A Study to Evaluate the Pharmacokinetics, Safety and Tolerability of ABBV-553 in Healthy Volunteers and in Subjects With Psoriasis and Efficacy of ABBV-553 in Subjects With Psoriasis

Inclusion Criteria: - Male or female and between 18 and 55 years of age, inclusive, for
Substudy 1, OR between 18 and 75 years of age, inclusive, for Substudy 2.

- If female, participant must be of non-child bearing potential defined as either:

a. Postmenopausal: Age > 55 years with no menses for 12 or more months without an
alternative medical cause. Postmenopausal: Age <= 55 years with no menses for 12 or
more months without an alternative medical cause AND a follicle stimulating hormone
(FSH) level >= 40 IU/L (OR) b. Permanently surgical sterile (bilateral oophorectomy,
bilateral salpingectomy or hysterectomy).

- Non-postmenopausal females must have a negative urine pregnancy test result at
Screening, and a negative serum pregnancy test result on Day -2 or Day -1.

- Male participants who are sexually active with women of child bearing potential
(WOCBP), even if the male participant has undergone a successful vasectomy, must agree
to use condoms from Day 1 through at least 30 days after the last dose of study drug,
and male participant agrees not to donate sperm at least 30 days after the last dose
of study drug.

- Body Mass Index (BMI) >= 18.0 to <= 29.9 kg/m2 after rounding to the tenths decimal
for Substudy 1 OR BMI >= 18.0 to <= 34.9 kg/m2 after rounding to the tenths decimal
for Substudy 2. BMI is calculated as weight measured in kilograms (kg) divided by the
square of height measured in meters (m).

- In the opinion of the Investigator, that the participant is in 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).

- Must voluntarily sign and date each informed consent form, approved by an
Institutional Review Board (IRB), prior to the initiation of any screening or
study-specific procedures and be willing to comply with the requirements of this study
protocol.

Additional criteria for Substudy 2:

- Has a clinical diagnosis of chronic plaque psoriasis (with a disease duration of at
least 6 months).

- Has a Psoriasis Area and Severity Index (PASI) score ≥ 12.

- Has a Static Physician's Global Assessment (sPGA) score ≥ 3.

- Has a Body Surface Area (BSA) affected by Ps ≥ 10%. Exclusion Criteria: - Male
participant who is considering fathering a child or donating sperm during the study or
through 30 days after the last dose of study drug.

- History of clinically significant sensitivity to any drug.

- History of epilepsy, any clinically significant cardiac (including any family history
of long-QT syndrome or unexplained sudden death), respiratory (except mild asthma),
renal, hepatic, gastrointestinal, hematologic or psychiatric disease or disorder, or
any uncontrolled medical illness.

- History of gastric surgery (except pyloromyotomy for pyloric stenosis during infancy),
vagotomy, bowel resection or any surgical procedure that might interfere with
gastrointestinal motility, pH or absorption.

- Infection(s) requiring treatment with intravenous (IV) anti-infectives within 30 days
prior to the start of confinement (Day -2 or Day -1) or oral anti-infectives within 14
days prior to the start of confinement (Day -2 or Day -1)..

- Requirement for any over-the-counter and/or prescription medication, vitamins and/or
herbal supplements on a regular basis.

- Use of any medications, vitamins and/or herbal supplements within the 2-week period
prior to study drug administration. For Substudy 2, medications used to treat chronic,
stable medical conditions are allowed during screening and participation in the study
unless the medication is specifically prohibited.