PF-04447943 is a potent, selective brain penetrant PDE9 inhibitor that increased indicators of hippocampal synaptic plasticity and improved cognitive function in a variety of cognition models in both rats and mice. PF-04447943 was found to have high affinity (Ki of 2.8, 4.5 and 18 nM) for human, rhesus and rat recombinant PDE9 respectively and high selectivity for PDE9 versus PDEs1-8 and 10-11. Inhibition of phosphodiesterase 9 (PDE9) has been reported to enhance rodent cognitive function and may represent a potential novel approach to improving cognitive dysfunction in Alzheimer's disease.