1. Males or females, ages greater than or equal to 2 to less than or equal to 55 years, at the time of Screening.
2. Participant/parent/legal authorized representative (LAR) willing and able to give written informed consent/assent.
3. Diagnosis of LGS, including:

• Evidence of at least one type of countable major motor seizure.
• History of electroencephalogram (EEG) consistent with LGS (abnormal background activity, and one of the following: 1) slow spike-wave discharges [less than 2.5 Hz], or 2) paroxysmal fast activity during sleep).
• Abnormal cognitive development.
• Onset of seizures at 11 years of age or younger.

Key

1. Known sensitivity, allergy, or previous exposure to clemizole HCl.
2. Known history of long QT syndrome or any significant history of a serious abnormality of the electrocardiogram (ECG) (e.g., recent myocardial infarction, clinically significant arrhythmia).
3. Family history of sudden cardiac death, unexplained death, or death from a primary dysrhythmia potentially associated with QT prolongation in any family member.
4. Seizures secondary to illicit drug or alcohol use, infection, neoplasm, demyelinating disease, degenerative neurological disease, or progressive central nervous system disease, metabolic illness, recent anoxic episode within the last 6 months requiring resuscitation, or progressive degenerative disease or any other condition, which in the opinion of the investigator, could affect seizure control.
5. Epilepsy surgery planned during the study or epilepsy surgery within 6 months prior to Screening.
6. Concomitant use of fenfluramine.

7/ Prior or concomitant use of lorcaserin.