Ondexxya (Andexanet Alfa)

What Is Ondexxya and What Is It Used For?

Ondexxya (andexanet alfa) is a recombinant modified form of human factor Xa that acts as a decoy protein to reverse the anticoagulant effects of the direct factor Xa inhibitors apixaban and rivaroxaban. It is indicated for adult patients experiencing life-threatening or uncontrolled bleeding who require urgent reversal of anticoagulation.

Factor Xa inhibitors such as apixaban (Eliquis) and rivaroxaban (Xarelto) are among the most widely prescribed anticoagulants worldwide. They are used to prevent and treat blood clots in conditions such as atrial fibrillation, deep vein thrombosis, and pulmonary embolism. While these medications are highly effective at preventing thromboembolic events, they carry an inherent risk of bleeding — including life-threatening hemorrhage that may require urgent reversal of the anticoagulant effect.

Andexanet alfa was developed to address this critical unmet medical need. It is a genetically modified variant of human factor Xa that retains the ability to bind factor Xa inhibitors but lacks the catalytic activity needed to participate in the coagulation cascade. In essence, it functions as a molecular “sponge” that soaks up the anticoagulant drug molecules in the bloodstream, freeing native factor Xa to resume its normal role in blood clotting.

The clinical development of andexanet alfa was primarily supported by the ANNEXA-4 study, a prospective, open-label, single-arm trial that enrolled patients with acute major bleeding while taking apixaban or rivaroxaban. The study demonstrated effective hemostatic efficacy in approximately 82% of patients, with significant reductions in anti-FXa activity observed within minutes of administration. These results led to regulatory approvals by both the European Medicines Agency (EMA) and the U.S. Food and Drug Administration (FDA).

It is important to understand that Ondexxya is not effective against all anticoagulants. It specifically targets only the direct factor Xa inhibitors apixaban and rivaroxaban. For patients taking warfarin, vitamin K and prothrombin complex concentrates (PCC) remain the standard reversal approach. For dabigatran (Pradaxa), the specific reversal agent is idarucizumab (Praxbind). Ondexxya also does not reverse heparin or low-molecular-weight heparins.

What Should You Know Before Receiving Ondexxya?

Ondexxya should not be used in patients with known allergies to andexanet alfa, hamster proteins, or any excipients. It should also be avoided in patients currently receiving heparin, as it renders heparin anticoagulation ineffective. The primary risk of treatment is thromboembolic events due to reversal of anticoagulation.

Contraindications

There are specific situations in which Ondexxya must not be administered. Healthcare professionals will assess these before initiating treatment:

• Hypersensitivity: Known allergy to andexanet alfa or any of the excipients (tromethamine, tromethamine hydrochloride, L-arginine hydrochloride, sucrose, mannitol, polysorbate 80)
• Hamster protein allergy: Andexanet alfa is produced using Chinese hamster ovary (CHO) cell technology, and patients with known hypersensitivity to hamster proteins should not receive this medicine
• Concurrent heparin therapy: Ondexxya must not be administered to patients who are receiving heparin, as it renders heparin anticoagulation ineffective

Warnings and Precautions

Several important warnings apply to the use of Ondexxya. The treating physician will carefully weigh the benefits against these risks before administering the drug:

If side effects occur during the infusion (such as chills, elevated blood pressure, or shortness of breath), the healthcare team may slow the infusion rate, pause treatment temporarily, or administer antihistamine medications to manage the reaction. These infusion-related reactions are generally manageable with standard supportive care.

If a surgical procedure requiring heparin as an anticoagulant is planned, Ondexxya should be avoided. Because andexanet alfa binds to and neutralizes heparin, it would render perioperative heparin anticoagulation ineffective, potentially creating dangerous surgical conditions.

Pregnancy and Breastfeeding

Ondexxya is not recommended during pregnancy or in women of childbearing potential who are not using contraception. There are limited clinical data on the use of andexanet alfa during pregnancy, and the potential risks to the developing fetus are unknown.

It is also unknown whether andexanet alfa is excreted in human breast milk. As a precautionary measure, breastfeeding should be discontinued during treatment with Ondexxya. The decision to use this medicine in pregnant or breastfeeding women should be made only when the life-threatening nature of the bleeding justifies the potential risks.

Children and Adolescents

There is no clinical data on the use of Ondexxya in patients under 18 years of age. The safety and efficacy of andexanet alfa in pediatric populations have not been established. Factor Xa inhibitor use in children is uncommon, and the need for specific reversal agents in this population remains limited.

How Does Ondexxya Interact with Other Drugs?

Ondexxya is specifically designed to interact with factor Xa inhibitors (apixaban and rivaroxaban) by binding and neutralizing them. It renders heparin ineffective and should not be combined with heparin anticoagulation. No other significant drug interactions have been identified.

Andexanet alfa is a targeted reversal agent, and its drug interaction profile is relatively narrow. Because it functions as a decoy receptor for factor Xa inhibitors, it is intentionally designed to bind these specific molecules. The primary clinical concern regarding drug interactions involves its effect on anticoagulant therapies.

Beyond heparin and factor Xa inhibitors, Ondexxya is not expected to affect the efficacy of other medications, and other drugs are not expected to influence the pharmacokinetics or pharmacodynamics of andexanet alfa. However, healthcare professionals should always be aware of the patient's full medication list when making treatment decisions in emergency bleeding situations.

It is worth noting that while edoxaban is also a factor Xa inhibitor, Ondexxya has not been approved for reversal of edoxaban, as clinical efficacy data for this specific indication are limited. Healthcare teams managing patients on edoxaban who experience major bleeding should consult hematology or toxicology specialists for the most appropriate management strategy.

What Is the Correct Dosage of Ondexxya?

Ondexxya is administered as an IV bolus followed by a continuous 2-hour infusion. The dose depends on which factor Xa inhibitor the patient was taking, the dose of that inhibitor, and the time elapsed since the last dose. Two dosing regimens exist: low dose (400 mg bolus + 480 mg infusion) and high dose (800 mg bolus + 960 mg infusion).

Andexanet alfa dosing is determined by the specific factor Xa inhibitor being reversed, the last dose taken, and the time since that last dose. Healthcare professionals calculate the appropriate regimen based on standardized dosing tables provided in the prescribing information. If the strength of the last anticoagulant dose or the time since dosing is unknown, no specific dose recommendation can be made.

Measurement of baseline anti-FXa activity levels should support the clinical decision to initiate treatment, provided the result is available within an acceptable timeframe. In emergency situations, treatment should not be delayed while waiting for laboratory results if clinical assessment supports the need for reversal.

Dosing Regimens

Dosing for Apixaban Reversal

Dosing for Rivaroxaban Reversal

Administration

Ondexxya is administered exclusively in hospital settings by trained healthcare professionals. The medicine is supplied as a lyophilized (freeze-dried) powder that must be reconstituted with sterile water for injection before administration. Key preparation and administration details include:

• Each vial is reconstituted with 20 mL of sterile water for injection, yielding a 10 mg/mL solution
• The reconstituted solution should be gently swirled (not shaken) until fully dissolved, typically within 3–5 minutes
• A 0.2 or 0.22 micrometer polyethersulfone (PES) in-line filter must be used during administration
• Administration can be via syringe pump or IV bag (polyolefin or PVC)
• The bolus and infusion should ideally be prepared in separate containers to ensure correct administration rates
• Reconstituted solution is stable for up to 8 hours at room temperature but should be used immediately for microbiological safety

Missed Dose

The concept of a missed dose does not apply to Ondexxya in the traditional sense, as it is a single-use emergency treatment administered in a hospital setting. If the infusion is interrupted for any reason, the treating physician will determine whether to resume or restart treatment based on the clinical situation and ongoing assessment of hemostasis.

Overdose

There is limited clinical experience with overdose of andexanet alfa. Because the drug reverses anticoagulation, an overdose could theoretically increase the risk of thromboembolic events beyond what is expected with standard dosing. No specific antidote for andexanet alfa overdose exists. Management would be supportive, with close monitoring for signs of thrombosis and consideration of re-anticoagulation if clinically appropriate.

What Are the Side Effects of Ondexxya?

The most significant side effects of Ondexxya are thromboembolic events (blood clots), which occur because reversing anticoagulation restores the patient's underlying thrombotic risk. Common side effects include stroke, heart attack, blood clots, and fever. Infusion-related reactions can also occur.

Like all medicines, Ondexxya can cause side effects, although not everyone who receives it will experience them. The side effect profile is primarily related to the pharmacological action of the drug — by reversing anticoagulation, patients are re-exposed to their baseline risk of forming blood clots. This is an expected consequence of treatment rather than an unexpected adverse reaction.

Clinical trials and post-marketing surveillance have identified the following side effects in patients with acute major bleeding. It is important to note that many of these patients had serious underlying conditions that independently increased their risk of these events.

The risk of thromboembolic events is an inherent consequence of reversing anticoagulation in patients who are being treated with blood thinners precisely because they have conditions that predispose them to blood clots. The treating physician will carefully balance the immediate need to stop life-threatening bleeding against the longer-term risk of clotting complications.

Infusion-related reactions, when they occur, are generally manageable by slowing the infusion rate, pausing the infusion temporarily, or administering antihistamine medications. Severe infusion reactions are uncommon. Healthcare teams are trained to monitor patients closely during and after administration for signs of both thromboembolic events and infusion-related reactions.

How Should Ondexxya Be Stored?

Ondexxya must be stored in a refrigerator at 2°C to 8°C (36°F to 46°F) and must not be frozen. After reconstitution, the solution should be used immediately, though it is stable for up to 8 hours at room temperature.

As a hospital-only medicine, Ondexxya is stored and managed by hospital pharmacy departments. Patients will not need to handle or store this medication themselves. The following storage requirements are maintained by healthcare facilities:

• Unopened vials: Store in a refrigerator at 2°C to 8°C (36°F to 46°F)
• Do not freeze: Freezing may damage the protein structure and compromise drug efficacy
• After reconstitution: Use immediately for microbiological safety. Chemical and physical stability has been demonstrated for up to 8 hours at 25°C (77°F)
• Expiry date: Do not use after the expiry date printed on the vial and carton (last day of the stated month)
• Visual inspection: The reconstituted solution should be clear, colorless, or slightly yellowish. Do not use if opaque particles or discoloration are present
• Keep out of sight and reach of children

Unused reconstituted solution, used syringes, needles, and vials should be disposed of according to local hospital waste management protocols for pharmaceutical products.

What Does Ondexxya Contain?

Each vial of Ondexxya contains 200 mg of the active substance andexanet alfa as a white to off-white powder. The excipients include tromethamine, tromethamine hydrochloride, L-arginine hydrochloride, sucrose, mannitol, and polysorbate 80.

Understanding the composition of Ondexxya is important both for clinical use and for identifying potential allergen concerns. The complete composition of each vial is as follows:

Active Substance

• Andexanet alfa — 200 mg per vial. Andexanet alfa is a recombinant modified form of human coagulation factor Xa produced in Chinese hamster ovary (CHO) cells by recombinant DNA technology

Excipients (Inactive Ingredients)

• Tromethamine (Tris) — buffer to maintain pH stability
• Tromethamine hydrochloride — buffer component
• L-arginine hydrochloride — protein stabilizer
• Sucrose — cryoprotectant (protects protein during freeze-drying)
• Mannitol — bulking agent and cryoprotectant
• Polysorbate 80 — surfactant (2 mg per vial, equivalent to 0.1 mg/mL). Note: polysorbates can cause allergic reactions in sensitive individuals

Packaging

Ondexxya is supplied in glass vials as a white to off-white lyophilized powder. Each vial contains 200 mg of andexanet alfa. Cartons contain either 4 or 5 vials. Not all pack sizes may be marketed in every country. After reconstitution with 20 mL of sterile water for injection, each vial yields a clear, colorless to slightly yellowish solution at a concentration of 10 mg/mL.