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Long-term Safety and Efficacy Study of JR-141 (Pabinafusp Alfa) for Male Patients with Hunter Syndrome (Mucopolysaccharidosis II)

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What is this trial about?

A plain-language summary of the goals, design and what participants do

This clinical trial is focused on studying a rare genetic disorder called Mucopolysaccharidosis Type II, also known as Hunter Syndrome. This condition affects the body's ability to break down certain complex sugars, leading to various health issues. The study involves a treatment called JR-141, which is a medication in the form of a lyophilized powder that is prepared for injection. The active substance in JR-141 is pabinafusp alfa, a type of protein. The medication is administered through an intravenous injection, which means it is given directly into a vein.

The purpose of this study is to evaluate the long-term effects of JR-141 on symptoms affecting the central nervous system in individuals with Hunter Syndrome. Participants in this study will have previously completed a related study and will continue to receive JR-141 to assess its safety and effectiveness over an extended period. The study will monitor various health aspects, including liver and spleen size, shoulder movement, and walking ability, as well as other health indicators like heart and lung function.

Throughout the study, participants will undergo regular assessments to track their quality of life, sleep patterns, and overall health. These assessments will help researchers understand how JR-141 impacts the symptoms of Hunter Syndrome over time. The study aims to provide valuable insights into the long-term benefits and safety of JR-141 for individuals living with this condition.

The research process

The trial runs in 5 steps – from screening to follow-up. Each step says what happens and what the team monitors.

  1. Step 1

    Joining the study

    Participation in this study requires completion of a previous study, JR-141-GS31, and the absence of safety concerns as determined by the principal investigator.

    Written informed consent must be obtained from the participant or their legally acceptable representative. If the participant is underage or unable to provide consent due to intellectual disability, consent can be provided by a guardian.

  2. Step 2

    Treatment administration

    The treatment involves the administration of JR-141, which is a lyophilized powder prepared for injection.

    The medication is administered through an intravenous injection. The specific dosage and frequency are determined by the study protocol and the healthcare team.

  3. Step 3

    Monitoring and assessments

    Regular monitoring includes assessments of cerebrospinal fluid (CSF) concentrations, neuropsychological evaluations, and quality of life assessments.

    Additional evaluations may include liver and spleen volume, shoulder range of motion, a six-minute walk test, and pulmonary function tests if applicable.

  4. Step 4

    Safety evaluations

    Safety is continuously assessed through laboratory tests, vital signs monitoring, electrocardiograms (ECG), and physical examinations.

    The presence of anti-JR-141 antibodies and any adverse events are also monitored throughout the study.

  5. Step 5

    Study duration

    The study is expected to continue until April 30, 2029.

    Participants will be involved in regular assessments and follow-ups as outlined in the study protocol.

Who can join the trial?

4 criteria

  • The patient must have participated in the previous study (JR-141-GS31) and completed the necessary assessments at specific weeks, with no safety concerns according to the main doctor in charge.
  • The patient or their legal representative must sign a written consent form approved by an ethics committee. If the patient is under 18 years old (or under 16 in the UK) or cannot confirm their willingness to participate due to intellectual disability from MPS II, a parent or guardian can sign on their behalf. If possible, the patient should also provide written agreement to participate.
  • If the patient is a female who can have children or a male with a female partner who can have children, they must agree to use a highly effective method of birth control from the time they sign the consent form. This applies from the start of menstruation until menopause unless they are permanently unable to have children.
  • If the patient has hearing problems and needs hearing aids, they should agree to use them during the study and on days when cognitive tests are done. The study team will encourage the use of working hearing aids.

Who cannot join the trial?

9 criteria

  • Patients who have a different condition than Mucopolysaccharidosis II (also known as Hunter Syndrome) cannot participate.
  • Patients who are not within the specified age range for the study cannot participate.
  • Patients who do not meet the specific health requirements set by the study cannot participate.
  • Patients who are unable to follow the study procedures or instructions cannot participate.
  • Patients who have other medical conditions that might interfere with the study cannot participate.
  • Patients who are taking medications that might affect the study results cannot participate.
  • Patients who have participated in another clinical trial recently cannot participate.
  • Patients who are pregnant or breastfeeding cannot participate.
  • Patients who have a history of allergic reactions to the study medication cannot participate.
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Investigated drugs

JR-141 is a medication being studied for its long-term safety and effectiveness in treating symptoms of Mucopolysaccharidosis Type II, also known as Hunter Syndrome. This medication is specifically being evaluated for its impact on central nervous system symptoms in male patients with this condition.

What is already known about the treatment

JR-141 – JR-141 is administered through intravenous infusion, allowing the medication to be delivered directly into the bloodstream. It is currently being studied in clinical trials, with a focus on its long-term safety and effectiveness for treating Mucopolysaccharidosis II, also known as Hunter Syndrome. This medication is designed to address symptoms affecting the central nervous system in individuals with this condition. At the molecular level, JR-141 works by delivering a modified enzyme that can cross the blood-brain barrier, aiming to replace the deficient enzyme in patients. It is classified pharmacologically as an enzyme replacement therapy.

Investigated diseases

Mucopolysaccharidosis II – This is a rare genetic disorder caused by a deficiency of the enzyme iduronate-2-sulfatase. It leads to the accumulation of glycosaminoglycans in the body's cells, which can cause a variety of symptoms. These symptoms often include developmental delays, distinctive facial features, and joint stiffness. Over time, individuals may experience progressive physical and neurological decline. The disease primarily affects males and can vary in severity. It is also known as Hunter Syndrome.
Trial detailsLast updated 2 Oct 2026
Age18+ yearsPhasePhase IIITrial ID2022-503142-41-00Protocol codeJR-141-GS32Estimated enrolment79 patientsSponsorJcr Pharmaceuticals Co. Ltd.

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On this site, “treatment” means an investigational medicine being studied in a clinical trial. Its safety and efficacy for the use being studied have not yet been confirmed, some participants may receive a placebo or a comparator medicine, and taking part does not guarantee any health benefit. The decision to take part is made by the doctor at the research site. This site is for information only and does not replace medical advice.

This service is not affiliated with the European Commission, the EMA, or the official CTIS system. Most information comes from publicly available international clinical-trial registries, supplemented by data from academic sites, national regulators and commercial sponsors. On this site, “treatment” and “therapy” mean a medicine being tested in a clinical trial. Its safety and effectiveness in the use being studied are not yet confirmed, some participants may receive a placebo or a comparator, and taking part does not guarantee a health benefit. The doctor at the research site decides who can take part. This site provides information, not medical advice. Certain content and visual elements on this website have been generated or enhanced using artificial intelligence (AI).