The objectives of this study are to assess safety and tolerability of the new ABBV-668 ER tablets, to assess the oral bioavailability of the ABBV-668 ER tablets relative to the ABBV-668 IR capsules, and to assess the pharmacokinetics of the ER tablets under fasting and fed conditions in healthy adults.

Inclusion Criteria:

• BMI is ≥ 18.0 to ≤ 32.0 kg/m^2 after rounding to the tenths decimal at screening.
• A condition of general good health, based upon the results of a medical history, physical examination, vital signs, laboratory profile and a 12-lead ECG.

Exclusion Criteria:

• History of any clinically significant sensitivity or allergy to any medication or food.
• History of any investigational drug within 30 days or 5 half-lives of the drug (whichever is longer) prior to the first dose of study drug or is currently enrolled in another clinical study or was previously enrolled in this study.
• History of epilepsy, any clinically significant cardiac, respiratory (except mild asthma as a child), renal, hepatic, gastrointestinal, hematologic or psychiatric disease or disorder, or any uncontrolled medical illness.
• Participant using any medication, vitamins and/or herbal supplements (including traditional Chinese medicine), with the exception of hormonal replacement therapies for females, on a regular basis.
• Any use of tobacco or nicotine-containing products within 180 days prior to the first dose of study drug.