Andexanet Alfa Reduces Anti-Factor Xa Activity for Patients With Acute Major Bleeding

Factor Xa (FXa) inhibitors, such as the direct oral anticoagulants rivaroxaban and apixaban, are safe and effective for the treatment of venous thromboembolism and stroke in patients with atrial fibrillation, though enthusiasm about their use has been tempered by the risk of major bleeding and the lack of reversal agents. Preliminary results from the ongoing ANNEXA-4 (Andexanet Alfa, a Novel Antidote to the Anticoagulation Effects of FXA Inhibitors) study suggest that andexanet alfa, a recombinant modified human factor Xa decoy protein, can rapidly reverse anti-FXa activity and lead to hemostasis in patients with acute major bleeding.

“Andexanet has been designed and developed specifically to reverse the effects of both direct and indirect FXa inhibitors,” study authors, led by Stuart J. Connolly, MD, of the Population Health Research Institute at McMaster University in Canada, wrote in their report, which was published in the New England Journal of Medicine. “On the basis of a descriptive preliminary analysis, an initial bolus and subsequent two-hour infusion of andexanet substantially reduced anti-factor Xa activity in patients with acute major bleeding associated with factor Xa inhibitors.”

The ongoing, multicenter, prospective, open-label, single-group ANNEXA-4 study is evaluating the use of andexanet in patients with acute major bleeding that is potentially life-threatening. Patients were enrolled starting on April 10, 2016, from 20 centers in the United States, one center in the United Kingdom, and one center in Canada; the current analysis describes the 67 patients for whom data were complete as of June 17, 2016.

Adult patients were eligible for the study if they received apixaban, rivaroxaban, edoxaban, or enoxaparin within the previous 18 hours (at a dose of at least
1 mg/kg of body weight per day for enoxaparin) for acute major bleeding. Acute major bleeding was defined as:

  • potentially life-threatening acute overt bleeding with signs or symptoms of hemodynamic compromise (e.g., severe hypotension, poor skin perfusions, mental confusion, or low cardiac output that could not otherwise be explained)
  • acute overt bleeding associated with a decrease in hemoglobin level of ≤8 g/dL if no baseline hemoglobin level was available (or an investigator’s opinion that the hemoglobin level would fall to ≤8 g/dL)
  • acute symptomatic bleeding in a critical area or organ (e.g., retroperitoneal, intra-articular, pericardial, intracranial, or intramuscular with the compartment syndrome)

Patients were excluded from the study if they had surgery scheduled within 12 hours after presentation; a major thrombotic event within two weeks prior to enrollment; or had received a vitamin K antagonist, dabigatran, prothrombin complex concentrate, or whole blood or plasma within seven days prior to screening.

Patients received an andexanet bolus for 15 to 30 minutes followed by a two-hour infusion of andexanet. Doses were adjusted based on the anticoagulant used:

  • For patients receiving apixaban or rivaroxaban >7 hours before andexanet, the bolus was 500 mg and the infusion was 480 mg.
  • For those who received enoxaparin, edoxaban, or rivaroxaban ≤7 hours before andexanet or at an unknown time, the bolus was 800 mg and the infusion was 960 mg.

Dr. Connolly and investigators assessed patients at three hours and at 15 minutes prior to administration of andexanet (baseline); at the end of bolus administration (15-30 minutes after initiation); at the end of the two-hour infusion; at four, eight, and 12 hours after infusion; and at three and 30 days thereafter.

The mean patient age was 77 years, and all patients had a history of thrombotic events and cardiovascular disease. Thirty-two patients received rivaroxaban (median daily dose = 20 mg), 31 patients received apixaban (median daily dose = 5 mg), and four patients received enoxaparin (median daily dose = 90 mg).

The site of bleeding was most often gastrointestinal (n=33; 49%) or intracranial (n=28; 42%); other bleeding sites included nasal, pericardial, pleural or retroperitoneal, genital or urinary, or articular. The mean time from presentation in the emergency department to initiation of andexanet bolus was 4.8 hours.

Forty-seven patients were included in the efficacy analysis (this only included patients whose baseline anti-FXa activity was later determined to be ≥75 ng/mL, or <0.5 IU/mL for enoxaparin).

“The clinical response 12 hours after the andexanet infusion was excellent or good in 79 percent of the patients (n=37) and was consistent across a variety of subgroups (95% CI 64-89),” Dr. Connolly and colleagues reported.

“Excellent” clinical response was defined as a cessation of bleeding within one hour after infusion for visible bleeding or pain relief, unequivocal improvement in objective signs of bleeding, and no increase in swelling within four hours after infusion for musculoskeletal bleeding; “good” clinical response was defined as a cessation of bleeding within four hours after infusion and for visible bleeding or pain relief, unequivocal improvement in objective signs of bleeding, and no increase in swelling within one hour after infusion for musculoskeletal bleeding.

The median reductions in anti-FXa activity (the study’s primary endpoint) from baseline until the end of bolus administration and the two-hour infusion are reported in TABLE. Patients treated with apixaban had the greatest reduction in anti-FXa activity, with a relative decrease from baseline to infusion of 92 percent. “Baseline anti-FXa activity was somewhat higher for patients receiving rivaroxaban than for those receiving apixaban, which may be related to a higher median daily dose of rivaroxaban (20 mg) than of apixaban (5 mg),” Dr. Connolly and authors noted. “Rates of hemostatic efficacy were similar for patients who were receiving the two agents.”

The decreases in anti-FXa activity continued through four hours after andexanet administration:

  • For patients treated with rivaroxaban, the relative decrease was 39 percent from baseline (95% CI 27-45).
  • For patients treated with apixaban, the relative decrease was 30 percent from baseline (95% CI 23-46).
  • For the patient treated with enoxaparin, the decrease was 25% from baseline.

Anti-FXa activity levels also remained similar at eight and 12 hours after treatment with andexanet.

“This finding supports the idea that prolonged reversal of FXa inhibition may not be necessary to achieve a good hemostatic response,” the authors wrote.

All patients were included in the safety analysis, and no infusion reactions or development of antibodies to FXa or FX were reported. Eighteen percent of patients experienced thrombotic events (n=12), including myocardial infarction (n=1), stroke (n=5), deep-vein thrombosis (n=7), and pulmonary embolism (n=1). Ten patients died (15%), with six deaths related to cardiovascular events. Anticoagulation was resumed in 18 patients (27%) within 30 days.

The study is limited by its small patient population and its non-blinded, non-randomized design. The authors also noted that, “a controlled study would be required to assess whether the frequency of these events exceeded that expected in patients at increased risk for thrombotic events.”

The study is ongoing and expected to enroll patients until there are 162 participants available for the efficacy analysis and 230 for the safety analysis.


Connolly SJ, Milling TJ, Eikelboom JW, et al. Andexanet alfa for acute major bleeding associated with factor Xa inhibitors. N Engl J Med. 2016;375:1131-41.

TABLE. Reductions in Anti-Factor Xa Activity After Andexanet Infusion
  Baseline anti-FXa activity End of bolus Percent change (95% CI) End of infusion Percent change (95% CI)
Rivaroxaban-treated patients (n=26) 297.0±171.0 ng/mL 16.8 ng/mL −89% (−58 to −94) 30.6 ng/mL −86% (−55 to −93)
Apixaban-treated patients (n=20) 174.5±97.0 ng/mL 10.3 ng/mL −93% (−87 to −94) 12.5 ng/mL −92% (−85 to −94)
Enoxaparin-treated patients (n=1) 0.6 IU/mL 0.15 IU/mL NA 0.19 IU/mL NA