This is a Phase II, prospective, multicenter, randomized, double-blind, placebo-controlled, dose finding trial in parallel-groups with primary endpoint at Week 16 (visit V9) to investigate the efficacy, tolerability, safety, pharmacokinetics and immunogenicity of ABY-035 in adult patients with PsA.

The study evaluates two dose levels of ABY-035, in comparison to placebo, in subjects with psoriatic arthritis. The clinical trial duration is 72 weeks (Screening - last FU visit) comprised of up to 4 weeks of screening, 44 weeks of double-blind-treatment, and 24 weeks of follow-up.

Treatment Periods are:

• Treatment Period I: from V1 (Week 0) to V9 (Week 16)
• Treatment Period II: from V9 (Week 16) to V16 (Week 44: last dosing)

At visit V1 (Week 0), patients who meet the entry criteria will be randomly assigned in equal rates (1:1:1) to one of three parallel groups (A, B, C).

The treatment will be administered once every 2 weeks (Q2W).

Patients assigned to groups A and B will receive active treatment from V1 to V16 (group A = 40 mg ABY-035; group B = 80 mg ABY-035). Patients initially assigned to placebo will switch to active treatment at visit V9 in a blinded fashion (group C = Placebo from V1 to V8 and 80 mg ABY-035 from V9 to V16).

Inclusion criteria:

• Patient who has given his / her signed declaration of consent and data protection declaration
• At least 18 years and less than 75 years of age at Screening visit
• Psoriatic arthritis with inflammatory musculoskeletal disease (joint, spine, or entheseal) with the presence of ≥3 points from the five categories of the Classification Criteria for Psoriatic Arthritis (CASPAR) at any timepoint in medical history.
• Active psoriatic arthritis defined by:
• ≥3 swollen joints out of 66 joints (SJC66) at Screening visit and Baseline visit
• ≥3 tender joints out of 68 (TJC68) at Screening visit and Baseline visit
• Precedent failure or insufficient treatment response to or contraindication or intolerability to nonsteroidal anti-inflammatory drug (NSAID), csDMARD (i.e. MTX, sulfasalazine, leflunomide, hydroxychloroquine, cyclosporine A), and/or TNFi (e.g. adalimumab, infliximab, etanercept, golimumab, certolizumab)
• Rheumatoid factor (RF) and anti-CCP antibody negative
• Presence or history of plaque psoriasis

Exclusion Criteria:

• Underlying conditions which significantly immunocompromise the patient and / or place the patient at unacceptable risk for receiving an immunomodulatory therapy
• History of or current relevant autoimmune diseases (e.g. rheumatoid arthritis, primary ankylosing spondylitis, systemic lupus erythematosus) other than psoriasis or psoriatic arthritis
• History of or current fibromyalgia or pain syndrome
• Uncontrolled inflammatory bowel disease
• Presence or history of recurrent or medically important infections in the last 6 months prior to Baseline visit
• Clinically relevant Candida infection requiring systemic treatment within the last 6 months prior to Baseline visit
• History or any signs of lymphoproliferative disease, or a known malignancy or a history of malignancy within the previous 5 years
• Insufficiently controlled heart failure
• Current uncontrolled arterial hyper- or hypotension