A Study to Assess the Safety and Tolerability of ZX008 in Children and Young Adults With Dravet Syndrome or Lennox Gastaut Syndrome Currently Taking Cannabidiol

Key Inclusion Criteria: * Subject has a clinical diagnosis of DS or LGS, where convulsive seizures (generalized tonic clonic seizures, tonic seizures, atonic seizures, tonic/atonic seizures, focal seizures with clear observable motor signs) are not completely controlled by current AEDs. * All medications or interventions for epilepsy (including ketogenic diet \[KD\] and vagal nerve stimulation \[VNS\]) must be stable for at least 4 weeks prior to first ZX008 dose and are expected to remain stable throughout the study. * Subjects must be taking a stable dose of CBD for at least 4 weeks prior to first ZX008 dose. * Subject's source of CBD is expected to be consistent for at least 3 months during study participation. Key Exclusion Criteria: * Subject has current or past history of cardiovascular or cerebrovascular disease, myocardial infarction or stroke. * Subject with current cardiac valvulopathy or pulmonary hypertension that the Investigator, parent/caregiver, Independent Cardiac Advisory Board, Independent Data and Safety Monitoring Committee, or sponsor deems clinically significant. * Subject has a current or past history of glaucoma.