A study testing fitusiran to prevent bleeding in boys aged 1 to 11 years with hemophilia A or B with or without inhibitors

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What is this study about?

This study is looking at hemophilia, which is a condition where blood does not clot properly because the body does not make enough of certain clotting proteins. The study focuses on two types of this condition: hemophilia A, where the body lacks enough of a clotting protein called Factor VIII, and hemophilia B, where there is not enough of a clotting protein called Factor IX. Some people with hemophilia develop substances in their blood called inhibitory antibodies that work against these clotting factors, making treatment more difficult. The study will test a medicine called SAR439774, which contains the active substance fitusiran. This medicine is given as an injection under the skin.

The purpose of this study is to see how well fitusiran works to prevent bleeding episodes in boys between 1 and less than 12 years old who have hemophilia A or B, whether or not they have inhibitory antibodies. The study will compare how often bleeding happens when participants use fitusiran versus when they use their usual treatment, which is called standard of care and may include clotting factor concentrates or bypassing agents. Bypassing agents are special medicines used when inhibitory antibodies are present because they help blood clot in a different way.

During the study, participants will receive fitusiran treatment for a period of time, and there will also be a period where they use their standard of care treatment so that doctors can compare the results. The study will look at how many bleeding episodes happen during these different treatment periods, including bleeding that happens on its own without injury and bleeding that happens in the joints. Doctors will also check for any side effects and will measure changes in physical activity, pain levels, and overall quality of life. Some participants may be joining this study after being in a previous study with fitusiran, and they will have a different treatment schedule. The study will last several years to gather enough information about how safe and effective fitusiran is for preventing bleeding in young boys with hemophilia.

1 Initial treatment period with standard care

At the beginning of the study, you will continue receiving your current treatment for hemophilia. Hemophilia is a condition where blood does not clot properly.

If you have inhibitors, which are antibodies that block clotting factor treatments, you will continue using bypassing agents. These are special medications that help blood clot without using regular clotting factors.

If you do not have inhibitors, you will continue using clotting factor concentrates. These are medications that replace the missing clotting factor in your blood.

This period is called the standard of care period and serves as a comparison to the treatment with fitusiran.

2 Treatment with fitusiran

After the standard of care period, you will begin receiving fitusiran, the medication being studied in this trial.

Fitusiran is given as an injection under the skin, similar to how some vaccines are given.

The medication comes as a solution for injection and is also known as SAR439774.

This treatment period with fitusiran lasts for 160 weeks, which is approximately three years.

During this time, the study will measure how many bleeding episodes you experience that require treatment. This is called the annualized treated bleeding rate.

3 Monitoring and assessments throughout treatment

Throughout the study, various measurements will be taken to assess how well the treatment works and how safe it is.

The number of spontaneous bleeding episodes will be recorded. These are bleeding episodes that happen without any obvious injury or cause.

The number of joint bleeding episodes will be tracked. These occur when bleeding happens inside your joints, such as knees, elbows, or ankles.

Your physical activity levels will be assessed to see if there are any changes during treatment.

Pain intensity will be measured to determine if the treatment affects your pain levels.

Your health-related quality of life will be evaluated. This means assessing how the treatment affects your daily activities and overall well-being.

Blood samples will be taken at various points during the study. Adequate access to veins for blood draws is required for participation.

4 Safety monitoring

Throughout the entire study, any adverse events will be recorded. These are any unwanted or unexpected medical occurrences that happen during the study.

The severity and seriousness of these events will be assessed, as well as whether they might be related to the study medication.

This monitoring continues throughout all treatment periods to ensure your safety.

5 Assessment of target joints

Target joints are joints where you have had repeated bleeding episodes. The study will assess whether these joints improve during treatment.

This assessment looks at whether the frequency of bleeding in these specific joints decreases or resolves completely.

6 Completion of study

The study is expected to continue until the end of 2031.

Throughout the study, all measurements and assessments will be completed according to the study schedule.

The total duration of your participation will depend on when you enter the study and which treatment periods you complete.

Who Can Join the Study?

The participant must be between 1 year old and younger than 12 years old at the time of joining the study
The participant must have severe hemophilia A or B, which means their blood has very low levels of certain clotting factors (proteins that help blood clot). Specifically, Factor VIII less than 1 percent or Factor IX equal to or less than 2 percent, confirmed by a laboratory test
The participant must fall into one of two groups: either having inhibitors (substances in the blood that block clotting factor treatment) or not having inhibitors
If the participant has inhibitors, they must have been using bypassing agents (special medicines that help blood clot when regular treatments do not work) for at least the last 3 months, and must meet specific test results showing inhibitor levels in their blood
If the participant does not have inhibitors, they must have been using clotting factor concentrates (medicines containing the missing clotting proteins) for at least the last 3 months, have low inhibitor levels on testing, and have not used bypassing agents in the last 3 months
The participant must have adequate veins in their arms or hands that allow the doctors to draw blood samples as needed for the study
The participant’s parent or legal guardian (the adult legally responsible for the child) must provide written permission for the child to participate, and the child must also agree to participate according to local rules

Who Cannot Join the Study?

The study is only for male patients, so female patients cannot participate
The study is limited to specific age groups, so patients outside the designated age range cannot participate
Patients who have previously received fitusiran, which is the study medicine being tested, cannot participate in certain parts of the study
Patients must have hemophilia, which is a condition where blood does not clot properly, to be eligible for this study

Where you can join this trial?

Verified and Recommended Sites

No sites found in this category

Verified Sites

Site Name	City	Country
Azienda Ospedaliero Universitaria Careggi	Florence	Italy
IRCCS Humanitas Research Hospital	Rozzano	Italy
Uniwersytecki Szpital Kliniczny Im Jana Mikulicza Radeckiego We Wroclawiu	Wroclaw	Poland

Other Sites

Site Name	City	Country
Azienda Ospedaliera di Padova	Padua	Italy
Spitalul Clinic De Urgenta Pentru Copii Sfanta Maria Iasi	Iasi	Romania
Fondazione IRCCS Ca Granda Ospedale Maggiore Policlinico	Milan	Italy
Hospital Sant Joan De Deu Barcelona	Esplugues De Llobregat	Spain
Uniwersyteckie Centrum Kliniczne Warszawskiego Uniwersytetu Medycznego	Warsaw	Poland
University Childrens Hospital Queen Fabiola	Brussels	Belgium
University Of Debrecen	Debrecen	Hungary
Spitalul Clinic De Urgenta Pentru Copii Louis Turcanu Timisoara	Timisoara	Romania
Ludwig Maximilian University Of Munich	Munich	Germany
Fundeni Clinical Institute	Bucharest	Romania
Cyzwnouql Uigkraowzjvfgk Slmnleptd	Woluwe-Saint-Lambert	Belgium
Sduydvl Kxblpkghg Ibx Kswjox Jvzyowqra Ufmhglfjjidl Mdnecjdnmn Iyj Kacnjd Mojdgzjoypafwit W Pqbpdyqx	Poznan	Poland
Fnafklwui Pviq Lr Imcvpecxtrwth Blijeahio Dbj Hlvthjng Unkhenbuhuqzb La Pzx	Madrid	Spain

Trial status

Country	Status	Recruitment Start
Belgium	Not yet recruiting	19.12.2025
Germany	Not yet recruiting	19.12.2025
Hungary	Not yet recruiting	19.12.2025
Italy	Not yet recruiting	19.12.2025
Poland	Not yet recruiting	19.12.2025
Romania	Not yet recruiting	19.12.2025
Spain	Recruiting	19.12.2025

Trial locations

Investigated drugs:

Fitusiran

Fitusiran is an investigational medication being studied as a preventive treatment for hemophilia A or B in male children. It works by helping the blood to clot more effectively in people whose natural clotting process does not work properly. This medication is being tested to see if it can reduce bleeding episodes when given regularly as a preventive therapy.

Standard of care treatment refers to the usual medications that patients with hemophilia currently receive to prevent or treat bleeding. This may include factor replacement therapies or other treatments that doctors normally prescribe for managing hemophilia. In this study, participants will receive their standard treatment during certain periods for comparison with the investigational medication.

Hemophilia – Hemophilia is a rare bleeding disorder in which the blood does not clot properly due to a lack of certain clotting factors. People with this condition experience prolonged bleeding after injuries, surgery, or dental procedures. Spontaneous bleeding can occur without an obvious cause, particularly into joints and muscles. Joint bleeding is especially common and can happen repeatedly in the same joints, called target joints. Over time, repeated bleeding into joints can lead to joint damage and reduced mobility. The condition is usually inherited and passed down through families.

Trial ID:

2025-521858-42-00

Protocol code:

EFC17905

Trial Phase:

Therapeutic confirmatory (Phase III)

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A study testing fitusiran to prevent bleeding in boys aged 1 to 11 years with hemophilia A or B with or without inhibitors

What is this study about?

Who Can Join the Study?

Who Cannot Join the Study?