The purpose of this study is to evaluate adverse events and tolerability of single and multiple doses of ABBV-903, and to assess how the drug moves through the body in healthy adult volunteers.

Inclusion Criteria:

• Body Mass Index (BMI) is => 18.0 to <= 32 kg/m2 after rounded to the nearest tenth, at Screening and upon initial confinement.
• A condition of general good health, based upon the results of a medical history, physical examination, vital signs, laboratory profile and a 12-lead electrocardiogram (ECG).

Exclusion Criteria:

• History of any clinically significant illness/infection/major febrile illness, hospitalization, or any surgical procedure within 30 days prior to the first dose of study drug.
• 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.
• Evidence of dysplasia or history of malignancy (including lymphoma and leukemia) other than successfully treated non-metastatic cutaneous squamous cell, basal cell carcinoma or localized carcinoma in situ of the cervix.