Andexanet alfa: a recombinant mimetic of human factor Xa for the reversal of anticoagulant therapies

Unexpected Dynamic Binding May Rescue the Binding Affinity of Rivaroxaban in a Mutant of Coagulation Factor X

A novel coagulation factor X (FX) Tyr319Cys mutation (Y99C as chymotrypsin numbering) was identified in a patient with severe bleeding. Unlike the earlier reported Y99A mutant, this mutant can bind and cleave its specific chromogenetic substrate at a normal level, suggesting an intact binding pocket. Here, using molecular dynamics simulations and MM-PBSA calculations on a FX-rivaroxaban (RIV) complex, we confirmed a much stronger binding of RIV in Y99C than in Y99A on a molecular level, which is actually the average result of multiple binding poses in dynamics. Detailed structural analyses also indicated the moderate flexibility of the 99-loop and the importance of the flexible side chain of Trp215 in the different binding poses. This case again emphasizes that binding of ligands may not only be a dynamic process but also a dynamic state, which is often neglected in drug design and screening based on static X-ray structures. In addition, the computational results somewhat confirmed our hypothesis on the activated Tyr319Cys FX (Y99C FXa) with an impaired procoagulant function to bind inhibitors of FXa and to be developed into a potential reversal agent for novel oral anticoagulants (NOAC).

Andexanet alfa for reversal of factor Xa inhibitor-associated anticoagulation

Background:

Review of clinical data on andexanet alfa for the reversal of factor Xa (FXa) inhibitor associated anticoagulation.

Data sources:

In the present review, we identified articles via PubMed using the combined keywords andexanet alfa, apixaban, enoxaparin, edoxaban, and rivaroxaban. Additional online searches via PubMed, Google Scholar, and Lexicomp were conducted for both prescribing and cost information. Portola Pharmaceuticals was contacted for information used for United States Food and Drug Administration approval of andexanet alfa.

Study selection and data extraction:

English-language clinical trials and reviews published between January 2008 and April 2019 were included for review. Bibliographies of selected articles were reviewed manually for relevant publications, focusing on reversal strategies for apixaban, enoxaparin, edoxaban, or rivaroxaban associated anticoagulation using andexanet alfa. Review articles were excluded.

Data synthesis:

The safety and tolerability of andexanet alfa were evaluated in one phase I, two phase II, and one phase III clinical trials. The use of andexanet alfa for reversing FXa inhibitor-associated anticoagulation were evaluated in the phase III ANNEXA-4 study.

Conclusions:

Studies evaluating laboratory parameters for coagulation show that andexanet alfa rapidly neutralizes the anticoagulant effects of apixaban, enoxaparin, edoxaban, and rivaroxaban. Clinical studies show that andexanet alfa improves markers related to coagulation, and reverses major bleeding in healthy volunteers and patients with life-threatening bleeding. Interruption of anticoagulation may result in thromboembolic and ischemic events. The use of andexanet alfa requires close monitoring for signs and symptoms of thromboembolic events, ischemic events, and cardiac arrest. Furthermore, anticoagulation should be resumed following the administration of andexanet alfa as soon as medically appropriate.

A Systematic and Evidence-Based Review of Published and Pending Reports of Andexanet Alfa

Andexanet alfa is the newly approved factor Xa inhibitor reversal agent for the treatment of life-threatening or uncontrolled bleeding from apixaban or rivaroxaban. This decoy protein directly binds factor Xa inhibitors reversing their action. A systematic and evidence-based evaluation of the available clinical trials is lacking in the literature. This research will provide a systematic and evidence-based evaluation of published clinical trials for andexanet alfa. It will also present an evaluation of active research. Reports of research were identified through multiple databases using search terms andexanet alfa, andexanet, or PRT064445. The level of evidence and strength of recommendation were accomplished using the Strength of Recommendation Taxonomy. There are 2 published articles presenting 3 clinical trials. Two trials had a low level of evidence and the third trial, a preliminary report of results, showed a moderate level of evidence. The review of active research found 3 unique studies and a preliminary results study. The level of evidence is low for 2 of these studies, moderate for a third, and potentially high for the fourth. The strength of recommendation for all 6 studies is a C. Four of the studies present disease-oriented evidence resulting in a low level of evidence. Another study is unblinded and uncontrolled but presents patient-oriented evidence resulting in a moderate level of evidence. Only one study could score high because the outcome is patient-oriented evidence, and it could achieve follow-up of 80%. There is a need for well-controlled and blinded evaluation to improve the recommendation strength.