Phosphodiesterase-4: Selective and Dual-Specificity Inhibitors for the Therapy of Chronic Obstructive Pulmonary Disease

Phosphodiesterase-4 isoenzymes have absolute specificity for cyclic adenosine-3',5'-monophosphate and are considered potential therapeutic targets for the treatment of chronic inflammatory disorders, such as chronic obstructive pulmonary disease, with small-molecule inhibitors. Several selective phosphodiesterase-4 inhibitors are in clinical trials of chronic obstructive pulmonary disease, including cilomilast and roflumilast. Despite some encouraging data from phase III clinical trials, the current generation of phosphodiesterase-4 inhibitors is hampered by a low therapeutic ratio. Indeed, a major obstacle is their propensity to evoke non-steroid-like side effects, of which nausea, diarrhea, abdominal pain, vomiting, and dyspepsia are the most common. In addition, a particularly worrying potential toxicity of phosphodiesterase-4 inhibitors, also shared by phosphodiesterase-3 inhibitors and other vasodilators, is arteritis/periarteritis. One potential means of improving the therapeutic ratio and safety of phosphodiesterase-4 inhibitors may lie in the development of compounds that have broader phosphodiesterase specificity. Of the 11 phosphodiesterase families that have been unequivocally identified, dual-specificity compounds that inhibit phosphodiesterase-4 and phosphodiesterase-1, phosphodiesterase-3, or phosphodiesterase-7 may offer the best opportunities to enhance clinical efficacy.