A Study to Assess the Safety of Marstacimab and How Well it Works in People With Mild to Moderate Hemophilia A or B Without Inhibitors

Inclusion Criteria

* Male or female, 12 to less than 75 years of age and weighing at least 35 kg.
* Moderate hemophilia A or B (factor VIII or factor IX activity ≥1% to ≤5%), or Mild hemophilia A or B (factor VIII or factor IX activity \>5% to ≤15%).
* Females with severe hemophilia A or B may be eligible on a case-by-case basis following discussion with sponsor.
* No history of factor VIII or factor IX inhibitors within the 5 years before screening.
* At least 12 months of medical records and/or bleeding diary data documenting hemophilia-related bleeding episodes and factor VIII or factor IX replacement therapy prior to screening.
* Participants receiving on-demand factor replacement therapy must have had at least 2 treated bleeding episodes during the 12 months before screening (surgical or procedural bleeds do not count).
* Participants receiving routine prophylactic factor replacement therapy must have received a stable prophylaxis regimen for at least 12 months with ≥80% compliance and at least 1 treated bleeding episode in the 12 months before screening (excluding surgical or procedural bleeds).

Exclusion Criteria

* Current evidence of a congenital or acquired thrombophilic disorder, including antithrombin III deficiency, Factor V Leiden mutation, prothrombin G20210A mutation, protein C deficiency, protein S deficiency, or antiphospholipid syndrome.
* Known coronary artery disease, history of venous or arterial thrombosis, or other clinically significant ischemic disease.
* Any bleeding disorder other than hemophilia A or hemophilia B.
* Any medical, psychiatric, or laboratory abnormality that, in the investigator's opinion, could increase risk or interfere with study participation.
* Current use of bypassing agents, non-factor replacement therapies for hemophilia, or prior hemophilia gene therapy.
* Regular treatment with immunomodulatory medications.
* Use of prohibited medications, including systemic antifibrinolytic agents, medications that increase bleeding risk, or certain nonsteroidal anti-inflammatory drugs (NSAIDs), as specified by the protocol.
* Use of estrogen-containing hormonal therapies (including contraceptives and HRTs) within 30 days before or during the study, or depot medroxyprogesterone acetate within 6 months before or during the study.
* Receipt of another investigational drug or vaccine within 30 days or 5 half-lives before study treatment (whichever is longer), or planned participation in another investigational study during this study.
* Significant renal or hepatic impairment or clinically significant laboratory abnormalities at screening.
* Platelet count \<100,000/μL, hemoglobin \<10 g/dL (with limited exceptions), fibrinogen below the lower limit of normal, or prolonged prothrombin time.
* CD4 count ≤200 cells/μL in participants with HIV infection.
* Clinically significant electrocardiogram abnormalities at screening.
* Planned surgical procedure during the study period.
* Known hypersensitivity to hamster protein or any component of the study treatment.
* Study site personnel, sponsor personnel directly involved in the conduct of the study, or their immediate family members.