Inclusion Criteria:

• Subject is aged 2 to 18 years inclusive, as of the day of the core study Screening Visit.
• Subject has satisfactorily completed the core study in the opinion of the investigator and the Sponsor.
  • NOTE: Those subjects who do not complete the 12-week Maintenance Period of the core study may, on a case-by-case basis, be eligible for entrance after consideration of the circumstances of the early termination and the potential benefit-risk of continued participation in a ZX008 trial. The decision whether to permit open-label extension study participation resides solely with the Sponsor, who may consult with the site investigator, the IPCAB and/or the IDSMC.
• Subject is male or non-pregnant, non-lactating female. Female subjects of childbearing potential must not be pregnant or breast-feeding. Female subjects of childbearing potential must have a negative urine pregnancy test. Subjects of childbearing or childfathering potential must be willing to use medically acceptable forms of birth control, which includes abstinence, while being treated on this study and for 90 days after the last dose of study drug.
• Subject has documented medical history to support a clinical diagnosis of DS, where convulsive seizures are not completely controlled by current antiepileptic drugs.
• Subject has been informed of the nature of the study and informed consent has been obtained from the legally responsible parent/guardian.
• Subject has provided assent in accordance with IRB/IEC requirements, if capable.
• Subject’s caregiver is willing and able to be compliant with diary completion, visit schedule and study drug accountability.
• Subject’s parent/caregiver has been compliant with diary completion during the core study, in the opinion of the investigator (e.g., at least 90% compliant).
• Subjects entering from study ZX008-1504 must be receiving a therapeutically relevant and stable dose of CLB and/or VPA, and STP (Cohort 1 dose regimen 3 and Cohort 2 only) for at least 4 weeks prior to screening and are expected to remain stable throughout the study.
• Subjects who are > 18 to ≤ 35 years of age at the time of screening and did not participate in one of the core studies must meet criteria 3 to 7 above and the following criteria below in order to be considered for participation. Participation is at the discretion of the Sponsor:
  • Onset of seizures in the first year of life in an otherwise healthy infant;
  • A history of seizures that are either generalized tonic-clonic or unilateral clonic or bilateral clonic, and are prolonged;
  • Initial development is normal;
  • History of normal brain MRI without cortical brainmalformation;
  • Lack of alternative diagnosis;
  • Meets one of the following 3 confirmatory diagnostic criteria:
    • Emergence of another seizure type, including myoclonic, generalized tonic-clonic, tonic, atonic, absence and/or focal has developed after the first seizure type;
    • Prolonged exposure to warm temperatures induces seizures and/or seizures are associated with fevers due to illness or vaccines, hot baths, high levels of activity and sudden temperature changes and/or seizures are induced by strong naturaland/or fluorescent lighting, as well as certain visual patterns;
    • Genetic test results consistent with a diagnosis of Dravet syndrome (pathogenic, likely pathogenic, variant of unknown significance, or inconclusive but unlikely to support an alternative diagnosis);
  • Subject has been approved for study inclusion by the Epilepsy Study Consortium;
  • Subject does not have an exclusionary cardiovascular or cardiopulmonary abnormality based on ECHO, ECG or physical examination and is approved for entry by the central cardiac reader. Exclusionary abnormalities include, but are not limited to:
    • Mild or greater mitral or aortic valve regurgitation in subjects > 18 yrs of age;
    • Possible signs of pulmonary hypertension with abnormal or greater than upper limit of normal values iii. Evidence of diastolic dysfunction;
    • Subject must have had ≥ 4 convulsive seizures (tonic, tonic-atonic, tonicclonic, clonic) per 4-week period for past 12 weeks prior to screening, by parent/guardian report to investigator or investigator medical notes.
  • 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 screening and are expected to remain stable throughout the study.
• Subject’s parent/caregiver is willing and able to be compliant with diary completion, visit schedule and study drug accountability.

Exclusion Criteria:

• Subject has a known hypersensitivity to fenfluramine or any of the excipients in the study medication.
• Subject has current or past history of cardiovascular or cerebrovascular disease, myocardial infarction or stroke.
• Subject from one of the core studies with current cardiac valvulopathy or pulmonary hypertension that the investigator, parent, IPCAB, IDSMC, or Sponsor deems clinically significant and warrants discontinuation of study medication.
• For de novo subjects: possible signs of pulmonary hypertension with abnormal or greater than upper limit of normal values.
• Subject has 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 is at imminent risk of self-harm or harm to others, in the investigator’s opinion, based on clinical interview and responses provided on the Columbia-Suicide Severity Rating Scale (C-SSRS). Subjects must be excluded if they report suicidal behavior as measured by the C-SSRS Since Last Visit, which includes suicidal ideation with intent and plan (Item #5). If a subject reports suicidal ideation on Item 4 without specific plan, and the investigator feels that the subject is appropriate for the study considering the potential risks, the investigator must document appropriateness for inclusion, and discuss with the parent/caregiver to be alert to mood or behavioral changes, especially around times of dose adjustment.
• Subject has a current or past history of glaucoma.
• Subject has moderate or severe hepatic impairment. Asymptomatic subjects with mild hepatic impairment (elevated liver enzymes < 3x upper limited of normal [ULN] and/or elevated bilirubin < 2 x ULN) may be entered into the study after review and approval by the Medical Monitor in conjunction with the Sponsor, in consideration of comorbidities and concomitant medications.
• 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; atomoxetine, or other centrally-acting noradrenergic agonist; cyproheptadine, and/or cytochrome P450 (CYP) 2D6/3A4/2B6 inhibitors/substrates.
  • Note: Short-term medication requirements will be handled on a per case basis by the Medical Monitor.
• Subject is currently taking carbamazepine, oxcarbazepine, eslicarbazepine, phenobarbital, or phenytoin, or has taken any of these within the past 30 days, as maintenance therapy.
• For subjects entering from core studies ZX008-1501, ZX008-1502, or ZX008-1504 (Cohort 1/dose regimens 1 & 2): Subject is currently receiving or has received STP in the past 21 days prior to core study Visit 1.
• Subject is unwilling to refrain from large or daily servings of grapefruits and/or Seville oranges, and their juices beginning with Visit 1 and throughout the study.
• Subject has positive result on urine tetrahydrocannabinol (THC) Panel or whole blood cannabidiol (CBD) at Visit 1.
• Subject is unwilling or unable to comply with scheduled visits, drug administration plan, laboratory tests, other study procedures, and study restrictions.
• Subject has a clinically significant condition, or has had clinically relevant symptoms or a clinically significant illness in the 4 weeks prior to Visit 1, other than epilepsy, that would negatively impact study participation, collection of study data, or pose a risk to the subject, including chronic obstructive pulmonary disease, interstitial lung disease, or portal hypertension.
• Subject has participated in another clinical trial within the past 30 days (i.e., the last visit of the previous study was in the past 30 days), with the exception of the core studies.