Severe hemophilia

About 60% of people with hemophilia have a severe form of the condition. The severity level of hemophilia is a measure of clotting factor activity in the blood. People with severe hemophilia have less than 1% clotting factor activity. By comparison, normal factor activity is above 50% and as high as 150 to 200%.

Treatment for severe hemophilia

Current standard care is replacement of clotting factor. For some people, the frequency and intravenous (IV) administration route of replacement therapy can make it a burden. For people who have developed inhibitors, the standard replacement therapy is not effective.


About 15–20% of people with hemophilia develop inhibitors that prevent standard hemophilia treatments from working effectively. The body thinks the replacement clotting factor is a foreign substance and tries to destroy it. People with inhibitors require special treatment, which can be complicated and costly.

CDC fact sheet about inhibitors


Research is underway to find treatments that may be more effective for all hemophilia patients, with or without inhibitors. Treatment options that work differently than factor replacement therapy and work against inhibitors may help improve compliance and reduce bleeding episodes.

The BASIS study is working to find a hemophilia treatment that may be more convenient and more effective than current standard treatments. If you have severe hemophilia A or B, with or without inhibitors, you may be able to help with this research.

Clinical Trial Reference Number


If you’re interested in the BASIS study, you can print this page and show it to your doctor or simply give this trial reference number to your doctor.

Why is the BASIS study being done?

BASIS is testing a study drug to see if it is safe and what effects it may have on hemophilia in adolescents (12 to 17 years old) and adults (less than 75 years old) living with severe hemophilia A or B, with or without inhibitors. *

This study is different from your regular medical care. The purpose of regular medical care is to improve or otherwise manage your health, but the purpose of research is to gather information to advance science and medicine.

* Some people develop inhibitors that prevent standard hemophilia treatments from working.

Who can participate?

The study team determines eligibility of each potential participant, but the following parameters should give you some idea of whether this trial may be an option for you.

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At least 12 but less than 75 years old

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Severe hemophilia A or B, with or without inhibitors

What is the study drug?

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The study drug, called PF-06741086 or marstacimab, is:

  • A subcutaneous (under the skin) injection
  • Intended for people with hemophilia A or B, with or without inhibitors
  • Taken once a week
  • Investigational (not approved for uses other than research)

What happens in this clinical trial?

Before joining the BASIS study, you will review all aspects of the trial with the study team and ask as many questions as you want. After having all your questions answered, if you want to participate in the study, you will sign the informed consent document and continue with the study screening, to see if this study is right for you. If you join the study, your participation will last about 20 months. You will visit the study site 12 times and have 9 phone calls with the study team. You may be reimbursed for your time and travel.

During study visits and phone calls, you will answer questions and review your study eDiary with the study team. At some visits, procedures such as measurement of your vital signs, blood and urine tests, and physical examinations will be required.

Optional extension study

If you complete 12 months of study drug injections, you may have the opportunity to participate in a 12-month extension study and continue study drug injections.

1. Screening (up to 5 weeks)
  • Visit the study site 1 time
  • Answer questions, have a physical examination, have vital signs taken, and provide blood samples
2. Standard Hemophilia Treatments (6 months)
  • Continue your current hemophilia treatments for 6 months
  • Visit the study site 1 time and have 3 phone calls with the study team
  • Use the study eDiary to record bleeds and hemophilia medications
3. Study Drug (12 months)
  • Take the study drug once a week for 12 months
  • Visit the study site 10 times and have 5 phone calls with the study team
  • Use the study eDiary to record bleeds, hemophilia medications, and study drug injections
4. Safety Follow-up (1 month)

You will have a final phone call with the study team about 1 month after your last dose of study drug.

Before participating in a clinical trial, you need to understand what will happen. The study team will discuss all aspects of the trial with you, and you will have the opportunity to ask as many questions as you want. The following common questions cover some of the basics.

Clinical trials, also called clinical studies, help determine if study drugs, medical devices, and procedures are safe and effective for treating or preventing diseases and other health conditions.
Clinical trials follow a specific set of standards and are closely regulated to help protect study participants. An independent Ethics Committee or Institutional Review Board reviews and monitors each study closely to safeguard the rights and welfare of participants. Nevertheless, all clinical trials and investigational products have risks, including the potential to make participants sick or uncomfortable. Although safety precautions are put in place to protect people who participate in clinical trials, your condition or health could remain the same, improve, or get worse. Any known risks and side effects will be discussed during the informed consent process, but there may be unknown risks.
You will review the informed consent document with the study team. You should ask as many questions as needed to make sure you understand what will happen during your study participation. Throughout your study participation, you should talk to the team when you have questions or concerns about any aspects of the study.
The study team will assess if this study is right for you. You will be asked about your medical history, treatments and medications, and other issues that may affect your ability to participate in the study. You may also need to undergo health assessments to determine if this study is right for you.
Clinical trials rely on the cooperation of volunteer participants. While you are taking part in a trial, you will need to:
  • Follow instructions from the study team
  • Ask questions if you have concerns or need clarification
  • Attend all scheduled study visits (whether at a study clinic or by phone) and complete all required activities
  • Tell the study team about any changes to your health, even if you think the changes are not important
  • Talk with the study doctor before taking any new medications or supplements
Visits may occur at a study location or by phone. Activities can vary from visit to visit and may include the following:
  • Study treatment (have treatment at the visit or receive study drug to take at home)
  • Discussions about how you are feeling and about other medications you may be taking
  • Review of any self-assessments you may complete at home between visits
  • Questionnaires about your health and your quality of life
  • Medical procedures, such as measurement of your vital signs, blood and urine tests, electrocardiograms (ECGs), and physical examinations

Study visits and phone calls are also an opportunity for you to ask questions.

If you choose to participate in a clinical trial, we encourage you to communicate freely with the study team throughout the trial. If you have questions or concerns about any aspect of the trial, you should feel comfortable discussing them with the study team at any time.
Taking part in a clinical trial is voluntary. If the study team determines that this study is right for you, based on a screening process, you may choose to participate or not participate. You will be given time and opportunity to ask questions before deciding what you want to do. Your decision will in no way affect current or future treatment you receive outside the trial. If you choose to participate, you will need to follow instructions and complete trial-related activities BUT you may withdraw from the trial at any time for any reason without penalty or loss of benefits to which you are otherwise entitled.
You can visit any doctor to meet your health needs during a clinical trial. You should tell other medical professionals you see that you are participating in a clinical trial and always communicate openly with the study team regarding other care you receive.