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Testing marstacimab for treating hemophilia in children

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

Phase 3 Interventional Pfizer · NCT05611801

This study is testing a new medication called marstacimab to see if it can help children with severe hemophilia control their bleeding and improve their health.

Quick facts

PhasePhase 3
Study typeInterventional
Enrollment100 (estimated)
Ages1 Year to 17 Years
SexMale
SponsorPfizer Industry-sponsored
Drugs / interventionsemicizumab, concizumab, rituximab, marstacimab
Locations34 sites (Mendoza and 33 other locations)
Trial IDNCT05611801 on ClinicalTrials.gov

What this trial studies

This clinical trial aims to evaluate the safety and efficacy of marstacimab in pediatric patients diagnosed with severe Hemophilia A or moderately severe to severe Hemophilia B. The study will enroll participants aged 1 to 17 years in a sequential manner, starting with adolescents and then including younger children. Participants must have documented medical histories of their hemophilia treatment and bleeding events for at least one year prior to enrollment. The trial will assess the effects of the medication on bleeding control and overall health in these patients.

Who should consider this trial

Good fit: Ideal candidates are male pediatric patients aged 1 to 17 years with severe Hemophilia A or moderately severe to severe Hemophilia B.

Not a fit: Patients with mild hemophilia or those who do not meet the specific inclusion criteria may not benefit from this study.

Why it matters

Potential benefit: If successful, this treatment could significantly improve bleeding management and quality of life for children with hemophilia.

How similar studies have performed: Other studies have shown promise in using similar approaches for hemophilia treatment, indicating potential for success.

Eligibility criteria

Show full inclusion / exclusion criteria 
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 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

Exclusion Criteria:

* 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

Where this trial is running

Mendoza and 33 other locations

Study contacts

How to participate

  1. Review the eligibility criteria above with your treating physician.
  2. Visit the official trial page on ClinicalTrials.gov for the most current contact information and recruitment status.
  3. Contact the listed study coordinator or principal investigator to request pre-screening. Pre-screening is free and never obligates you to enroll.
View on ClinicalTrials.gov → Find more trials like this →
Conditions Hemophilia AHemophilia BFactor VIII InhibitorFactor IX InhibitorPF-06741086MarstacimabAnti-TFPIFactor VIII
Last reviewed 2026-06-13 by the Find a Trial editorial team. Information on this page is for educational purposes and is not medical advice. Always consult qualified healthcare professionals about clinical trial participation.