---
title: "Abelacimab (MAA868): A Once-Monthly Factor XI Antibody Aiming to Prevent Clots Without the Bleeding (September 18, 2026) | PepTracker Pro Blog"
url: https://peptrackerpro.com/blog/abelacimab-maa868-anthos-novartis-fully-human-dual-factor-xi-xia-monoclonal-antibody-anticoagulant-stroke-prevention-atrial-fibrillation-azalea-timi-71-rivaroxaban-bleeding-lilac-timi-76-cancer-associated-vte-aster-apixaban-magnolia-dalteparin-hemostasis-sparing-september-18-2026
description: "Abelacimab (MAA868) is an investigational once-monthly antibody that blocks Factor XI, a clotting protein high in the coagulation cascade. The idea, grounded in human genetics, is to prevent strokes and clots while causing far less bleeding than today's blood thinners. In the Phase 2b AZALEA-TIMI 71 trial it cut bleeding so sharply versus rivaroxaban that the study was stopped early; now Phase 3 trials in anticoagulation-ineligible atrial fibrillation and cancer-associated clots must prove it prevents clots as well as it avoids bleeds."
lang: en
---

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Research & Compounds

Research & Compounds

# Abelacimab (MAA868): A Once-Monthly Factor XI Antibody Aiming to Prevent Clots Without the Bleeding (September 18, 2026)

PepTracker Pro Research Team September 18, 2026 9 min read

## Table of Contents

- A blood thinner designed to spare the bleeding
- Why Factor XI is such an appealing target
- How abelacimab works: locking Factor XI in the 'off' position
- AZALEA-TIMI 71: a bleeding result big enough to stop the trial
- The open question: does it prevent strokes as well as it prevents bleeds?
- A second front: clots in cancer
- Who is behind it, and where it sits in the field
- What to watch, and the usual caution

## A blood thinner designed to spare the bleeding

Every anticoagulant in wide use today carries the same trade-off: the same action that stops dangerous clots also makes ordinary bleeding worse. Warfarin and the newer direct oral anticoagulants (DOACs) such as apixaban and rivaroxaban prevent strokes in atrial fibrillation and treat clots in the legs and lungs, but each raises the risk of serious bleeding - and that fear keeps many people who would benefit, especially older or frail patients, from being treated at all. Abelacimab, known by its development code MAA868, is an investigational antibody built to break that trade-off. It targets Factor XI, a clotting protein that sits high in the body's coagulation 'cascade,' and it is being developed as a once-monthly injection that could prevent clots while causing far less bleeding than the drugs we use now.

## Why Factor XI is such an appealing target

The case for Factor XI comes straight from human biology. People born with a shortage of Factor XI are protected from many venous and arterial clots, yet they bleed only mildly; people missing the factor just above it, Factor XII, are protected from clots with essentially no bleeding problem at all. That pattern tells researchers something important: the 'intrinsic' branch of the clotting cascade, where Factors XII and XI live, matters a great deal for the abnormal clots behind strokes and deep-vein thrombosis but relatively little for the everyday hemostasis that seals a cut. Blocking Factor XI, then, might 'uncouple' clot prevention from bleeding risk - the reason the whole Factor XI class is described as potentially 'hemostasis-sparing.' Abelacimab is one of the most advanced tools for testing whether that promise holds in real patients.

## How abelacimab works: locking Factor XI in the 'off' position

Abelacimab is a fully human monoclonal antibody, and it is often called a 'dual' inhibitor of Factor XI and its activated form, Factor XIa. It binds the catalytic domain of Factor XI and holds the protein in its inactive, or zymogen, shape. In that locked state, Factor XI can no longer be switched on by its usual triggers - Factor XIIa or thrombin - and any activated Factor XIa is neutralized too. Because it is an antibody with a long half-life on the order of a month, a single subcutaneous injection (an intravenous loading dose is used in some settings) produces deep, steady suppression of Factor XI for weeks, a very different rhythm from the daily peaks and troughs of an oral pill. The flip side of that durability is that the effect cannot be switched off quickly, which matters for surgery, trauma or unexpected bleeding.

## AZALEA-TIMI 71: a bleeding result big enough to stop the trial

The data that put abelacimab on the map came from the Phase 2b AZALEA-TIMI 71 trial. It enrolled 1,287 patients with atrial fibrillation and moderate-to-high stroke risk and randomized them to abelacimab (150 mg or 90 mg subcutaneously once monthly) or to the standard DOAC rivaroxaban (20 mg orally once daily). In September 2023 the study's independent data-monitoring committee recommended stopping it early because of an 'overwhelming' reduction in bleeding on abelacimab. At the 150 mg dose, major or clinically relevant non-major bleeding fell by roughly 67%, major bleeding by about 74%, and major gastrointestinal bleeding by about 93% compared with rivaroxaban, and the findings were later published in the New England Journal of Medicine. It was a striking demonstration of the hemostasis-sparing idea - but with an important caveat built into the design.

## The open question: does it prevent strokes as well as it prevents bleeds?

AZALEA-TIMI 71 was a safety study focused on bleeding, not an efficacy study, so it was not large enough to prove that abelacimab prevents strokes as effectively as the anticoagulants it might replace. That is the decisive question, and it is exactly what the Phase 3 program is built to answer. LILAC-TIMI 76 (NCT05712200) is testing abelacimab against placebo in roughly 1,900 atrial-fibrillation patients who are considered unsuitable for, or unable to tolerate, conventional oral anticoagulation - a real-world group that today is often left untreated and unprotected. Its primary endpoint is ischemic stroke or systemic embolism, and because it is event-driven, results are anticipated in 2026. A positive readout would show not just that abelacimab bleeds less, but that it meaningfully protects a population with few good options.

## A second front: clots in cancer

Abelacimab is also being tested where the clotting-versus-bleeding dilemma is sharpest: cancer-associated thrombosis. Cancer patients are both more likely to form clots and more likely to bleed on standard anticoagulation, so a hemostasis-sparing drug is especially attractive. The ASTER study (NCT05171049) compares abelacimab with the oral anticoagulant apixaban, and MAGNOLIA (NCT05171075) compares it with the low-molecular-weight heparin dalteparin in gastrointestinal and genitourinary cancers; together they enroll roughly 2,700 patients. The FDA granted abelacimab Fast Track designation in this setting, and readouts are expected in 2026. These trials will test whether the antibody can cut clot recurrence while keeping bleeding low in some of the most challenging patients to treat.

## Who is behind it, and where it sits in the field

Abelacimab was developed by Anthos Therapeutics, a company launched in 2019 by Blackstone Life Sciences and Novartis. In a notable vote of confidence, Novartis reacquired Anthos in a deal that closed on April 3, 2025 - about $925 million upfront and up to roughly $3.1 billion with milestones - folding abelacimab back into its late-stage cardiovascular pipeline. Abelacimab is not alone in chasing the Factor XI target: small-molecule Factor XIa inhibitors such as asundexian and milvexian, and the antisense agent fesomersen, are pursuing the same hemostasis-sparing goal by different routes. What sets abelacimab apart is its format - a long-acting antibody given once a month - and the size of its Phase 2 bleeding signal, which made it a benchmark for the entire class.

## What to watch, and the usual caution

The next year is pivotal. The LILAC-TIMI 76 and cancer-VTE readouts expected in 2026 should reveal whether abelacimab's remarkable safety translates into real clot prevention - the result that would determine whether Factor XI inhibitors become a genuine next generation of anticoagulants or remain a promising idea. A few caveats belong alongside the optimism. Even a hemostasis-sparing anticoagulant can still cause bleeding, and there is no widely established rapid reversal agent for abelacimab, which matters for emergencies. Its long duration complicates the timing of surgery and procedures. And, importantly, abelacimab is investigational and not approved anywhere; anything sold as 'abelacimab' or 'MAA868' outside a legitimate clinical trial is unverified and unsafe. This article is background on an investigational medicine and its evidence, not medical advice - anticoagulation decisions should always be made with a qualified clinician, and no one should start, stop or change a blood thinner on their own.

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## Citations

1. [1] Abelacimab versus Rivaroxaban in Patients with Atrial Fibrillation (AZALEA-TIMI 71) - New England Journal of Medicine Source (https://www.nejm.org/doi/full/10.1056/NEJMoa2406674)
2. [2] Safety and Tolerability of Abelacimab (MAA868) vs. Rivaroxaban in Atrial Fibrillation (AZALEA-TIMI 71; NCT04755283) - ClinicalTrials.gov Source (https://clinicaltrials.gov/study/NCT04755283)
3. [3] Abelacimab in High-risk AF Patients Deemed Unsuitable for Oral Anticoagulation (LILAC-TIMI 76; NCT05712200) - ClinicalTrials.gov Source (https://clinicaltrials.gov/study/NCT05712200)
4. [4] Abelacimab vs Apixaban in Cancer-Associated VTE (ASTER; NCT05171049) - ClinicalTrials.gov Source (https://clinicaltrials.gov/study/NCT05171049)
5. [5] Abelacimab vs Dalteparin in GI/GU Cancer-Associated VTE (MAGNOLIA; NCT05171075) - ClinicalTrials.gov Source (https://clinicaltrials.gov/study/NCT05171075)
6. [6] Pharmacokinetics and Pharmacodynamics of Abelacimab (MAA868), a Novel Dual Inhibitor of Factor XI and Factor XIa - Journal of Thrombosis and Haemostasis Source (https://www.sciencedirect.com/science/article/pii/S1538783622028008)
7. [7] Novartis bolsters late-stage cardiovascular pipeline with agreement to acquire Anthos Therapeutics for USD 925 million upfront - Novartis Source (https://www.novartis.com/news/media-releases/novartis-bolsters-late-stage-cardiovascular-pipeline-agreement-acquire-anthos-therapeutics-usd-925-million-upfront)
8. [8] FDA Grants Fast Track Designation to Abelacimab for Thrombosis Associated With Cancer - OncLive Source (https://www.onclive.com/view/fda-grants-fast-track-designation-to-abelacimab-for-thrombosis-associated-with-cancer)

**Disclaimer:** This article is for educational purposes only and does not constitute medical advice. Always consult a licensed healthcare provider. Read full research disclaimer → (https://peptrackerpro.com/research-disclaimer)

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