A 2-Part Study to Investigate the Dose-Ranging Safety and Pharmacokinetics, Followed by the Efficacy and Safety of ZX008 (Fenfluramine Hydrochloride) Oral Solution as an Adjunctive Therapy in Children ≥ 2 Years Old and Young Adults With Dravet Syndrome

Key Inclusion Criteria: * Subject must be male or non-pregnant, non-lactating female, aged 2 to 18 years (inclusive). * Subject must have documented medical history to support a clinical diagnosis of Dravet syndrome, where convulsive seizures are not completely controlled by current antiepileptic drugs. * Subject must be receiving a therapeutically relevant and stable dose of stiripentol (STP) plus clobazam (CLB) and/or valproate (VPA), and for at least 4 weeks prior to screening and be expected to remain stable throughout the study (Cohort 2 only). * Subject must be receiving a stable dose of CLB and VPA, administered twice daily (BID), to be eligible for Dose Regimen 1 and 2, or subject must be receiving a stable dose of CLB, VPA, and STP, administered BID, to be eligible for Dose Regimen 3 (Cohort 1 only). Key Exclusion Criteria: * Subject has a known hypersensitivity to fenfluramine or any of the excipients in the study medication. * Subject has pulmonary arterial hypertension. * Subject has a current or past history of cardiovascular or cerebrovascular disease, such as cardiac valvulopathy, myocardial infarction, or stroke. * Subject has a current or recent history of anorexia nervosa, bulimia, or depression within the prior year that required medical treatment or psychological treatment for a duration greater than 1 month. * Subject has a current or past history of glaucoma. * Subject is receiving concomitant therapy with: centrally acting anorectic agents; monoamine-oxidase inhibitors; any centrally acting compound with clinically appreciable amount of serotonin agonist or antagonist properties, including serotonin reuptake inhibition; triptans, atomoxetine, or other centrally acting noradrenergic agonists; cyproheptadine, and/or cytochrome P450 (CYP) 2D6/3A4/2B6 inhibitors/substrates. * Subject is currently taking carbamazepine ,oxcarbazepine, eslicarbazepine, phenobarbital, or phenytoin, or has taken any of these within the past 30 days, as maintenance therapy. * Subject has a positive result on urine tetrahydrocannabinol (THC) panel or whole blood cannabidiol (CBD) at the Screening Visit. * Subject has a clinically significant condition, or has had clinically relevant symptoms or a clinically significant illness in the 4 weeks prior to the Screening Visit, other than epilepsy, that would negatively impact study participation, collection of study data, or pose a risk to the subject.