PDE4A and Alzheimer disease: On the other hand, PDE4 inhibitors were promising to a great extent.95 HT-0712, a 3,5-disubstituted piperidinone (Fig. 8), has completed a phase II study, showing enhancement in the long-term memory of patients with age-related memory impairment.96 It is now under another clinical trial study designed to further evaluate its effect on AD improvement.97