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A Phase 3 Study of Fenfluramine Hydrochloride for Safety and Efficacy in Patients with Rett Syndrome

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What is this trial about?

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

The study focuses on Rett syndrome, a rare neurological condition that usually appears in early childhood and leads to difficulties with movement, speech, and learning. The medication being tested is an oral liquid called fenfluramine hydrochloride, which is given by mouth at a dose based on body weight. A matching placebo with no active ingredient is also used for comparison.

The main goal of the trial is to find out whether the study drug is safe and can improve the symptoms of Rett syndrome. Participants are randomly assigned to receive either the medication or the placebo, and neither the participants nor the researchers know which one is being given. The treatment period lasts about three months, during which caregivers and clinicians complete questionnaires about behavior, sleep, and communication, and routine heart checks are performed. After the initial phase, participants may continue in an open‑label extension where all receive the active medication, allowing further observation of long‑term safety and benefit.

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

    Baseline visit and randomization

    After joining the study, you attend a baseline visit where eligibility is confirmed and initial measurements are taken. assessments include the rett syndrome behaviour questionnaire, sleep disturbance questionnaire, observer‑reported communication ability questionnaire, a 12‑lead ecg, and a doppler echocardiogram.

    Based on the baseline data, you are randomly assigned to receive either fenfluramine hydrochloride oral solution or a matching placebo.

  2. Step 2

    Start of study medication

    You begin taking the assigned study medication once daily by mouth.

    The active medication is fintepla 2.2 mg/ml oral solution containing fenfluramine hydrochloride. the prescribed dose is 0.8 mg per kilogram of body weight each day. the volume of oral solution is calculated from the concentration (2.2 mg per ml) to achieve the correct dose.

    If you are assigned to the placebo group, you take a matching oral solution that contains no active substance, using the same dosing schedule.

  3. Step 3

    Regular monitoring visits during the 14‑week treatment period

    Throughout the 14‑week core phase, you attend scheduled study visits. at each visit, safety is checked, any adverse events are recorded, and questionnaires are completed to track changes in behaviour, sleep, and communication.

    The study team may perform repeat ecg and echocardiogram examinations as needed to monitor heart function.

  4. Step 4

    Week 14 evaluation

    At the end of week 14, you complete the final set of questionnaires, including the rett syndrome behaviour questionnaire, sleep disturbance questionnaire, observer‑reported communication ability questionnaire, and caregiver global impression of change for seizures.

    A follow‑up ecg and doppler echocardiogram are performed to assess any changes in heart rhythm or valve function.

    The study team evaluates all collected data to determine the primary and secondary outcomes.

  5. Step 5

    Open‑label extension

    After the week‑14 assessment, you may enter an open‑label extension in which all participants receive fenfluramine hydrochloride oral solution.

    The same daily dose of 0.8 mg/kg is continued, and ongoing safety monitoring and periodic assessments are performed for as long as the extension continues.

Who can join the trial?

9 criteria

  • Be diagnosed with Rett Syndrome that matches the typical or classic form as defined by the RettSearch Consortium criteria.
  • Have a known disease‑causing change (mutation) in the MECP2 gene, which is the gene linked to Rett Syndrome.
  • Have been in a stable “post‑regression” phase for at least six months, meaning you have not lost or gotten worse in walking or standing, hand use, speech (including babbling or words), or non‑verbal communication such as eye contact and social attentiveness.
  • Score between 10 and 36 (inclusive) on the Rett Syndrome Clinical Severity Scale (RTT‑CSS), a tool doctors use to measure how severe the condition is.
  • Receive a rating of 4 or higher on the Clinical Global Impression‑Severity (CGI‑S) score, which reflects a moderate to severe level of illness as judged by a clinician.
  • Have a legal representative (such as a parent or guardian) who can sign a written informed consent form agreeing to take part in the study.
  • Be between 5 and 35 years old (inclusive) at the time of receiving the first study medication.
  • Be either male or female.
  • Have a regular caregiver who is at least 18 years old and can complete the study assessments, and the same caregiver should remain involved for the whole study.

Who cannot join the trial?

13 criteria

  • You cannot join the study if you have had cancer such as lymphoma, leukemia, or any other type in the last 5 years, except for certain skin cancers (basal cell or squamous cell) that were completely removed and have not spread for at least 3 years.
  • You cannot join if your doctor finds that your vital signs (like blood pressure, heart rate, temperature) are unusually abnormal and considered important for your health.
  • You cannot join if a heart or lung test (such as an echocardiogram (ECHO), electrocardiogram (ECG), or physical exam) shows any of the following serious problems that are not approved by the study’s central heart expert:
    • More than a tiny leak in the aortic valve (aortic valve regurgitation).
    • More than a mild leak in the mitral valve (mitral valve regurgitation).
    • Signs of pulmonary arterial hypertension (PAH), which means high pressure in the arteries of the lungs, measured as a pulmonary artery pressure of 35 mmHg or higher.
    • Weakness of the left side of the heart (left ventricular dysfunction), whether it does not pump well when the heart beats (systolic) or when it relaxes (diastolic).
    • Any major heart structural problem, such as a floppy mitral valve (mitral valve prolapse), holes between the heart chambers (atrial or ventricular septal defects), or a persistent opening called a patent ductus arteriosus that allows blood to flow the wrong way (right‑to‑left shunt).
    • You cannot join if you have a serious medical condition that could affect the study, including:
      • Chronic obstructive pulmonary disease (COPD) – a long‑term lung disease that makes breathing difficult.
      • Interstitial lung disease – scarring or inflammation of lung tissue.
      • Portal hypertension – high blood pressure in the vein that carries blood to the liver.
      • Need for a breathing machine that goes directly into the airway (such as a tracheostomy).
      • Recent (within the past 4 weeks) any illness or symptoms that could make it unsafe or hard to take part in the study.
      • You cannot join if you are currently taking more than four seizure medicines at the same time (rescue medicines used for emergencies do not count).
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Investigated drugs

Fintepla 2.2 mg/ml oral solution is a liquid medicine taken by mouth that contains the active ingredient fenfluramine hydrochloride. In this trial it is being tested to see if it can help people with Rett syndrome, a condition that affects brain development and causes severe physical and cognitive challenges. The study will observe how well the medicine works to improve symptoms and will also monitor any side effects to assess its safety.

What is already known about the treatment

Fintepla 2.2 mg/ml oral solution - Fintepla 2.2 mg/ml oral solution is taken by mouth as a liquid, usually dosed at 0.8 mg per kilogram of body weight. It is an approved orphan drug for seizure disorders and is being studied in Rett syndrome, a rare neurodevelopmental condition. The medicine works by increasing the levels of serotonin, a brain chemical that helps calm nerve activity. It belongs to the class of serotonergic agents that act as serotonin‑releasing agents.

Investigated diseases

Rett syndrome - Rett syndrome is a rare neurodevelopmental disorder that mainly affects girls. Children usually develop normally for the first six to eighteen months, after which they begin to lose previously acquired skills such as purposeful hand use and spoken language. As the condition progresses, they may experience difficulties with coordination, balance, and breathing patterns. Communication abilities often become limited, and repetitive hand movements may appear. Over time, the disorder can lead to ongoing challenges with movement and social interaction.
Trial detailsLast updated 2 Oct 2026
Age18+ yearsPhasePhase IIITrial ID2025-523157-34-00Protocol codeEP0247Estimated enrolment221 patientsSponsorUCB Biosciences Inc.

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