PDE4A and psoriatic arthritis: PDE4 is implicated in multiple disease settings, with 3 FDA-approved PDE4 inhibitors marketed for the treatment of atopic dermatitis, chronic obstructive pulmonary disorder, psoriasis, and psoriatic arthritis; several PDE4B-selective inhibitors are in development (Li, Zuo, and Tang 2018; Tibbo and Baillie 2020).