This is a study to demonstrate the clinical efficacy and safety of the nanobody® sonelokimab administered subcutaneously (sc) compared with placebo in the treatment of adult participants with active psoriatic arthritis. The study includes adalimumab treatment as an active reference arm.

Patients will be randomized to receive one of three sonelokimab treatment regimes, adalimumab or placebo. Primary efficacy evaluation will take place at Week 12. Patients will be allocated to a further 12 weeks of treatment with sonelokimab or adalimumab based on response assessment at week 12. In certain countries, treatment will end at week 12.

Inclusion Criteria:

1. Participant is ≥18 years of age;
2. Participant has a confirmed diagnosis of PsA per the 2006 Classification criteria for Psoriatic Arthritis (CASPAR) with symptoms for ≥6 months prior to the Screening Visit;
3. Participant has active disease (defined by a TJC68 of ≥3 and a SJC66 of ≥3);
4. Participant has either current active PsO or a dermatologist confirmed history of PsO;
5. Participant tests negative for rheumatoid factor (RF) at the Screening Visit;
6. Participant tests negative for anti-cyclic citrullinated peptide (CCP) antibodies at the Screening Visit;
7. Participant must be, in the opinion of the investigator, a suitable candidate for treatment with adalimumab per approved local product information.

Exclusion Criteria:

1. Participant with known hypersensitivity to sonelokimab or any of its excipients;
2. Participant with known hypersensitivity to adalimumab or any of its excipients;
3. Participant who has previously failed on anti-interleukin (IL)-17 therapy;
4. Participant who has previously failed on anti-tumor necrosis factor alpha (TNFα) therapy;
5. Participant who has had previous exposure to more than 2 biologic agents of any type to treat PsA prior to the Screening Visit;
6. Participant who has a diagnosis of chronic inflammatory conditions other than PsO or PsA;
7. Participant who has a diagnosis of arthritis mutilans