Study of TAK-935 as an Adjunctive Therapy in 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.