New anticoagulants: Moving on from scientific results to clinical implementation

Abstract

Vitamin K antagonists (VKA) are the only registered oral anticoagulants for the treatment of venous thromboembolism (VTE). VKA have an unpredictable and highly variable effect on coagulation, with a high risk of under- and over-treatment. Novel anticoagulants, such as dabigatran and rivaroxaban, could be a very welcome replacement for VKA, as they show a predictable anticoagulant effect. Results of several phase II and III studies have shown the efficacy and safety of dabigatran and rivaroxaban in the prophylaxis and treatment of VTE, and for the prevention of stroke in atrial fibrillation. It remains to be shown whether these new anticoagulants have the same safety profile in daily clinical practice, where more vulnerable patients will be treated. Lack of information on the proper monitoring method or antidote in case of bleeding may also hinder the translation from science to clinical practice.

Key words::

• Anticoagulants
• antidote
• clinical trials
• factor Xa
• monitoring
• thrombin
• thrombosis

Abbreviations
APTT	=	Activated Partial Thrombin Time
DTI	=	Direct Thrombin Inhibitor
DTT	=	Diluted Thrombin Time
ECT	=	Ecarin Clotting Time
ED50	=	The dose required to produce a defined therapeutic response in 50% of the population
ETP	=	Endogenous Thrombin Potential
FIIa	=	Factor IIa, thrombin
FXa	=	Factor Xa
INR	=	International Normalized Ratio
LMWH	=	Low Molecular Weight Heparin
PCC	=	Prothrombin complex concentrate
PT	=	Prothrombin Time
rFVIIa	=	Recombinant Factor VIIa
TAT	=	Thrombin Antithrombin Complex
TGT	=	Thrombin Generation Test
TT	=	Thrombin Time
UFH	=	Unfractionated Heparin

Key messages

• Dabigatran and rivaroxaban are novel oral anticoagulants that could replace the standard therapy for the treatment of venous thromboembolism, because they can be administered in a fixed dose, have beneficial efficacy and safety, and do not require frequent monitoring.

• The knowledge on the pharmacology of dabigatran and rivaroxaban mainly comes from animal experiments and studies in healthy, young, Caucasian subjects; prescribing these compounds to a wider patient population may lead to unknown side-effects or alterations of their pharmacokinetics and pharmacodynamics.

• There is at present no antidote for dabigatran and rivaroxaban, and information regarding the appropriate monitoring method is lacking; these issues need to be resolved for specific situations such as severe bleeding or emergency surgery, before both drugs can be applied safely in daily practice.

Introduction

For over fifty years, oral vitamin K antagonists (VKA) have been the most commonly used anticoagulants for the treatment of venous thromboembolism (VTE) (1). Nevertheless, their pharmacological characteristics cause substantial clinical challenges. Their slow onset and offset of action often require bridging with parenteral or subcutaneous anticoagulant drugs. Furthermore, VKA have a narrow therapeutic window as well as interindividual variability in dose response, and they show interactions with other drugs and food. The risk of under-treatment is therefore high, which requires routine monitoring and dose adjustments. The risk of over-treatment contributes to under-utilization (1). Other drugs often prescribed for the prevention and treatment of VTE are unfractionated heparin (UFH) and low-molecular-weight heparin (LMWH). UFH is usually administered intravenously and needs monitoring because of variability in anticoagulant effect. It is notorious for its rare but serious complication of heparin-induced thrombocytopenia (HIT) (2). LMWH is generally preferred over UFH, because it does not require routine monitoring, can be administered subcutaneously, and results in less major bleeding and fewer recurrent thrombotic events (3). For long-term antithrombotic therapy, however, oral drugs are favourable. New oral anticoagulants might be a much-desired replacement for the VKA. Dabigatran (Boehringer-Ingelheim, Ingelheim, Germany) and rivaroxaban (Bayer, Leverkusen, Germany) are two new antithrombotic agents that have been registered in Europe and Canada in 2008 for the prevention of VTE after elective orthopaedic surgery in adults. Recently, dabigatran was also approved for the treatment of patients with atrial fibrillation for the prevention of stroke. Dabigatran and rivaroxaban exhibit a predictable pharmacological profile. Consequently, no regular monitoring of the anticoagulant effect is required (4). However, there is a need for a proper method of monitoring in special circumstances, such as extreme body-weight or renal failure. Furthermore, in case of bleeding, there is no method of reversal available (5). This article describes the pharmacology of dabigatran and rivaroxaban, gives an overview of current clinical trial results, and briefly discusses different monitoring methods and possible antidotes.

Pharmacology

Dabigatran and rivaroxaban have a predictable pharmacodynamic and pharmacokinetic profile. They show little interaction with other drugs and are mildly influenced by the recipient's life-style and food. The pharmacologic properties of dabigatran and rivaroxaban are described below.

Dabigatran

Thrombin is a key player in the coagulant cascade: it activates platelets, leading to platelet aggregation, and causes the formation of fibrin. Dabigatran is a competitive direct thrombin inhibitor (DTI) that interacts specifically with the active site of thrombin, and also inactivates fibrin-bound thrombin. The drug has a steady-state pharmacologic profile and can therefore be prescribed in fixed doses. According to in-vitro animal studies, its inhibitory constant, reflecting its binding to human thrombin, is stable at 4.5 nmol/L. Thrombin leads to platelet aggregation, and this is inhibited for 50% by 10 nmol dabigatran. In the phase III PETRO study in which patients with atrial fibrillation (AF) were treated with both dabigatran and aspirin, plasma concentrations were calculated in a pharmacokinetic model. At a normal kidney function, the dose of dabigatran 220 mg once daily gave median plasma concentrations of 37 ng/mL to 183 ng/mL (trough to maximum values, respectively). Adjusted for its molecular weight, this leads to average plasma concentrations of 200–300 nmol/L. Maximum plasma concentrations for 150 mg twice daily were comparable (6). Thrombus formation is completely reversed at a dose of active dabigatran 0.1 mg/kg (7,8). As it is administered as a prodrug, dabigatran etexilate, the absolute bioavailability is low (5%). The drug reaches its peak after about 2 hours and has a half-life of 14–17 hours (Table I). After 3 days of treatment, steady-state levels are achieved (9). Dabigatran is excreted 80% via the kidneys; the remaining part is conjugated and removed via the biliary system. Its metabolization does not involve cytochrome P450. Protein binding studies with radiolabelled dabigatran showed that approximately one-third of the drug is plasma protein-bound (9–12).

Table I. Main characteristics of dabigatran and rivaroxaban.

	Dabigatran	Rivaroxaban
Binding site	FIIa, thrombin	FXa
Time to peak	2 hours	2–4 hours
Half life	14–17 hours	5–9 hours: young subjects 9–13 hours: elderly subjects
Steady state levels	After 3 days	After the first dosage
Excretion	80% renal, remaining part biliary	65% renal, remaining part biliary
Drug interactions	CYP3A4 inducers or inhibitors; P-glycoprotein inducers or inhibitors	CYP3A4 inducers or inhibitors; P-glycoprotein inducers or inhibitors
Possible monitoring method	ECT, TT, DTT, ETP	PT, Heptest, ETP, anti-FXa assay
Possible antidote	PCC reversed bleeding time and blood loss, but not coagulation assays^a rFVIIa gave contradicting results, from partial reversal to no reversal^b	PCC reversed bleeding time and TAT levels^a rFVIIa partially reversed the bleeding time and coagulation assays^b

^aBased on animal models, no data on studies with humans available yet.

^bBased on animal models and spiked pooled human plasma.

Renal impairment. There are no data regarding the influence of renal impairment in subjects receiving dabigatran. Based on a dose response study among almost 300 patients undergoing hip and knee surgery, an increase in drug exposure of 11% was predicted for every decrease in creatinine clearance of 10 mL/min (13). Dabigatran should not be used in patients with a creatinine clearance < 30 mL/min.

Hepatic impairment. In a study with 12 patients with moderate hepatic impairment, Child–Pugh classification B, similar pharmacokinetic and pharmacodynamic profiles for dabigatran were found as in healthy volunteers (n = 12) (10). However, more severe hepatic dysfunction was an exclusion criterion in the clinical trials and is a contra-indication for the use of dabigatran.

Life-style and diet. The stable pharmacokinetic profile of dabigatran is not influenced by weight, gender, smoking, or alcohol (13). In a single-dose study in healthy male subjects, the only influence of a high-fat, high-calorie breakfast was a delay in absorption of dabigatran for 2 hours. Once dabigatran has reached steady-state levels, this effect becomes clinically irrelevant. But if an immediate anticoagulant effect is needed, no food should be taken together with the first dosage of dabigatran (9).

Drug interactions. Dabigatran etexilate has a mild affinity for the efflux transporter P-glycoprotein (P-gp), which is mainly restricted to the prodrug form. Caution is therefore required for concomitant treatment with strong P-glycoprotein inhibitors (verapamil, clarithromycin) or inducers (rifampicine, St John's-wort), and CYP3A4 inhibitors or inducers. Co-administration with atorvastatin or digoxin has no clinically relevant effect on the availability of both drugs. Given simultaneously with amiodarone, a rise of dabigatran's maximum concentration (Cmax) and AUC was seen of 50% and 60%, respectively, without influencing the absorption of amiodarone. Treatment of dabigatran together with P-glycoprotein inhibitor quinidine is contra-indicated (13). The interaction between dabigatran and pantozol, as found in studies with healthy volunteers (13), does not seem to have clinical consequences. Patients in the RE-LY study with proton-pump inhibitors had the same efficacy and safety of dabigatran as patients without this medication (15). Finally, diclofenac does not influence the pharmacokinetic properties of dabigatran (14).

Pregnancy/lactation. Based on the results from animal studies, dabigatran should not be given to pregnant women, as it passes the placenta, leads to a higher risk of pre-implantation loss, and gives an increase in foetal mortality at toxic doses. There are no data regarding dabigatran and lactation (14).

Rivaroxaban

Rivaroxaban binds to the active site of activated coagulation factor X (FXa) and blocks the interaction with its substrate. Rivaroxaban inhibits free FXa and FXa bound to the surface of platelets, other clotting factors, or a clot. Factor Xa plays an important role in the formation of fibrin (16). By binding directly to all forms of FXa, rivaroxaban inhibits its substrate more than 10,000-fold compared to other serine proteases (13). Like dabigatran, it operates with stable pharmacokinetics and pharmacodynamics and can be administered orally. In a rat venous thrombosis model, intravenous rivaroxaban reduced thrombus formation with an ED₅₀ of 0.1 mg kg ⁻¹. At a dosage of 0.3 mg kg ⁻¹ there was almost no thrombus formation (13). Rivaroxaban has a half-life of 5–9 hours in young subjects and 9–13 hours in elderly (Table I). Its time to peak is 2–4 hours, and steady-state levels are achieved after the first dosage (17). Metabolization of rivaroxaban operates mainly via oxidative degradation of the morpholinone moiety, and partially via CYP3A4/3A5 and CYP2J2. Rivaroxaban is excreted 65% via the kidneys, with 36% in an unchanged form. The remaining part is removed via the faecal/biliary routes, with a disposal of 7% in an unchanged form. Plasma protein binding of rivaroxaban in humans is high, estimated at 95% (18).

Renal impairment. Decrease of the creatinine clearance increases rivaroxaban plasma concentrations, from 44% to 64% for clearances from 50–79 mL/min to < 30 mL/min. It also delays the time to maximum plasma concentration with 1 hour, without influencing the maximum concentration itself (19). For VTE prophylaxis, rivaroxaban should not be used in patients with a creatinine clearance < 15 mL/min. In the ROCKET trial, performed for the prevention of stroke in atrial fibrillation, a creatinine clearance of < 30 mL/min was an exclusion criterion (20).

Hepatic impairment. In a non-randomized, non-blinded study, eight patients with mild (Child–Pugh A) and eight patients with severe (Child–Pugh B) hepatic impairment were given one dose of 10 mg rivaroxaban. The area under the plasma concentration–time curve and maximum plasma concentration of rivaroxaban were increased in comparison to healthy subjects (LS-mean ratios (90% CI) 2.27 (1.63–3.07) and 1.27 (0.99–1.63), respectively), together with a mild prolongation of the prothrombin time (PT) (exact numbers not mentioned) (21). Severe hepatic dysfunction was an exclusion criterion in prevention and treatment studies and should be a contra-indication for rivaroxaban use.

Life-style and diet. Gender and obesity have no influence on the area under the curve, Cmax, or half-life of rivaroxaban. Low body-weight (≤ 50 kg) leads to a prolongation of the PT of only 15% and has therefore been considered as clinically irrelevant. It should be noted that this PT prolongation is reagent-specific, and specific references and cut-off values for the PT are not available (22). In two food interaction studies, one with a high-fat dish and one with a high-calorie meal, the time to reach the maximal concentration of rivaroxaban was delayed by 1.5 hours. Both meals increased the AUC and maximal concentration likewise (respectively 25% and 41%). These studies were conducted with healthy subjects; there are no results from large clinical trials to underline if rivaroxaban should be taken in a fasting state (17).

Drug interactions. Treatment of rivaroxaban together with strong CYP3A4 or P-glycoprotein inducers or inhibitors is contra-indicated. Co-administration of ketoconazol should be done with caution, as it causes a 2.6-fold increase of the AUC and a 1.7-fold increase of the Cmax of rivaroxaban (23). Both antacids and ranitidine show no interactions with rivaroxaban (24). Similar results were obtained for the concomitant use of atorvastatin and digoxin (18). In the RECORD studies, 70% of all rivaroxaban recipients used naproxen, resulting in similar relative bleeding rates (1.08; 95% CI 0.89–1.30) as the subjects on rivaroxaban alone (1.08; 95% CI 0.88–1.34). Naproxen can therefore be prescribed together with rivaroxaban without any restrictions. (23).

Pregnancy/lactation. In animal models, rivaroxaban passes the placenta and is secreted into breast-feeding milk. Rivaroxaban should therefore not be taken by pregnant and lactating women (18).

Conclusion

Dabigatran and rivaroxaban are new antithrombotic drugs that block a specific step in the coagulation pathway by selectively binding to, respectively, clotting factors IIa and Xa. As a result, they have shown a stable effect on coagulation, with little drug and food interaction. This knowledge, however, comes mainly from in-vitro studies and clinical trials. Post-marketing surveillance will be necessary to assess a better side-effect profile.

Clinical studies of dabigatran and rivaroxaban

The efficacy and safety of rivaroxaban and dabigatran have been assessed in phase II and III trials, designed in patients at risk or in need of anticoagulant treatment for arterial and venous thrombosis. The following chapter provides a summary of these studies (see also Table II).

Table II. Results from phase II and III clinical trials for dabigatran and rivaroxaban.

		Dabigatran		Rivaroxaban
	Primary outcome	Primary outcome	Major bleedings	Primary outcome	Major bleedings
VTE prevention after hip replacement^a	Total VTE events	6.0%^e versus 6.7% (P < 0.001 for non-inferiority)	2.0%^e versus 1.6% (P = 0.6)	1.1% versus 3.7% (P < 0.001)	0.3% versus 0.1% (P = 0.18)
VTE prevention after knee replacement^a	Total VTE events	36.4%^e versus 37.7% (P = 0.017 for non-inferiority)	1.5%^e versus 1.3% (P = 0.82)	9.6% versus 18.9% (P = 0.001)	0.6% versus 0.5% (P = 0.77)
VTE treatment^b	VTE recurrence	2.4% versus 2.1% (P < 0.001 for non-inferiority)	1.6% versus 1.9% (HR 0.82; 95% CI 0.45–1.48)	HR 0.68; 95% CI 0.44–1.04 Extended treatment^c HR 0.18; 95% CI 0.09–0.39	8.1% for both treatment arms Extended treatment^c 0.7% v none (P = 0.11)
Stroke prophylaxis in AF^b	Stroke, systemic embolism	1.1%^f versus 1.7% (RR 0.7; 95% CI 0.5–0.8)	3.1%^f versus 3.4% (P = 0.31)	2.12% versus 2.42% (P = 0.117)	3.6% versus 3.45% (P = 0.58)
ACS secondary events prevention^d	ACS recurrence	RE-DEEM, no results available		ATLAS ACS-TIMI 46 and ATLAS ACS-TIMI 51, no results available

VTE = venous thromboembolism; AF = atrial fibrillation; ACS = acute coronary syndrome.

Specific dosages of both dabigatran and rivaroxaban per clinical trial are mentioned in the corresponding chapter.

^aVersus enoxaparin 40 mg once daily.

^bVersus LMWH/VKA (target INR 2–3).

^cVersus placebo.

^dVersus placebo, additional to aspirin and (stratified or at the physician's discretion) a thienopyridine.

^eFor dabigatran 220 mg once daily.

^fFor dabigatran 150 mg twice daily.

Dabigatran

Prevention of VTE after elective knee and hip surgery. Several phase III studies have investigated dabigatran 150 mg and 220 mg once daily (od) in patients undergoing planned knee or hip surgery (Table II) (25). The RE-MODEL trial proved both dosages of dabigatran non-inferior to enoxaparin for preventing VTE in patients planned for knee arthroplasty. In this randomized, double-blind, non-inferiority study, 1,500 subjects were treated with either dabigatran (150 mg or 220 mg q.i.d.), or with enoxoparin (40 mg q.i.d.) for 6–10 days. The occurrence of major bleeding events was comparable for all three groups (26). In the RE-NOVATE study, a double-blind, phase III trial, about 3,500 patients undergoing total hip replacement were randomized for treatment with either dabigatran or enoxaparin for 1 month, with RE-MODEL dosages. Both dabigatran dosages again had similar risks of VTE and similar major bleeding rates as enoxaparin (27). The RE-MOBILIZE study had a different outcome, as dabigatran was compared with a higher dosage of enoxaparin 30 mg twice daily. The primary outcome of total VTE and death occurred more often in patients in the dabigatran 150 mg and 220 mg group (33.7% and 31.1%, respectively), than in the enoxaparin group (25.3%) (28). Nevertheless, since enoxaparin 40 mg is commonly used for VTE prophylaxis, the results from other phase III trials were sufficient for dabigatran (220 mg q.i.d.) to be licensed in 2008 for the prevention of VTE after elective orthopaedic surgery.

Treatment of VTE. The efficacy and safety of a therapeutic dose of dabigatran of 150 mg twice daily was compared with warfarin (target international normalized ratio (INR) 2–3) in the double-blind RE-COVER trial. Over 2,500 patients with acute VTE were treated for 6 months. All patients also initially received LMWH-treatment. Recurrence rates of VTE were similar for dabigatran and warfarin, as was the risk of major bleeding (Table II) (29).

Prevention of stroke in atrial fibrillation. The RE-LY study included over 18,000 patients with non-valvular AF. Patients were randomized to either dabigatran 110 mg twice daily, dabigatran 150 mg twice daily (t.d.), or dose-adjusted warfarin (target INR 2–3) for 2 years. In comparison to warfarin, dabigatran 110 mg t.d. was found non-inferior, and dabigatran 150 mg t.d. superior, in preventing stroke or systemic embolism. The incidence of major bleeding was similar for warfarin and dabigatran in the highest dose and significantly lower for dabigatran 110 mg twice daily (Table II). Dyspepsia occurred in 12% of the dabigatran patients, and no evidence of hepatotoxicity was reported. In conclusion, with the RE-LY study dabigatran 150 mg twice daily was shown to be safe and superior in stroke prophylaxis for patients with AF, compared to warfarin. The FDA therefore approved dabigatran 150 mg t.d. for all AF patients, with the exception of those suffering from severe renal impairment. Severe renal impairment was, however, an exclusion criterion of the RE-LY, and the suggested dose adjustment (dabigatran 75 mg t.d.) was not based on scientific results from this trial (14).

Treatment of acute coronary syndrome. For patients with an acute myocardial infarct and at least one cardiovascular risk factor, dabigatran is being evaluated in the RE-DEEM trial, a phase II, randomized trial with dabigatran as an addition to standard antiplatelet therapy. Using dabigatran from 50 mg twice daily to 150 mg twice daily, this study is designed to find the dosage with the lowest acceptable risk of bleeding. Primary results have shown that the addition of dabigatran gives a dose-dependent increase in bleeding, with an occurrence of major bleeding of 0.5% in the placebo arm, and from 0.3% for the dabigatran 75 mg t.d. to 2.0% for the 110 mg t.d. group. Final results have not been published yet (30).

Rivaroxaban

Prevention of VTE after elective knee and hip surgery. After several promising phase II trials (31), the phase III RECORD studies underlined the safety and efficacy of rivaroxaban in the prevention of VTE after total knee and total hip surgery. Two randomized, double-blind, non-inferiority trials were performed, with approximately 7,000 patients planned for total hip arthroplasty. Subjects were treated for 5 weeks with 10 mg rivaroxaban q.i.d. or with 40 mg enoxaparin q.i.d. for 5 weeks (RECORD 1) or for 2 weeks (RECORD 2). These studies demonstrated that rivaroxaban was superior in the prevention of symptomatic VTE in comparison with enoxaparin. There was no difference in the occurrence of major bleeding between the rivaroxaban and enoxaparin group (Table II) (32,33). The efficacy of rivaroxaban in preventing VTE after total knee arthroplasty was analysed in the RECORD 3 and 4 studies. Over 5,500 patients either received rivaroxaban or enoxaparin for 10–14 days in similar dosages as mentioned previously. VTE occurred significantly less for rivaroxaban, and the risk of major bleeding was similar to enoxaparin (34,35). In a recent meta-analysis by Cao et al. of eight RCTs, involving 15,586 patients, rivaroxaban was associated with significantly fewer VTE and lower all-cause mortality in comparison with enoxaparin (RR 0.56; 95% CI 0.39–0.80). The incidence of major (RR 1.65; 95% CI 0.93–2.93) and clinically relevant non-major bleeding was comparable (RR 1.21; 95% CI 0.98–1.50). The percentage of all bleeding events (both major and clinically relevant non-major) caused by rivaroxaban was 7.2% versus 6.5% for enoxaparin (P = 0.14 for overall effect) (36). As a result, rivaroxaban 10 mg q.i.d. was registered in Europe and Canada for VTE prevention after planned knee or hip surgery.

Treatment of VTE. Rivaroxaban was also investigated for the treatment of acute DVT in the Einstein-DVT study. In this phase III study 3,449 patients were randomized to rivaroxaban t.d. for 3 weeks followed by 20 mg q.i.d., or warfarin (target INR 2–3). Rivaroxaban was administered without bridging, because the novel anticoagulant has the favourable feature of developing steady-state levels immediately. As warfarin was started with at least 5 days of enoxaparin, the study was open label. Treatment period varied between 3 and 12 months. Rivaroxaban was non-inferior to warfarin in preventing recurrent symptomatic VTE, and there was no difference in major bleeding in both treatment arms (Table II).

In the Einstein-extension study, 20 mg rivaroxaban q.i.d. was compared with placebo for the extended treatment of VTE for 6–12 months. Subjects had either participated in the Einstein-DVT study or had been treated with VKA for 6–12 months prior. The net clinical benefit, the composite of recurrent VTE or major bleedings, was 2.0% for the rivaroxaban group and 7.1% for the placebo group (HR 0.28; 95% CI 0.15–0.53; P < 0.001), which suggests that rivaroxaban may be beneficial for the long-term treatment of VTE after 6–12 months (37).

The Einstein-PE study investigating patients with pulmonary embolism is still on-going.

Atrial fibrillation. In the ROCKET AF study, subjects were treated with either rivaroxaban 20 mg q.i.d. or warfarin, for the prevention of stroke caused by atrial fibrillation. What set the trial apart from similar studies was the double-blind, double-dummy design and the co-morbidity of patients. The average age of the 14,000 participants was 73 years, and the CHADS 2 score was 3 or higher for 90% of all subjects. Rivaroxaban was proven non-inferior to warfarin for the prevention of stroke and systemic embolism and as safe in terms of major bleeding. The discontinuation of subjects was high (22%), and the percentage of the warfarin-treated subjects with an INR in the therapeutic range was low (less than 60%) (20).

Treatment of acute coronary syndrome. The benefit of adding rivaroxaban to the standard treatment of acute coronary syndrome was investigated in the ATLAS ACS-TIMI 46 study. Almost 3,500 patients were divided into two arms of either aspirin alone or aspirin plus a thienopyridine. All patients in both arms were then randomized between placebo and different doses of rivaroxaban. The addition of rivaroxaban in dosages from 5 to 20 mg once or twice daily to either aspirin, or to aspirin plus a thienopyridine, decreased the primary outcome of death, myocardial infarction, stroke, and/or severe recurrent ischemia significantly in comparison to placebo (HR 0.69; 95% CI 0.50–0.96; P = 0.0270). However, the occurrence of clinically significant bleeding increased with a dose response relationship (HR 2.21, 95% CI 1.25–3.91 for rivaroxaban 5 mg versus placebo; HR 5.06, 95% CI 3.45–7.42 for 20 mg; P < 0.0001). Clinically significant bleeding rates were 1.67%–6.69% for rivaroxaban 5–20 mg q.i.d. in combination with aspirin alone versus 3.28%–5.12% for rivaroxaban plus both aspirin and a thienopyridine (38). The phase III ATLAS ACS-TIMI 51 trial, designed in a similar manner, will therefore investigate low-dose rivaroxaban 5 mg q.i.d. as adjunctive therapy for acute coronary syndrome (31).

Conclusion

Clinical studies with both dabigatran and rivaroxaban suggest that these compounds are beneficial and safe for the prevention and treatment of VTE, and for the prevention of stroke in patients with atrial fibrillation. Results from trials for the treatment of acute coronary syndrome are on their way. Current findings come from studies with a patient population that mainly consists of Caucasians with a normal renal clearance and normal body-weight. Whether or not these conclusions can be extrapolated to subjects with obesity or impaired renal functioning is currently unknown.

Laboratory monitoring of new anticoagulants

Dabigatran and rivaroxaban have shown a predictable and stable dose response without the need of monitoring (39). In the various clinical trials as mentioned previously, no routine monitoring of the treatment effect was performed. Clinical practice will sometimes require monitoring of the anticoagulant effect, as patients who would have been disqualified for such studies may have a higher bleeding risk (40). In a recent debate about the need for laboratory monitoring of the new antithrombotic drugs, both sides agreed that monitoring of dabigatran and rivaroxaban is not indicated for the majority of patients but may be useful in certain situations (41,42). Examples such as renal impairment, acute surgical intervention, and a high risk of bleeding are already applicable for LMWH therapy (43). Also, since routine monitoring is not needed, it will be more difficult to determine compliance, for instance in case of recurrent thrombosis. In search of the proper monitoring method, the results of various phase II and III trials provide information on various coagulation assays and show a close correlation between dosage, plasma concentrations, and the degree of anticoagulation. These trials have, however, not compared the efficacy and safety of these novel anticoagulants in association with monitoring methods. Nor do these studies provide information on what the preferable references are for the most suitable assays (44).

Clotting assays (summarized in Table I)

According to in-vitro studies in humans and animals, rivaroxaban gave a concentration-dependent prolongation of PT and activated partial thrombin time (aPTT) (17,45). The PT seemed to be more sensitive than the aPTT, which is also known for other direct FXa inhibitors (16) but is not seen with indirect FXa inhibitors. A possible explanation is that indirect FXa inhibitors can only inhibit free factor FXa, whereas rivaroxaban also binds to FXa in the prothrombinase complex, which is more efficient in generating thrombin. If necessary, rivaroxaban exposure could therefore be determined by using the PT. However, results of the PT vary with the reagents used, and the PT would inevitably require standardization. This problem could be overcome by expressing the PT in plasma concentrations of rivaroxaban with a standard calibration curve, rather than seconds. At present, no reference ranges or cut-off levels of the PT are present for rivaroxaban (44,46). Dabigatran added to human or animal blood prolonged the PT and the aPTT in vitro and ex vivo in a concentration-dependent mode. Dose-escalating studies with orally administered dabigatran in healthy subjects found that the aPTT had a linear relationship with the square root of the plasma concentration. A flattening response of the aPTT at higher plasma concentrations was unfortunately observed and found in several clinical trials (6). As a result of this plateau effect and the lack of standardization of aPTT reagents, the aPTT is not recommended for measuring the precise quantitative effect of dabigatran.

Ecarin is a snake venom protease that exclusively generates meizothrombin from prothrombin. Direct thrombin inhibitors inhibit the thrombin-like activity of meizothrombin. The ecarin clotting time (ECT) is described as a simple, fast, and precise quantitative measurement tool for DTIs (6). In a dose escalation study with healthy male subjects, the ECT increased in direct proportion to plasma concentrations of dabigatran. The ECT had a greater sensitivity in precise measurement of anticoagulation by dabigatran than the aPTT and thrombin time (TT), results confirmed by a phase II trial for thromboprophylaxis after hip surgery (47,48).

The Heptest (Haemachem, St. Louis, MO, USA) is a clotting test based on inhibition of factor Xa. The time to clot is measured after incubation with exogenous factor Xa, recalcification, and addition of a bovine plasma fraction rich in factor V and fibrinogen. In-vitro studies with rivaroxaban showed that the Heptest was highly sensitive to rivaroxaban over a broad concentration range (16), results confirmed by a randomized placebo controlled study with healthy male subjects (17).

To measure the TT, excess thrombin is added to plasma. In a dose escalation study with dabigatran etexilate, given to healthy male subjects (n = 80), the TT increased in direct proportion to plasma concentration levels of dabigatran. Maximum TT prolongations of almost 30-fold were observed, and the maximum measurement time was regularly exceeded (48). Although high dosages of dabigatran were applied, these results suggest that the TT might be too sensitive for monitoring dabigatran.

Specifically developed clotting assays

To adjust the thrombin time for therapeutic drug monitoring of direct thrombin inhibitors, Love et al. diluted one volume of patient plasma into three volumes of normal human plasma (49). Both in-vivo and ex-vivo investigations with dabigatran showed a linear relationship between the specific diluted thrombin time DTT Hemoclot (Hyphen BioMed, Neuville-sur-Oise, France) and dabigatran concentrations, which makes this test a potential laboratory monitoring assay for dabigatran (50). The Hemoclot has also been studied in vitro with paediatric patients and showed comparable results in children as in adults (51).

Thrombin generation assays

A thrombin generation curve (thrombogram) can be obtained by using a fluorogenic thrombin substrate and continuous calibration of each individual sample. It produces more information than a clotting time assay, since over 95% of all thrombin forms after the process of clotting. In normal platelet-poor plasma spiked with dabigatran and rivaroxaban, both drugs influenced the parameters of the thrombin generation test (TGT) and gave a decrease of the ETP (52). In a rat model of venous thrombosis, oral and intravenous dabigatran inhibited thrombin generation in a concentration-dependent manner (7). Similar findings came from a randomized placebo-controlled study with single doses (5 or 30 mg) of rivaroxaban (45).

Direct measurement of the inhibition of coagulation factors

Anti-FXa chromogenic assays have been used in several studies with rivaroxaban, showing a dose and concentration-dependent inhibition of factor Xa activity (16). Currently available anti-FXa chromogenic assays for LMWH are not specific for direct FXa inhibitors, and therefore a new specific and highly sensitive anti-FXa assay has been developed. The results of this assay were comparable to the direct concentration measurements in a multicentre quality assessment study (52). There are few data on the use of anti-factor IIa chromogenic assays and direct thrombin inhibitors. The chromogenic anti-factor IIa assay was found to monitor successfully bivalirudin levels during cardiopulmonary bypass (53). Chromogenic assays are factor-specific and unaffected by other coagulation disorders. However, reproducibility of these assays can vary (44).

Conclusion

Only few studies have compared monitoring methods for dabigatran and rivaroxaban (54). Different indications for laboratory monitoring may require different assays. For anticoagulant activity of these agents, for example to assess compliance, conventional tests or other qualitative assessments will be sufficient. When there is a need for risk assessment regarding an invasive procedure, or in case of a major bleeding, a precise estimation of the anticoagulant level might be necessary, by means of a quantitative assay. Conventional assays should be interpreted with caution, since references for new anticoagulants are unknown, and the variation in reagents might lead to difficulties with standardization.

Reversal of novel anticoagulants

Although both dabigatran and rivaroxaban have a predictable and stable anticoagulant effect, immediate reversal may be necessary for major bleeding or emergency surgery. Crowther and Warkentin performed a systematic evaluation of the literature available at the beginning of 2009 and found no references to the treatment of bleeding in subjects receiving dabigatran or rivaroxaban (55). Little has changed since their publication, and animal experiments are still the main studies performed to assess the proper antidote for dabigatran and rivaroxaban. With the general knowledge based on current standard anticoagulation therapy and the results of animal studies, possible antidotes for the new anticoagulants are listed hereafter.

Fresh frozen plasma

Transfusion of fresh frozen plasma is a recommended strategy for the reversal of bleeding caused by anticoagulant agents. A disadvantage of this therapy is the requirement of large amounts of plasma, requiring several hours of infusion before reaching its beneficial effect. This strategy has not been investigated for the reversal of dabigatran and rivaroxaban.

Prothrombin complex concentrates (activated or non-activated)

Prothrombin complex concentrates (PCCs) are often used for acute reversal of VKA. They have an immediate onset of action after administration (1). PCCs are human plasma products, containing coagulation factors II, VII, IX, X (56). There are animal studies suggesting a beneficial effect of PCCs for the reversal of bleeding with dabigatran or rivaroxaban. In a kidney injury bleeding model in rabbits, PCC (Beriplex 50 IU/kg; CSL Behring, Marburg, Germany) showed a dose-dependent reversal of bleeding time and amount of blood loss, caused by dabigatran (57). In a similar animal study, anaesthetized rats were given a standardized tail incision to measure the bleeding time. Activated PCC (100 IU/kg Feiba; Baxter AG, Vienna, Austria) reversed the bleeding time prolongation caused by dabigatran but had no influence on the prolonged aPTT (58). However, as previously mentioned, the aPTT is not a precise monitoring method for the use of dabigatran. After cutting a mesenteric vessel and administration of rivaroxaban, PCC (Beriplex 50 IU/kg) caused complete normalization of the prolonged bleeding time in a rat model (59). No studies in humans are available at present.

Recombinant factor VIIa

Recombinant factor VIIa (rFVIIa) has shown its potential for the reversal of current and novel anticoagulant drugs in animal models, but information from daily clinical practice is anecdotal (1,60). In the previously mentioned rat model, 0.5 mg/kg rFVIIa had a more pronounced effect than aPCC, reducing the bleeding time and partially reversing the aPTT prolongation caused by dabigatran (58). However, an in-vitro study proved otherwise. Platelet-rich plasma from healthy volunteers was spiked with dabigatran, resulting in a suppression of thrombin generation that was unchanged after adding rFVIIa (10 nmol). In the same study rFVIIa partially reversed the effect of rivaroxaban on thrombin generation (61). A study with baboons showed that the prolongation of bleeding time and PT measured 15 minutes after high-dose rivaroxaban was partially reduced (36% and 26% respectively) by a single bolus of rFVIIa (210 μg/kg) (62). rFVIIa therefore seems to counteract both dabigatran and rivaroxaban, but only partially.

Haemodialysis

As rivaroxaban is mainly plasma protein-bound, haemodialysis is not an option for acute reversal of the anticoagulant (18). Haemodialysis could be effective for the reversal of dabigatran, when other alternatives are insufficient. There is, however, only one animal study to support this choice (63).

Conclusion

Dabigatran and rivaroxaban have the potential to serve a wide target population, with clinical indications ranging from the prophylaxis and treatment of VTE to stroke prevention in atrial fibrillation. With their predictable pharmacodynamic and pharmacokinetic profiles, they have the potential to replace the current oral anticoagulants, the vitamin K antagonists. Nevertheless, both drugs still have an increased risk of bleeding. At present, a sufficient method of reversal has not been found, and there is a lack of information on monitoring. Since these new anticoagulants can already be prescribed in clinical practice, solving these issues is crucial.

Declaration of interest: The authors state no conflict of interest and have received no payment in preparation of this manuscript.