RS-127445(MT 500) is a selective 5-HT2B receptor antagonist with pKi of 9.5 and pIC50 of 10.4 [1].

5-HT2B receptor belongs to the 5-HT2 receptor family which binds the neurotransmitter serotonin. It plays important role relevant to serotonin, including neuronal sensitization to tactile stimuli.

RS 127445 is a selective serotonin 5-HT2B receptor inhibitor. RS-127445 is found to have nM affinity and 1000 fold selectivity for the 5-HT2B receptor [2]. RS-127445 potently blocks the 5-HT evoked increase in inositol phosphate formation and blocks the 5-HT evoked increases in intracellular calcium concentrations with a potency 1000 times greater than that of yohimbine.

In rat model, oral administration of RS-127445 at 1 to 10 mg/kg resulted in significant inhibition of visceral hypersensitivity up to 35 to 74% triggered by restraint stress [3]. Moreover, 3 to 30 mg/ kg of RS-127445 resulted in a significant inhibition of TNBS-induced visceral hypersensitivity varied from 15 to 62% [3].

References:
[1].  O'Mahony, S M et al. 5-HT2B receptors modulate visceral hypersensitivity in a stress-sensitive animal model of brain-gut axis dysfunction. Neurogastroenterology & Motility, 2012, 22(5): 573-578.
[2].  Ohashi-Doi K et al. A selective, high affinity 5-HT2B receptor antagonist inhibits visceral hypersensitivity in rats. Neurogastroenterology & Motility, 2010, 22(2): 69-76.
[3].  Bassil A K et al. Inhibition of colonic motility and defecation by RS-127445 suggests an involvement of the 5-HT2B receptor in rodent large bowel physiology. British Journal of Pharmacology, 2009, 158(1): 252-258.