So, the purpose of the study is to analyse the effects of blinded MTX-co-medication on outcome in patients treated with UST: Non-inferiority at week 24 of UST monotherapy compared to add-on to MTX in patients with active PsA and at least 12 weeks of MTX treatment prior to screening or who are actually not treated with MTX and do not have prior inadequate response to MTX-treatment for PsA will be demonstrated.