To allow evaluation of emerging non-clinical data (see detailed description below)

The purpose of this study is to characterize the safety and tolerability of TAK-137 when administered as multiple oral doses in adults with attention-deficit/hyperactivity disorder (ADHD).

The drug being tested in this study is called TAK-137. TAK-137 is being tested to find a safe and well-tolerated dose and to assess how TAK-137 is metabolized in people with attention-deficit/ hyperactivity disorder (ADHD). This study will look at side effects and lab results in people who take TAK-137. This study is designed as a randomized, sequential-cohort, multiple rising dose study.

Therefore, the TAK-137 2 mg Cohort will not start until the TAK-137 0.5 mg Cohort has completed, etc.

This trial will be conducted in the United States. The overall time to participate in this study is up to 42 days. Participants will make at least 2 visits to the clinic, including one 9-day period of confinement to the clinic. All participants will be contacted by telephone 7 days after the last dose of study drug for a follow-up assessment.

A decision was made to terminate this study so that emerging data from preclinical studies could be further assessed.

Inclusion Criteria:

1. Is a male or female adult who is 18 to 55 years of age, inclusive.
2. Weighs at least 45 kg and has a body mass index (BMI) between 18 and 30.0 kg/m^2, inclusive at Screening.
3. Has a documented diagnosis of attention-deficit/hyperactivity disorder (ADHD) for a minimum of 1 year.
4. Is willing to discontinue all medications to treat adult ADHD (eg, stimulants, antidepressants) and all other medications and dietary products as specified in the protocol, from Day -7 until Follow-up phone call (Day 14).

Exclusion Criteria:

1. Has received any investigational compound within 30 days prior to the first dose of study medication.
2. Has uncontrolled, clinically significant neurologic (including mildly abnormal or significantly abnormal EEG at screening), cardiovascular, pulmonary, hepatic, renal, metabolic, gastrointestinal, urologic, immunologic, endocrine disease, or psychiatric disorder (other than ADHD), or other abnormality, which may impact the ability of the participant to participate or potentially confound the study results.
3. Has previously had a seizure or convulsion (lifetime), including absence seizure and febrile convulsion.
4. Has a positive urine drug result for drugs of abuse other than amphetamines or other medications to treat ADHD or positive result for alcohol at Screening or Check-in (Day -1).
5. Has taken any excluded medication, supplements, or food products listed in the Excluded Medications and Dietary Products.
6. Is pregnant or lactating or intending to become pregnant before, during, or within 12 weeks after participating in this study; or intending to donate ova during such time period.
7. Has used nicotine-containing products (including but not limited to cigarettes, pipes, cigars, chewing tobacco, nicotine patch or nicotine gum) within 28 days prior to Check-in (Day -1).

Safety analysis set was defined as all participants who received at least one dose of study drug.

Treatment-emergent adverse events are adverse events that started after the first dose of double-blind study drug and no more than 14 days (or 30 days for a serious adverse event) after the last dose of double-blind study drug (Day 7).

At each visit the investigator had to document any occurrence of adverse events and abnormal laboratory findings. Any event spontaneously reported by the participant or observed by the investigator was recorded, irrespective of the relation to study treatment.