Factor XI inhibitors: The promise of a truly safe anticoagulant?


The quest to find an anticoagulant that can prevent strokes, cardiovascular events, and venous thrombosis without significantly increasing risk of bleeding is something of a holy grail in cardiovascular medicine. Could the latest focus of interest in this field – the factor XI inhibitors – be the long–sought-after answer?

Topline results from the largest study so far of a factor XI inhibitor – released on Sep. 18 – are indeed very encouraging. The phase 2 AZALEA-TIMI 71 study was stopped early because of an “overwhelming” reduction in major and clinically relevant nonmajor bleeding shown with the factor XI inhibitor abelacimab (Anthos), compared with apixaban for patients with atrial fibrillation (AFib).

Very few other data from this study have yet been released. Full results are due to be presented at the scientific sessions of the American Heart Association in November. Researchers in the field are optimistic that this new class of drugs may allow millions more patients who are at risk of thrombotic events but are concerned about bleeding risk to be treated, with a consequent reduction in strokes and possibly cardiovascular events as well.

Why factor XI?

The hope that factor XI inhibitors will prevent pathologic thrombosis with a lower bleeding risk, compared with other anticoagulants, comes down to the role of factor XI in the coagulation cascade.

In natural physiology, there are two ongoing processes: hemostasis – a set of actions that cause bleeding to stop after an injury – and thrombosis – a pathologic clotting process in which thrombus is formed and causes a stroke, MI, or deep venous thrombosis (DVT).

In patients prone to pathologic clotting, such as those with AFib, the balance of these two processes has shifted toward thrombosis, so anticoagulants are used to reduce the thrombotic risks. For many years, the only available oral anticoagulant was warfarin, a vitamin K antagonist that was very effective at preventing strokes but that comes with a high risk for bleeding, including intracranial hemorrhage (ICH) and fatal bleeding.

The introduction of the direct-acting anticoagulants (DOACs) a few years ago was a step forward in that these drugs have been shown to be as effective as warfarin but are associated with a lower risk of bleeding, particularly of ICH and fatal bleeding. But they still cause bleeding, and concerns over that risk of bleeding prevent millions of patients from taking these drugs and receiving protection against stroke.

John Alexander, MD, professor of medicine at Duke University Medical Center, Durham, N.C., a researcher active in this area, notes that “while the DOACs cause less bleeding than warfarin, they still cause two or three times more bleeding than placebo, and there is a huge, unmet need for safer anticoagulants that don’t cause as much bleeding. We are hopeful that factor XI inhibitors might be those anticoagulants.”

The lead investigator the AZALEA study, Christian Ruff, MD, professor of medicine at Brigham and Women’s Hospital, Boston, explained why it is thought that factor XI inhibitors may be different.

“There’s a lot of different clotting factors, and most of them converge in a central pathway. The problem, therefore, with anticoagulants used to date that block one of these factors is that they prevent clotting but also cause bleeding.

“It has been discovered that factor XI has a really unique position in the cascade of how our body forms clots in that it seems to be important in clot formation, but it doesn’t seem to play a major role in our ability to heal and repair blood vessels.”

Another doctor involved in the field, Manesh Patel, MD, chief of cardiology at Duke University Medical Center, added, “We think that factor XI inhibitors may prevent the pathologic formation of thrombosis while allowing formation of thrombus for natural hemostasis to prevent bleeding. That is why they are so promising.”

This correlates with epidemiologic data suggesting that patients with a genetic factor XI deficiency have low rates of stroke and MI but don’t appear to bleed spontaneously, Dr. Patel notes.


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