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Safety and tolerability of intrathecal S233107 in participants with spinocerebellar ataxia type 3: a Phase 1b/2a first‑in‑human randomized study

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What is this trial about?

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

Spinocerebellar ataxia type 3 is a rare inherited disorder that gradually impairs coordination, balance and speech. The study tests a new medicine called S233107, which is given by injection directly into the fluid surrounding the spinal cord (intrathecal administration). Some participants receive a harmless substance (placebo) for comparison.

The purpose of the study is to assess the safety and tolerability of the drug. In the first part, participants receive several doses that increase in amount over time; in the second part, all participants continue to receive the drug in an open‑label phase where both researchers and participants know the treatment. Throughout the trial, researchers monitor for side effects (AEs) and perform routine checks such as a heart test (ECG), laboratory analysis of blood and the fluid around the brain and spine (CSF), vital signs, weight and mental health screening. Blood and fluid samples are also taken to understand the drug’s pharmacokinetics, meaning how it is absorbed, distributed, and cleared from the body.

Participants attend regular clinic visits over several months, during which they receive the study medication, undergo the safety checks described above, and provide samples for analysis. The study does not provide any guarantee of benefit, and its primary aim is to gather information on how the drug is tolerated in people with this condition.

The research process

The trial runs in 8 steps – from screening to follow-up. Each step says what happens and what the team monitors.

  1. Step 1

    Randomization and baseline assessments

    After joining the study, the participant is assigned by chance to receive either the study medication or a matching placebo.

    Baseline measurements are taken, including heart rhythm (ecg), blood tests, spinal fluid tests, vital signs, weight, and assessment of mood and safety.

  2. Step 2

    First intrathecal injection

    The participant receives a single injection into the fluid surrounding the spinal cord (intrathecal injection).

    The study medication is supplied as a solution for injection with a concentration of 15 mg/mL or 4 mg/mL; the placebo looks identical but contains no active substance.

  3. Step 3

    Immediate safety monitoring

    After the injection, vital signs, heart rhythm, and any symptoms are checked for a short period to ensure there are no immediate adverse effects.

  4. Step 4

    Multiple ascending dose phase (part 1)

    The participant receives additional intrathecal injections on scheduled visits.

    Each subsequent injection may use a higher dose of the study medication (dose escalation) according to the study plan.

    After each injection, safety checks (ecg, blood and spinal fluid tests, vital signs, weight, and mood assessment) are performed.

  5. Step 5

    Transition to open‑label extension (part 2)

    Once the multiple ascending dose phase is completed, the participant may enter the open‑label extension.

    In this phase, all participants receive the active study medication (s233107) regardless of prior assignment.

  6. Step 6

    Ongoing intrathecal administrations in part 2

    The participant continues to receive intrathecal injections of s233107 at the concentration defined by the study protocol.

    Injections are given at regular intervals as specified by the trial schedule.

  7. Step 7

    Continuous safety and pharmacokinetic monitoring

    Throughout both parts of the trial, blood and spinal fluid samples are collected to measure the amount of s233107 in the body.

    Regular assessments of heart rhythm, laboratory tests, vital signs, weight, and mood are performed to track safety.

  8. Step 8

    Final study visit and overall assessment

    At the end of the trial, a comprehensive evaluation is performed, summarizing safety findings and the behavior of the medication in the body.

Who can join the trial?

6 criteria

  • Be between 18 and 65 years old at the time of the screening visit.
  • Have a confirmed genetic diagnosis of Spinocerebellar ataxia type 3 (SCA3) with a repeat of the DNA letters C‑A‑G (called a CAG repeat) that is 60 or longer on one copy of the ATXN3 gene, as shown by a test from an accredited laboratory.
  • Be able to walk on your own without needing a permanent walking aid (ankle‑foot braces are allowed) and without having to hold onto a support arm while walking outside (this matches an adapted Klockgether stage 1). In addition, you must score at least 3 on the overall Scale of Assessment and Rating of Ataxia (SARA) and have a gait (walking) sub‑score between 1 and 4 at screening.
  • Either have never taken medication for SCA3 symptoms (treatment‑naive) or be on a steady dose of allowed medicines (such as amantadine, buspirone, riluzole, dalfampridine, TRH analogues, varenicline, valproic acid, acetazolamide, or trehalose) for at least 3 months before the baseline visit.
  • Maintain a stable non‑drug therapy routine (for example, physical therapy, speech therapy, or transcranial brain stimulation) for at least 3 months before the baseline visit.
  • Be judged by the study doctor to be medically stable at the start of the trial, based on your medical history, physical exam, laboratory test results, and a 12‑lead electrocardiogram (ECG) (a test that records the heart’s electrical activity).

Who cannot join the trial?

12 criteria

  • You cannot join the study if you have any other type of ataxia (loss of coordination) besides SCA3, such as ataxia caused by alcohol use, head injury, multiple sclerosis, certain brain diseases, or stroke.
  • You are excluded if you have medical reasons that make an MRI (magnetic resonance imaging scan) unsafe, for example certain metal implants or a pacemaker.
  • A history of epilepsy (a condition that causes repeated seizures) or any seizure occurring within the past three years prevents participation.
  • If you have had ongoing problems with alcohol or other drug misuse, abuse, or dependence in the last two years, you cannot take part.
  • Any condition that could interfere with evaluating SCA3 symptoms, such as severe muscle‑bone problems, arthritis (joint inflammation), or nerve injuries, makes you ineligible.
  • Severe vision or hearing loss that cannot be corrected with glasses or hearing aids, and that would affect study tests, is an exclusion reason.
  • Significant spasticity (muscle stiffness) or dystonia (involuntary muscle twists) that would prevent the SARA test (a rating scale for ataxia) from accurately measuring your condition excludes you.
  • If genetic testing shows a CAG repeat length of 60 or more in the ATXN3 gene on both copies of the gene (called homozygous), you cannot participate.
  • People whose SCA3 symptoms began before age 18 are not eligible.
  • Abnormalities on a brain MRI that are not typical for SCA3, or any finding that could increase risk or affect your health, exclude you.
  • A past experience of a moderate or severe post‑dural puncture headache (headache after a spinal needle procedure) that required hospitalization or a blood patch (a procedure to seal the leak) disqualifies you.
  • If you have any medical reason that makes a lumbar puncture (spinal tap) or intrathecal (IT) administration (delivery of medication into the spinal fluid) unsafe, you cannot join the study.
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Investigated drugs

S233107 is an experimental medicine being tested for the first time in people with spinocerebellar ataxia type 3. It is given directly into the fluid that surrounds the spinal cord (intrathecal injection). In this study the drug is administered several times to see if it is safe and well‑tolerated, and to learn how the body processes it. The researchers are looking to find out whether S233107 might help manage the symptoms of this type of ataxia, but the main focus of the trial is to make sure the medicine does not cause harmful side effects.

What is already known about the treatment

  • S233107 Solution for injection 15 mg/mL

    This investigational drug is supplied as a clear liquid that is injected directly into the spinal fluid (intrathecal injection). It is currently in a first‑in‑human Phase 1b/2a trial and has not yet received any regulatory approval. The study is testing it for spinocerebellar ataxia type 3, a rare disorder that causes loss of coordination. S233107 is thought to work by binding to a specific protein that reduces harmful cellular stress, classifying it as an experimental neuroprotective agent.

  • S233107 Solution for injection 4 mg/mL

    This formulation is also a sterile liquid given by intrathecal injection into the cerebrospinal fluid. Like the higher‑strength version, it is still experimental and being evaluated in early‑stage clinical trials. It is being studied for the same condition, spinocerebellar ataxia type 3, to see if it can improve nerve cell function. The drug’s action involves blocking a molecular pathway that leads to protein buildup in nerve cells, placing it in the category of investigational neuroprotective compounds.

Investigated diseases

Spinocerebellar ataxia type 3 - Spinocerebellar ataxia type 3 is a hereditary disorder that affects the brain and spinal cord, leading to loss of coordination and balance. It usually begins in adulthood and gradually worsens over time. Patients may notice unsteady walking, difficulty with fine hand movements, and slurred speech. As the disease advances, muscle stiffness and weakness can develop, and eye movement problems may appear. The condition progresses at different rates, but symptoms typically become more pronounced over many years.
Trial detailsLast updated 7 Oct 2026
Age18+ yearsPhasePhase I/IITrial ID2025-525111-17-00Protocol codeS233107-284Estimated enrolment62 patientsSponsorInstitut De Recherches Internationales Servier IRIS

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