Factor Xa Reversal Agent Pregnancy: There are no adequate and well-controlled studies of andexanet alfa in pregnant women to inform patients of associated risks, and animal reproductive and developmental studies have not been conducted. The safety and effectiveness during labour and delivery have not been evaluated.

Andexanet Alfa

Brand names: Ondexxya

Andexanet alfa is a recombinant modified human factor Xa protein indicated as a targeted reversal agent in adults with life-threatening or uncontrolled bleeding during treatment with a factor Xa inhibitor (apixaban or rivaroxaban).

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: Two regimens. LOW DOSE: 400 mg initial intravenous bolus at a target rate of 30 mg/min, followed by a continuous intravenous infusion of 4 mg/min for 120 minutes (480 mg). HIGH DOSE: 800 mg initial intravenous bolus at a target rate of 30 mg/min, followed by a continuous intravenous infusion of 8 mg/min for 120 minutes (960 mg).

Route: For intravenous use only — bolus then continuous infusion, given through a 0.2 or 0.22 micron in-line polyethersulfone or equivalent low protein-binding filter

Frequency: Single treatment course. Start the continuous infusion within two minutes of completing the bolus and run it for 120 minutes. The safety and effectiveness of more than one dose have not been evaluated.

Max: The high-dose regimen (800 mg bolus plus 8 mg/min for 120 minutes, total 960 mg) is the highest regimen described; the safety and efficacy of an additional dose have not been established.

DOSE SELECTION — CRITICAL GAP: the regimen is chosen on the specific FXa inhibitor, the dose of that FXa inhibitor, and the time since the patient's last dose. The dose-selection table (Table 2) was captured INCOMPLETELY in this bundle — the fetched text contains only fragments reading 'FXa Inhibitor Last Dose 10 mg or Unknown: High Dose' and 'Apixaban: 5 mg or less, Low Dose; more than 5 mg or Unknown, High Dose', with the rivaroxaban rows and the entire time-since-last-dose column missing. Do NOT select low versus high dose from this draft — the clinician must read the complete Table 2 in the label. VIALS: low dose = 5 x 200 mg vials (2 for the bolus, 3 for the infusion); high dose = 9 x 200 mg vials (4 for the bolus, 5 for the infusion). Reconstituted concentration is 10 mg/mL (200 mg vial reconstituted with 20 mL Sterile Water for Injection); reconstituted product is stable at room temperature for up to eight hours, or up to 24 hours at 2 to 8 degrees C in vials. RESTARTING ANTICOAGULATION: reversing FXa inhibitor therapy exposes patients to the thrombotic risk of their underlying disease — resume anticoagulant therapy as soon as medically appropriate. PAEDIATRIC: safety and efficacy in the paediatric population have not been studied. SOURCE: no UK SPC was fetched in this bundle — this draft is distilled from the US ANDEXXA prescribing information and must be verified against the UK SPC (the UK product and its dosing tables may differ).

Dose auto-extracted from US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

None stated — the US label section 4 Contraindications reads 'None'

Side effects

Urinary tract infection — most common (at least 5%) in bleeding patients
Pneumonia — most common (at least 5%) in bleeding patients
Arterial and venous thromboembolic events, ischaemic events and cardiac events including sudden death — a thrombotic event occurred in 45 of 419 patients (10.7%) in ANNEXA-4, median time to first event 10 days
Unresponsiveness to unfractionated heparin following administration
Re-elevation or incomplete reversal of anticoagulant activity

Interactions

Unfractionated heparin — do not use following andexanet alfa administration; unresponsiveness has occurred, characterised by non-prolongation of activated clotting times and a requirement for increased heparin dosing
Anti-FXa activity assays — current commercial clinical assays are unsuitable for measuring FXa activity after andexanet alfa; high sample dilution dissociates the inhibitor and gives erroneously elevated anti-FXa levels, underestimating the reversal effect

Clinical monograph

How it works

Acting as a high-affinity decoy, it binds factor Xa inhibitor molecules and neutralises their activity, allowing endogenous factor Xa to resume thrombin generation and restore haemostasis.

Prescribing in practice

There is a recognised risk of thromboembolic and ischaemic events following reversal, so use should be restricted to serious bleeding and anticoagulation reinstated as soon as it is clinically safe.
Administration is a bolus then a continuous infusion, with low- or high-dose regimens chosen by the offending agent, its last dose and timing.
Anti-factor Xa activity assays are unreliable for monitoring during and shortly after treatment and should not guide management.

Monitoring

Observe closely for recurrent bleeding and for symptoms or signs of arterial or venous thrombosis after dosing.

Counselling the patient

It is an injectable treatment given in hospital for severe bleeding.
Tell staff about any signs of a new clot, such as chest pain, breathlessness or limb swelling.
The team will decide when it is safe to restart your usual blood thinner.

Evidence & guidelines

Evidence from the ANNEXA-4 study underpins its licensed use as a specific factor Xa inhibitor reversal agent, with NICE and MHRA guidance applying.

Reference: NICE TA697 (Andexanet alfa for reversing anticoagulation from apixaban or rivaroxaban, 2021); ANNEXA-4 trial (NEJM 2019); BSH guidance on reversal of anticoagulants; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Drugs

Pathways

DOAC overdose / reversalRecommended
BSH 2018; ESC 2021; TOXBASE/NPIS; SmPC Praxbind/Ondexxya

Same specialty