Antidote reverses FXa inhibitor anticoagulation after acute major bleeding

Final Study Report of Andexanet Alfa for Major Bleeding With Factor Xa Inhibitors

Literature - Milling TJ Jr, Middeldorp S, Xu L, et al. - Circulation. 2023 Feb 20. doi: 10.1161/CIRCULATIONAHA.121.057844

Introduction and methods


In patients with nonvalvular AF and venous thromboembolism, treatment with factor Xa (FXa) inhibitors reduces thrombotic events, but this comes at the cost of an increased bleeding risk. Andexanet alfa, a modified recombinant inactive FXa, was specifically designed to reverse the anticoagulation induced by FXa inhibitors [1]. Andexanet alfa was given conditional approval for the reversal of anticoagulation in life-threatening or uncontrolled bleeding by the US Food and Drug Administration (in 2018) and European Medicines Agency (in 2019); in Japan, full market approval was obtained (in 2022).

Previously, results from 2 analyses (in 67 and 352 patients, respectively) of the ANNEXA-4 (Andexanet Alfa, a Novel Antidote to the Anticoagulation Effects of Factor Xa Inhibitors) study were published [2,3]. The current analysis comprised the final cohort (n=479).

Aim of the study

The study aim was to assess the safety and efficacy of andexanet alfa and the relationship among anti-FXa activity level, mortality, and hemostatic efficacy in patients with FXa inhibitor–related major bleeding.


The ANNEXA-4 study was an international, prospective, open-label, phase 3b/4, single-group cohort study in which 479 patients presenting with acute major bleeding within 18 hours of receiving an FXa inhibitor (apixaban, rivaroxaban, edoxaban, or enoxaparin) were treated with andexanet alfa (administered as a 15–30-minute bolus and subsequently a 2-hour infusion).

Anti-FXa activity and the unbound fraction of the FXa inhibitor in plasma before andexanet alfa treatment were measured in blood samples at the end of the bolus administration, at the end of the infusion, and at 4, 8, and 12 hours after infusion. Patients with intracranial hemorrhage (ICH) underwent follow-up imaging. An independent adjudication committee assessed whether patients met major bleeding criteria and adjudicated hemostatic efficacy, thrombotic events, and cause of death.


The 2 co-primary endpoints were percent change from baseline in anti-FXa activity during andexanet alfa treatment and percentage of patients with good or excellent hemostatic efficacy (based on prespecified criteria, such as hematoma volume expansion for ICH) at 12 hours after andexanet alfa infusion. Primary safety endpoints were death, thrombotic events, and development of antibodies to andexanet alfa or to native FX and FXa between baseline and day 30. A secondary endpoint was the endogenous thrombin potential at baseline and across the follow-up period.

Main results

Anti-FXa activity

  • Median anti-FXa activity level decreased from 146.9 ng/mL at baseline to 10.0 ng/mL at the on-treatment nadir (median reduction: 93%; 95%CI: 94%–93%) in patients treated with apixaban (n=172), from 214.6 to 10.8 ng/mL (median reduction: 94%; 95%CI: 95%–93%) in rivaroxaban-treated patients (n=132), from 121.1 to 24.4 ng/mL (median reduction: 71%; 95%CI: 82%–65%) in edoxaban-treated patients (n=28), and from 0.48 to 0.11 IU/mL (median reduction: 75%; 95%CI: 79%–67%) in enoxaparin-treated patients (n=17).
  • Decreased anti-FXa activity was seen after administration of the andexanet alfa bolus (within 2 minutes after completion) for all 4 FXa inhibitors, which was sustained through the end of the infusion period. After the infusion ended, anti-FXa activity gradually increased back to anticipated normal clearance levels.

Hemostatic efficacy

  • Efficacy analyses only included patients with baseline anti-FXa activity ≥75 ng/mL (for apixaban and rivaroxaban), ≥40 ng/mL (for edoxaban), or ≥0.25 IU/mL (for enoxaparin) who had adjudication committee–confirmed major bleeding at presentation. Of the 342 eligible patients, 274 (80%; 95%CI: 75%–84%) showed good or excellent hemostatic efficacy.
  • Hemostatic efficacy ranged from 79% for apixaban-treated patients and edoxaban-treated patients to 88% for edoxaban-treated patients.
  • Hemostatic efficacy was 79% (95%CI: 74%–84%) for patients with ICH (n=331), 82% (95%CI: 72%–90%) for those with gastrointestinal bleeding (n=109), and 82% (95%CI: 60%–95%) for those with other types of bleeding (n=39).


  • During the 30-day follow-up period, 50 patients (10.4%) experienced ≥1 thrombotic events; 16 events occurred after patients were restarted on anticoagulation therapy with nonoral agents.
  • None of the 130 patients (27.1%) who were restarted on oral anticoagulant therapy had a thrombotic event during the 30-day study period, whereas 11 (8.5%) had a thrombotic event before restart of oral anticoagulation.
  • There were 75 deaths (15.7%): 40 CV deaths related to the index bleeding, 18 CV deaths not related to the index bleeding, 15 non-CV deaths, and 2 uncertain/unknown deaths.
  • None of the patients developed neutralizing antibodies to andexanet alfa, FX, or FXa.
  • There was one severe infusion reaction (consisting of rigors, severe chills, hypertension, oxygen desaturation, fever, agitation, and confusion), which began 75 minutes after initiating andexanet alfa treatment and resolved 80 minutes later with appropriate therapy.

Thrombin generation

  • For all FXa inhibitors, median endogenous thrombin potential returned to the normal range by the end of administration of the andexanet alfa bolus through 24 hours.

Relationship among anti-FXa activity, hemostatic efficacy, and mortality

  • Correlations between anti-FXa activity, hemostatic efficacy, and mortality were examined in exploratory analyses. There was no significant association between anti-FXa activity level at nadir and mortality. However, in patients <75 years of age, median anti-FXa activity level at nadir was correlated with lower mortality (unadjusted P=0.003; adjusted P=0.022).
  • In the entire cohort, there was no significant association between change in anti-FXa activity level from baseline to nadir and hemostatic efficacy. For ICH patients (excluding those on enoxaparin because of different units of activity levels), reduction of anti-FXa activity predicted hemostatic efficacy (area under receiver operating characteristic curve: 0.62; 95%CI: 0.54–0.70).
  • In all patients, there was a correlation between higher hemostatic efficacy and lower mortality (P<0.001).


In the final ANNEXA-4 study cohort of 479 patients with FXa inhibitor–related acute major bleeding, andexanet alfa reduced anti-FXa activity up to 94% and was associated with good or excellent hemostatic efficacy in 80% of patients.


1. Lu G, DeGuzman FR, Hollenbach SJ, Karbarz MJ, Abe K, Lee G, Luan P, Hutchaleelaha A, Inagaki M, Conley PB, et al. A specific antidote for reversal of anticoagulation by direct and indirect inhibitors of coagulation factor Xa. Nat Med. 2013;19:446–451. doi: 10.1038/nm.3102

2. Connolly SJ, Milling TJ Jr, Eikelboom JW, Gibson CM, Curnutte JT, Gold A, Bronson MD, Lu G, Conley PB, Verhamme P, et al; ANNEXA-4 Investigators. Andexanet alfa for acute major bleeding associated with factor Xa inhibitors. N Engl J Med. 2016;375:1131–1141. doi: 10.1056/NEJMoa1607887

3. Connolly SJ, Crowther M, Eikelboom JW, Gibson CM, Curnutte JT, Lawrence JH, Yue P, Bronson MD, Lu G, Conley PB, et al; ANNEXA-4 Investigators. Full study report of andexanet alfa for bleeding associated with factor Xa inhibitors. N Engl J Med. 2019;380:1326–1335. doi: 10.1056/NEJMoa1814051

Find this article online at Circulation.

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