This is a study to demonstrate the clinical efficacy and safety of sonelokimab administered subcutaneously compared with placebo in the treatment of adult patients with active psoriatic arthritis who are naive to biologic disease-modifying antirheumatic drug therapy.

M1095-PSA-301 is a Phase 3, multicenter, randomized, parallel-group, double-blind, 3-arm, placebo-controlled study to investigate the efficacy and safety of sonelokimab 60 mg every 4 weeks (with and without an induction regimen) versus placebo in adults with active psoriatic arthritis who are naive to biologic disease-modifying antirheumatic drug therapy.

Inclusion Criteria:

1. Participants must be ≥18 years of age .
2. Participants have a confirmed diagnosis of psoriatic arthritis (PsA) per the 2006 Classification for Psoriatic Arthritis (CASPAR) criteria with symptoms for ≥6 months before the Screening Visit.
3. Participants have active disease (defined by a 68 tender joint count [TJC68] of ≥3 and a 66 swollen joint count [SJC66] of ≥3).
4. Participants have current active plaque psoriasis (PsO) or a dermatologist-confirmed history of plaque PsO.
5. Participants test negative for both rheumatoid factor and anti-cyclic citrullinated peptide at the Screening Visit.

Exclusion Criteria:

1. Participants with a known hypersensitivity to sonelokimab or any of its excipients.
2. Participants who have a diagnosis of chronic inflammatory conditions other than PsO or PsA.
3. Participants with a diagnosis of inflammatory bowel disease.
4. Participants who have experienced a period of ≥3 consecutive weeks of unexplained diarrhea in the 24 weeks before the Baseline Visit.
5. Participants who have an established diagnosis of arthritis mutilans.
6. Previous exposure to sonelokimab.
7. Participants who have ever received any biologic immunomodulating agents for PsA or PsO, whether investigational or approved.