As previously mentioned, Roflumilast (Table 2) was the first marked PDE4DI with IC50 values of 0.68 nM [118,119,120,121], while compounds HT-072 (PDE4D IC50 = 150 nM), Difamilast (PDE4D IC50 = 73.8 nM), and LEO29102 (PDE4D IC50 = 5 nM) are currently in clinical trials for the treatment of atopic dermatitis, psoriasis, and AAMI, as reported in Table 3 [81,83,85]. Here, PDE4D is linked to psoriasis.