A Clinical Trial of Study Medicine (Marstacimab) in Pediatric Patients With Hemophilia A or Hemophilia B

Inclusion Criteria:

• 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.

• Participants aged 6 to 11 years must be at least 19 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 severehemophilia B

• Participants must have at least 1 year of diary and/or medical records available inwhich exogenous FVIII or FIX replacement or bypass agent infusions and hemophilicbleeding episodes were consistently documented over the 12 months prior to the timeof 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 5years 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 prophylaxisregimen 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 titerinhibitor (<5 BU/mL) refractory to FVIII or FIX replacement and with FVIII or FIXrecovery <60% of expected within previous 12 months prior to the time of consent

• Participants who have documented inhibitors while on factor-replacement therapy butwho do not meet the high quantitative inhibitor criteria described in the priorbullet at the time of screening (eg, participant with a previously documentedhigh-titer inhibitor ≥5 BU/mL) and whose condition precludes re-challenge with FVIIIor FIX replacement may be considered for eligibility on a case-by-case basis withdiscussion and agreement from the Pfizer medical monitor.

• Hemophilia A participants with on-demand treatment regimen with ≥12 bleedingepisodes or hemophilia B participants with on-demand treatment regimen with ≥8bleeding episodes (spontaneous or traumatic) necessitating treatment with bypassfactor in the 12 months prior to informed consent

• Participants must be on an on-demand bypass treatment regimen during the 12 monthsprior to informed consent

Exclusion Criteria:

• Known coronary artery, thrombotic, or ischemic disease, or current evidence ofcongenital or acquired thrombophilic disease such as Anti-thrombin III deficiency,Factor V Leiden mutation, prothrombin 20210 mutation, protein C deficiency, proteinS deficiency and antiphospholipid syndrome.

• Known planned surgical procedure during the planned study period

• Known hemostatic defect other than hemophilia A or B

• Abnormal hematology, renal or hepatic function laboratory results at screening

• Other acute or chronic medical or psychiatric condition that may increase the riskassociated with study participation or investigational product administration or mayinterfere with the interpretation of study results and, in the judgment of theinvestigator

• Individuals with known allergic reaction or hypersensitivity to hamster protein orother components of the study intervention

• Current routine prophylaxis with bypassing agent, non-coagulation non-factorreplacement therapy (eg, emicizumab), or any previous treatment with a gene therapyproduct for treatment of hemophilia

• Participants with inhibitors who are being treated using a prophylaxis treatmentregimen with a bypass agent, and, participants who have previously receivednon-factor-based hemophilia therapy (eg, fitusiran, concizumab, emicizumab) will beconsidered on a case-by-case basis, only after discussion and agreement between theinvestigator and the Pfizer medical monitor

• Regular use of immunomodulatory medications (eg, IVIG, routine systemiccorticosteroids, rituximab)

• Use of systemic antifibrinolytics, medications that may increase the risk ofbleeding, and certain non-steroidal anti-inflammatory drugs within 120 hours offirst dose of study intervention and while on study

• Ongoing or planned use of ITI, or prophylaxis with FVIII or FIX replacement at anytime after initiation of treatment with study intervention

• Participation in other studies involving investigational drug(s) or investigationalvaccine(s) within 30 days (or as determined by local requirements) or 5 half-livesprior to study entry or during study participation

• Previous exposure to marstacimab during participation in other marstacimab clinicalstudies

• CD4 cell count ≤200/uL if HIV-positive

• Abnormal ECG of clinical relevance that may affect participant safety orinterpretation of study results

• Investigator site staff members directly involved in the conduct of the study andtheir family members, site staff members otherwise supervised by the investigator,and sponsor and sponsor delegate employees directly involved in the conduct of thestudy and their family members