A brain-permeable PDE inhibitor selective for PDE9A (IC50 = 12 nM) over other PDEs (IC50s = >940 nM); over 1,000-fold more selective for PDE9A over 79 non-PDE targets, with the exception of the melatonin MT3/ML2 receptor (Ki = 3,800 nM); inhibits rat and rhesus monkey PDE9A (Kis = 18.1 and 4.5 nM, respectively); increases neurite outgrowth and the density of synapsin puncta in primary rat hippocampal neurons (0.03 μM); improves scopolamine-induced deficits in episodic memory in rats and increases the time spent exploring the novel object in the NOR test (1 mg/kg); improves spatial memory in the MWM and decreases the distance traveled to the hidden platform (3.2 mg/kg)