Inclusion Criteria:

• The patient and patient’s legal representative (i.e., parent or legal guardian) are willing and able to read, understand, and sign the informed consent form and assent, if applicable.
• Male and female patients aged ≥ 2 and ≤ 17 years at the time of informed consent and the first dose of study drug
• Clinical diagnosis of LGS and a history of, on average, ≥ 4 drop seizures per month during the 3 months immediately prior to Screening based on historical information, and the patient has ≥ 4 drop seizures during a minimum of 4 weeks of seizure data collection during the prospective Baseline Period or Clinical diagnosis of Dravet syndrome and a history of, on average, ≥ 3 convulsive seizures per month during the past 3 months based on the historical information, and the patient has ≥ 3 convulsive seizures during a minimum of 4 weeks of seizure data collection during the prospective Baseline Period.
• Weight of ≥ 10 kg at the Screening visit (Visit 1).
• Currently taking 1 to 4 AEDs at a stable dose for 4 weeks prior to the Screening visit (Visit 1); benzodiazepines used chronically (on daily frequency) to treat seizures are considered AEDs.
• If using a VNS, must have VNS placed at least 3 months prior to the Screening visit with stable settings for > 1 month; VNS parameters must remain constant throughout the study (VNS will not be counted as an AED).
• If on a ketogenic diet, must have started the ketogenic diet at least 3 months prior to the Screening visit (Visit 1), diet should be stable for 4 weeks before the Screening visit (Visit 1); and should continue through the duration of the study (ketogenic diet will not be counted as an AED).
• Failed to become and remain seizure free with trials of at least 2 AEDs.
• The patient and patient’s legal representative (i.e., parent or legal guardian) are willing to keep the AED, VNS, and ketogenic diet regimen(s) stable throughout the study.
• The patient is able to carry out all appropriate assessment and take study drug in the opinion of the Investigator and parent/caregiver.
• The patient has a documented clinical diagnosis of Dravet syndrome or LGS.

Diagnosis of Dravet syndrome supported by:

• Onset of seizures around 6 months of age.
• Initial seizures:
  • Fever-induced or fever-triggered seizures;
  • Hemiclonic or generalized tonic seizures;
  • Prolonged seizures (approximately 15 minutes or longer, some > 30 min).
• Between 1 and 5 years, other seizure types emerge:
  • Myoclonic seizures;
  • Focal awareness altered;
  • Absence;
  • Non-convulsive status (absence or myoclonic);
  • Convulsive status epilepticus;
  • Development normal within 1st year of life, then intellectual disability emerges;
  • May regress with recurrent status epilepticus.

Diagnosis of Lennox-Gastaut syndrome supported by:

• History of abnormal EEG consistent with LGS
  • EEG with slow and/or disorganized background AND one of the following:
    • EEG with bursts of generalized 2.5 Hz (or less) spike and wave activity; or
    • Generalized paroxysmal fast activity (GPFA).
  • Greater than 1 type of generalized seizure for at least 6 months:
    • At least 1 seizure type with drop seizures.
  • Less than 11 years of age at the onset of LGS:
    • Evidence of development delay or regression OR history of special education classed OR measured IQ < 70.
• Sexually active female patients of childbearing potential (defined as first menarche) must agree to use a highly effective method of birth control during the study and for 30 days following the last dose of study drug.

Exclusion Criteria:

• The patient has been admitted to a medical facility and intubated for treatment of status epilepticus 2 or more times in the 3 months immediately prior to the screening visit.
• Non-epileptic events that cannot be reliably distinguished from epileptic seizures (e.g., gastroesophageal reflux, muscle cramps, etc.).
• Patients with history of confirmed cataract (untreated with surgery).
• Unstable, clinically significant neurologic (other than the disease being studied), psychiatric, cardiovascular, ophthalmologic, pulmonary, hepatic, renal, metabolic, gastrointestinal, urologic, immunologic, hematopoietic, endocrine disease, malignancy including progressive tumors, or other abnormality, which may impact the ability to participate in the study or that may potentially confound the study results. It is the responsibility of the Investigator to assess the clinical significance; however, consultation with the Medical Monitor may be warranted.
• Any history of alcohol, opioid, or other drug use disorder, as per the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-V), within the 2 years immediately prior to the Screening visit (Visit 1).
• Suicide attempt within the last year, at significant risk of suicide (either in the opinion of the Investigator or defined as ‘yes’ to suicidal ideation question 4 or 5 on the Columbia-Suicide Severity Rating Scale [C-SSRS] at Screening) or appearing suicidal per Investigator judgment.
• History of human immunodeficiency virus (HIV) infection (patient who has tested positive for human immunodeficiency virus antibodies (HIV)-1/2Ab), or history of active hepatitis B, or active hepatitis C infection. (Note that patients who have been vaccinated against hepatitis B [hepatitis B surface antibody {Ab}-positive] who are negative for other markers of prior hepatitis B infection [e.g., negative for hepatitis B Core Ab] are eligible).
• Abnormal and clinically significant ECG abnormality at Screening:
  • QT interval with Fridericia’s correction method (QTcF) > 450 ms (males) or > 470 ms (females), confirmed with one repeat testing, at the Screening visit.
• Abnormal clinical laboratory test results at the Screening visit that suggest a clinically significant underlying disease that would compromise the well-being of the patient (if the patient has alanine aminotransferase [ALT] and/or aspartate aminotransferase [AST] > 2.5 × the upper limit of normal [ULN], the Medical Monitor should be consulted).
• Participated in a clinical study involving another study drug in the previous month (or 5 half-lives of the study drug, whichever is longer) or currently receiving another study drug.
• Received TAK-935 in a previous clinical study or as a therapeutic agent.
• Immediate family members, or in a dependent relationship with a study site employee who is involved in the conduct of this study (e.g., child, sibling).
• Known hypersensitivity to any component of the TAK-935 formulation.