiodarone is discontinued because of amiodarone's long half-life. In the population pharmacokinetics study from RE-LY, no important changes in dabigatran trough levels were observed in patients who received amiodarone.
Quinidine: Quinidine was given as 200 mg dose every 2 hours up to a total dose of 1000 mg. Dabigatran etexilate was given over 3 consecutive days, the last evening dose on Day 3 with or without quinidine pre-dosing. Concomitant quinidine administration increased dabigatran's AUC and Cmax by 53% and 56%, respectively.
Clarithromycin: Coadministered clarithromycin had no impact on the exposure to dabigatran.
Clopidogrel: When dabigatran etexilate was given concomitantly with a loading dose of 300 mg or 600 mg clopidogrel, the dabigatran AUC and Cmax increased by approximately 30% and 40%, respectively. The concomitant administration of dabigatran etexilate and clopidogrel resulted in no further prolongation of capillary bleeding times compared to clopidogrel monotherapy. When comparing combined treatment and the respective mono-treatments, the coagulation measures for dabigatran's effect (aPTT, ECT, and TT) remained unchanged, and inhibition of platelet aggregation (IPA), a measurement of clopidogrel's effect, remained unchanged.
Enoxaparin: Enoxaparin 40 mg given subcutaneously for 3 days with the last dose given 24 hours before a single dose of PRADAXA had no impact on the exposure to dabigatran or the coagulation measures aPTT, ECT, or TT.
Diclofenac, Ranitidine, and Digoxin: None of these drugs alters exposure to dabigatran.
In RE-LY, dabigatran plasma samples were also collected. The concomitant use of proton pump inhibitors, H2 antagonists, and digoxin did not appreciably change the trough concentration of dabigatran.
Impact of Dabigatran on Other Drugs
In clinical studies exploring CYP3A4, CYP2C9, P-gp and other pathways, dabigatran did not meaningfully alter the pharmacokinetics of amiodarone, atorvastatin, clarithromycin, diclofenac, clopidogrel, digoxin, pantoprazole, or ranitidine.
The clinical evidence for the efficacy of PRADAXA was derived from RE-LY (Randomized Evaluation of Long-term Anticoagulant Therapy), a multi-center, multinational, randomized parallel group trial comparing two blinded doses of PRADAXA (110 mg twice daily and 150 mg twice daily) with open-label warfarin (dosed to target INR of 2 to 3) in patients with non-valvular, persistent, paroxysmal, or permanent atrial fibrillation and one or more of the following additional risk factors:
- Previous stroke, transient ischemic attack (TIA), or systemic embolism
- Left ventricular ejection fraction < 40%
- Symptomatic heart failure, ≥ New York Heart Association Class 2
- Age ≥ 75 years
- Age ≥ 65 years and one of the following: diabetes mellitus, coronary artery disease (CAD), or hypertension
The primary objective of this study was to determine if PRADAXA was non-inferior to warfarin in reducing the occurrence of the composite endpoint, stroke (ischemic and hemorrhagic) and systemic embolism. The study was designed to ensure that PRADAXA preserved more than 50% of warfarin's effect as established by previous randomized, placebo-controlled trials of warfarin in atrial fibrillation. Statistical superiority was also analyzed.
A total of 18,113 patients were randomized and followed for a median of 2 years. The patient's mean age was 71.5 years and the mean CHADS2 score was 2.1. The patient population was 64% male, 70% Caucasian, 16% Asian, and 1% black. Twenty percent of patients had a history of a stroke or TIA and 50% were Vitamin K antagonist (VKA) na´ve, defined as less than 2 months total lifetime exposure to a VKA. Thirty-two percent of the population had never been exposed to a VKA. Concomitant diseases of patients in this trial included hypertension 79%, diabetes 23%, and CAD 28%. At baseline, 40% of patients were on aspirin and 6% were on clopidogrel. For patients randomized to warfarin, the mean percentage of time in therapeutic range (INR 2 to 3) was 64%.
Relative to warfarin and to PRADAXA 110 mg twice daily, PRADAXA 150 mg twice daily significantly reduced the primary composite endpoint of stroke and systemic embolism (see Table 4 and Figure 2).
Table 4 : First Occurrence of Stroke or Systemic
Embolism in the RE-LY Study
|PRADAXA 150 mg twice daily||PRADAXA 110 mg twice daily||Warfarin|
|Patients (%) with events||134 (2.2%)||183 (3%)||202 (3.4%)|
|Hazard ratio vs. warfarin (95% CI)||0.65 (0.52, 0.81)||0.90 (0.74, 1.10)|
|P-value for superiority||0.0001||0.3|
|Hazard ratio vs. PRADAXA 110 mg (95% CI)||0.72 (0.58, 0.90)|
|P-value for superiority||0.004|
Figure 2 : Kaplan-Meier Curve Estimate of Time to First Stroke or Systemic Embolism
The contributions of the components of the composite endpoint, including stroke by subtype, are shown in Table 5. The treatment effect was primarily a reduction in stroke. PRADAXA 150 mg twice daily was superior in reducing ischemic and hemorrhagic strokes relative to warfarin.
Table 5 : Strokes and
Systemic Embolism in the RE-LY Study
|PRADAXA 150 mg twice daily||Warfarin||Hazard ratio vs. warfarin (95% CI)|
|Stroke||122||186||0.64 (0.51, 0.81)|
|Ischemic stroke||103||134||0.75 (0.58, 0.97)|
|Hemorrhagic stroke||12||45||0.26 (0.14, 0.49)|
|Systemic embolism||13||21||0.61 (0.30, 1.21)|
In the RE-LY trial, the rate of all-cause mortality was lower on dabigatran 150 mg than on warfarin (3.6% per year versus 4.1% per year). The rate of vascular death was lower on dabigatran 150 mg compared to warfarin (2.3% per year versus 2.7% per year). Non-vascular death rates were similar in the treatment arms.
The efficacy of PRADAXA 150 mg twice daily was generally consistent across major subgroups (see Figure 3).
Figure 3 : Stroke and Systemic Embolism Hazard Ratios
by Baseline Characteristics
In RE-LY, a higher rate of clinical myocardial infarction was reported in patients who received PRADAXA (0.7 per 100 patient-years for 150 mg dose) than in those who received warfarin (0.6).
Last reviewed on RxList: 5/14/2013
This monograph has been modified to include the generic and brand name in many instances.
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