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Study Comparing Sepiapterin and Sapropterin for Treating Phenylketonuria in Patients Aged 2 Years and Older

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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 condition called Phenylketonuria (PKU), which is a rare genetic disorder that affects how the body processes a substance called phenylalanine, found in many foods. The study will compare two treatments: Sepiapterin and Sapropterin. Sepiapterin is a new treatment being tested, while Sapropterin is an existing medication used to help manage PKU. The purpose of the study is to see how effective Sepiapterin is compared to Sapropterin in reducing the levels of phenylalanine in the blood of people with PKU.

Participants in the study will be randomly assigned to receive either Sepiapterin or Sapropterin for a certain period, and then they will switch to the other treatment. This type of study is called a crossover study. The study will last for several weeks, during which participants will take the medication in the form of either soluble tablets or powder for oral use. The researchers will monitor the participants' blood phenylalanine levels to see how well each treatment works. Some participants may also receive a placebo during the study.

The study aims to provide valuable information about the effectiveness of Sepiapterin compared to Sapropterin in managing PKU. By participating, individuals with PKU can contribute to research that may improve treatment options for this condition in the future. The study is open to both male and female participants who are at least two years old and have been diagnosed with PKU. Participants will need to continue their current diet and follow the study procedures as instructed by the research team.

The research process

The trial runs in 6 steps – from screening to follow-up. Each step says what happens and what the team monitors.

  1. Step 1

    Joining the study

    Upon joining the study, informed consent is required. For children or those unable to consent due to mental impairment, a parent or legal representative must provide consent.

    Eligibility includes being at least 2 years old and having a clinical diagnosis of phenylketonuria (PKU) with specific blood phenylalanine (Phe) levels.

  2. Step 2

    Initial assessment

    An initial assessment is conducted to confirm eligibility, including a review of medical history and blood Phe levels.

    Women of childbearing potential must have a negative pregnancy test and agree to use effective contraception during the study.

  3. Step 3

    Treatment period 1

    Participants receive sapropterin dihydrochloride in the form of Kuvan 100 mg soluble tablets, taken orally as an oral solution.

    The treatment aims to reduce blood Phe levels, and the dosage and frequency are determined by the study protocol.

  4. Step 4

    Crossover to treatment period 2

    After completing the first treatment period, participants switch to the second treatment, receiving sepiapterin.

    This phase also involves oral administration, with the dosage and frequency specified by the study protocol.

  5. Step 5

    Monitoring and assessments

    Throughout the study, regular monitoring of blood Phe levels is conducted to assess the effectiveness of the treatments.

    Additional assessments include physical examinations, vital sign checks, ECGs, and routine laboratory tests.

  6. Step 6

    Completion of study

    The study concludes with a final assessment of blood Phe levels and overall health.

    Participants are required to continue their current diet unchanged during the study.

Who can join the trial?

8 criteria

  • Participants must provide informed consent. If the participant is a child or has mental impairment due to the disease, consent must be given by a parent or legally designated representative.
  • Participants can be male or female and must be at least 2 years old.
  • Participants must have a blood Phe level of at least 360 micromoles per liter (μmol/L) on their current therapy during the screening and when averaging the 3 most recent Phe levels from their medical history.
  • Participants must have a clinical diagnosis of PKU (Phenylketonuria) with a history of at least 2 blood Phe measurements of 600 μmol/L or higher.
  • Women who can have children must have a negative pregnancy test at screening and agree to abstain from sex or use a highly effective form of birth control during the study and for up to 90 days after the last dose of the study drug.
  • Men who are sexually active with women who can have children and have not had a vasectomy must agree to use a barrier method of birth control during the study and for up to 90 days after the last dose of the study drug. They must also refrain from donating sperm during this time. Men who are abstinent do not need to use birth control unless they become sexually active. Men who have had a vasectomy at least 16 weeks ago do not need to use birth control.
  • Participants must be willing and able to follow the study protocol and procedures.
  • Participants must be willing to keep their current diet unchanged while participating in the study.

Who cannot join the trial?

4 criteria

  • Individuals who are not diagnosed with Phenylketonuria (PKU) cannot participate. PKU is a genetic condition that affects how the body processes a certain amino acid called phenylalanine.
  • Participants must be within the specified age range for the study. If you are outside this age range, you cannot participate.
  • Both males and females are eligible, but if you do not identify as either, you may not be able to participate.
  • If you belong to a vulnerable population, such as children or individuals with certain disabilities, you may not be eligible to participate.
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Investigated drugs

  • Sepiapterin

    is being studied to see how well it can lower blood phenylalanine levels in people with phenylketonuria (PKU). This medication is being compared to another treatment to determine its effectiveness in managing PKU.

  • Sapropterin

    is a medication already used to help reduce blood phenylalanine levels in people with PKU. It is being used as a comparison in this study to evaluate the effectiveness of sepiapterin.

What is already known about the treatment

  • Sepiapterin

    Sepiapterin is administered orally and is currently being studied in clinical trials for its effectiveness in treating phenylketonuria (PKU). It is not yet widely used in medical practice, as it is still under investigation. The main therapeutic indication for sepiapterin is to help reduce blood phenylalanine levels in individuals with PKU. At the molecular level, sepiapterin works by enhancing the production of tetrahydrobiopterin, a cofactor that helps break down phenylalanine. It is classified as a pharmacological agent that targets metabolic pathways.

  • Sapropterin

    Sapropterin is an oral medication that is already approved and used in the treatment of phenylketonuria (PKU). It is well-documented in medical literature and is used to lower blood phenylalanine levels in patients with PKU. The drug acts by increasing the activity of the enzyme phenylalanine hydroxylase, which helps convert phenylalanine into other substances the body can use. Sapropterin is classified as a synthetic form of tetrahydrobiopterin, a cofactor involved in amino acid metabolism.

Investigated diseases

Phenylketonuria – Phenylketonuria, often abbreviated as PKU, is a rare genetic disorder that affects the body's ability to break down an amino acid called phenylalanine. This amino acid is found in many protein-containing foods and some artificial sweeteners. In individuals with PKU, the enzyme needed to process phenylalanine is either missing or not working properly, leading to a buildup of phenylalanine in the blood. If untreated, this can cause a variety of health problems, including intellectual disabilities and other neurological issues. The condition is typically identified through newborn screening, allowing for early dietary management to prevent complications. PKU requires lifelong management to maintain normal phenylalanine levels in the body.
Trial detailsLast updated 2 Oct 2026
Age18+ yearsPhasePhase IIITrial ID2023-506238-61-00Protocol codePTC923-PKU-301Estimated enrolment173 patientsSponsorPTC Therapeutics Inc.

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