TNF and psoriatic arthritis: Given that PDE4 inhibitors were clinically efficacious in psoriasis and psoriatic arthritis patients (IL‐17/IL‐23 axis dominant diseases) [20, 21], but showed less effect in rheumatic arthritis patients [22] (TNFα axis dominant disease) further supports the hypothesis that PDE4 inhibition has a major impact on the pathogenic TH17 cell pathway.