“In the long-term extension study treatment with marstacimab demonstrates sustained or improved efficacy for treated and total annualized bleeding rates (ABR) in adults and adolescents with hemophilia A or hemophilia B in this data set of patients without inhibitors,” first author Shamsah Kazani, MD, of Pfizer, Cambridge, Massachusetts, said in presenting the findings at the 2024 annual meeting of the European Hematology Association (EHA) in Madrid.
“The majority of the patients from the pivotal study chose to transition into the long-term extension, and we are finding that these patients are highly compliant with their weekly marstacimab dose, with more than 98% compliance,” Dr. Kazani said.
Marstacimab targets the tissue factor pathway inhibitor, a natural anticoagulation protein that prevents the formation of blood clots, and is administered as a once-weekly subcutaneous injection.
The therapy has been granted fast-track and orphan drug status in the United States, in addition to orphan drug status in the European Union for the prevention of hemophilia bleeding episodes.
If approved, the therapy would become the first once-weekly subcutaneous therapy for either hemophilia A or B. Emicizumab, which also is administered subcutaneously, is only approved to prevent or reduce bleeding in hemophilia A.
The latest findings are from an interim analysis of a long-term extension study involving 107 of 116 patients who were in the non-inhibitor cohort in the pivotal BASIS trial. Data from that trial, involving patients aged 12-75 previously showed favorable outcomes in the non-inhibitor cohort receiving marstacimab, and a cohort of patients with inhibitors is ongoing.
Participants entering the extension study were continuing on 150-mg subcutaneous doses of marstacimab, which had been administered in the BASIS study for 12 months after a loading dose of 300 mg.
Of the patients, 89 (83%) were adult and 18 (17%) were adolescents. Overall, they had a mean age of 29 years; 83 (76%) patients had hemophilia A, while 24 (22.4%) had hemophilia B.
Prior to switching to marstacimab treatment, 32 patients had been treated with factor replacement therapy on demand, while 75 received the therapy as routine prophylaxis.
With a mean additional duration of follow-up of 12.5 months in the extension study (range, 1-23.1 months), the overall rate of compliance was very high, at 98.9%.
In the pivotal and extension studies combined, 21% of patients had their marstacimab dose increased from 150 mg to 300 mg weekly, which was an option if patients had 2 or more spontaneous bleeds in a major joint while on the 150-mg dose.
In the hemophilia A and B groups combined, those previously treated with on-demand factor replacement therapy (n = 33) had substantial reductions in estimated ABR for treated bleeds from the baseline of 38.0 prior to initiating marstacimab, to 3.2 after 12 months of the treatment in the trial (P < .001).That reduction was sustained at an ABR of 3.7 after the mean additional 12.5 months in the extension study.
The corresponding estimated ABR rates in the routine prophylaxis group (n = 83) were 7.9 at baseline, 5.1 at the end of the trial, and 2.8 in the extension study analysis interim cutoff.
The authors then further stratified the results based on hemophilia A or B groups: Among patients with hemophilia A (n = 26), the on-demand subgroup had a baseline ABR of 40.6, which dropped substantially to just 3.6 after 12 months on marstacimab in the pivotal trial and was sustained at 5.3 in the extension study.
Similar trends were observed in the hemophilia A group who received routine prophylaxis (n = 65), with an ABR of 9.2 at baseline; 5.3 after the trial, and 3.1 at the extension study interim.
The trends were similar among those with hemophilia B, albeit with lower numbers of patients, consistent with hemophilia B being more rare.
The mean ABR at baseline in the on-demand group of those patients (n = 7) was 28.7, which was reduced to just 1.7 after the 12-months of active marstacimab treatment and sustained at 1.8 by the interim analysis of the extension study.
Of hemophilia B patients previously on routine prophylaxis (n = 18), the mean ABR at baseline was 3.3 and was at 4.7 at the end of the trial. The rate declined to 2.3 in the extension phase.
“We see that these trends of improvement with marstacimab are sustained into the long-term extension study, both in the on-demand group and in the routine prophylaxis groups,” Dr. Kazani said.
Importantly, she noted that marstacimab continued to be well tolerated and safe in the long-term extension study, with no reports of thromboembolic events, which had been a concern with the drug.
Commenting on the study, Margaret Ragni, MD, MPH, a professor of medicine and clinical and translational research in the Division of Hematology/Oncology at the University of Pittsburgh, Pittsburgh, Pennsylvania, noted that marstacimab could represent an important addition in the prevention of bleeds in hemophilia. “[If marstacimab is approved], hemophilia B patients [will] have a drug that can be given subcutaneously weekly to rebalance hemostasis, reducing bleeds, just as hemophilia A patients have with emicizumab.”
Dr. Ragni underscored, however, that caveats include the important point that “neither [marstacimab nor emicizumab] treats bleeds. For that, standard factor replacement therapy or bypass for inhibitors, would be required.”
Also, “a limitation with marstacimab is the lack of weight-dependent dosing. All use one dose [however, in the studies they did use 150 mg or 300 mg]. ... Furthermore, emicizumab can be given weekly, biweekly, or monthly, while that [variation in dosing] is not yet studied with marstacimab.”
And while no thromboembolic events occurred during the trial, Dr. Ragni underscored that “longer-term follow-up is needed.”
The marstacimab long-term extension study is designed to extend to 7 years of follow-up.
The study was sponsored by Pfizer, and Dr. Kazani is an employee of Pfizer. Dr. Ragni reported no disclosures.