Extension Study to Evaluate How Safe and Tolerable NBI-921352 is as an Adjunctive Therapy for Participants With SCN8A-DEE

This is more likely than not to read as positive because it is a long-term extension study focused on safety and tolerability, not a randomized efficacy test with a hard statistical hurdle. Extension studies are usually enriched for participants who reached rollover and could remain on therapy, which lowers the chance of an outright negative outcome versus an initial de novo cohort. The primary endpoint is also practical and safety-oriented: serious TEAEs, discontinuations due to TEAEs, and fatal TEAEs through Week 168. In a severe pediatric epileptic encephalopathy, an acceptable safety profile can be enough for a favorable interpretation if no clear drug-related safety signal emerges. Risks remain meaningful: this is a medically fragile population, exposure is very long, and even a few serious events in a rare-disease sample can make tolerability look poor. Status near primary completion supports operational progress, but disclosure can still be uneven for extension studies. Netting these factors, I favor positive but only with moderate confidence.

This is an open-label extension study (NCT05226780) of the KAYAK Phase 2 trial for NBI-921352 in SCN8A-DEE. Multiple critical negative signals exist: (1) The parent KAYAK study has been reported as discontinued by the SCN8A patient advocacy group (The Cute Syndrome Foundation). (2) NBI-921352 already failed its Phase 2 FOS study in Nov 2023, showing no meaningful seizure reduction. (3) Neurocrine's Q4 2025 earnings and 2026 focus make no mention of NBI-921352, focusing instead on INGREZZA, CRENESSITY, osavampator, and direclidine. (4) Neurocrine is advancing a next-generation dual NaV1.2/1.6 inhibitor (NBI-921355), suggesting they've moved past the NBI-921352 program. While the extension study's primary endpoint is safety/tolerability (TEAEs), a discontinued parent study strongly suggests the overall program failed. The estimated primary completion date has already passed (March 2026) with no positive results announced. Probability of positive resolution is low at ~20%.

This is a Phase 2 extension study focused on safety and tolerability of NBI-921352 in SCN8A-DEE patients. The primary endpoint measures serious TEAEs, discontinuations, and fatal events through Week 168. Extension studies typically enroll patients who already demonstrated tolerability in the parent study, providing an inherent safety signal. The sponsor (Neurocrine Biosciences) is experienced in CNS drug development. The primary completion date has passed, suggesting data may be mature. For a safety study, 'positive' typically means no unexpected serious safety signals, which has higher intrinsic probability than efficacy endpoints. However, the binary threshold and ambiguity around what constitutes 'positive' results (safety vs. efficacy) introduces uncertainty. The 60% confidence reflects moderate conviction given the safety-focused design and prior patient selection.

This is an open-label extension study (NCT05226780) designed to evaluate long-term safety and tolerability of NBI-921352 as adjunctive therapy in SCN8A-DEE, a rare genetic epilepsy. Extension studies typically have a high probability of reporting positive safety results, as they enroll participants who have already tolerated the drug in prior controlled trials. The primary endpoint is incidence of serious TEAEs, discontinuations, and fatalities over 168 weeks—a standard safety assessment. Neurocrine Biosciences is an experienced CNS sponsor. The main risk is that long-term exposure in a pediatric population with severe epilepsy could reveal new safety signals, but the prior Phase 2 data likely supported initiating this extension. The study is 'Active Not Recruiting,' indicating completion of enrollment and ongoing follow-up. Given the design and typical outcomes for extension studies, intrinsic odds favor a positive safety report.

Open-label extension study assesses long-term safety/tolerability of NBI-921352 as adjunctive therapy in SCN8A-DEE, a rare severe epilepsy with high baseline morbidity and AE rates. Primary endpoint—incidence of serious TEAEs, discontinuations, and fatalities to Week 168—is descriptive, standard for extensions but limited by small n (rare population), no control arm, and low power for rare events. Rollover design implies parent study tolerability threshold met. Endpoint quality moderate: composite captures key risks (e.g., cardiac for NaV1.6 blocker) but historical comparator needed for context. Neurocrine (NBIX) has strong neurology execution track record. Operational: primary completion overdue 32 days, yet active not recruiting signals no major safety halts, data maturing. Disclosure risk minimal—sponsors report safety fully, even if neutral. Extensions rarely fail outright if initiated; low attributable serious AE/discontinuation rates likely, supporting positive readout for advancement.

This Phase 2 extension study (NCT05226780) for NBI-921352 in SCN8A-DEE has a primary completion date of March 2026, indicating data collection is finalized. The primary endpoint focuses exclusively on safety and tolerability (incidence of TEAEs) over 168 weeks, rather than efficacy. Extension studies inherently select patients who tolerated the drug in the parent trial, creating a strong selection bias toward favorable safety profiles. The 'Active Not Recruiting' status confirms operational success without early termination. SCN8A-DEE is a severe rare disease with high unmet need, where regulators often accept wider safety margins. Given the survival of the cohort through the parent study and the non-comparative nature of the endpoint, the probability of a 'positive' (safe/tolerable) result is high, driven by patient selection and study design rather than therapeutic novelty.

This Phase 2 extension study evaluates safety/tolerability of NBI-921352 in SCN8A-DEE over 168 weeks. The primary endpoint focuses on serious TEAEs, discontinuations, and fatalities—favorable safety bar for an extension study where participants have already tolerated initial treatment. SCN8A-DEE is a severe, genetic epilepsy with high unmet need; prior Phase 1/2 data (NCT04669223) showed meaningful seizure reduction with manageable safety. The study completed primary completion 32 days ago with 'Active Not Recruiting' status, suggesting clean data collection without safety holds. Extension studies in rare diseases with positive parent trials typically report favorable safety profiles. Disclosure risk is moderate given Neurocrine's pattern of conference presentations. No competing therapies create favorable regulatory context.

NBI-921352 is a NaV1.6-selective sodium channel inhibitor developed by Neurocrine Biosciences. This prediction market asks if the Phase 2 open-label extension study (NCT05226780) in patients with SCN8A-DEE will yield positive results. The intrinsic probability of positive results is close to zero because the sponsor has discontinued the asset's clinical development. In late 2023, Neurocrine announced that NBI-921352 failed to demonstrate a meaningful reduction in seizure frequency in a Phase 2 trial for focal onset seizures. Following this failure, the company halted development in that indication and placed the SCN8A-DEE program under review. Consequently, the main Phase 2 efficacy trial (NCT04873869, the KAYAK study) was prematurely terminated due to sponsor decision, and patient advocacy groups confirmed the trial's discontinuation. Since the parent study was terminated and development of the drug has been abandoned, the extension study cannot fulfill the criteria for a positive resolution, which typically requires meeting endpoints, confirming safety, and moving to the next development stage. Therefore, no positive data readout will emerge, and the market should resolve NO.

The trial is an extension study evaluating the safety and tolerability of NBI-921352 as an adjunctive therapy for participants with SCN8A-DEE. Given that the study is active but not recruiting, it implies that the initial phases have shown promise, reducing the likelihood of severe safety concerns. The primary endpoint focuses on the incidence of serious adverse events over an extended period, which is a critical aspect for a condition with significant unmet medical needs. The sponsor, Neurocrine Biosciences, has a vested interest in the drug's success, indicated by their continued investment in the trial. While the probability of positive results cannot be certain, the structured and lengthy nature of the trial suggests a favorable outcome is plausible.