Abstract

Ibodutant (MEN15596, [1-(2-phenyl-1R-{[1-(tetrahydropyran-4-ylmethyl)-piperidin-4-ylmethyl]-carbamoyl}-ethylcarbamoyl)-cyclopentyl]-amide) is a tachykinin NK₂ receptor (NK₂R) antagonist currently under phase II clinical trials for irritable bowel syndrome. This study focuses on the ibodutant pharmacodynamic profile at the human NK₂R and compares it with two other antagonists, nepadutant (MEN11420, (cyclo-{[Asn(β-d-GlcNAc)-Asp-Trp-Phe-Dpr-Leu]cyclo(2β-5β)}) and saredutant [SR48968, (S)-N-methyl-N[4-(4-acetylamino-4-phenylpiperidino)-2-(3,4-dichlorophenyl)butyl]benzamide]. In functional experiments (phosphatidylinositol accumulation) in Chinese hamster ovary cells expressing the human NK₂R, ibodutant potency measured toward concentration-response curves to neurokinin AaspK_B was 10.6, and its antagonism mechanism was surmountable and competitive. In the same assay, antagonism equilibration and reversibility experiments of receptor blockade indicated that ibodutant quickly attains equilibrium and that reverts from receptor compartment in a slower manner. Kinetic properties of ibodutant were assessed through competitive binding kinetics experiments performed at [³H]nepadutant and [³H]saredutant binding sites. Determined K_on and K_off values indicated a fast association and slow dissociation rate of ibodutant at the different antagonist binding sites. Last, by radioligand binding experiments at some mutated human tachykinin NK₂Rs, the amino acidic determinants crucial for the high affinity of ibodutant were identified at the transmembrane (TM) level: Cys167 in TM4; Ile202 and Tyr206 in TM5; Phe270, Tyr266, and Trp263 in TM6; and Tyr289 in TM7. These results indicated an extended antagonist binding pocket in the TM portion of the receptor, which is conceived crucial for TM3 and 6 arrangement and leads to G protein-coupled receptor activation. By combining this information and molecular modeling, the docking mode of ibodutant-human NK₂R complex is proposed.