A Phase 2 Study to Investigate the Safety, Tolerability and Efficacy of ABT-122 in Subjects With Active Psoriatic Arthritis (PsA) Who Have an Inadequate Response to Methotrexate (MTX)

Inclusion Criteria: * PsA diagnosis of at least 3 months duration prior to the date of first screening with ClASsification of Psoriatic ARthritis (CASPAR) confirmed diagnosis at Screening. * Have active psoriasis defined by at least 1 psoriasis lesion \>= 2 cm diameter in areas other than the axilla or groin. * Have active arthritis defined by minimum disease activity criteria: 1. \>= 3 swollen joints (based on 66 joint counts) at Screening 2. \>= 3 tender joints (based on 68 joint counts) at Screening * On a stable dose of methotrexate (MTX) defined as: 1. Oral or parenteral treatment \>= 3 months 2. On a stable dose with an unchanged mode of application for at least 4 weeks prior to baseline 3. Stable MTX dose of \>= 10 mg/week and \<= the upper limit of the applicable approved local label 4. Can also be on stable doses of nonsteroidal anti-inflammatory drugs, sulfasalazine and/or hydroxychloroquine as long as they are also on methotrexate Exclusion Criteria: * Up to 30% (approximately 66 subjects) with prior exposure to a TNF inhibitor may be enrolled if the TNF inhibitor was not discontinued due to lack of efficacy or safety concerns. Subjects must be washed out for at least 5 half-lives of these drugs prior to the Baseline visit. * Subjects on prior adalimumab may not be enrolled in the study * Prior exposure to other non-TNF inhibitor biological disease-modifying antirheumatic drugs (DMARDs) will be permitted if the subject is washed out at least 5 half-lives of these drugs prior to the baseline visit. * Current treatment with traditional oral/intramuscular DMARDs, including conventional synthetic DMARDs (csDMARDs; except for concomitant treatment with sulfasalazine and/or hydroxychloroquine in addition to MTX). Oral DMARDs must be washed out for at least 5 half-lives of a drug apart from MTX prior to the Baseline visit. a. Subject could have been exposed to prior Janus kinase (JAK) or phosphodiesterase type 4 (PDE4) inhibitors so long as they have been off therapy for at least 5 half-lives. * Stable prescribed dose of oral prednisone or prednisone equivalent \> 10 mg/day within the 30 days of the Baseline visit. * Intra-articular or parenteral administration of corticosteroids in the preceding 4 weeks of the Baseline visit. Inhaled corticosteroids for stable medical conditions are allowed. * Laboratory values of the following at the Screening Visit: 1. Confirmed hemoglobin \< 9 g/dL for males and \< 8.5 g/dL for females 2. Absolute neutrophil count (ANC) \< 1500 mm\^3, (or \< 1200 cells/µL for subjects of African descent who are black) 3. Aspartate aminotransferase or alanine aminotransferase \> 1.5 x the upper limit of normal (ULN) or bilirubin \>= 3 mg/dL 4. Serum creatinine \> 1.5 x the ULN 5. Platelets \< 100,000 cells/\[mm\^3\] (10\^9/L), 6. Clinically significant abnormal screening laboratory results as evaluated by the Investigator