PDE8, similar to PDE4, is a cAMP-specific hydrolase with a cAMP affinity 40-100 times greater than that of PDE4.20 Research enrolling AD patients reported that PDE8B, one of the 2 subtypes of PDE8, was the only isozyme with significantly elevated high levels in cerebrocortical areas and parts of the hippocampal formation at Braak stages III-VI.21 Meanwhile, animal research indicated that inactivation of the PDE8B gene can enhance learning and memory functions in mice.22 These findings indicate that PDE8 may participate in the pathogenesis of AD. This evidence concerns the gene PDE4A and Alzheimer disease.