A Study to Learn About How Changing Therapy From Emicizumab to Marstacimab Affects People With the Severe Hemophilia A.

Inclusion Criteria: 1. Male and 12 to \<75 years of age with a minimum body weight of 35 kg at the time of signing the informed consent. 2. Diagnosis of severe hemophilia A (FVIII activity \<1%) without inhibitors. 3. On emicizumab therapy at a standard clinical dose for ≥6 months. Exclusion Criteria: 1. Previous or current treatment for or history of coronary artery diseases, venous or arterial thrombosis, or ischemic disease. 2. Any medical or psychiatric condition including recent (within the past year) or active suicidal ideation/behavior or laboratory abnormality that may increase the risk of study participation or, in the investigator's judgment, make the participant inappropriate for the study. 3. Known hemostatic defect other than hemophilia A. 4. Current use of any prohibited concomitant medication(s) or unwillingness or inability to use a required concomitant medication(s). 5. Previous administration of an investigational product (drug or vaccine) within 30 days or 5 half-lives preceding the first dose of study intervention used in this study (whichever is longer). Participation in studies of other investigational products (drug or vaccine) at any time during participation in this study. 6. Platelet count \<100,000/μl or hemoglobin \<10 g/dL. 7. Clinically significant renal or hepatic function abnormality based on laboratory results at screening, or known kidney or liver disease. 8. CD4 cell count ≤200/μl if HIV positive. 9. Screening 12-lead ECG that demonstrates clinically significant abnormalities that, in the opinion of the investigator, may affect participant safety or interpretation of study results. 10. Known planned surgical procedure. 11. Hypersensitivity or allergic reaction to hamster protein or other components of the study intervention. 12. Investigator site staff directly involved in the conduct of the study and their family members, site staff otherwise supervised by the investigator, and sponsor and sponsor-delegate employees directly involved in the conduct of the study and their family members.