Absorption, emission, and chiroptical spectra of neurokinin 1 tachykinin receptor antagonists: the role of charge-transfer states on the biological activity

3D structure, dynamics, and activity of synthetic analog of the peptaibiotic trichodecenin I

In this contribution, we report on the conformational preferences of synthetic analogs of the antimicrobial peptide trichodecenin I in solution. This 6-amino acid residue long peptide is characterized by a single, strongly helicogenic Aib residue in the central part of the sequence and is rich in the conformationally mobile Gly residues. It has been reported that, in CHCl3 solution and in the crystal state, this peptaibiotic adopts a non-helical, multiple β-turn conformation, whereas a 310 /α-helical structure was obtained from an X-ray diffraction study on a trichodecenin I analog (TDT4W6) containing the fluorescent Trp residue in position 6 (replacing Ile) and an equally helicogenic TOAC residue in position 4 (replacing Aib). In this work, we applied spectroscopic techniques and molecular-dynamics calculations, in particular, on the fluorescent TDT4W6 trichodecenin I analog with the aim at investigating its 3D-structural and dynamical features in solution. Our results revealed that TDT4W6 can be described by an ensemble of conformers quickly interconverting in the nanosecond time scale. The most populated cluster has a conformation similar to the NMR structure of native trichodecenin I in CHCl3 . However, also helical-like conformers are present, even if poorly populated and less stable under the analytical conditions.

Pharmacology of MEN 11467: a potent new selective and orally- effective peptidomimetic tachykinin NK(1) receptor antagonist

We have investigated the pharmacological properties of MEN 11467, a novel partially retro-inverse peptidomimetic antagonist of tachykinin NK(1) receptors. MEN 11467 potently inhibits the binding of [(3)H] substance P (SP) to tachykinin NK(1) receptors in the IM9 limphoblastoid cell line (pK(i) = 9.4 +/- 0.1). MEN 11467 is highly specific for the human tachykinin NK(1) receptors, since it has negligible effects (pK(i) <6) on the binding of specific ligands to tachykinin NK(2) or NK(3) receptors and to a panel of 30 receptors ion channels unrelated to tachykinin receptors. The antagonism exerted by MEN 11467 at tachykinin NK(1) receptors is insurmountable in saturation binding experiments, both K(D) and B(max) of SP were significantly reduced by MEN 11467 (0.3-10 nM). In the guinea-pig isolated ileum, MEN 11467 (0.03-1 nM) produced a nonparallel rightward shift of the concentration-response curve to SP methylester with a concomitant reduction of the Emax to the agonist (pK(B) = 10.7 +/- 0.1). Moreover the antagonist activity of MEN 11467 was hardly reversible despite prolonged washout. In vivo, MEN 11467 produced a long lasting (> 2-3h) dose-dependent antagonism of bronchoconstriction induced by the selective tachykinin NK(1) receptor agonist, [Sar(9), Met(O(2))(11)]SP in anaesthetized guinea-pigs (ID(50)s' = 29+/-5, 31+/-12 and 670+/-270 microg/kg, after intravenous, intranasal and intraduodenal administration, respectively), without affecting bronchoconstriction induced by methacholine. After oral administration MEN 11467 produced a dose-dependent inhibition of plasma protein extravasation induced in guinea-pig bronchi by [Sar(9), Met(O(2))(11)] (ID(50) = 6.7 +/- 2 mg/kg) or by antigen challenge in sensitized animals (ID(50) = 1.3 mg/kg). After i.v. administration MEN 11467 weakly inhibited the GR 73632-induced foot tapping behaviour in gerbil (ED(50) = 2.96 +/- 2 mg/kg), indicating a poor ability to block central tachykinin NK(1) receptors. These results demonstrate that MEN 11467 is a potent, highly selective and orally effective insurmountable pseudopeptide antagonist of peripheral tachykinin NK(1) receptors with a long duration of action.

A spectroscopic and molecular modeling study on novel pseudopeptides exhibiting biological activity

A series of pseudopeptides, containing two fluorophores, such as naphthalene (N) and indole (I), and exhibiting interesting biological activity as tachykinin receptor antagonists, were investigated by electronic absorption, CD and steady-state fluorescence experiments. In polar solvents (e.g. methanol), bioactivity is coupled with a stacked, charge-separated complex between I and N, the amount of which depends on the stereochemical features and conformational mobility of the central scaffold in the molecules examined. This agrees with the idea that dipolar charged, spatially close, aromatic moieties are important topochemical elements in the mechanism of action of these receptor antagonists. Molecular mechanics calculations allowed us to build up hypothetical, low-energy conformations of a few representative pseudopeptides, whose structural features are consistent with the experimental findings.

Correlation between binding characteristics and functional antagonism in human glioma cells by tachykinin NK1 receptor antagonists

Binding characteristics and functional antagonism exerted by two structurally related tachykinin NK1 receptor antagonists, MEN 11467 ((1R,2S)-2N[1(H)indol-3-yl-carbonyl]-1-N-{Nalpha(p-tolylacetyl+ ++)-Nalpha(methyl)-D-3-(2-naphthyl)alanyl}diaminocyclohexane) and FK888 (N2-[(4R)-4-hydroxy-1-(1-methyl-1H-indol-3-yl)carbonyl-L-prolyl]-N-methy l-N-phenylmethyl-L-3-(2-naphthyl)alaninamide), were investigated in the human astrocytoma cell line U373 MG. In radioligand binding studies, conducted with [3H]substance P and intact cells at 37 degrees C, MEN 11467 bound to tachykinin NK1 receptors in an irreversible manner with a Ki value of 1.2+/-0.5 nM while FK888 bound in competitive manner with a Ki value of 4.6+/-2.2 nM. Receptor blockade by both antagonists resulted in a powerful and complete inhibition of functional responses induced by substance P, such as accumulation of the second messenger inositol monophosphate or interleukin-6 release. However, MEN 11467 showed a greater potency for blocking functional responses than FK888 despite their similar affinity for human tachykinin NK1 receptors. Moreover, MEN 11467 was more potent in inhibiting late rather than early phases of substance P-induced inositol monophosphate accumulation and its antagonism was enhanced by drug preincubation and barely affected by removal of unbound drug from the external medium, suggesting that MEN 11467 bound in a tight manner to the receptor. Such behaviour was not observed with the competitive and rapidly reversible antagonist FK888. These data indicate that the small differences in the chemical structure of MEN 11467 and FK888 determine the different binding characteristics at the tachykinin NK1 receptor and which are responsible for the greater potency of MEN 11467 for blocking functional responses. The Ki value obtained in binding studies may be inadequate to reveal the real potency of tachykinin NK1 receptor antagonists.