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New Anticoagulants Enhance “Clot-Busting” Ability of the “Clot-Former” Factor Xa

By April 3, 2019No Comments

If you, loved ones or friends suffer from diseases that may involve abnormal blood clotting, such as heart attack, stroke, atherosclerosis, deep vein thrombosis, pulmonary embolism or even cancer, commercials on “blood thinners” (anticoagulants) may catch your attention more than other drugs while watching your favorite TV show. Drug advertisements are everywhere! They often promise us clinical benefits that outweigh side effects, even better outcome than “traditional” drugs used for decades to mitigate the same condition/disease. Regardless of the length of clinical use, drugs are continuously under investigation in quest of a deeper understanding of their mechanism of action and novel applications.

A recent publication from the Pryzdial lab at the Centre for Blood Research (CBR), took a deeper look at two relatively new clotting factor Xa (FXa)-directed oral anticoagulants (DOACs) known as rivaroxaban (Xarelto®) and apixaban (Eliquis®). You have likely seen their advertisements of over the past few years. Both were approved by the U.S. Food and Drug Administration in 2011 and 2012 respectively. This class of anticoagulant is the first to be approved since the advent of warfarin in 1954. Unlike warfarin, which simultaneously inhibits several clotting factors including FXa, these DOACs function by specifically blocking only the active site of FXa. This reduces the generation of thrombin (a key clot-forming enzyme) and thus prevents clot formation. DOACs are an improvement over the traditionally administered warfarin because they target only one clotting factor, requiring less frequent laboratory tests and no dietary restrictions. Furthermore, in May 2018, an antidote named andexanet alfa (andexanet) was approved if rapid reversal was required due to the requirement of emergency surgery or to curtail bleeding side effects.

So, what else do DOACs do in addition to inhibiting clot formation? Before answering, let’s do a very brief review of how clot-formation and clot-busting happens in our blood vessels.

Maintaining the circulation of blood requires extensive molecular surveillance orchestrated by many enzymes and cofactors. They help us form healthy clots to prevent us from bleeding and to rapidly clear obstructive clots. These dynamic events are broadly classified as coagulation (clot-forming) and fibrinolysis (clot-busting). Clot-forming can be initiated through two mechanistic arms forming complexes called “extrinsic tenase” upon tissue injury to initiate FXa production and “intrinsic tenase” to amplify FXa production. Think of these events as two “waterfalls” merging together to create a big splash. In this case, FXa!

Various coagulation factors including FXa play “double agent” where they perform an additional task opposing their primary role. When bound to an anionic phospholipid-containing membrane (eg. activated platelets), FXa is sequentially cleaved by plasmin (a key clot-busting protein) from its intact form, FXaα, to FXaβ and then to Xa33/13. Although, plasmin generation was enhanced in the presence of either FXaβ or Xa33/13, the former resulted in superior plasmin generation in plasma.

The Pryzdial lab previously showed that irreversible chemical modification of FXaβ reduced the formation of Xa33/13 and promoted clot-busting. This observation lead to the hypothesis that FXa-blocking DOACs enhance clot-busting ability by stabilizing FXaβ and inhibiting further fragments.

Dr. Rolinda Carter, the lead author of “Rivaroxaban and Apixaban Induce Clotting Factor Xa Fibrinolytic Activity” explains her work in this brief interview:

Knowledge Translation Committee (KT): What is the specific finding of your publication?

R: The study identified a new mechanism by which FXa-directed DOACs can enhance clot dissolution.

KT: Does this finding affect our current clinical understanding of DOACs?

R: It’s hard to really give clinical conclusions because a clinical study was not done. Nevertheless, it would go back to the positive use of FXa-DOACs to treat deep vein thrombosis and thus prevent post thrombotic syndrome which has links to ineffective clot dissolution.

KT: Are more factors acting as “double agents” in coagulation and fibrinolysis currently being studied? Do you think our most common understanding of fibrinolysis and coagulation as separate entities is coming to end?

R: More and more researchers are finding that elements of the clotting cascade have multiple functions. Studies are ongoing in the Pryzdial laboratory to look at the role clotting factor Va has in clot-dissolution. Further investigation is also ongoing to optimize the clot-busting activity of FXa derivatives.

KT: What was unexpected during your investigation? Did it deviate from the initial hypothesis?

R: The initial hypothesis was that anticoagulants would bind to the active site of FXa, preventing the formation of Xa33/13 and thus enhance clot lysis. This enhancement was not due to reduced thrombin generation and the consequent effects on its numerous functions but its ability to bind to the active site of FXa and promote plasmin generation. Thus, our findings were in line with the original hypothesis.

Visit the “Publications of the Week” section of Science in the City for more information on this publication: