Validation of the PsASon ULtrasound Scores in Patients With Psoriatic Arthritis Undergoing TReatment With Apremilast

Inclusion Criteria: 1. Male or female patient ≥18 years and \<90 years of age 2. PsA according to CASPAR criteria 3. Peripheral manifestation (arthritis, tenosynovitis, dactylitis and/or enthesitis) 4. Active disease as defined by a DAPSA \>14 and clinical indication for treatment with Apremilast (as per approved indication for PsA, including failure to methotrexate) 5. Written informed consent Exclusion Criteria: 1. Inability to perform US at any site included in the PsASon22 or PsASon13 score (f.e. due to complete destruction of a joint) 2. Planned surgery within the study period or history of surgery of any of the joints to be investigated clinically or by sonography. 3. Contraindication to Apremilast (as per patient information leaflet) 4. Current severe medical illness requiring hospitalization 5. Pregnancy or lactation 6. Inability of the patient to follow the treatment protocol 7. Fulfillment of the MDA Criteria or DAPSA≤14 8. Current treatment with any investigational drug 9. Current treatment with glucocorticoids at a prednisone equivalent \>10mg 10. Intra-articular glucocorticoid injection in one of the joints to be investigated clinically or by sonography, or intra-muscular glucocorticoid injection within 8 weeks before baseline 11. Change, including dosage changes or discontinuation, of csDMARD treatment (with the exception of leflunomide) in the last 4 weeks before baseline 12. Change, including dosage changes or discontinuation of leflunomide treatment in the last 8 weeks before baseline. (Exception: If patients stop leflunomide and complete an 11 day treatment with cholestyramine (8g, 3 x daily), prior to the baseline visit, they may enter the study.) 13. Current bDMARD, tsDMARD treatment 14. Prior bDMARD or tsDMARD treatment without a minimal washout period before baseline (the minimal washout period is twice the half-life of the respective drug)