PDE5A and Alzheimer disease: 121−125 The potential of herbal extracts or isolated natural compounds withPDE inhibitory activity in contrasting AD progression was reviewedby Kumar et al. Those authors clearly listed several examples of naturalplants with cAMP-specific, cGMP-specific, or dual inhibitory activity.126 The flavonoid icariin (Figure 3) is a known natural PDE5 inhibitor from Epimedium brevicornum.