Evaluation of a series of lactam heterocyclic analogues of cilostamide (2) as inhibitors of cyclic AMP phosphodiesterase derived from both human platelets and rat heart in comparison with their corresponding methoxy-substituted heterocycles has revealed that the N-cyclohexyl-N-methyl-4-oxybutyramide side chain of 2 is an important lipophilic and/or steric pharmacophore. Attachment of this side chain to the parent heterocycle of the potent cyclic AMP phosphodiesterase inhibitor anagrelide (3) afforded the hybrid structure RS-82856 (1), shown to be more potent than either of its progenitors as an inhibitor of cyclic AMP phosphodiesterase or of ADP-induced platelet aggregation. The available in vitro data suggest that 1 possesses potentially useful antithrombotic and cardiotonic properties.