A Phase 3 interventional study of marstacimab in Hemophilia A or B, sponsored by Pfizer. Recruiting at 1 site in United States. Open to participants aged 12 Years to 74 Years. Per ClinicalTrials.gov, last updated 2026-11-09.
Sponsored by Pfizer · Phase 3, Interventional, and Treatment
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.
844 studies on the registry are indexed under Hemophilia A; 138 are open to participants now.
This study's planned enrollment of 95 is above the median of 30 across 499 interventional studies indexed under Hemophilia A.
Browse Hemophilia A studies →Pfizer is the lead sponsor of 3,334 studies on the registry; 149 are open to participants now.
Of its 601 completed or terminated interventional studies of FDA-regulated products, 391 (65%) have results posted.
Counted across the registry records on this site, refreshed daily.
Go through the criteria one by one — yes, no or not sure — and take the list to your doctor or the study team. Kept in this browser only.
Exclusion Criteria
Underlined terms in the criteria open these too. Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Judged by the study doctor to matter for your health or for the study. It is a doctor’s decision, so ask the study team if you are unsure whether something counts.
Ways of preventing pregnancy. “Highly effective” methods fail less than 1% of the time a year when used correctly — for example an IUD or an implant.
Criteria usually say which methods count and how long to keep using them after the last dose, sometimes for male participants too. Some studies accept a partner’s vasectomy if it was confirmed successful and that person is the participant’s only sexual partner.
A drug or device being studied in a clinical trial.
It may be new, or already approved and being studied for another use, dose or form. Criteria often leave out people who took part in another study of one within a recent period, commonly 30 days or five half-lives.
Blood cells that help blood clot. Too few raises the risk of bleeding and bruising. A usual adult range is about 150–400 × 10⁹/L (150,000–400,000 per µL); each lab has its own reference range.
The protein in red blood cells that carries oxygen. A low level is anaemia, which can cause tiredness and breathlessness. Measured in g/dL or g/L (9 g/dL is 90 g/L).
Blood tests of how long blood takes to clot. The INR is the prothrombin time (PT) put on a common scale; about 1 is usual for someone not taking a blood thinner.
Criteria often make an exception for people on a blood thinner such as warfarin, whose INR is raised on purpose.
The number of CD4 cells, a type of white blood cell, in a given amount of blood. It is used to follow how well the immune system is doing in HIV.
The human immunodeficiency virus, which weakens the immune system.
Many studies now accept people whose HIV is controlled on treatment, judged by the viral load and the CD4 count. Check the limits this study sets.
A recording of the heart’s electrical activity, taken with stickers on the chest, arms and legs. It takes a few minutes and does not hurt.
An allergy, or another reaction of the immune system, to a substance. “Hypersensitivity to the study drug” means a known allergic reaction to it — not every side effect a medicine can have.
Clinically significant
Judged by the study doctor to matter for your health or for the study. It is a doctor’s decision, so ask the study team if you are unsure whether something counts.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Contraception
Ways of preventing pregnancy. “Highly effective” methods fail less than 1% of the time a year when used correctly — for example an IUD or an implant.
Criteria usually say which methods count and how long to keep using them after the last dose, sometimes for male participants too. Some studies accept a partner’s vasectomy if it was confirmed successful and that person is the participant’s only sexual partner.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Investigational drug or device
A drug or device being studied in a clinical trial.
It may be new, or already approved and being studied for another use, dose or form. Criteria often leave out people who took part in another study of one within a recent period, commonly 30 days or five half-lives.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Platelets
Blood cells that help blood clot. Too few raises the risk of bleeding and bruising. A usual adult range is about 150–400 × 10⁹/L (150,000–400,000 per µL); each lab has its own reference range.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Hemoglobin (Hb)
The protein in red blood cells that carries oxygen. A low level is anaemia, which can cause tiredness and breathlessness. Measured in g/dL or g/L (9 g/dL is 90 g/L).
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
INR, PT and aPTT (clotting tests)
Blood tests of how long blood takes to clot. The INR is the prothrombin time (PT) put on a common scale; about 1 is usual for someone not taking a blood thinner.
Criteria often make an exception for people on a blood thinner such as warfarin, whose INR is raised on purpose.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
CD4 count
The number of CD4 cells, a type of white blood cell, in a given amount of blood. It is used to follow how well the immune system is doing in HIV.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
HIV
The human immunodeficiency virus, which weakens the immune system.
Many studies now accept people whose HIV is controlled on treatment, judged by the viral load and the CD4 count. Check the limits this study sets.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
ECG (electrocardiogram)
A recording of the heart’s electrical activity, taken with stickers on the chest, arms and legs. It takes a few minutes and does not hurt.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Hypersensitivity
An allergy, or another reaction of the immune system, to a substance. “Hypersensitivity to the study drug” means a known allergic reaction to it — not every side effect a medicine can have.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
All enrolled participants will be assigned to treatment with marstacimab (PF-06741086) administered as once weekly SC injection.
Drug: marstacimab
* Participants with mild to moderate hemophilia will receive 150 mg subcutaneous (SC) injection once weekly. * Female participants with severe hemophilia will receive 300 mg SC loading dose followed by 150 mg SC once weekly. A dose of 300 mg administered SC once weekly may be prescribed for participants who meet the dose-escalation criteria.
Also known as: PF-06741086
Annualized Bleeding Rate (ABR) of Treated Bleeding Events
Derived for each subject for each period (historical and study treatment) by using the following formula: ABR = number of bleeds requiring treatment / (days on treatment period / 365.25)
Time frame: Baseline (up to 12 months prior to informed consent) up to end of 12-month treatment phase
Incidence and Severity of Adverse Events and Serious Adverse Events
Time frame: Screening to up to end of 12-month treatment phase including 30-day follow up (up to 15 months)
Incidence and Severity of Thrombotic Events
Time frame: Baseline up to end of 12-month treatment phase including 30-day follow up (up to 13 months)
Incidence and Severity of Thrombotic Microangiopathy
Time frame: Baseline up to end of 12-month treatment phase including 30-day follow up (up to 13 months)
Incidence and Severity of Disseminated Intravascular Coagulation/Consumption Coagulopathy
Time frame: Baseline to end of 12-month treatment phase including 30-day follow up (up to 13 months)
Incidence and Severity of Injection Site Reactions
Time frame: Baseline up to end of 12-month treatment phase including 30-day follow-up (up to 13 months)
Incidence of Severe Hypersensitivity Reactions, including Anaphylaxis
Time frame: Baseline up to end of 12-month treatment phase including 30-day follow-up (up to 13 months)
Immunogenicity: Incidence of ADA and Clinically Significant Persistent NAb to Marstacimab
Time frame: Baseline up to end of 12-month treatment phase including 30-day follow-up (up to 13 months)
ABR of Treated Joint Bleeds
Time frame: Baseline (up to 12 months prior to informed consent) through the end of the 12-month treatment phase
ABR of Treated Spontaneous Bleeds
Time frame: Baseline (up to 12 months prior to informed consent) through the end of the 12-month treatment phase
ABR of Treated Traumatic Bleeds
Time frame: Baseline (up to 12 months prior to informed consent) through the end of the 12-month treatment phase
ABR of Total Bleeds (Treated and Untreated)
Time frame: Baseline (up to 12 months prior to informed consent) through the end of the 12-month treatment phase
Number of Target Joints
Time frame: Baseline up to end of 12-month treatment phase
ABR of Target Joint Bleeds
Time frame: 12-month treatment phase (up to 12 months)
ABR of Treated Target Joint Bleeds
Time frame: 12-month treatment phase (up to 12 months)
Change from Baseline in joint Health as Measured by the Hemophilia Joint Health Score (HJHS)
Time frame: Baseline up to end of 12-month treatment phase
Change From Baseline in Hemophilia Early Arthropathy Detection With Ultrasound (HEAD-US) Score
Time frame: Baseline up to end of 12-month treatment phase (up to 12 months)
Change from Baseline in the Hemophilia Activities List (HAL)
Time frame: Baseline up to end of 12-month treatment phase
Change from Baseline in the Pediatric Hemophilia Activities List (pedHAL)
Time frame: Baseline up to end of 12-month treatment phase
Patient Global Impression of Change-Hemophilia (PGIC-H)
Time frame: Baseline up to end of 12-month treatment phase
Change from Baseline in EuroQoL-5 Dimensions - 5 Levels (EQ-5D-5L)
Time frame: Baseline up to end of 12-month treatment phase
Change from Baseline in Hemophilia Treatment Experience Measure (Hemo-TEM)
Time frame: Baseline up to end of 12-month treatment phase
Percentage of Participants With an Overall Treatment Preference for Marstacimab, Prior Hemophilia Therapy, or No Preference, as Assessed by a Treatment Preference Questionnaire
Time frame: Day 60 and Day 120 during the 12-month treatment phase (up to 4 months)
Change From Baseline in Menstrual Bleeding Questionnaire (MBQ)
Time frame: Baseline up to end of 12-month treatment phase
Pictorial Blood Assessment Chart (PBAC) Score
Time frame: 12-month treatment phase (up to 12 months)
ABR of Treated Bleeding Events During the Long-Term Extension Phase
Time frame: Baseline (up to 12 months prior to informed consent) and from Month 13 up to Month 48
ABR of Treated Joint Bleeds During the Long-Term Extension Phase
Time frame: Baseline (up to 12 months prior to informed consent) and from Month 13 up to Month 48
ABR of Treated Spontaneous Bleeds During the Long-Term Extension Phase
Time frame: Baseline (up to 12 months prior to informed consent) and from Month 13 up to Month 48
ABR of Treated Traumatic Bleeds During the Long-Term Extension Phase
Time frame: Baseline (up to 12 months prior to informed consent) and from Month 13 up to Month 48
ABR of Total Bleeds (Treated and Untreated) During the Long-Term Extension Phase
Time frame: Baseline (up to 12 months prior to informed consent) and from Month 13 up to Month 48
Number of Target Joints During the Long-Term Extension Phase
Time frame: Month 13 up to Month 48
ABR of Treated Target Joint Bleeds During the Long-Term Extension Phase
Time frame: Month 13 up to Month 48
Change from Baseline in joint Health as Measured by the Hemophilia Joint Health Score (HJHS) During the Long-Term Extension Phase
Time frame: Baseline and from Month 13 up to Month 48
Change From Baseline in Hemophilia Early Arthropathy Detection With Ultrasound (HEAD-US) Score During the Long-Term Extension Phase
Time frame: Baseline and Month 13 up to Month 48
Incidence and Severity of Adverse Events and Serious Adverse Events During the Long-Term Extension Phase
Time frame: Month 13 up to Month 48
Incidence and Severity of Thrombotic Events During the Long-Term Extension Phase
Time frame: Month 13 up to Month 48 including 30-day follow-up
Incidence and Severity of Thrombotic Microangiopathy During the Long-Term Extension Phase
Time frame: Month 13 up to Month 48 including 30-day follow-up
Incidence and Severity of Disseminated Intravascular Coagulation/Consumption Coagulopathy During the Long-Term Extension Phase
Time frame: Month 13 up to Month 48 including 30-day follow-up
Incidence and Severity of Injection Site Reactions During the Long-Term Extension Phase
Time frame: Month 13 up to Month 48
Incidence of Severe Hypersensitivity Reactions, including Anaphylaxis During the Long-Term Extension Phase
Time frame: Month 13 up to Month 48
Immunogenicity: Incidence of ADA and Clinically Significant Persistent NAb to Marstacimab During the Long-Term Extension Phase
Time frame: Month 13 up to Month 48
Change from Baseline in the Hemophilia Activities List (HAL) During the Long-Term Extension Phase
Time frame: Baseline and Month 13 up to Month 48
Change from Baseline in the Pediatric Hemophilia Activities List (pedHAL) During the Long-Term Extension Phase
Time frame: Baseline and Month 13 up to Month 48
Patient Global Impression of Change-Hemophilia (PGIC-H) During the Long-Term Extension Phase
Time frame: Month 13 up to Month 48
Change from Baseline in EuroQoL-5 Dimensions - 5 Levels (EQ-5D-5L) During the Long-Term Extension Phase
Time frame: Baseline and Month 13 up to Month 48
Change from Baseline in Hemophilia Treatment Experience Measure (Hemo-TEM) During the Long-Term Extension Phase
Time frame: Baseline and Month 13 up to Month 48
Change From Baseline in Menstrual Bleeding Questionnaire (MBQ) During the Long-Term Extension Phase
Time frame: Baseline and Month 13 up to Month 48
Pictorial Blood Assessment Chart (PBAC) Score During the Long-Term Extension Phase
Time frame: Month 13 up to Month 48
Plan to share: Yes — Pfizer will provide access to individual de-identified participant data and related study documents (e.g. protocol, Statistical Analysis Plan (SAP), Clinical Study Report (CSR)) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions. Further details on Pfizer's data sharing criteria and process for requesting access can be found at: https://www.pfizer.com/science/clinical_trials/trial_data_and_results/data_requests.
Data sharing details ↗ · www.pfizer.com
From the registry record's own update history. This site started tracking changes on Sep 25, 2026; for anything earlier, see the record history on ClinicalTrials.gov ↗
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Studies of the drug tested here: Marstacimab