Neurokinin receptors antagonists: old and new

Neurokinin receptor subtypes characterized by biological assays

Neurokinin receptors have been characterized by biological assays using naturally occurring and selective agonists as well as peptide and non peptide antagonists. Six preparations have been used: the rabbit vena cava and the rat urinary bladder, treated with a NK-2 receptor antagonist for the NK-1 receptor, the rabbit pulmonary artery and the hamster urinary bladder for the NK-2, the rat portal vein and the guinea pig ileum, treated with a NK-1 receptor antagonist, for the NK-3. Treatment with antagonists was required because of the presence (in some preparations) of two functional sites contributing to the biological effect. Differences in the order of potency of agonists between each couple of receptors have been demonstrated, especially with tachykinins and the selective agonists. Such differences are even more evident with antagonists, some of which show apparent affinity (pA2) values 1.5 to 3 log units higher in one than in the other member of each couple. Based on data obtained in pharmacological experiments, it is concluded that NK-1, NK-2 and NK-3 receptors show differences strong enough to justify the assumption that their coding and/or expression diverge among species.

Synthesis of a potent antagonist of substance P by replacing the CH2SCH3 and the alpha-carboxamide groups of the methionine at [Orn6]-SP6-11 by benzyl ester groups

Analogues of [Orn6]-SP6-11 have been synthesized in which the CH2SCH3 group of Met11 is replaced by a COOCH3 or a COOBzl group. These analogues, which were tested for agonist and antagonist activity in three in vitro preparations representative of NK-1, NK-2 and NK-3 receptor types, were full agonists at NK-1 receptors, showed very weak agonist activity at NK-2, receptors and were weak antagonists at NK-3 receptors. The above analogues were modified by substituting the alpha-carboxamide of residue 11 by a COOCH3 and a COOBzl group, respectively. The resulting analogues were found to be devoid of agonist activity in each of the functional assays. However, they showed weak antagonist activity at each receptor subtype, with the exception of the dibenzyl analogue, which was a potent and selective NK-1 receptor antagonist. It is concluded that appropriate modification of the side chain of Met11 and its alpha-carboxamide leads to a potent and selective at NK-1 receptor antagonist.

Synthesis and in vitro activities of new tryptophan-modified and thiomethylene-containing pseudopeptide antagonists of the neurokinins

A series of pseudopeptide analogs of the substance P-like hexapeptide Ava-Phe-Phe-Gly-Leu-Met-NH2 was produced by N alpha-protection, introduction of the thiomethylene bond, of D- and non-proteinogenic amino acids, and alteration of the side chain of tryptophan. Synthesis of the pseudopeptides on a solid phase was successfully improved by direct formation of the CH2-S bond on the resin. However, while thiomethylene formation between leucine and norleucine led to the expected SS diastereoisomer, the major product of the similar coupling between two phenylalanines was the SR isomer. An improved resistance of the analogs to proteolysis was observed, which could be related to the structural changes. Interestingly, these modifications led to three water-soluble and potent neurokinin antagonists on classical in vitro bioassays.

Substance P (NK-1 receptor) antagonists: in vivo and in vitro activities in rats and guinea pigs

NK-1 receptor subtypes have been identified by the use of CP-96,345 and RP-67,580, two non-peptide antagonists. These and other antagonists have been tested in vivo and in vitro in guinea pigs and rats to counteract the hypotensive and contractile (urinary isolated bladder) effects of a) SP, b) the NK-1 selective agonist [Sar9,Met(O2)11]SP and c) other neurokinins. CP-96,345 has been found to be more active on the guinea pig and RP-67,580 more active on the rat by at least 1 log unit both in vivo and in vitro. Both compounds are selective NK-1 antagonists and RP-67,580 appears to be weaker than CP-96,345. Two in vitro preparations, the guinea pig and rat urinary bladder are proposed as bioassays for the NK-1A (guinea pig) and NK-1B (rat) receptor subtypes, which have been shown to mediate smooth muscle contraction and hypotension, resulting from peripheral vasodilatation. CP-96,345 and RP-67,580 are more potent antagonists than spantide, its homologous octapeptide and the Fujisawa tri or dipeptides.

Characterization of NK-1 receptors in guinea pig and rat brain membranes with NK-1 peptides and a non-peptide antagonist

The major finding of the present investigation is the demonstration of different NK-1 receptors in rat and guinea pig brain membranes with CP 96345 (non-peptide NK-1 antagonist) and R-544 (NK-1 peptide antagonist). We used [3H][Sar9,Met(O2)11]SP, the highly selective ligand for NK-1 receptor to compare NK-1 binding sites in rat and guinea pig brain membranes. Scatchard analysis revealed the existence of a single population of [3H][Sar9,Met(O2)11]SP binding sites in both preparations. The affinity and the maximal number of binding sites were found closely similar in rat (Kd 2 nM, Bmax = 37 fmol/mg protein) and guinea pig brain membranes (Kd = 3 nM, Bmax = 25 fmol/mg of protein). The order of potency of neurokinins to inhibit [3H][Sar9,Met(O2)11]SP binding from rat brain (SP > NKA > NKB) was found different of that observed on guinea pig brain (SP > NKB > NKA). Results obtained with [Sar9,Met(O2)11]SP, [beta Ala8]NKA(4-10) and [MePhe7]NKB suggest that selective agonists cannot discriminate between NK-1 receptors of different species. Using the non-peptide antagonist CP 96345 and the tripeptide R-544, we found that these two NK-1 antagonists discriminate between rat and guinea pig [3H][Sar9,Met(O2)11]SP binding sites.

Effects of the tripeptide substance P antagonist, FR113680, on airway constriction and airway edema induced by neurokinins in guinea-pigs

FR113680 is a newly developed tripeptide substance P (SP) receptor antagonist. The effects of FR113680 on airway constriction and airway edema induced by neurokinins were investigated in guinea-pigs. In in vitro experiments, FR113680 inhibited the contraction of isolated guinea-pig trachea induced by SP and neurokinin A (NKA) in a dose-dependent manner with IC50 values of 2.3 x 10(-6) and 1.5 x 10(-5) M, respectively. The tracheal contraction induced by histamine and acetylcholine was not affected by FR113680. FR113680 (5 x 10(-5) M) also significantly inhibited the atropine-resistant contraction of isolated guinea-pig bronchi induced by electrical field stimulation. In in vivo experiments, FR113680 given i.v. inhibited SP-induced airway constriction in guinea-pigs at doses of 1 and 10 mg kg-1. However, FR113680 only inhibited NKA- and capsaicin-induced airway constriction by 40-50% even at a dose of 10 mg kg-1. FR113680 also inhibited SP-induced airway edema in guinea-pigs with the same potency as it inhibited SP-induced airway constriction. Histamine-induced airway constriction and airway edema were not affected at a dose of 10 mg kg-1. These results suggest that FR113680 preferentially inhibits responses induced by NK1 receptor activation (SP-induced airway constriction and airway edema), but is less effective on a NK2 receptor-induced response (airway constriction by NKA and neurogenic stimulation).

Selectivity and specificity of new, non-peptide, quinuclidine antagonists of substance P

Two members of a new class of non-peptide antagonists of substance P, (+-)-cis-3-(2-methoxybenzylamino)-2-benzhydrylquinuclidine [(+/-)-CP-96,345; I] and (+-)-cis-3-(2-chlorobenzylamino)-2-benzhydrylquinuclidine [II], were tested for their ability to antagonize neurokinin-induced contractions of the rabbit cava and jugular veins (NK-1), the rabbit pulmonary artery (NK-2) and the rat portal vein (NK-3 system). Compound 1 is the most potent NK-1 receptor antagonist identified until now; its apparent affinity (pA2 = 9.52) is at least two log units higher than those of other NK-1 antagonists. Compound II is less active. Both compounds have been found to be almost inactive as NK-2 and NK-3 antagonists and should, therefore, be considered as selective for the NK-1 receptor. The new compounds have no direct myotropic effects and are specific for neurokinin (NK-1) receptors since they do not affect the myotropic effects of angiotensin, noradrenaline and bradykinin in the rabbit cava and jugular veins.