PDE4A and fragile X syndrome: Both cAMP and cyclic guanosine monophosphate (cGMP) are essential to support long- and short-term memory consolidation, and PDE4 inhibition has been shown to improve learning and memory in FXS patients, by increasing cAMP availability.15 Ibudilast is a broad-spectrum PDE inhibitor that allows for the maintenance of both cAMP and cGMP levels.