RS 23597‐190: a potent and selective 5‐HT4 receptor antagonist
Authors: Eglen R.M. , Bley K. , Bonhaus D.W. , Clark R.D. , Hegde S.S. , Johnson L.G. , Leung E. , Wong E.H.F. 
Published: July 2012

The pharmacological properties of RS 23597‐190 (3‐(piperdine‐1‐yl)‐propyl‐4‐amino‐5‐chloro‐2‐methoxy benzoate hydrochloride) have been studied in vitro and in vivo.
RS 23597‐190 competitively antagonized 5‐HT4 receptor‐mediated relaxations of rat, carbachol precontracted oesophageal muscularis mucosae, (pA2 = 7.8 ± 0.1; Schild slope = 1.2 ± 0.2). Affinity estimates (−log KB) at 5‐HT4 receptors using either renzapride or SC‐53116 as agonists yielded a −log KB value of 8.0 ± 0.01. In contrast, RS 23597‐190 failed to antagonize contractile responses to 5‐HT of guinea‐pig ileal 5‐HT3 receptors, even at concentrations up to 10 μm.
Increases in short‐circuit current, induced by 5‐HT, were studied in guinea‐pig ileal mucosal sheets. Concentration‐response curves to 5‐HT were biphasic, with the high potency phase to 5‐HT inhibited by RS 23597‐190 and mimicked by 5‐methoxytryptamine. The −log KB value for RS 23597‐190 at the high potency phase was 7.3 confirming that 5‐HT4 receptors mediated the high potency phase.
In rat isolated vagus nerve, 5‐HT elicited a slow, maintained depolarization at low concentrations and a rapid, transient depolarization at higher concentrations. The high potency, slow depolarizing phase to 5‐HT was abolished selectively in the presence of 1 μm RS 23597‐190 and the low potency phase was abolished selectively in the presence of 1 μm ondansetron. These data confirm that 5‐HT4 and 5‐HT3 receptors mediated slow and fast depolarization responses, respectively.
At 5‐HT3 binding sites in membranes from NG 108‐15 cells, labelled by [3H]‐quipazine, RS 23597‐190 exhibited an apparent affinity (−log Ki) of 5.7 ± 0.1. At 5‐HT3 receptors in membranes from rat cerebral cortex, labelled by [3H]‐RS 42358‐197, the apparent affinity (−log Ki) of RS 23597‐190 was also 5.7 ± 0.1. In both studies, Hill coefficients were not significantly different from unity. At 5‐HT1A, 5‐HT2, muscarinic M15 M2, M3, M4 and dopamine D1 and D2 receptors, RS 23597‐190 exhibited low apparent affinities, with all −log Ki values less than 5.5.
Intravenous infusion of RS 23597‐190 in the conscious, restrained rat antagonized the von Bezold Jarisch reflex induced by 2‐methyl 5‐HT, with an ID50 of 300 μg kg−1 min−1, i.v. In the anaesthetized, bilaterally vagotomized micropig, RS 23597‐190 (6 mg kg−1, i.v.) antagonized 5‐HT‐induced tachycardia with a half‐life of 77 (63–99) min. Transient arrhythmic effects were noted after administration of the compound.
In conclusion, RS 23597‐190 acts as a high affinity, selective competitive antagonist at 5‐HT4 receptors. Thus, the compound appears to be a useful tool for 5‐HT4 receptor identification in vitro. In vivo, the compound is rapidly metabolized in pigs such that 5‐HT4 blockade is not maintained. However, in the rat, when given by infusion, RS 23597‐190 antagonizes 5‐HT3 mediated responses, at doses consistent with a low affinity 5‐HT3 receptor. These data suggest that, under appropriate experimental conditions, RS 23597‐190 may also be used in vivo to characterize further 5‐HT4 receptor function.