Andexanet alfa is a recombinant modified factor XA protein approved by the FDA in May 2018 for the reversal of apixaban and rivaroxaban in patients with life-threatening or uncontrolled bleeding..
Factor XA inhibitors have been shown to be as effective as warfarin with a better safety profile in terms of bleeding. They are also convenient in that less routine blood monitoring is required than with the use of warfarin. However, emergency room visits and hospital admissions for bleeding due to factor XA inhibitors have risen with increasing use of these agents. Andexanet alfa is the first FDA approved reversal agent for factor XA inhibitors. It is not currently approved for the reversal of edoxaban, fondaparinux, or low-molecular-weight heparins due to lack of sufficient data in patients on these agents.
Andexanet alfa was approved as breakthrough therapy and as an orphan drug based on the results of two phase 3 trials, annexa-A and annexa-R. These trials evaluated the efficacy and safety of andexanet alfa in reversing of apixaban and rivaroxaban, respectively, in healthy volunteers. The FDA also considered the interim results of an on-going trial, ANNEXA-4, which analyzed reversal of apixaban, rivaroxaban, edoxaban or enoxaparin in patients presenting with acute major bleeding within 18 hours of the last dose of a factor XA inhibitor. 
Andexanet alfa acts as a decoy and sequesters rivaroxaban or apixaban, inhibiting them from binding to natural factor XA. The increase in available factor XA reduces anticoagulant action which can be measured by anti-factor XA activity, thrombin generation, or unbound factor XA inhibitor plasma concentration from baseline. In clinical trials, the median decline in anti-factor XA activity for apixaban or rivaroxaban was 88% or higher. Binding of andexanet alfa to factor XA inhibitors is dose-dependent. There is a rapid decrease in anti-FXa activity after the bolus has been given. This decrease in anti-FXa activity is sustained during the infusion. After the infusion, anti-XA activity increases and peaks 4 hours after the infusion. After the peak, anti-FXa activity decreases at a rate similar to that of the clearance of FXa inhibitors. In ANNEXA-A, andexanet alfa restored thrombin generation in 100% healthy volunteers who were on apixaban therapy. Thrombin generation was restored in 96% of healthy volunteers on rivaroxaban therapy who received andexanet alfa in ANNEXA-R.
Andexanet alfa also binds tissue factor inhibitor pathway (TPFI), a peptide that inhibits factor XA. Binding TPFI reduces its activity, thus increasing the formation of thrombin. Tissue factor pathway inhibition is sustained for at least 22 hours after andexanet alfa therapy.
The volume of distribution is almost equivalent to blood volume at 5 liters, clearance is about 4.3 liters per hour, and the half-life is 5 to 7 hours. 
Andexanet alfa is available as a lyophilized powder in 100 mg, single-use vials with no preservatives. Vials should be refrigerated at 2 to 8 degrees C. Reconstitute each 100 mg vial with 10 mL of sterile water for injection to make a concentration of 10 mg/mL. Dissolution takes approximately 3 to 5 minutes. The reconstituted drug is stable for 8 hours at room temperature and for 24 hours at 2 to 8 degrees C in the vial. The reconstituted solution can also be transferred to polyolefin or polyvinyl intravenous bags where it is stable for 8 hours at room temperature or for 16 hours at 2 to 8 degrees C.
Two dosing regimens (low and high) are based on which factor XA inhibitor is to be reversed, when it was last taken, and the dose that was given.
Andexanet alfa should be given intravenously, using a 0.2 or 0.22-micron in-line filter. Administering more than one dose has not been tested.
Adverse effects include deep vein thrombosis, arterial thrombosis, pulmonary embolism, ischemic stroke, acute myocardial infarction, and death. Other effects include cardiogenic shock, heart failure exacerbations, urinary tract infections, pneumonia, acute respiratory failure, and infusion-related reactions.
In a 2018 interim report of the ANNEXA-4 clinical trial, 6 of 227 (2.6%) had experienced thrombosis within 3 days and 24 patients (11%) within 30 days. Twenty-seven of 227 (12%) patients died at 30 days. One hundred thirty-nine of 227 (61%) received andexanet alfa for intracranial hemorrhage, and 16 of these 139 patients (12%) died within 30 days. The mean age of patients in this trial was 77 years, and 78% of them had atrial fibrillation. 
It is important to re-initiate anticoagulation therapy as soon as it is clinically appropriate to reduce the risk of thrombosis after andexanet alfa therapy. Thrombosis may occur even after re-initiation of anticoagulation.
There is no clinical trial data on the use of andexanet alfa in pregnancy, labor, delivery or lactation. Safety has not been tested in patients who have experienced thrombosis or disseminated intravascular coagulation within 14 days before bleeding that requires treatment with andexanet alfa. It has also not been tested in patients who received prothrombin complex concentrates, recombinant factor VIIa, or whole blood products within 7 days before bleeding requiring andexanet alfa treatment. 
Drug effects can be measured by anti-XA activity, free fraction of apixaban or rivaroxaban, and thrombin generation. Anti-Xa activity returns to placebo levels about 2 hours after a bolus or infusion, but tissue factor pathway inhibitors persist for about 22 hours after the drug is given.
Increase in tissue-factor-initiated thrombin generation is maintained during the infusion and persists for about 22 hours. Monitor patients for signs and symptoms of thrombosis.
Reversal of apixaban or rivaroxaban with andexanet alfa in patients who are bleeding requires coordination with a multidisciplinary team. Licensed independent practitioners should contact pharmacy and nursing immediately to coordinate the process of drug preparation and administration. Nurses play a vital role in monitoring for signs and symptoms of thrombosis. Inform patients that thrombotic events may occur within 30 days after andexanet alfa treatment.
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