TNF and rheumatoid arthritis: Our results correspond with other studies in other diseases treated with MTX monotherapy, such as juvenile idiopathic arthritis, rheumatoid arthritis (RA) or psoriasis, which also found a pre-existent higher concentration of TNFα in non-responders,20 or lack of effect of MTX monotherapy on IL-23 or IL-17 serum or plasma concentrations.29 30 Together, this may explain why MTX is not effective in patients with PsA with primarily IL-23-driven disease.