RS-127445 (MT500) is a selective 5-HT2B receptor antagonist with pKi of 9.5 and pIC50 of 10.4, exhibits >1000-fold selectivity against other 5-HT receptors.

5-HT2B:10.4(pIC50), 5-HT2B:9.5(pKi)

RS-127445是一种新型高亲和力、选择性5-HT2B受体拮抗剂，没有可检测的固有活性。RS-127445对5-HT2B受体的亲和力达到nM级别，具有1000倍的选择性。RS-127445能强效阻断5-HT引起的肌醇磷酸生成增加，并以比约汉宾高1000倍的效能阻断5-HT引起的细胞内钙浓度增加。[1]

RS-127445经口服和腹膜内给药后迅速吸收，不受剂量或给药方式限制，血浆浓度在给药后15分钟内达到峰值。血浆中RS-127445的浓度与给药剂量成正比。5 mg/kg剂量下，RS-127445的生物利用度约为腹膜内给药的60%和口服的14%。预计，RS-127445在血浆中的浓度足以完全饱和大鼠可接触的5-HT2B受体，并可维持该状态。口服1到10 mg/kg的RS-127445能显著抑制由束缚应激引起的内脏过敏性，抑制率达35%至74%。口服RS-127445能显著抑制TNBS诱导的内脏过敏性（3到30 mg/kg时的抑制率为15%到62%）。口服1到30 mg/kg的RS-127445还能剂量依赖性地减少原生及TNBS处理的大鼠由束缚应激引起的排便。[2]。RS-127445抑制结肠运动和排便。[3]

Radioligand binding: The selectivity of RS-127445 for 5-HT2B receptors is examined by testing the compound for affinity at over 100 additional ion channel or receptor binding sites. 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 and centrifugation (48,000×g for 15 min). 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 [3 H]-ketanserin at 32 °C 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 [3 H]-5-HT at 48 °C for 120 min. Nonspecific binding is determined using 10 μM 5-HT. For the 5-HT2Creceptor, membranes from 3×10 5 cells are incubated with 0.5 nM [3 H]-mesuler -gine at 32 °C for 60 min. Nonspecific binding is determined using 10 μM methysergide. Assays are terminated by vacuum filtration through glass fibre filters(GF/B) which has 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.

RS-127445, vehicle or other antagonists are pre-incubated with 240 μl of HEK-293 cells expressing the human 5-HT2B receptor suspension at 37 °C for 20 min. HEK-293 cells are incubated with[3H]-myoinositol (1.67 μCi/ml) in 162 cm2 flasks overnight at 37 °C in an inositol free Ham's F12 medium containing 10% dialyzed foetal bovine serum. The cells are harvested, washed five times with phosphate bufffered saline and resuspended in inositol free Ham's F12 media at density of approximately 3×103 cells/ml. 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 160μl 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 has 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] (Only for Reference)