Marstacimab for Pediatric Hemophilia A or B
This study is testing a medicine called marstacimab to see if it is safe and effective for children and teenagers with hemophilia A or hemophilia B. Researchers want to understand how marstacimab affects bleeding rates and if it causes any side effects, including blood clots. The study plans to enroll about 100 male participants between 1 and 17 years old who have severe hemophilia A or moderately severe to severe hemophilia B. Enrollment will happen in stages, starting with older teenagers, then younger children, and finally the youngest children. To be considered, participants must have accurate records of their past treatments and bleeding events for at least one year.
- Study design
- This is an interventional study planning to enroll 100 participants. It will enroll participants in a sequential manner, starting with older age groups.
- What's involved
- Not specified in the trial record.
- Compensation
- Not stated in the trial record.
- Follow-up
- Participants will be followed for safety for approximately 14 months after the study treatment period ends.
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NCT05611801
A Clinical Trial of Study Medicine (Marstacimab) in Pediatric Patients With Hemophilia A or Hemophilia B
Recruiting
PHASE3Ages 1–17InterventionalTreatmentPfizer
~100 participants
Updated 2026-08-10 on ClinicalTrials.gov
What's tested:marstacimab
At a glance
Recruiting sites
60 of 64 listed sites are recruiting right now
RecruitingSuspended, closed, or not yet open
What they're measuring
Annualized bleeding rate (ABR) of treated bleeding events
Measured over Baseline to end of 12-month treatment period
+7 more outcomes measured
Conditions
Where it's being run
64 sites across 46 statesTurkey (Türkiye)8
United Kingdom4
Hubei2
Czechia2
Denmark2
Italy2
Slovakia2
Gauteng2
Study leadership
- Pfizer CT.gov Call Center · STUDY_DIRECTOR · Pfizer
Who to contact
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Do you actually qualify for this trial?
Add a private profile and we'll compare every criterion below against your situation — and tell you which ones are met, uncertain, or excluding.
Eligibility criteria
Inclusion
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 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.
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
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
Exclusion
Known coronary artery, thrombotic, or ischemic disease, or current evidence of congenital or acquired thrombophilic disease such as Anti-thrombin III deficiency, Factor V Leiden mutation, prothrombin 20210 mutation, protein C deficiency, protein S 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 risk associated with study participation or investigational product administration or may interfere with the interpretation of study results and, in the judgment of the investigator
Individuals with known allergic reaction or hypersensitivity to hamster protein or other components of the study intervention
Current routine prophylaxis with bypassing agent, non-coagulation non-factor replacement therapy (eg, emicizumab), or any previous treatment with a gene therapy product for treatment of hemophilia
Participants with inhibitors who are being treated using a prophylaxis treatment regimen with a bypass agent, and, participants who have previously received non-factor-based hemophilia therapy (eg, fitusiran, concizumab, emicizumab) will be considered on a case-by-case basis, only after discussion and agreement between the investigator and the Pfizer medical monitor
Regular use of immunomodulatory medications (eg, IVIG, routine systemic corticosteroids, rituximab)
Use of systemic antifibrinolytics, medications that may increase the risk of bleeding, and certain non-steroidal anti-inflammatory drugs within 120 hours of first dose of study intervention and while on study
Ongoing or planned use of ITI, or prophylaxis with FVIII or FIX replacement at any time after initiation of treatment with study intervention
Participation in other studies involving investigational drug(s) or investigational vaccine(s) within 30 days (or as determined by local requirements) or 5 half-lives prior to study entry or during study participation
Previous exposure to marstacimab during participation in other marstacimab clinical studies
CD4 cell count ≤200/uL if HIV-positive
Abnormal ECG of clinical relevance that may affect participant safety or interpretation of study results
Investigator site staff members directly involved in the conduct of the study and their family members, site staff members otherwise supervised by the investigator, and sponsor and sponsor delegate employees directly involved in the conduct of the study and their family members
What this trial measures
- Annualized bleeding rate (ABR) of treated bleeding eventsBaseline to end of 12-month treatment period
Derived for each subject for each period (historical and study treatment) by using the following formula: ABR = number of bleeds requiring treatments/ (days on treatment period / 365.25)
- Incidence of adverse events and serious adverse eventsScreening through end of follow-up period (approximately 14 months)
- Incidence and severity of thrombotic eventsBaseline to end of 12-month treatment period
- Incidence and severity of thrombotic microangiopathyBaseline to end of 12-month treatment period
- Incidence and severity of disseminated intravascular coagulation/consumption coagulopathy eventsBaseline to end of 12-month treatment period
- Immunogenicity (incidence of ADA and clinically significant persistent NAb against marstacimab)Baseline to end of 12-month treatment period
- Incidence and severity of injection site reactionBaseline to end of 12-month treatment period
- Incidence of severe hypersensitivity and anaphylactic reactionsBaseline to end of 12-month treatment period