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

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 a…