Candidates in development
The pharmaceutical industry is on the case with several factor XI inhibitors now in clinical development. At present, three main candidates lead the field. These are abelacimab (Anthos), a monoclonal antibody given by subcutaneous injection once a month; and two small molecules, milvexian (BMS/Janssen) and asundexian (Bayer), which are both given orally.
Phase 3 trials of these three factor XI inhibitors have recently started for a variety of thrombotic indications, including the prevention of stroke in patients with AFib, prevention of recurrent stroke in patients with ischemic stroke, and prevention of future cardiovascular events in patients with acute coronary syndrome (ACS).
Dr. Alexander, who has been involved in clinical trials of both milvexian and asundexian, commented: “We have pretty good data from a number of phase 2 trials now that these factor XI inhibitors at the doses used in these studies cause a lot less bleeding than therapeutic doses of DOACs and low-molecular-weight heparins.”
He pointed out that, in addition to the AZALEA trial with abelacimab, the phase 2 PACIFIC program of studies has shown less bleeding with asundexian than with apixaban in patients with AFib and a similar amount of bleeding as placebo in ACS/stroke patients on top of antiplatelet therapy. Milvexian has also shown similar results in the AXIOMATIC program of studies.
Dr. Ruff noted that the biggest need for new anticoagulants in general is in the AFib population. “Atrial fibrillation is one of the most common medical conditions in the world. Approximately one in every three people will develop AFib in their lifetime, and it is associated with more than a fivefold increased risk of stroke. But up to half of patients with AFib currently do not take anticoagulants because of concerns about bleeding risks, so these patients are being left unprotected from stroke risk.”
Dr. Ruff pointed out that the AZALEA study was the largest and longest study of a factor XI inhibitor to date; 1,287 patients were followed for a median of 2 years.
“This was the first trial of long-term administration of factor XI inhibitor against a full-dose DOAC, and it was stopped because of an overwhelming reduction in a major bleeding with abelacimab, compared with rivaroxaban,” he noted. “That is very encouraging. It looks like our quest to develop a safe anticoagulant with much lower rates of bleeding, compared with standard of care, seems to have been borne out. I think the field is very excited that we may finally have something that protects patients from thrombosis whilst being much safer than current agents.”
While all this sounds very promising, for these drugs to be successful, in addition to reducing bleeding risk, they will also have to be effective at preventing strokes and other thrombotic events.
“While we are pretty sure that factor XI inhibitors will cause less bleeding than current anticoagulants, what is unknown still is how effective they will be at preventing pathologic blood clots,” Dr. Alexander points out.
“We have some data from studies of these drugs in DVT prophylaxis after orthopedic surgery which suggest that they are effective in preventing blood clots in that scenario. But we don’t know yet about whether they can prevent pathologic blood clots that occur in AFib patients or in poststroke or post-ACS patients. Phase 3 studies are now underway with these three leading drug candidates which will answer some of these questions.”
Dr. Patel agrees that the efficacy data in the phase 3 trials will be key to the success of these drugs. “That is a very important part of the puzzle that is still missing,” he says.
Dr. Ruff notes that the AZALEA study will provide some data on efficacy. “But we already know that in the orthopedic surgery trials there was a 70%-80% reduction in VTE with abelacimab (at the 150-mg dose going forward) vs. prophylactic doses of low-molecular-weight heparin. And we know from the DOACs that the doses preventing clots on the venous side also translated into preventing strokes on the [AFib] side. So that is very encouraging,” Dr. Ruff adds.