PDE4A and psoriatic arthritis: Owing to its broad anti-inflammatory activities, it has been investigated that PDE4 inhibitors can be applied for treating various skin disorders or rheumatic diseases, like psoriasis, PsA, and AD.511 Apremilast, an orally administered PDE4 inhibitor, was approved in the USA in 2014 for adult patients with active psoriatic arthritis or patients with moderate-to-severe plaque psoriasis who were candidates for phototherapy or systemic therapy.