NCT07923981RecruitingPhase 3

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

AN OPEN-LABEL STUDY TO EVALUATE THE EFFICACY AND SAFETY OF MARSTACIMAB PROPHYLAXIS IN ADOLESCENT AND ADULT PARTICIPANTS WITH MILD (FVIII OR FIX ACTIVITY >5% TO ≤15%) TO MODERATE (≥1% TO ≤5%) HEMOPHILIA A OR B WITHOUT INHIBITORS

Source: ClinicalTrials.gov · Last updated 9 Nov 2026

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Brief summary (registry, in English)

This is an open-label, multicenter study evaluating the efficacy and safety of marstacimab prophylaxis in approximately 95 male and female adolescents and adults aged 12 to <75 years with mild to moderate hemophilia A or B without inhibitors who have a severe bleeding phenotype. Eligible participants must have baseline factor VIII or factor IX activity levels >1% to ≤15% and a history of clinically significant bleeding despite prior treatment with either on-demand factor replacement therapy or routine factor prophylaxis. The study will compare bleeding outcomes and other efficacy measures during a historical 12-month period of prescribed factor replacement therapy prior to study entry with outcomes observed during a 12-month treatment phase in which participants receive marstacimab 150 mg administered subcutaneously once weekly. Participants will be enrolled into 2 cohorts based on their treatment regimen before study entry: a prior on-demand treatment cohort and a prior routine prophylaxis treatment cohort. Female participants with severe hemophilia A or B without inhibitors may also be enrolled on a case-by-case basis. Data from these participants will contribute to safety assessments, and separate descriptive analyses of efficacy outcomes will be performed for all female participants. The study will also evaluate the long-term safety and efficacy of marstacimab during a long-term extension (LTE) treatment phase of up to 36 months. Participants who complete the 12-month treatment phase may continue into the LTE phase. Participants may be in the study for approximately 50 months, including a 12-month treatment phase and a long-term extension phase of up to 36 months.

Status (registry): Recruiting
Phase (registry): Phase 3
Study type: Interventional
Enrollment: 95 (estimated)
First submitted: 21 Oct 2026
Start date: 23 Oct 2026
Primary completion date: 2 Jul 2029
Lead sponsor: Pfizer
Diseases: Haemophilia
NCT number: NCT07923981

What you commit to

Study type: Interventional · Phase 3
Planned participants: 95
From 23 Oct 2026 to the primary completion date, 2 Jul 2029
1 site in 1 country
Visits and tests: ask the trial team, the registry does not list them

Built from the registry fields of ClinicalTrials.gov. It describes the study design.

AI-kivonat: who the study is looking for

Machine-read from the registry text on this page, so it can be wrong. The registry text below and the trial team decide who can take part.

The trial is looking for people aged 12 to under 75 with mild or moderate hemophilia A or B who have no inhibitors and have had bleeds despite factor replacement treatment.

Age: 12 to under 75 years; weight at least 35 kg
Sex: Males and females. Females with severe hemophilia A or B may join case by case after talking with the sponsor.
Disease stage: Mild hemophilia A or B (factor VIII or factor IX activity above 5% up to 15%) or moderate (1% to 5%).
Earlier treatment: At least 12 months of records of bleeds and factor replacement treatment. On-demand group: at least 2 treated bleeds in the 12 months before screening. Routine prophylaxis group: a stable regimen for at least 12 months with 80% or more compliance and at least 1 treated bleed.
Biomarker or test result: No factor VIII or factor IX inhibitors in the 5 years before screening.

Main reasons for not taking part

Clotting disorder that raises clot risk, or past blood clot, coronary artery disease or other significant ischemic disease
Any bleeding disorder other than hemophilia A or B
Use of bypassing agents, non-factor therapies or prior hemophilia gene therapy
CD4 count of 200 cells/μL or lower in people with HIV
Planned surgery during the study

Read by machine on 10 Nov 2026.

Who cannot take part

Copied verbatim from ClinicalTrials.gov and grouped by topic. Only the trial team decides who can take part.

Prior treatment

Current use of bypassing agents, non-factor replacement therapies for hemophilia, or prior hemophilia gene therapy.
Regular treatment with immunomodulatory medications.
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.
Planned surgical procedure during the study period.

Other illness

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.
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.
Known hypersensitivity to hamster protein or any component of the study treatment.

Lab values

Any medical, psychiatric, or laboratory abnormality that, in the investigator's opinion, could increase risk or interfere with study participation.
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.

Other

Clinically significant electrocardiogram abnormalities at screening.
Study site personnel, sponsor personnel directly involved in the conduct of the study, or their immediate family members.

Who the study is looking for

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).

Age in the registry: 12 years to 74 years · Sex: All · Does not accept healthy volunteers

Full criteria on ClinicalTrials.gov

Sites (1)

Sites in registry order. Save your city in your profile to sort them by distance.

Site	City	Country	Site status
Southern Specialty Clinic	Flowood	United States	Recruiting

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