Evaluation of Long-Term Safety and Efficacy of PTC518 in Huntington's Disease: A Phase 2b Double-Blind, Randomized Extension Study

The primary objective of this Phase 2b, double-blind, randomized extension study is to evaluate the long-term **safety** of PTC518 in participants with **Huntington's Disease**. Additionally, the study aims to assess the **efficacy** of PTC518 by evaluating its pharmacodynamic effects through the reduction in blood total huntingtin (tHTT) levels. These objectives are clinically relevant as they address the potential of PTC518 to provide a therapeutic benefit in managing Huntington's Disease, a progressive neurodegenerative disorder characterized by motor dysfunction, cognitive decline, and psychiatric symptoms.

Secondary objectives include assessing the long-term effects of PTC518 on changes in caudate volume using volumetric magnetic resonance imaging (vMRI) and evaluating changes in the composite Unified Huntington’s Disease Rating Scale (cUHDRS). Additionally, the study aims to determine the long-term effect of PTC518 on mutant Huntingtin (mHTT) protein levels in cerebrospinal fluid (CSF) and blood. These secondary objectives are crucial for understanding the broader impact of PTC518 on disease progression and its potential to modify the underlying pathophysiology of Huntington's Disease.

The clinical trial involves a total of **186 participants** diagnosed with **Huntington's Disease**. The study population includes both male and female subjects, aged 25 years and older, who have completed the Treatment Period in Study PTC518-CNS-002-HD. Participants are required to be ambulatory and capable of providing informed consent, ensuring compliance with all protocol requirements. The trial population was selected based on their previous involvement in related studies, and specific lifestyle considerations such as the use of effective contraception methods are mandated for women of childbearing potential and sexually active males. The study does not specify any particular dietary or physical activity requirements. The trial includes a vulnerable population, indicating a careful selection process to ensure participant safety and adherence to ethical standards.

The clinical trial is a **Phase 2b**, double-blind, randomized extension study designed to evaluate the long-term safety and efficacy of **PTC518** in participants with **Huntington's Disease**. The trial employs a controlled design with participants randomly assigned to receive either the investigational product, PTC518, or a placebo. The study aims to assess the safety profile through the characterization of treatment-emergent adverse events (TEAEs), laboratory abnormalities, vital signs, physical examinations, and the Columbia-Suicide Severity Rating Scale (C-SSRS). Additionally, the trial will evaluate the pharmacodynamic effects of PTC518 by measuring reductions in blood total huntingtin (tHTT) levels over time.

The trial is expected to run until June 2027, with participant recruitment having commenced in August 2023. Participants will be involved in the study for a maximum treatment period of 24 months. The sequence of study visits includes an initial screening visit to confirm eligibility, followed by regular follow-up visits to monitor safety and efficacy parameters. The end-of-study visit will conclude the participant's involvement, during which final assessments will be conducted. Participants are required to be ambulatory males or females aged 25 years and older who have completed the treatment period in a prior study, PTC518-CNS-002-HD. They must be willing to provide informed consent and comply with all protocol requirements.

Inclusion criteria necessitate that women of childbearing potential use highly effective contraception during dosing and for six months after stopping the study medication. Similarly, sexually active and fertile males must use a condom during intercourse while taking the study drug and for six months post-treatment. Conditions that may lead to early termination from the study include non-compliance with protocol requirements or the occurrence of significant adverse events. The trial's primary endpoints focus on safety and the evaluation of blood tHTT protein levels, while secondary endpoints include changes in caudate volume, cUHDRS scores, and CSF mHTT protein levels over time.

The clinical trial involves the administration of **PTC518**, an experimental medication developed by PTC Therapeutics, Inc. **PTC518** is a small molecule chemical compound formulated as a tablet for **oral use**. The trial includes two dosage regimens of **PTC518**: a 10 mg tablet and a 20 mg tablet. The maximum daily dose for the 10 mg tablet is 10 mg, while for the 20 mg tablet, it is 20 mg. The treatment period for both dosages is up to 24 weeks. The primary objective of the trial is to evaluate the long-term safety and efficacy of **PTC518** in participants with Huntington's disease, focusing on the reduction of blood total huntingtin (tHTT) levels. Participant compliance with the dosing schedule will be monitored throughout the study.

In addition to the experimental medication, the study employs a placebo control to ensure the validity of the trial results. The placebo is designed to match the 5 mg and 20 mg **PTC518** tablets in appearance but contains no active substance. The placebo is administered in the same manner as the experimental medication, ensuring that the study remains double-blind. This means neither the participants nor the investigators know who is receiving the active medication or the placebo, which helps to eliminate bias in the assessment of the treatment's efficacy and safety.

The efficacy of PTC518 in the treatment of **Huntington's Disease** will be assessed through a series of pharmacodynamic evaluations. The primary efficacy endpoint involves measuring the reduction in blood total huntingtin (tHTT) protein levels over time. This will be complemented by secondary endpoints, which include changes in caudate volume as assessed by volumetric MRI (vMRI), changes in composite Unified Huntington's Disease Rating Scale (cUHDRS) scores, and evaluations of cerebrospinal fluid (CSF) and blood mutant huntingtin (mHTT) protein levels over time.

These efficacy parameters will be collected and analyzed at specified intervals throughout the trial duration. The use of validated laboratory tests and imaging techniques will ensure the accuracy and reliability of the data collected. The trial is designed to provide a comprehensive assessment of the long-term efficacy of PTC518 in modifying disease progression in participants with Huntington's Disease.