Peak Thrombin and D-Dimer Levels in Subjects with Severe Hemophilia Receiving Acute Treatment for Bleeding Episodes Experienced during Prophylactic Marstacimab Treatment

Introduction: Marstacimab is a fully humanized monoclonal immunoglobulin G1 that targets the shared K2 domains of tissue factor pathway inhibitor (TFPI)α and (TFPI)β and is currently in phase 3 development. The intended indication is routine prophylaxis treatment to prevent or reduce the frequency of bleeding episodes in patients with hemophilia A or B (with or without inhibitors). Factor replacement or bypass treatment for bleeding events may lead to increased levels of peak thrombin and D-dimer associated with an increased risk of thrombosis . In this post hoc analysis of data from a phase 2 study in patients with hemophilia with and without bleeding episodes, receiving prophylactic marstacimab treatment, peak thrombin and D-dimer levels were investigated to assess the changes in these biomarker levels observed after bleeding episodes.

Methods: Individual subject data from the phase 2 study (clinicaltrials.gov identifier: NCT02974855) were used for this analysis. Biomarker data for healthy volunteers who received single doses of marstacimab in a phase 1 dose escalation study (clinicaltrials.gov identifier: NCT02531815) were used as control data, as these subjects represent an intact and uncompromised coagulation system. Study subjects in the phase 2 study received subcutaneous (SC) marstacimab at doses of (1) 150 mg once weekly (QW), with a loading dose of 300 mg, (2) 300 mg QW, and (3) 450 mg QW. All subjects with bleeding episodes were identified, along with on-demand treatment administered for each bleeding episode. Treatments permitted for bleeding episodes included activated coagulation factor VIIa, factor VIII, or factor IX; use of activated prothrombin complex concentrate was prohibited. D-dimer and peak thrombin data collected within 3 days after each bleeding episode were used for this analysis. Time profiles of peak thrombin and D-dimer levels were analyzed to assess the effect of bleed treatment. Biomarker profiles were compared between subjects with and without bleeding episodes, as well as with the data from healthy volunteers (n=41).

Results: A total of 15 bleeding episodes were reported in 8 of 26 subjects during the study (excluding screening and follow-up). No subject participating in the study showed any relevant increases in D-dimer levels after receiving on-demand treatment for a bleeding episode while receiving regular prophylaxis with marstacimab, compared with levels seen in subjects who did not experience a bleeding episode. Based on the peak thrombin data (see Figure), 150 nM was observed as the upper limit for 18 of 26 subjects who did not experience any bleeding episodes, which was approximately 50% of the 300 nM observed in healthy volunteer controls treated with 450 mg intravenous marstacimab. Transient increases in peak thrombin of >150 nM were observed at several time points in 3 of 8 subjects who experienced bleeding episodes. The highest peak thrombin level reported was approximately 211 nM in one subject receiving marstacimab 300 mg SC QW and factor VIII concentrate on demand during the study.

Conclusions: No transient increases in D-dimer could be attributed to the administration of bleeding episode treatment. The transient increases in peak thrombin levels following on-demand treatment for bleeding episodes did not exceed peak thrombin levels seen in subjects without bleeding events or the levels seen in healthy volunteer controls receiving single doses of marstacimab. Based on peak thrombin and D-dimer levels observed in this post hoc analysis, there does not appear to be any indication of an increased risk of thrombosis post administration of acute on-demand bleeding episode treatment while on prophylactic marstacimab therapy at the doses studied.