AN OPEN-LABEL STUDY IN PEDIATRIC (<18 YEARS OF AGE), SEVERE HEMOPHILIA A PARTICIPANTS (COAGULATION FACTOR ACTIVITY <1%) WITH OR WITHOUT INHIBITORS OR MODERATELY SEVERE TO SEVERE HEMOPHILIA B PARTICIPANTS (COAGULATION FACTOR ACTIVITY ≤2%) WITH OR WITHOUT INHIBITORS COMPARING 12 MONTHS OF HISTORICAL STANDARD TREATMENT TO MARSTACIMAB PROPHYLAXIS
The purpose of this clinical trial is to learn about the safety and effects of the study medicine (called marstacimab) for the potential treatment of hemophilia in pediatric patients. This study will enroll pediatric participants from ages 1 to 17 years in a sequential manner. The study will open enrollment to adolescent participants aged 12 to 17 years first. Then children aged 6 to 11 years will be permitted to enroll. Lastly, children aged 1 to 5 years will be permitted to enroll. This study will enroll participants who: * have severe Hemophilia A or moderately severe to severe Hemophilia B (with or without inhibitors) * have accurate historical records documenting all factor VIII, factor IX, or bypass agent infusions and hemophilia bleed events for at least 1 year prior to entering the study * if a non-inhibitor patient, must be on a stable routine prophylaxis regimen with factor VIII or factor IX replacement products for at least 12 months prior to study entry * if an inhibitor patient, must be on an on-demand bypass treatment regimen during the 12 months prior to study entry All participants in this study will receive marstacimab to use prophylactically. Marstacimab will be given once a week as a subcutaneous (under the skin) shot. The first dose of marstacimab will be given at the study site by the study site staff. During the 12-month treatment period, weekly doses of marstacimab can be given at home, or if preferred, the doses may be given by the study site staff. To help us determine if the study medicine is safe and effective, we will compare participant experiences when they are taking the study medicine to a historical period when they were not. Researchers want to see if the study medicine works to prevent the bleeding episodes commonly experienced by patients with Hemophilia. Participants will be in this study for about 14 months (approximately 1 month in a Screening period, 12 months receiving treatment, and 1 month in a follow-up period) during which they will visit the study site at least 10 times. If preferred, and if local regulations allow it, 2 of the study visits can be completed at the participant's home instead of at the study site. There will also be 6 scheduled telephone calls approximately every 2 months.
• Male participants of appropriate age and required minimum weight
• Participants aged 12 to 17 years must be at least 25 kgs at time of consent.
• Minimum weight requirement for participants aged 1 to 5 years is to be determined.
• Participants with a diagnosis of severe hemophilia A or moderately severe to severe hemophilia B
• Participants must have at least 1 year of diary and/or medical records available in which exogenous FVIII or FIX replacement or bypass agent infusions and hemophilic bleeding episodes were consistently documented over the 12 months prior to the time of consent.
⁃ Participants who are enrolled into the Non-Inhibitor Cohort must also meet the following criteria:
• No current detectable inhibitor and no documented history of inhibitors in the 5 years prior to consent
• Must have at least 50 exposure days to FVIII/FIX replacement products
• Must be at least 80% compliant with a stable and effective routine prophylaxis regimen with FVIII/FIX replacement products, for at least 12 months prior to consent
⁃ Participants who are enrolled into the Inhibitor Cohort must also meet the following criteria:
• Documentation of current high titer inhibitor (≥5 BU/mL); or current low titer inhibitor (\<5 BU/mL) refractory to FVIII or FIX replacement and with FVIII or FIX recovery \<60% of expected within previous 12 months prior to the time of consent
• Participants who have documented inhibitors while on factor-replacement therapy but who do not meet the high quantitative inhibitor criteria described in the prior bullet at the time of screening (eg, participant with a previously documented high-titer inhibitor ≥5 BU/mL) and whose condition precludes re-challenge with FVIII or FIX replacement may be considered for eligibility on a case-by-case basis with discussion and agreement from the Pfizer medical monitor.
• Hemophilia A participants with on-demand treatment regimen with ≥12 bleeding episodes or hemophilia B participants with on-demand treatment regimen with ≥8 bleeding episodes (spontaneous or traumatic) necessitating treatment with bypass factor in the 12 months prior to informed consent
• Participants must be on an on-demand bypass treatment regimen during the 12 months prior to informed consent