A Study to Investigate LP352 in Children and Adults With Developmental and Epileptic Encephalopathies (DEE)

This (DEEp OCEAN Study) is a double-blind, randomized, placebo-controlled, multicenter study to investigate the efficacy, safety, and tolerability of LP352 in the treatment of seizures in children and adults with DEE. The study consists of 3 main phases: Screening, Titration period, Maintenance period, followed by a Taper period and Follow-Up. The total duration of the study will be approximately 24 months.

Inclusion Criteria: * Participants who are characterized as having Lennox-Gastaut Syndrome (LGS) must fulfill all of the following criteria: * Onset of seizures at ≤8 years old * History of tonic/tonic-atonic seizures plus at least 1 of the following seizure type(s): atypical absence, atonic, myoclonic, focal impaired awareness, generalized tonic-clonic, nonconvulsive status epilepticus, or epileptic spasms * Presence of developmental plateauing or regression * History of electroencephalogram (EEG) showing generalized slow (\<2.5 Hertz \[Hz\]) spike-and-wave complexes * Participants who are characterized as having DEE (Other) must fulfill all of the following criteria: * Does not meet criteria for LGS * Onset of seizures at ≤5 years old * Presence of developmental plateauing or regression * History of multiple seizure types * History of interictal EEG background showing diffuse or multifocal slowing (with or without epileptiform activity) * The participant has a current occurrence of at least 1 of the following countable motor seizure types: generalized tonic-clonic, tonic (bilateral), clonic (bilateral), atonic (bilateral) with truncal/leg involvement, focal motor (including hemiclonic), and focal to bilateral tonic-clonic. * The participant has demonstrated an average of at least 4 countable motor seizures per month for each of the 3 months prior to Screening. * The participant has been taking 1 to 4 antiseizure medications (ASMs) at a stable dose for at least 4 weeks prior to Screening. * The participant, parent, or caregiver is willing and able (in the judgment of the investigator) to comply with completion of the diaries throughout the study. * The participant must be willing and able to provide written informed consent; in instances where the participant is unable to provide consent, an appropriate legal representative. Exclusion Criteria: * The participant has a diagnosis of Dravet Syndrome (DS) or has a mutation of the Sodium channel protein type 1 subunit alpha (SCN1A) gene consistent with DS. * The participant has been admitted to a medical facility for treatment of status epilepticus requiring mechanical ventilation within 3 months prior to Screening. * The participant has a neurodegenerative disorder as indicated by magnetic resonance imaging or genetic testing. * The participant has an acquired lesion/injury unrelated to the primary etiology that could contribute as a secondary cause of seizures. * The participant is receiving exclusionary medications. * The participant is currently using any cannabis product or cannabidiol that is not in oral solution/capsule/tablet form, not obtained from a government-approved dispensary, or contains ≥50% Delta-9-tetrahydrocannabinol (THC). * The participant has unstable, clinically significant neurologic (other than the disease being studied; eg, recurrent strokes), psychiatric, cardiovascular (eg, pulmonary arterial hypertension, cardiac valvulopathy, orthostatic hypotension/tachycardia), pulmonary, hepatic, renal, metabolic, gastrointestinal, urologic, immunologic, hematopoietic, or endocrine disease or other abnormality which may impact the ability of the participant to participate or potentially confound the study results. * The participant is unable or unwilling to comply with any of the study requirements or timelines.