Inclusion Criteria:  Subject has a confirmed pathogenic or likely pathogenic mutation in the CDKL5 gene and a clinical diagnosis of CDD with epilepsy onset in the first year of life, plus motor and developmental delays. Subject is male or female, aged 1 to 35 years, inclusive, as of the day of the Screening Visit. Subject must have failed to achieve seizure control despite previous or current use of 2 or more AETs. Subject is currently receiving at least 1 concomitant antiseizure treatment: antiseizure medication (ASM), vagus nerve stimulation (VNS), responsive neurostimulation (RNS), or ketogenic diet (KD). All medications or interventions for epilepsy (including VNS, RNS, and KD) must be stable prior to screening and are expected to remain stable throughout the study. At the Screening Visit, parent/caregiver reports that subject has ≥ 4 countable motor seizures(CMS) per week.  Exclusion Criteria:  Subject has a known hypersensitivity to fenfluramine or any of the excipients in the study drug. Subject has a diagnosis of pulmonary arterial hypertension. Subject has a clinically significant medical condition, including chronic obstructive pulmonary disease, interstitial lung disease, or portal hypertension, or has had clinically relevant symptoms or a clinically significant illness currently or 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. Subject has current or past history of cardiovascular or cerebrovascular disease, such as cardiac valvulopathy, myocardial infarction or stroke, severe ventricular arrhythmias, or clinically significant structural cardiac abnormality, including but not limited to mitral valve prolapse, atrial or ventricular septal defects, patent ductus arteriosus, and patent foramen ovale with reversal of shunt. (Note: Patent foramen ovale or a bicuspid aortic valve are not considered exclusionary). Subject has moderate to severe hepatic impairment. Subject has current eating disorder that suggests anorexia nervosa or bulimia. Subject has a current or past history of glaucoma. Subject is taking > 4 concomitant ASMs. Rescue medications are not included in the count. Subject is receiving concomitant treatment with cannabidiol (CBD) other than Epidiolex/Epidyolex or is being actively treated with tetrahydrocannabinol (THC) or any marijuana product for any condition. Subject has participated in another interventional clinical trial within 30 days of the Screening Visit or is currently receiving an investigational product. Subject has previously been treated with Fintepla® (fenfluramine) prior to the Screening Visit.