Development of Agonists/Antagonists for Protease-Activated Receptors (PARs) and the Possible Therapeutic Application to Gastrointestinal Diseases

Abstract

Protease-activated receptors (PARs), a family of G-protein-coupled seven-transmembrane-domain receptors, are activated by proteolytic unmasking of the N-terminal cryptic tethered ligand by certain serine proteases. Among four PAR family members cloned to date, PAR-1, PAR-2, and PAR-4 can also be activated through a non-enzymatic mechanism, which is achieved by direct binding of exogenously applied synthetic peptides based on the tethered ligand sequence, known as PARs-activating peptides, to the body of the receptor. Various peptide mimetics have been synthesized as agonists for PARs with improved potency, selectivity, and stability. Some peptide mimetics and/or nonpeptide compounds have also been developed as antagonists for PAR-1 and PAR-4. PARs are widely distributed in the mammalian body, especially throughout the alimentary systems, and play various roles in physiological/pathophysiological conditions, i.e., modulation of salivary, gastric, or pancreatic glandular exocrine secretion, gastrointestinal smooth muscle motility, gastric mucosal cytoprotection, suppression/facilitation of visceral pain and inflammation, etc. Thus PARs are now considered novel therapeutic targets, and development of selective agonists and/or antagonists for PARs might provide a novel strategy for the treatment of various diseases that are resistant to current therapeutics.