CDP 840 hydrochloride is a potent and selective inhibitor of phosphodiesterase 4 (PDE4). This compound competitively inhibits all PDE4 isoenzymes, which in turn prevents the cleavage of phosphodiester bonds found in secondary messenger molecules. CDP-840 had been in phase II clinical trials for the treatment of asthma. However, CDP840 did not lead to changes in baseline forced expiratory volume in one second (FEV1) as compared to placebo. CDP 840 research has been discontinued.