T1305 A Novel Type of Therapeutic Potential in a Traditional Herbal Medicine Kakkonto Against Gastrointestinal Anaphylaxis By Reducing Mucosal Mast Cell-Surface Expression of FCɛRi

Abstract

Racemic verapamil, primarily a cardiovascular agent, has been widely used off-label in patients with irritable bowel syndrome (IBS). Initial observations of its usefulness followed recognition of a high incidence of constipation with use in cardiovascular conditions. The enantiomers of verapamil are known to differ in cardiovascular potency, the S-isomer being much more potent than the R-isomer. In addition we found the S-isomer to be equiactive in relaxing vascular and colonic smooth muscle but the R-isomer to be 5-times more potent in relaxing colonic than vascular muscle. This selectivity led us to develop R-verapamil (Rezular) as a gut selective treatment in IBS and we have shown doses from 60mg/day to 240mg/day to greatly improve symptoms in non-constipation IBS patients. To better understand the mechanism by which Rverapamil improved the symptoms of IBS, we undertook an in-vitro screen of binding of R-verapamil to 147 receptors/receptor sub-types. Specific ligand binding was initially assessed using 10x-5 M verapamil and if there was greater than 50% inhibition of control specific binding, then binding at 8 different concentrations was tested and IC50 values (concentration for half-maximal inhibition of control specific binding (x10 -7M)) calculated. The therapeutic plasma concentration range of free R-verapamil was conservatively set at 0.1-3x10-7 M. Within this range R-verapamil showed affinity for 3 receptors: melatonin (MT1)(IC50 0.6), 5-HT2b (IC50 1.1) and L-type calcium channel (IC50 2.4). In addition compared with S-verapamil, R-verapamil showed stereoselectivity (x40)for MT1 binding , whereas S-verapamil showed stereoselectivity (x 3)for L-type calcium channel binding. R-Verapamil was selective for 5-HT2b relative to other 5-HT receptor sub types and affinity was low for 5-HT3(IC 50 3,400) or 5-HT4(>100) receptors.It was also highly selective for MT1(IC50 0.6) versus MT2 (IC50 >100)receptors. We conclude that Rverapamil most likely exerts its therapeutic effects in IBS via a previously unrecognized mechanism involving combined effects at melatonin receptors, serotonin receptors and Ltype calcium channels.