Phosphodiesterase 4 inhibitors for the treatment of asthma and COPD

Type 4 cyclic nucleotide phosphodiesterases (PDE4s) are metallo-hydrolases which specifically hydrolyze cAMP to AMP in various cells types. The catalytic core is a bimetallic ion center composed of a tightly bound Zn2+ and a loosely bound Mg2+, which plays a dictating role in eliciting cAMP binding and catalysis activation. An invariant glutamine positioned opposite to the ion center serves as the substrate recognition determinant and synergizes the transient Mg-oxo-phosphate interaction in the substrate complex. The Mg2+ binding is activated by a PKA-mediated serine phosphorylation and modulated through protein-protein interactions, thus, providing efficient mechanisms in the temporal regulation of cAMP signaling. Several PDE4 inhibitors including roflumilast, cilomilast and rolipram also rely on the interaction with the glutamine and metallic ion center for binding, with their affinity enhanced dramatically by the presence of the Mg2+ ion. Recent studies have provided new insights into the role of this enzyme in inflammatory settings, CFTR regulation, long term potentiation, and its importance in immune surveillance. The major inflammatory cytokines which are modulated with PDE4 inhibitors include TNF alpha, IL-2, IFN gamma, IL-12, GM-CSF and LTB4. The role of PDE4 inhibitors in modulating cytokines, lipid mediators and in mucociliary clearance, along with clinical efficacy in asthma and/or COPD demonstrated with roflumilast and cilomilast, suggest a broad anti-inflammatory spectrum for these compounds. Presently, the major impediment to approval of these novel therapies has been the mechanism based gastrointestinal adverse events which has limited the dosing and the ultimate efficacy with these novel therapeutic agents.