according to Jeffrey I. Weitz, MD.
These strategies could pick up where direct-acting oral anticoagulants leave off, he suggested during a presentation at the biennial summit of the Thrombosis & Hemostasis Societies of North America.
“We all know that the direct oral anticoagulants – the DOACs – are an advance over vitamin K antagonists,” said, professor of medicine and biochemistry at McMaster University, Hamilton, Ontario.
Not only are DOACs at least as effective as vitamin K antagonists such as warfarin for stroke prevention in atrial fibrillation or for treatment of venous thromboembolism (VTE), but they also reduce intracranial bleeding and major bleeding risk in those settings, respectively, and they are more convenient to administer because they can be delivered using fixed doses without the need for coagulation monitoring, he added.
Still, new targets are needed, he said, explaining that, although DOACs moved closer to the goal of attenuating thrombosis without increasing the risk of bleeding, annual rates of major bleeding remain at 2%-3% in the atrial fibrillation population, and rates of major and clinically relevant nonmajor bleeding are about 10%.
“The fear of bleeding leads to underuse of anticoagulants for eligible patients with atrial fibrillation and inappropriate use of low-dose [non–vitamin K antagonist oral anticoagulant] regimens, which can leave patients unprotected from thrombotic complications,” he said.
That’s where Factor XI (FXI) may come in, Dr. Weitz said.
Current anticoagulants target enzymes, including FXa or thrombin, in the common pathway of coagulation, but the intrinsic pathway at the level of FXI and FXII has attracted attention in recent years.
The intrinsic pathway is activated when blood comes into contact with medical devices like stents, mechanical heart valves, or central venous catheters, but evidence also suggests that it plays a role in clot stabilization and growth, he explained, noting additional evidence of attenuation of thrombosis in mice deficient in FXI or FXII and in animals with FXI or FXII inhibitors.
“There is no bleeding with congenital FXII deficiency, and patients with FXI deficiency rarely have spontaneous bleeding, although they can bleed with surgery or trauma,” he noted. “Therefore, the promise of contact pathway inhibition is that we can attenuate thrombosis with little or no disruption of hemostasis.”
The initiators of the intrinsic pathway are naturally occurring polyphosphates that can activate FXI and FXII, promote platelet activation, and lead to thrombosis. A number of agents are being investigated to target these enzymes – particularly FXI, for which the strongest epidemiological and other evidence of its link with thrombosis exists. He noted that FXI deficiency appears protective against deep-vein thrombosis (DVT) and ischemic stroke, whereas high levels are linked with an increased risk of venous and arterial thrombosis.
Investigative strategies include the use of antisense oligonucleotides to reduce hepatic synthesis of FXI, aptamers to bind FXI and block its activity, antibodies to bind FXI and block its activation or activity, and small molecules to bind reversibly to the active site of FXI and block its activity “much like the DOACs block the activity of FXa or thrombin.”
“We have to remember that the DOACs have taken over from vitamin K antagonists, like warfarin, for many indications, and as they go generic their uptake will increase even further,” Dr. Weitz said. “When we compare the FXI inhibitors with existing anticoagulants, we don’t necessarily want to go up against the DOACs – we’re looking for indications where [DOACs] have yet to be tested or may be unsafe.”
Potential indications include the following:
Prevention of major adverse cardiovascular events in patients with end-stage renal disease with or without atrial fibrillation.
Provision of a safer platform for antiplatelet therapy in patients with acute coronary syndrome.
Secondary stroke prevention.
Prevention or treatment of cancer-associated VTE.
Prevention of thrombosis associated with central venous catheters, left ventricular assist devices, or mechanical heart valves.
Agents in development
Of the FXI inhibitors in development, ISIS-FXIRx, an antisense oligonucleotide against FXI, is furthest along. In apublished in Blood, ISIS-FXIRx produced a dose-dependent and sustained reduction in FXI levels in healthy volunteers, and in a later published in The New England Journal of Medicine, it significantly reduced the incidence of DVT in patients undergoing voluntary total knee arthroplasty (30.4% with enoxaparin vs. 4.2% with ISIS-FXIRx at a dose of 300 mg). Bleeding rates were 8.3% and 2.6%, respectively.
The findings showed the potential for reducing thrombosis without increasing bleeding by targeting FXI, Dr. Weitz said, adding that ISIS-FXIRx was also evaluated in a small study of patients with end-stage renal disease undergoing hemodialysis and was shown to produce a dose-dependent reduction in FXI levels and to reduce the incidence of category 3 and 4 clotting in the air trap and dialyzer, compared with placebo, when given in addition to heparin.
This suggests that FXI knockdown can attenuate device-associated clotting to a greater extent than heparin alone, Dr. Weitz said.
The FXIa-directed inhibitory antibody osocimab has also been evaluated in both healthy volunteers and in patients undergoing total knee arthroplasty. In a 2019, a single IV injection showed a dose-dependent pharmacokinetic profile and produced FXI inhibition for about 1 month, and in the published in January in JAMA by Dr. Weitz and colleagues, osocimab was shown to reduce the incidence of symptomatic VTE, asymptomatic DVT, and VTE-related death up to day 10-13 after total knee arthroplasty.
Osocimab at doses ranging from 0.3-1.8 mg/kg given postoperatively or preoperatively were noninferior to enoxaparin (rates of 15.7%-23.7% vs. 26.3%), and osocimab at a preoperative dose of 1.8 mg/kg was superior to both enoxaparin and apixaban (11.5% vs. 26.3% and 14.5%, respectively), he said.
Bleeding rates ranged from 0%-5% with osocimab, compared with 6% with enoxaparin and 2% with apixaban
Currently ongoing studies of FXI-directed anticoagulation strategies include a study comparing ISIS-FXIRx with placebo in 200 patients with end-stage renal disease, a study comparing osocimab with placebo in 600 patients with end-stage renal disease, and a study comparing abelacimab – an antibody that binds to FXI and prevents its activation by either FXIIa or thrombin, with enoxaparin in 700 patients undergoing total knee arthroplasty, Dr. Weitz said.
Additionally, there is “considerable activity” with small molecule inhibitors of FXIa, including a phase 2, placebo-controlled, dose-ranging study looking at the novelagent for secondary stroke/transient ischemic attack prevention in 2,500 patients and a phase 2 study comparing it with enoxaparin for postoperative thromboprophylaxis in 1,200 patients undergoing total knee arthroplasty.
Parallel phase 2 studies are also underway to compare the novelsmall molecule inhibitor with placebo for stroke/transient ischemic attack prevention, with apixaban for atrial fibrillation, and for prevention of major adverse cardiovascular events in patients with acute MI.
These ongoing trials will help determine the risk-benefit profile of FXI inhibitors he said.
Session comoderator Anne Rose, PharmD, pharmacy coordinator at the University of Wisconsin, Madison, noted that these types of agents have been discussed “for quite some time” and asked whether they will be available for use in clinical practice in the near future.
Dr. Weitz predicted it will be at least a few years. The studies are just now moving to phase 2b and will still need to be evaluated in phase 3 trials and for appropriate new indications, he said.
Dr. Weitz reported research support from Canadian Institutes of Health research, Heart and Stroke Foundation, and Canadian Fund for Innovation, and he is a consultant and/or scientific advisory board member for Anthos, Bayer, Boehringer-Ingelheim, Bristol-Myers Squibb, Daiichi-Sankyo, Ionis Pharmaceuticals, Janssen, Merck, Novartis, Pfizer, Portola, Servier , and Thetherex.