Peptide-based tachykinin NK2 receptor antagonists

Successful bridging from a peptide to a non peptide antagonist at the human tachykinin NK-2 receptor

Non peptide products have been found to show nanomolar binding and functional affinities at the human tachykinin NK-2 receptor. The new antagonists do not possess stereogenic centers and their thermal behaviour in solution is featured by a peculiar set of conformational stereoisomers. A macroscopic viewpoint is preferentially adopted to rationalize the obtained results.

MEN 11420 (Nepadutant), a novel glycosylated bicyclic peptide tachykinin NK2 receptor antagonist

1. The pharmacological profile was studied of MEN 11420, or cyclo[[Asn(beta-D-GlcNAc)-Asp-Trp-Phe-Dap-Leu]cyclo(2beta-5beta )], a glycosylated derivative of the potent, selective, conformationally-constrained tachykinin NK2 receptor antagonist MEN 10627 (cyclo(Met-Asp-Trp-Phe-Dap-Leu)cyclo(2beta-5beta)). 2. MEN 11420 competitively bound with high affinity to the human NK2 receptor stably transfected in CHO cells, displacing radiolabelled [125I]-neurokinin A and [3H]-SR 48968 with Ki values of 2.5+/-0.7 nM (n = 6) and 2.6+/-0.4 nM (n = 3), respectively. 3. MEN 11420 showed negligible binding affinity (pIC50 < 6) at 50 different receptors (including tachykinin NK1 and NK3 receptors) and ion channels. 4. In the rabbit isolated pulmonary artery and rat urinary bladder MEN 11420 potently and competitively antagonized tachykinin NK2 receptor-mediated contractions (pK(B) = 8.6+/-0.07, n = 10, and 9.0+/-0.04, n = 12; Schild plot slope = -1.06 (95% c.l. = -1.3; -0.8) and -1.17 (95% c.l. = -1.3; -1.0), respectively). MEN 11420 produced an insurmountable antagonism at NK2 receptors in the hamster trachea and mouse urinary bladder. However, in both preparations, the effect of MEN 11420 was reverted by washout and an apparent pK(B) of 10.2+/-0.14, n = 9, and 9.8+/-0.15, n = 9, was calculated in the hamster trachea and mouse urinary bladder, respectively. 5. MEN 11420 showed low affinity (pK(B) < 6) at guinea-pig and rat tachykinin NK1 (guinea-pig ileum and rat urinary bladder) and NK3 (guinea-pig ileum and rat portal vein) receptors. On the whole, the affinities (potency and selectivity) showed by MEN 11420 for different tachykinin receptors, measured either in binding or in functional bioassays, were similar to those shown by the parent compound, MEN 10627. 6. The in vivo antagonism of the contractions produced by [betaAla8]neurokinin A(4-10) (1 nmol kg(-1)) was observed after intravenous (dose range: 1-10 nmol kg(-1)), intranasal (3-10 nmol kg(-1)), intrarectal (30-100 nmol kg(-1)) and intraduodenal (100-300 nmol kg(-1)) administration of MEN 11420. MEN 11420 was more potent (about 10 fold) and longer lasting than its parent compound MEN 10627, possibly due to a greater metabolic stability. 7. A dose of MEN 11420 (100 nmol kg(-1), i.v.), that produced potent and long lasting inhibition of the contraction of the rat urinary bladder induced by challenge with the NK2 selective receptor agonist [betaAla8]neurokinin A(4-10) (10-300 nmol kg(-1)), was without effect on the responses produced by the NK1 receptor selective agonist [Sar9]substance P sulphone (1-10 nmol kg(-1)). 8. These findings indicate that MEN 11420 is a potent and selective tachykinin NK2 receptor antagonist. The introduction of a sugar moiety did not produce major changes in the affinity profile of this antagonist as compared to MEN 10627, but markedly improved its in vivo potency and duration of action. With these characteristics, MEN 11420 is a suitable candidate for studying the pathophysiological significance of tachykinin NK2 receptors in humans.

Further evidence for the involvement of tachykinin receptor subtypes in formalin and capsaicin models of pain in mice

The intradermal (i.d.) injection of NK1 receptor antagonists GR 82334 and FK 888 (1-50 pmol/paw), in association with formalin, produced graded inhibition of the early but not the late phase of the formalin test. The NK2, SR 48968 and NK3 SR 142801 receptor antagonists (1-50 pmol/paw) were effective in inhibiting both phases of the formalin model. Co-injection of NK1, (FK 888, GR 82334), NK2 (SR 48968) or NK3 (SR 142801) receptor antagonists with capsaicin dose-dependently attenuated capsaicin-induced licking. In addition, all antagonists were more efficacious when compared with response in the formalin test. The antinociception caused by i.d. injection of the NK3 receptor antagonist SR 142801 against both phases of the formalin test, but not that of NK1 and NK2 receptor antagonists, was significantly reversed by intraperitoneal (i.p.) injection of naloxone (5 mg/kg). Intracerebroventricular (i.c.v.) injection of NK1, NK2 or NK3 receptor antagonists (15-500 pmol/site), all produced significant and dose-dependent inhibition of both phases of the formalin and capsaicin tests. With the exception of the response of SR 48968, which was equipotent in both models of nociception, FK 888, GR 82334 and SR 142801 were about 2-25-fold less potent at the ID50 level against the capsaicin-induced pain. The antinociception caused by i.c.v. injection of NK1, NK2 or NK3 receptor antagonists was reversed by i.p. injection of naloxone (5 mg/kg). These results indicate that tachykinin receptor antagonists, acting through NK1, NK2 and NK3 receptors, produce powerful antinociception when injected i.d. or by i.c.v. route against both formalin- and capsaicin-induced licking, being more efficacious against the latter model of nociception. The action of NK3 receptor antagonist given i.d. was mediated through an opioid mechanism sensitive to naloxone. However, when injected i.c.v., the antinociception caused by NK1, NK2 or NK3 receptor antagonists was largely reversed by naloxone when assessed in the formalin test, suggesting a distinct mechanism of action.

Effects of central administration of tachykinin receptor agonists and antagonists on plus-maze behavior in mice

This study assessed the effects of intracerebroventricular administration of selective agonists and antagonists for tachykinin NK1 and NK2 receptors on performance of mice in the elevated plus-maze, an ethological model of anxiety. Mice were treated with either vehicle (5 microliters) or 1, 10, 100 or 500 pmol of substance P, neurokinin A, the selective NK1 receptor agonist substance P methyl ester, or the selective NK2 receptor agonist, [beta-Ala8]neurokinin A-(4-10). Other mice received similar doses of FK 888, i.e., N2-[(4R)-4-hydroxy-1-(1-methyl-1 H-indol-3-y)carbonyl-L-prolyl]-N-methyl-N-phenylmethyl-3-(2-naphthyl)-L- alaninamide, or SR 48968, i.e., (S)-N-methyl-(N-[4-acetylamine-4-phenylpiperidine)-2-(3, 4-dichlorophenyl)buthyl]benzamide, selective antagonists of tachykinin NK1 and NK2 receptors, respectively. Injections of substance P, neurokinin A, substance P methyl ester or [beta-Ala8]neurokinin A-(4-10) significantly reduced the frequency of open arm entries, and [beta-Ala8]neurokinin A-(4-10) also enhanced the percentage of entries into enclosed arms. Conversely, the NK1 antagonist FK 888 and the NK2 antagonist SR 48968 each increased the time spent in the open arms, and SR 48968 also increased the frequency of entries into the open arms. None of the tachykinin receptor agonists or antagonists modified motor performance and coordination on the rotarod apparatus or ambulation in an activity cage. Together, these results suggest that centrally administered NK1 and NK2 receptor agonists and antagonists can modulate anxiety, as evaluated in the elevated plus-maze test in mice. Stimulation of either tachykinin NK1 or NK2 receptors induces anxiogenic-like responses, whereas the reverse occurs following their blockade. The anxiolytic-like profiles of action of both tachykinin NK1 and NK2 receptor antagonists suggest that central tachykinin mechanisms are tonically involved in the modulation of anxiety.