| Abstract: | The pharmacological outline of a novel and original antagonist at the human tachykinin NK2 receptor is presented, namely MEN13510 (N-N'-bis-[2-(1H-indol-3-yl)-ethyl]-N,N'-bis-(3-thiomorpholin-4-yl-propyl)-phthalamide). MEN13510 retained nanomolar affinity for the human tachykinin NK2 receptor (Ki 6.4 nM), and micromolar affinity for the human tachykinin NK1 and NK3 receptors. A competitive antagonism is indicated by the Schild analysis (pK(B) 7.8, slope -0.94) of concentration-response curves of NKA induced inositolphosphates accumulation in Chinese hamster ovary (CHO) cells expressing the human NK2 receptor in the presence of MEN13510 (30-300 nM concentration range). The MEN13510 interaction with the human NK2 receptor was evaluated by means of heterologous inhibition binding experiments, by using agonist and antagonist radioligands ([125I]NKA, [3H]nepadutant, [3H]saredutant) at a series of mutant receptors having single aminoacidic substitutions of residues located in transmembrane (TM) segments 3, 4, 5, 6, and 7. MEN13510 affinity was not affected by the mutations in TM 3 and 4 (Q109A, F112A, T171A, C167G), and it was reduced by 10-fold at the I202F mutant, but not at the Y206A (TM4). Amongst the investigated mutants bearing the mutated residues in TM6 (F270A, Y266F, W263A) only F270A decreased the MEN13510 affinity by 7-fold. Even mutations in TM7 did reduce MEN13510 affinity by 32-fold (Y289T, but not Y289F) and 13-fold (F293A). Studied mutations represent the human tachykinin NK2 receptor discriminants involved in the binding of previously reported peptidic and nonpeptidic antagonists, against which results obtained with MEN13510 are compared. Results indicate that the binding site of this antagonist is, at least in part, overlapping to that described for NKA or saredutant. Finally we show that MEN13510 retains nanomolar affinity for the recently discovered splice variant of the human tachykinin NK2 receptor, namely beta isoform, as it has been described for the nonpeptide antagonist saredutant. |
