Soticlestat in Dravet and Lennox-Gastaut Syndromes: Early Termination of a Phase III Study in Fenfluramine-Exposed Patients

Background and Rationale for the Study

Dravet syndrome (DS) and Lennox-Gastaut syndrome (LGS) are rare, severe childhood-onset epileptic encephalopathies marked by refractory seizures and significant developmental impairment. Soticlestat, an investigational cholesterol 24-hydroxylase (CH24H) inhibitor, has demonstrated promise in previous studies for reducing seizure burden. This Phase III trial (NCT06422377, TAK-935-3004), sponsored by Takeda, aimed to evaluate the efficacy and safety of soticlestat as add-on therapy in patients with DS or LGS who had prior or current exposure to fenfluramine—an established therapy in these populations. The design acknowledged the clinical need for additional effective and safe therapies for individuals with drug-resistant epileptic syndromes.

Study Design and Population

This open-label, single-arm, nonrandomized interventional study planned to enroll approximately 45 patients aged 2 to 65 years across multiple European sites. Eligibility required documented fenfluramine exposure and a confirmed diagnosis of DS or LGS. For LGS, participants needed a seizure history with an average of at least 12 major motor drop (MMD) seizures in the 90 days before screening and more than 4 MMD seizures in at least four weeks of baseline seizure data. The protocol excluded those with certain psychiatric risks or hypersensitivity to soticlestat.

Participants were to receive soticlestat orally, titrated from 100–300 mg (dose dependent on body weight) over four weeks, followed by up to 48 weeks of maintenance. The primary endpoints were percent change from baseline in convulsive seizure frequency (DS) or MMD seizure frequency (LGS) per 28 days during the first 12 weeks of maintenance. Patients continued on their standard antiseizure therapies alongside soticlestat.

Trial Conduct and Early Termination

Despite its robust design, the study enrolled only one participant. The sponsor terminated the trial early following a business decision unrelated to safety, as sufficient data for regulatory purposes had already been generated from preceding studies and additional data from this specific trial were deemed unnecessary. No safety concerns prompted the termination.

Due to the enrollment of a single participant, efficacy and safety outcomes remain undisclosed to maintain confidentiality and cannot provide actionable evidence regarding soticlestat’s benefit or risk profile in this particular cohort. The study protocol, statistical analysis plan, and other documents are available for qualified researchers through Takeda’s data-sharing platforms.

Endpoints and Methodology

The study’s primary endpoints—reduction in seizure frequency (convulsive for DS, MMD for LGS) from baseline—mirror regulatory expectations for antiepileptic drug trials in these disorders. The open-label, single-arm approach reflects the practical challenges of recruitment and retention in rare, refractory epilepsies, with participants often having tried and failed multiple therapies.

No meaningful demographic or adverse event data were reported, again due to the single-participant enrollment and early termination.

Reflections and Future Directions

While the early termination of this Phase III soticlestat trial limits direct clinical insights, the experience highlights key considerations in rare disease research: recruitment feasibility, trial design in heavily pretreated populations, and the necessity of adaptive business strategies in development pipelines.

Could more flexible, global, or adaptive trial designs facilitate recruitment and retention in these ultra-rare epileptic syndromes? How should future studies balance regulatory requirements, ethical considerations, and business realities to optimize data generation and access for patients and clinicians? What lessons can sponsors and CROs extract for the design and selection of future late-phase studies in refractory epilepsy?

This study underscores the ongoing challenges in advancing therapies for DS and LGS, emphasizing the need for robust planning and flexibility in rare disease clinical research.