• A diagnosis of CDKL5 deficiency, including molecular confirmation of a pathogenic or likely pathogenic CDKL5 variant and refractory seizures

• Male or female participants aged 6 months to less than 6 years

• Parent(s) or LAR willing to give written informed consent, after being properly informed of the nature and risks of the study and prior to engaging in any study-related procedures

• Failure to control seizures despite appropriate trial of 2 or more anti-seizure medications at therapeutic doses

• Have a history of at least 8 countable seizures per 4 weeks during the 8 weeks prior to screening. Countable seizures will be defined by the following:Seizures with or without impairment of consciousness with a clear motor component, including generalized tonic-clonic, focal to bilateral tonic-clonic, atonic, bilateral clonic, bilateral tonic, focal motor seizures with or without impaired awareness, or infantile spasms. Clusters of infantile spasms/tonic seizures will be counted as a single seizure.

• Participants should be on a stable regimen of anti-seizure medications for ≥ 4 weeks prior to the screening visit, without a foreseeable change in dosing for the duration of the DB phase.

• Vagus nerve stimulator (VNS) are permitted, but should be implanted for 1 year prior to screening and settings should be unchanged throughout the study.

• Parent/caregiver is able and willing to maintain an accurate and complete daily seizure eDiary for the duration of the study.