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Stroke and Systemic Embolism Prevention with REGN7508 and REGN9933 in Adults with Atrial Fibrillation Unsuitable for Oral Anticoagulation (Phase 3)

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What is this trial about?

A plain-language summary of the goals, design and what participants do

The study focuses on adults with Atrial Fibrillation, an irregular heart rhythm that raises the chance of blood clots forming in the heart. When clots travel to the brain they can cause an ischemic stroke, and when they travel to other parts of the body they can cause a systemic embolism. Some patients cannot use standard oral blood‑thinning medicines, so a new approach is being tested using two laboratory‑made antibodies that block a protein called Factor XI. The two test medicines are identified as REGN7508 and REGN9933, and they are given by injection. A control group will receive a placebo that looks the same but does not contain the active antibodies.

The purpose of the study is to determine whether the two antibodies can lower the risk of stroke, clot‑related events, and major bleeding compared with the placebo.

Participants are assigned at random to receive either one of the study medicines or the placebo and will receive a series of injections over several months. Throughout the study they will attend regular clinic visits where blood samples are taken and basic health checks are performed to watch for any side effects or problems. The trial continues until enough time has passed to see whether the medicines have an effect on the occurrence of clot‑related events.

The research process

The trial runs in 6 steps – from screening to follow-up. Each step says what happens and what the team monitors.

  1. Step 1

    Baseline visit and randomization

    After joining the study, you will attend a baseline visit where health information and test results are collected.

    Based on the study design, you will be randomly assigned to receive either the active medicines regn7508 and regn9933 or a matching placebo. the assignment is unknown to you and the study staff.

  2. Step 2

    First administration of study medication

    The assigned medication is given by injection. the product is a powder that is mixed with a solution to create an injectable form.

    Each dose contains 00 mg of the active substance (the exact amount is defined by the study protocol). the injection is performed in a clinic by trained staff.

  3. Step 3

    Regular dosing visits

    You will return to the clinic at predetermined intervals for additional injections of the same medication or placebo.

    The frequency and total duration of the injections follow the study schedule, which continues for the length of the trial unless you stop early.

  4. Step 4

    Routine safety and efficacy assessments

    At each visit, you will have blood samples taken to measure drug levels and to check clotting tests such as activated partial thromboplastin time (aptt) and prothrombin time (pt)/international normalized ratio (inr).

    You will also be asked about any new symptoms, especially signs of stroke, systemic embolism, or major bleeding.

  5. Step 5

    Monitoring for adverse events

    Throughout the trial, any treatment‑emergent adverse events (teae) are recorded, including their severity.

    If an adverse event occurs, you will be evaluated by the study team to determine whether the study medication should be continued.

  6. Step 6

    Study completion or early discontinuation

    The trial continues until the final scheduled visit, which may be up to several years after the first dose, or until you withdraw or are withdrawn for safety reasons.

    At the end of participation, a final assessment is performed and you receive information about the study results.

Who can join the trial?

4 criteria

  • Must have Atrial Fibrillation (AF) or atrial flutter, which are irregular heart rhythms; the condition should not be caused by something that can be fixed, and it can be occasional (paroxysmal) or ongoing (persistent).
  • Must be at moderate to high risk for stroke, measured with the CHA2DS2-VA score—a tool that adds points for heart failure, high blood pressure, age, diabetes, previous stroke, and blood‑vessel disease. A total score of 4 or higher meets the requirement, or a score of 3 plus at least one additional factor listed in the study.
  • Must and the doctor agree that the risk of bleeding from a blood‑thinning medicine called an Oral Anticoagulant (OAC) is greater than its benefit, making the patient unsuitable for those medicines.
  • Must meet any other specific requirements that are described in the study protocol.

Who cannot join the trial?

7 criteria

  • Having a mechanical heart valve prosthesis (an artificial heart valve made of metal or plastic) – note that a transcatheter aortic valve replacement does not count as an exclusion.
  • Having an ischemic stroke (a stroke caused by a blockage in a blood vessel in the brain) within 2 days before the study starts.
  • Having persistent, uncontrolled hypertension (high blood pressure that is not well controlled) as judged by the doctor.
  • Having a history of Central Nervous System (CNS) bleeding (bleeding in the brain or spinal cord) within 30 days before the study starts.
  • Having a life expectancy of less than 12 months (expected to live less than one year).
  • Having taken part in a previous Factor XI (FXI) inhibitor study (a study using a medicine that blocks a protein involved in blood clotting).
  • Any other reasons listed as protocol‑defined exclusion criteria in the study plan.
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Investigated drugs

  • REGN7508

    is a laboratory-made protein (monoclonal antibody) that targets a blood clotting factor called Factor XI. By blocking this factor, the medication aims to reduce the chance of blood clots that can cause strokes or other serious blockages in people with atrial fibrillation who cannot take regular oral blood thinners. It is given as an injection that is prepared from a powder mixed with liquid right before use.

  • REGN9933

    is another laboratory-made protein (monoclonal antibody) that also blocks Factor XI in the blood. Like REGN7508, its purpose is to lower the risk of strokes and other clot‑related events in patients with atrial fibrillation who are not suitable for standard oral anticoagulant medicines. This medication is also administered by injection, using a powder that is reconstituted into a solution before it is given.

What is already known about the treatment

  • REGN7508

    This medication is given by injection after the powder is mixed with liquid to form a solution for intravenous infusion. It is currently in an investigational phase 3 clinical trial and is not yet approved for general medical use. The drug is being studied to help prevent strokes and dangerous blood clots in people with atrial fibrillation who cannot take oral blood thinners. It works by using a laboratory‑made antibody that attaches to a protein called Factor XI, blocking its activity and reducing the formation of clots; it is classified as a Factor XI‑targeting anticoagulant monoclonal antibody.

  • REGN9933

    This medication is also administered by injection, using a powder that is reconstituted into a solution for intravenous infusion. It is an investigational product in a phase 3 trial and has not received market approval. The drug is being evaluated to lower the risk of ischemic stroke and systemic embolism in atrial fibrillation patients who cannot use oral anticoagulants. Its action involves a monoclonal antibody that binds to Factor XI, inhibiting the clotting cascade, and it belongs to the class of Factor XI inhibitor anticoagulant antibodies.

Investigated diseases

Atrial Fibrillation - A condition where the heart’s upper chambers beat irregularly and often rapidly. This irregular rhythm can cause the chambers to fill incompletely, leading to inefficient blood flow. Over time, the irregular beats may become more frequent or persist longer, and the heart may enlarge as it works harder. The condition can progress from occasional episodes to continuous (persistent) forms. It may also be associated with changes in the electrical pathways of the heart.
Trial detailsLast updated 7 Oct 2026
Age18+ yearsPhasePhase IIITrial ID2025-521618-25-00Protocol codeR9933-CVA-2495Estimated enrolment2 647 patientsSponsorRegeneron Pharmaceuticals Inc.

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