Cancer-Clot Trial for Abelacimab Terminated Before Reaching Its Primary Endpoint

The ASTER Phase 3 trial evaluating abelacimab — a fully human monoclonal antibody targeting Factor XI — versus apixaban for cancer-associated venous thromboembolism (VTE) has been terminated before reaching its primary endpoint, according to its ClinicalTrials.gov registry record (NCT05171049). No results have been posted, and neither Anthos Therapeutics nor its backer Novartis has publicly explained the reason for termination.

The Scientific Rationale

Factor XI inhibition represents a mechanistically distinct anticoagulation strategy. Factor XI sits one step upstream from the factor Xa target of apixaban and rivaroxaban, and unlike factor Xa, it plays a relatively limited role in primary hemostasis — the platelet-driven process that forms the initial clot at a wound site. The theoretical appeal is a narrower therapeutic window between anticoagulation and bleeding, particularly relevant in cancer patients who carry both high thrombotic risk and elevated bleeding risk due to malignancy, cancer-directed therapy, and procedures.

ASTER was designed to test that hypothesis specifically in a population with cancer-associated VTE, where apixaban is a guideline-recommended standard based on the ADAM-VTE and SELECT-D trials. Demonstrating non-inferiority or superiority to apixaban with a better bleeding profile would have been a significant clinical finding.

What Is and Is Not Known

The public registry shows ASTER’s status as terminated. The most common reasons for Phase 3 trial termination in the absence of a public safety signal are futility (failure to demonstrate anticipated benefit at an interim analysis), resource constraints, or a strategic program reassessment — but none of these can be confirmed from the registry record alone.

A trial termination without results is a data gap, not a negative result in the conventional sense. It removes one anticipated data source from the Factor XI inhibitor field in cancer-associated VTE — a population counting on the trial to clarify whether the mechanistic promise of Factor XI inhibition holds in this high-risk group.

Abelacimab continues in other trials, including AZALEA-TIMI 71 (NCT04755283) in atrial fibrillation, suggesting the broader program has not been abandoned. The cancer-associated VTE indication appears to have been discontinued.

NCT05171049 (ASTER, terminated). Anthos Therapeutics and Novartis abelacimab program. NCT04755283 (AZALEA-TIMI 71, active).