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A Study of Repinatrabit Compared to Placebo for Treatment of Patients with Phenylketonuria

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What is this trial about?

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

This study involves people with Phenylketonuria, which is a condition where the body cannot properly break down a substance called phenylalanine that is found in protein-containing foods. When phenylalanine builds up in the blood, it can cause various health problems. The study will test a medication called JNT-517, which is given as a film-coated tablet that is taken by mouth. Some people in the study will receive JNT-517 while others will receive placebo. The purpose of this study is to see how well JNT-517 works compared to placebo in lowering the levels of phenylalanine in the blood of people with Phenylketonuria.

The study is divided into two periods and will last up to 52 weeks. During the first period, which lasts 6 weeks, participants will be randomly assigned to receive either JNT-517 at a dose of 150 milligrams twice daily, JNT-517 at a dose of 75 milligrams twice daily, or placebo. Neither the participants nor the study doctors will know which treatment each person is receiving during this period. Blood samples will be collected regularly to measure phenylalanine levels. In the second period, all participants will receive JNT-517 at one of the two doses. Throughout the study, participants will need to maintain a stable diet and keep track of what they eat using diet diaries. Some participants may be able to gradually increase the amount of protein in their diet if their phenylalanine levels remain low enough.

The study will measure several things to determine if the treatment is working. The main measurement will be the change in blood phenylalanine levels after 6 weeks of treatment. The study will also look at how many people achieve certain target phenylalanine levels, whether people can eat more protein while keeping their phenylalanine levels controlled, and whether there are improvements in attention and thinking abilities. Safety will be monitored throughout the study by tracking any side effects or unwanted reactions that occur. Participants will attend regular visits where blood tests will be done, questionnaires will be completed, and any changes in health will be recorded.

The research process

The trial runs in 4 steps – from screening to follow-up. Each step says what happens and what the team monitors.

  1. Step 1

    Screening and baseline assessment

    Blood samples will be taken after fasting for at least 4 hours to measure phenylalanine levels in the blood. Phenylalanine is an amino acid that builds up in the body when a person has phenylketonuria. At least 3 blood samples will be taken during this period to establish baseline levels.

    The diet should remain stable in phenylalanine and total protein intake throughout the study. Dietary adjustments may be required according to specific guidelines provided.

    If currently taking sapropterin or large neutral amino acids such as PheBloc, NeoPhe, or PreKunil, the same dose must be maintained throughout the entire study.

  2. Step 2

    Period 1: Double-blind treatment phase

    This period lasts 6 weeks.

    Either JNT-517 or placebo will be assigned randomly. A placebo is an inactive substance that looks like the study medication but contains no active ingredient. Neither the participant nor the study staff will know which treatment has been assigned during this period.

    The medication is a film-coated tablet taken by mouth.

    If assigned to JNT-517, the dose will be either 75 mg twice daily or 150 mg twice daily.

    Blood samples will be taken at Week 2, Week 4, and Week 6 to measure phenylalanine levels.

    Assessments of attention and cognitive function may be performed using computerized tests.

    A 3-day diet diary will be completed to record food intake.

    The diet should continue to be maintained as stable, with adjustments made only as directed by the dietary management guidelines.

  3. Step 3

    Period 2: Extended treatment phase

    The duration of this period will be determined by the study protocol.

    Both dose groups will receive JNT-517 at either 75 mg twice daily or 150 mg twice daily.

    The medication continues to be taken as a film-coated tablet by mouth.

    Blood samples will continue to be taken regularly to monitor phenylalanine levels.

    Additional assessments of attention and cognitive function will be performed.

    For those participating in diet normalization, dietary phenylalanine and protein intake may be gradually increased while maintaining phenylalanine levels below 360 micromoles per liter.

    The goal of diet normalization is to determine if natural intact protein intake can be increased toward normal dietary levels while keeping phenylalanine levels controlled.

  4. Step 4

    Ongoing monitoring throughout the study

    Any side effects or health changes will be monitored and recorded throughout both periods.

    Body weight must remain above 40 kilograms throughout the study.

    If currently taking medications for psychiatric conditions, these must remain stable throughout the study.

    Contraception requirements must be followed as specified throughout the study and for 30 days after the last dose of study medication.

Who can join the trial?

16 criteria

  • You must be 18 years of age or older at the start of the study
  • You must have a clinical diagnosis of PKU, which is a condition where the body cannot properly break down a substance called phenylalanine found in protein
  • During the screening period, the average of at least 3 blood tests taken after fasting for 4 hours or more must show phenylalanine levels of 360 micromoles per liter or higher
  • You must not have used pegvaliase, which is a medication that helps break down phenylalanine, within 4 weeks before screening
  • If you are taking sapropterin or large neutral amino acids such as PheBloc, NeoPhe, or PreKunil at screening, you must have been on the same dose for 4 weeks before screening and continue this dose throughout the study
  • You must be willing and able to keep your diet stable in terms of phenylalanine and total protein, including both regular protein and medical food protein, and be able to adjust your diet as needed during the study
  • Your body weight must be more than 40 kilograms
  • If you have a psychiatric illness, which is a mental health condition, it must have been well-controlled for the past 6 months, and if you take medication for it, you must have been on the same medication for the past 3 months
  • You must be able to understand and sign an informed consent form, which is a document that explains the study, and you must be able to follow the study procedures
  • If you are a biological male, you must either not have sexual intercourse that could lead to pregnancy, or if you do, you must use 2 different birth control methods with at least 1 being highly effective from the start of the study until at least 30 days after the last dose of study medication, and you must not donate sperm during this time
  • If you are a biological male who had a vasectomy, which is a surgical procedure to prevent pregnancy, at least 4 months before screening with documentation, no birth control restrictions apply
  • If you are a biological female who can become pregnant, you must have a negative pregnancy test at screening and on the first day of the study
  • If you are a biological female who can become pregnant, you must either not have sexual intercourse that could lead to pregnancy, or if you do, you must use 2 different birth control methods with at least 1 being highly effective from screening until at least 30 days after the last dose of study medication
  • If you are a biological female taking birth control pills containing estrogen or progesterone, you must use 2 other birth control methods with at least 1 being highly effective, or not have sexual intercourse that could lead to pregnancy during the study
  • If you are a biological female who can become pregnant, you must not donate eggs during the study and for 30 days after the last dose of study medication
  • If you are a biological female who cannot become pregnant, this includes having had surgery such as removal of the uterus, ovaries, or fallopian tubes, or not having menstrual periods for at least 1 year with hormone testing confirmation, or not yet having had your first menstrual period

Who cannot join the trial?

3 criteria

  • The source data does not provide specific exclusion criteria, which are reasons why a patient cannot participate in the study
  • Without detailed exclusion criteria information, it is not possible to list the specific conditions or situations that would prevent someone from joining this clinical trial
  • Exclusion criteria typically include things like other medical conditions, medications being taken, pregnancy status, or previous treatments, but these details are not available in the provided information
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Investigated drugs

JNT-517 is an investigational medication being studied for the treatment of phenylketonuria, a genetic condition that affects how the body processes protein. This medication is being tested to see if it can help manage the symptoms and effects of this condition. In this study, participants will take this medication twice daily to evaluate how well it works compared to a placebo.

What is already known about the treatment

JNT-517 – JNT-517 is an investigational medication being studied in clinical trials for the treatment of phenylketonuria, a genetic disorder where the body cannot properly break down an amino acid called phenylalanine. This drug is administered orally at a dose of 150 mg twice daily and is currently in Phase 3 trials, meaning it is being tested in larger groups of patients to confirm its effectiveness and safety before potential approval. The study aims to evaluate whether JNT-517 can effectively lower phenylalanine levels in the blood compared to a placebo over a 6-week treatment period. While the exact mechanism of action at the molecular level is not specified in the available data, medications for phenylketonuria typically work by either helping the body break down phenylalanine more effectively or by reducing its absorption.

Investigated diseases

Phenylketonuria – Phenylketonuria is a genetic disorder that affects how the body processes a specific amino acid called phenylalanine, which is found in many protein-containing foods. People with this condition lack or have reduced activity of an enzyme needed to break down phenylalanine properly. As a result, phenylalanine builds up to harmful levels in the blood and body tissues. This accumulation can affect brain development and function, particularly if the condition is not managed from early childhood. Over time, elevated phenylalanine levels can lead to problems with attention, concentration, and cognitive abilities. The condition is present from birth and requires lifelong management to prevent complications.
Trial detailsLast updated 7 Oct 2026
Age18+ yearsPhasePhase IIITrial ID2024-519554-37-00Protocol codeJNT517-301Estimated enrolment120 patientsSponsorOtsuka Pharmaceutical Development & Commercialization Inc.

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