PDE4A and Alzheimer disease: In a phase 2, multicenter, randomized, blinded, excipient, parallel-group clinical study, topical small-molecule PDE4 inhibitor Hemay-808 ointment (1%, 3% or 7%) (Figure 3b) or a vehicle was to be applied twice daily for 28 days in 148 adult participants (18 to 65 years old) diagnosed with mild to moderate AD.