Study to Evaluate NBI-921352 as Adjunctive Therapy in Subjects With SCN8A Developmental and Epileptic Encephalopathy Syndrome (SCN8A-DEE)

Inclusion Criteria:

1. Male or female 2 to 21 years of age, inclusive.

2. Have a diagnosis of SCN8A-DEE supported by both clinical and genetic findings

3. Have on average at least 1 countable motor seizure per week and not be seizure-freefor more than 20 consecutive days

4. Being treated with at least 1 other antiseizure medication (ASM), but no more than 4ASMs

5. Have failed to achieve seizure freedom with at least 2 ASMs

6. Must be using a nocturnal alerting system or practice consistent with standards ofcare at the time of screening and continue to use this for the duration of the study

7. Must have an adequate rescue medication regimen per the investigator's judgment inplace at the time of screening and for the duration of the study

8. Have a body weight of at least 10 kg

9. The subject's parent/caregiver is able to accurately identify seizure types,especially countable motor seizures (defined as GTCS, tonic, atonic or FOS withnoticeable motor component) and is able to complete seizure diary

Exclusion Criteria:

1. Have previously been enrolled in this study and received blinded treatment

2. Have participated in an interventional clinical trial < 30 days prior to screening

3. Have symptoms that would be more consistent with another epilepsy disorder such asDravet syndrome (eg, fever-induced episodes of status epilepticus, frequentmyoclonic seizures, worsening on sodium channel blockers, absence seizures withgeneralized spike-and-wave EEG as the sole seizure type)

4. Are currently receiving cannabinoids or medical marijuana exceptEpidiolex/Epidyolex, unless approved by the Sponsor

5. Are currently taking systemic steroids (excluding inhaled medication for asthmatreatments and intranasal steroids for allergies). If subject has received thesemedications in the past, must be off these medications for at least 3 months priorto the screening visit and these drugs may not be initiated during the duration ofthe study. Intermittent steroids to treat nonepilepsy related diseases (such asallergies or dermatological conditions) are not exclusionary

6. Have a history of moderate or severe head trauma or other neurological disorders orsystemic medical diseases that are, in the investigator's opinion, likely to affectnervous system functioning

7. Have a clinically significant medical condition or chronic disease, that in theopinion of the investigator would preclude the subject from participating in andcompleting the study or that could confound interpretation of study outcome

8. Have clinically significant abnormal vital signs at the screening visit asdetermined by the investigator

9. Have one or more clinical laboratory test values outside the reference range, basedon blood samples taken at the screening visit, that are of potential risk to thesubject's safety as determined by the investigator

10. Have, at the screening visit, an electrocardiogram (ECG) finding of a corrected QTinterval using Fridericia's formula (QTcF) > 450 msec or presence of any significantcardiac abnormality.