• Participant is 15 years of age or older for first cohort; 12 years of age or older for second cohort. Note that age requirements for eligibility differ by cohort, as follows: the age limit for Cohort 1 is 15 years of age and above and the age limit for Cohort 2 may decrease to 12 years, pending a DSMB letter of recommendation, based on review of interim data analysis and concurrence with NINDS.

• Participant has medically intractable epilepsy defined as failure to achieve acceptable seizure control without unacceptable medication related side effects despite trials of 2 or more antiseizure medications.

• Participant had an average of ≥ 5 drop seizures per month in the 2 months preceding enrollment. A drop seizure is defined as an epileptic seizure (atonic, tonic, tonic-clonic, or myoclonic) involving the entire body, trunk, or head that leads or could lead to a fall, injury, or slumping in a chair.

• Participant's seizures are non-localized.

• Participant's scalp recorded EEG has features of LGS, such as multifocal spike, slow spike and wave discharges, and paroxysmal fast activity.

• Participant must (a) have a stable antiseizure medication (ASM) regimen for the 2 months preceding enrollment and (b) be willing to remain on the stable regimen, as medically able, through the Blinded Evaluation Period; rescue medication for acute seizure clusters are permitted. A stable ASM regimen is defined as no introduction or discontinuation of an ASM, and no change in an ASM dose of more than 25%.

• Participant is not on a therapeutic diet for epilepsy, or if participant is on a therapeutic diet for epilepsy must (a) have a stable diet for the 2 months preceding enrollment and (b) be willing to remain on the stable diet, as medically able, through the Blinded Evaluation Period.

• Participant does not have a vagus nerve stimulator (VNS), or if participant does have a VNS must (a) have had the VNS off for the 2 months preceding enrollment and (b) be willing to remain with the VNS off through the Blinded Evaluation Period.

• Participant is a male, or is a female of childbearing potential who is surgically sterile, 2 years postmenopausal, or practices a reliable method of contraception (hormonal, barrier method or abstention).

• Participant is willing to give informed consent (or assent, if a minor); if the participant assents or is not able to give informed consent, parent/legal guardian is willing to give informed consent.

• Participant is able to maintain a seizure log alone or with the assistance of a competent individual.

• Participant is able to attend study appointments in accordance with the study schedule.