Selectin antagonists : therapeutic potential in asthma and COPD

Asthma and COPD are chronic inflammatory conditions that affect hundreds of millions of patients worldwide. New therapeutics are desperately needed, especially those that target the underlying causes and prevent disease progression. Although asthma and COPD have distinct etiologies, both are associated with reduced airflow caused by excess infiltration of inflammatory cells into healthy lung tissues. As selectin-mediated adhesion of leukocytes to the vascular endothelium is a key early event in the initiation of the inflammatory response, selectin inhibition is thought to be a good target for therapeutic intervention. Three known selectins are expressed in distinct subsets of cells: P-selectin is presented on the surface of activated platelets and endothelial cells, L-selectin is constitutively expressed on leukocytes, and E-selectin synthesis is upregulated in activated endothelial cells. They mediate cell-cell adhesion in the shear flow of the bloodstream via specialized interactions with clusters of oligosaccharides presented on cell surface glycopeptide ligands. The role of selectin-ligand interactions in the inflammatory response has been demonstrated in various animal models, prompting considerable attention from the pharmaceutical industry.Drug discovery efforts have yielded many different classes of selectin inhibitors, including soluble protein ligands, antibodies, oligosaccharides and small molecules. Although many selectin inhibitors have shown activity in preclinical models, clinical progress of selectin-directed therapies has been slow. Early approaches employed carbohydrate-based inhibitors to mimic the natural ligand sialyl Lewis X; however, these compounds proved challenging to develop. Cytel's CY 1503, a complex oligosaccharide, progressed to phase II/III trials for reperfusion injury, but further development was halted when it failed to demonstrate clinical efficacy. Two protein-based selectin inhibitors have reached phase II development. These included Wyeth's recombinant soluble P-selectin ligand, TSI (PSGL-1), which was discontinued after disappointing results in myocardial infarction trials and Protein Design Labs' humanized anti-L-selectin monoclonal antibody, which is currently in development for trauma. Bimosiamose, discovered by Encysive Pharmaceutical and presently being developed by Revotar Biopharmaceuticals, is an 863 g/mol molecular weight dimer with minimal carbohydrate content and is, to date, the leading selectin inhibitor in clinical development. This compound has shown promise in a phase IIa 'proof of concept' trial in patients with asthma, reducing airway recruitment of eosinophils after intravenous administration. Further clinical development of an inhaled formulation is underway. Despite a significant need for new therapeutics, selectin inhibitors have not yet been explored for the treatment of COPD. Bimosiamose represents an important proof of principle, and hopefully continued success will spark renewed interest in selectin-directed therapeutics for respiratory diseases.