Beeline: A Phase 3 Study in GRIN-related Neurodevelopmental Disorder

Inclusion Criteria: Part A, Participant: * Diagnosed with GRIN-NDD with GRIN1, GRIN2A, GRIN2B, or GRIN2D gene variants known to result in GoF of the NMDA receptor * Phase 3 Cohort 1 (Qualifying Seizures Cohort) ONLY: Experiencing at least 1 CMS per week and ≥4 CMS (generalized or focal) during screening * With history of inadequate response to at least 2 standard antiseizure medications (ASMs) * Phase 3 Cohort 2 (Without Qualifying Seizures Auxiliary Cohort) ONLY: With significant neurodevelopmental symptoms and a GRIN-CGI-S score ≥4 * On a stable dose of standard ASMs for at least 4 weeks prior to screening and should remain on stable doses throughout study participation * On stable nonpharmacological treatments such as ketogenic diet and should remain stable throughout study participation Part B: \- Participant has completed Part A and is eligible to continue study participation according to the judgement of the investigator and sponsor. Exclusion Criteria: PART A, Participant: * Has clinically relevant medical, neurologic, or psychiatric condition and/or behavioral disorder (including those related to GRIN-NDD) that would preclude or jeopardize participant's safe participation or study drug administration or the conduct of the study according to the judgement of the investigator or sponsor. * Is receiving \>4 standard ASMs at screening * Has a body weight of less than 5 kg at screening Part B: \- Participant has clinically relevant medical, neurologic, or psychiatric condition and/or behavioral disorder (including those related to GRIN-NDD) that would preclude or jeopardize participant's safe participation of study drug administration or the conduct of the study according to the judgement of the investigator or sponsor.