Kinase experiment:

CHO-K1 cells expressing human 5-HT2A, 5-HT2B or 5-HT2C receptors are harvested using 2 mM EDTA in phosphate buffered saline. Cell membranes are prepared by four cycles of homogenization (Brinkman P10 disrupter) and centrifugation (48,000×g for 15 min). As previously described, each assay is established so as to achieve steady state conditions and to optimize specific binding. For the 5-HT2A receptor, membranes from 1×106 cells are incubated with 0.2 nM [3H]-ketanserin at 32℃ for 60 min. Nonspecific binding is determined using 10 μM methysergide. For the 5-HT2B receptor, membranes from 1.5×106 cells are incubated with 0.2 nM [3H]-5-HT at 4℃ for 120 min. Nonspecific binding is determined using 10 μM 5-HT. For the 5-HT2C receptor, membranes from 3×105 cells are incubated with 0.5 nM [3H]-mesulergine at 32℃ for 60 min. Nonspecific binding is determined using 10 μM methysergide. Assays are terminated by vacuum filtration through glass fibre filters (GF/B) which had been pretreated with 0.1% polyethyleneimine. Total and bound radioactivity is determined by liquid scintillation counting. Greater than 90% specific binding is achieved in each of these assays.The selectivity of RS-127445 for 5-HT2B receptors is further examined by testing the compound for affinity at over 100 additional ion channel or receptor binding sites. These studies are conducted by Cerep using standard protocols[1].

Cell experiment:

HEK-293 cells expressing the human 5-HT2B receptor are incubated with [3H]-myoinositol (1.67 μCi/mL) in 162 cm2 flasks overnight at 37℃ in an inositol free Ham's F12 medium containing 10% dialyzed foetal bovine serum. The cells are harvested, washed five times with phosphate buffered saline and resuspended in inositol free Ham's F12 media at density of approximately 3×106 cells/mL. RS-127445 (10 μM) is initially dissolved in 10% (v/v) DMSO with 90% inositol free Ham's F12 medium. Subsequent dilutions are made with inositol free Ham's F12 medium. 5-HT is dissolved in inositol free Ham's F12 medium containing 100 mM LiCl and 1 mM ascorbate. RS-127445, vehicle or other antagonists are pre-incubated with 240 μL of cell suspension at 37℃ for 20 min. The reactions are initiated by addition of 5-HT. Sixty minutes later, the reactions are terminated by adding 50 μL of ice-cold 20% perchloric acid, chilled in an ice-water bath for 10 min and then neutralized with 160 μL of 1 N KOH. Each sample is diluted with 2 ml of 50 mM Tris-HCl, pH 7.4 at room temperature. The aqueous portion (2.2 mL) is transferred onto Dowex AG1X8 columns (1 ml, 1 : 1, w/v) which had been washed with 5 ml of distilled water. The columns are then washed with 18 ml of distilled water and the inositol phosphates are eluted with 3 ml of 1 N HCl. The eluted radioactivity is determined by liquid scintillation spectroscopy using a Packard 1900CA analyzer[1].

Animal experiment:

Rats[1] Male Sprague-Dawley rats (200 g) are used. To compare the plasma kinetics of RS-127445 following different routes of administration, 90 rats are distributed into three treatment groups of 30 rats each. A single dose of RS-127445 (5 mg/kg) dissolved (2.5 mg/mL) in ethanol:propylene glycol : water (10 : 50 : 40, v:v:v), is administered to each rat. At 0.08, 0.25, 0.5, 1, 2, 4, 8 and 24 h after dosing, the rats are anaesthetized and blood samples are collected by cardiac puncture. Mice[2] Adult male C57BL/6J mice (25-30 g) are used. The effects of RS-127445 (1 nM-10 μM, single concentration per tissue, 15 min contact time) or vehicle (5 or 50 μL DMSO) are expressed as the percentage change in amplitude compared with the mean amplitude of four pre-drug, post-EFS contractile responses. The results are analysed using a two-sample equal variance t-test.

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.