The purpose of this study is to characterize the multiple-dose safety and tolerability profile of TAK-935 in adult participants with developmental and/or epileptic encephalopathies.

The drug being tested in this study is called TAK-935. TAK-935 is being tested to treat people who have developmental and/or epileptic encephalopathies. This study will look at safety, tolerability and pharmacokinetics of people who take TAK-935. Study drug will be administered in a double-blind manner in Part 1 and in an open-label manner in Part 2.

The study will enroll approximately 20 participants. Participants will be randomly assigned (by chance, like flipping a coin) to one of the two treatment groups in Part 1-which will remain undisclosed to the participant and study doctor during the study (unless there is an urgent medical need):

• TAK-935
• Placebo (dummy inactive pill) - this is a tablet that looks like the study drug but has no active ingredient

Participants will receive placebo or 100 milligram (mg) TAK-935 tablets, orally or through stable G-tube/PEG tube, BID, in Part 1 (Day 1) and dose will be increased to 200 mg (Day 11) BID and to 300 mg (Day 21) BID in dose titration period. All participants who complete the Double-Blind Treatment Period in Part 1 will have the option to continue directly into the Open-Label Treatment Period in Part 2 where they will receive TAK-935 as two 100 mg tablets (total dose is 200 mg TAK-935) orally or through G-tube/PEG tube, BID and dose will be increased to three 100 mg tablets (total dose is 300 mg TAK-935), orally, BID (Day 41). This dose level will be maintained until the final visit (Day 85) for the dose de-escalation phase.

This multi-center trial will be conducted in North America. The overall time to participate in this study is 121 days excluding screening period of 30-41 days. Participants will make multiple visits to the clinic, and a follow-up phone call will be conducted on Day 91 and at the end of the 30-day follow-up period (Day 121), participants will return to the clinic for a follow-up assessment.

Inclusion Criteria:

1. Has a documented clinical diagnosis of developmental and/or epileptic encephalopathies with countable bilateral motor seizures, defined as an average of greater than or equal to (>=) 2 per month during the past 3 months, based on the investigator's assessment, and a monthly average of >=1 per month during the Baseline Period, based on the seizure diary record.
2. Has been taking 1 to 4 antiepileptic drug (AEDs) at a stable dose for >=4 weeks before Screening and the participant or participant's legally acceptable representative is willing to keep the regimen(s) stable throughout the study.
3. Has an average of >=1 bilateral motor seizure per month during the 4-week Baseline Period (that is., drop seizures, tonic-clonic, tonic, bilateral clonic, atonic, myoclonic-atonic, myoclonic-tonic-clonic, focal seizures with bilateral hyperkinetic motor features).
4. Must agree to not post any participant's personal medical data related to the study or information related to the study on any web site or social media site (example, Facebook, Twitter) until the study has been completed.
5. For participants with G-tube/PEG tube, G-tubes/PEG tubes should have been placed and been functioning for at least 3 months prior to screening. Naso-gastric tubes are not allowed.

Exclusion Criteria:

1. Has received TAK-935 in a previous clinical study or as a therapeutic agent.
2. Was admitted to a medical facility for treatment of status epilepticus requiring mechanical respiration within 3 months before Screening.
3. Had a vagal nerve stimulator implanted within 6 months before Screening and settings have been changed within 1 month of the Screening Visit and/or anticipated to change during the study.
4. Is on ketogenic diet that has been started within 6 months of the Screening Visit, has been changed within 1 month of the Screening Visit, or is anticipated to change during the study.
5. Has degenerative eye disease.
6. Has a history of suicidal behavior or any suicidal ideation of type 4 or 5 on the Columbia-Suicide Severity Rating Scale (C-SSRS) at Screening. If the participant is unable to comply with the C-SSRS due to developmental status, a parent proxy may be used for the completion of the C-SSRS. The Investigator may also use clinical judgment, which must then be documented in the source document.
7. Positive for human immunodeficiency virus, hepatitis B, or hepatitis C infections. (Note that participants who have been vaccinated against hepatitis B [hepatitis B surface antibody (Ab)-positive] who are negative for other markers of prior hepatitis B infection [example, negative for hepatitis B core Ab] are eligible. Also note that participants who are positive for hepatitis C Ab are eligible as long as they have a negative hepatitis C viral load by quantitative polymerase chain reaction [qPCR]).
8. Has an abnormal and clinically significant ECG at Screening in the opinion of the investigator, for example, second or third degree heart block or a corrected QT interval (QTc) greater than (>) 450 millisecond (msec). Entry of any participant with an abnormal but not clinically significant ECG must be approved and documented by signature by the principal investigator or appropriately qualified delegate.
9. Has abnormal clinical laboratory test results at Screening that suggest a clinically significant underlying disease. If the participant has alanine aminotransferase (ALT) and/or aspartate aminotransferase (AST) >2.5*the upper limit of normal (ULN), the Medical Monitor should be consulted.
10. Has received any excluded medications, procedures, or treatments during the time periods.
11. Has any a history of alcohol, opioid, or other drug use disorder, as per the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, within the previous 2 years before Screening. Medical marijuana use is allowed.
12. Has unstable, clinically significant neurologic (other than the disease being studied), psychiatric, cardiovascular, pulmonary, hepatic, renal, metabolic, gastrointestinal, urologic, immunologic, hematopoietic, or endocrine disease or other abnormality, which may impact the ability of the participant to participate or potentially confound the study results.

Takeda provides access to the de-identified individual participant data (IPD) for eligible studies to aid qualified researchers in addressing legitimate scientific objectives (Takeda's data sharing commitment is available on https://clinicaltrials.takeda.com/takedas-commitment?commitment=5). These IPDs will be provided in a secure research environment following approval of a data sharing request, and under the terms of a data sharing agreement.

IPD from eligible studies will be shared with qualified researchers according to the criteria and process described on https://vivli.org/ourmember/takeda/. For approved requests, the researchers will be provided access to anonymized data (to respect patient privacy in line with applicable laws and regulations) and with information necessary to address the research objectives under the terms of a data sharing agreement.