Synthesis and biological evaluation of novel NK-1 tachykinin receptor antagonists: the use of cycloalkyl amino acids as a template

The Neurokinins: Peptidomimetic Ligand Design and Therapeutic Applications

The neurokinins are indisputably essential neurotransmitters in numerous pathoand physiological events. Being widely distributed in the Central Nervous System (CNS) and peripheral tissues, their discovery rapidly promoted them to drugs targets. As a necessity for molecular tools to understand the biological role of this class, endogenous peptides and their receptors prompted the scientific community to design ligands displaying either agonist and antagonist activity at the three main neurokinin receptors, called NK1, NK2 and NK3. Several strategies were implemented for this purpose. With a preference to small non-peptidic ligands, many research groups invested efforts in synthesizing and evaluating a wide range of scaffolds, but only the NK1 antagonist Aprepitant (EMENDT) and its prodrug Fosaprepitant (IVEMENDT) have been approved by the Food Drug Administration (FDA) for the treatment of Chemotherapy-Induced and Post-Operative Nausea and Vomiting (CINV and PONV, respectively). While non-peptidic drugs showed limitations, especially in side effect control, peptidic and pseudopeptidic compounds progressively regained attention. Various strategies were implemented to modulate affinity, selectivity and activity of the newly designed ligands. Replacement of canonical amino acids, incorporation of conformational constraints, and fusion with non-peptidic moieties gave rise to families of ligands displaying individual or dual NK1, NK2 and NK3 antagonism, that ultimately were combined with non-neurokinin ligands (such as opioids) to target enhanced biological impact.

Successful virtual screening for a submicromolar antagonist of the neurokinin-1 receptor based on a ligand-supported homology model

The neurokinin-1 (NK1) receptor belongs to the family of G-protein-coupled receptors (GPCRs), which represents one of the most relevant target families in small-molecule drug design. In this paper, we describe a homology modeling of the NK1 receptor based on the high-resolution X-ray structure of rhodopsin and the successful virtual screening based on this protein model. The NK1 receptor model has been generated using our new MOBILE (modeling binding sites including ligand information explicitly) approach. Starting with preliminary homology models, it generates improved models of the protein binding pocket together with bound ligands. Ligand information is used as an integral part in the homology modeling process. For the construction of the NK1 receptor, antagonist CP-96345 was used to restrain the modeling. The quality of the obtained model was validated by probing its ability to accommodate additional known NK1 antagonists from structurally diverse classes. On the basis of the generated model and on the analysis of known NK1 antagonists, a pharmacophore model was deduced, which subsequently guided the 2D and 3D database search with UNITY. As a following step, the remaining hits were docked into the modeled binding pocket of the NK1 receptor. Finally, seven compounds were selected for biochemical testing, from which one showed affinity in the submicromolar range. Our results suggest that ligand-supported homology models of GPCRs may be used as effective platforms for structure-based drug design.

Naphtho[2,1-b][1,5] and [1,2-f][1,4]oxazocines as selective NK1 antagonists

Previously we reported on the synthesis and properties of a series of highly potent piperidinyl 2-subsituted-3-cyano-1-naphthamide NK1 antagonists that includes 3 and 4. Here we report our efforts to alleviate a troublesome atropisomeric property of those derivatives by introduction of a tethering bridge that, in addition, could be used to lock the resulting cyclic derivatives in a purported NK1 pharmacophore conformation. Using 3 as a starting point, the naphtho[2,1-b][1,5]oxazocine, 17, was found to contain the optimal ring tether size (8) for retaining NK1 activity, was more NK1 versus NK2 selective, and reduced the number of atropisomers from four to two. Cyclic derivatives 29 and 32, which exist as essentially single atropisomers in the purported pharmacophore conformation, were prepared in the closely related naphtho[1,2-f][1,4]oxazocine series as part of an effort to use mono methyl substitution of the tethering bridge as a conformation stabilizing factor. Both 29 and 32 were found to be less active as NK1 antagonists than the non-methylated parent 28 possibly due to methyl group destabilization of receptor interaction. We discuss the above findings in the context of a previously proposed NK1 pharmacophore model and present a further refinement of that model.

Structural Analysis and Optimization of NK1 Receptor Antagonists through Modulation of Atropisomer Interconversion Properties

We have previously described a series of antagonists that showed high potency and selectivity for the NK(1) receptor. However, these compounds also had the undesirable property of existing as a mixture of interconverting rotational isomers. Here we show that alteration of the 2-naphthyl substituent can modulate the rate of isomer exchange. Comparisons of the NK(1) receptor affinity for the various conformational forms has facilitated the development of a detailed NK(1) pharmacophore model.

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.

Multiple-conformation and protonation-state representation in 4D-QSAR: the neurokinin-1 receptor system

Using a 4D-QSAR approach (software Quasar) allowing for multiple-conformation, orientation, and protonation-state ligand representation as well as for the simulation of local induced-fit phenomena, we have validated a family of receptor surrogates for the neurokinin-1 (NK-1) receptor system. The evolution was based on a population of 500 receptor models and simulated during 40 000 crossover steps, corresponding to 80 generations. It yielded a cross-validated r(2) of 0.887 for the 50 ligands of the training set (represented by a total of 218 conformers and protomers) and a predictive r(2) of 0.834 for the 15 ligands of the test set (70 conformers and protomers). A series of five "scramble tests" (with an average predictive r(2) of -0.438) demonstrates the sensitivity of the surrogate toward the biological data, for which it should establish a QSAR. On the basis of this model, the activities of 12 new compounds - four of which have been synthesized and tested in the meantime - are predicted. For most of the NK-1 antagonists, the genetic algorithm selected a single entity - out of the up to 12 conformers or protomers - to preferably bind to the receptor surrogate. Moreover, the evolution converged at an identical protonation scheme for all NK-1 antagonists. This indicates that 4D-QSAR techniques may, indeed, reduce the bias associated with the choice of the bioactive conformation as each ligand molecule can be represented by an ensemble of conformations, orientations, and protonation states.

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

A spectroscopic investigation, based on both electronic absorption and emission spectra as well as on chiroptical data, was performed on novel neurokinin 1 (NK1) tachykinin receptor antagonists, exhibiting interesting biological activity. These pseudopeptides have two fluorophores, i.e. indole (I) and naphthalene (N), and a central scaffold with different conformational mobility. Absorption spectra in methanol show the presence of a new band with respect to the sum spectrum of the isolated chromophores at around 285 nm, the intensity of which linearly increases as the bioactivity increases. This absorption disappears by using dioxane as solvent. It is ascribed to an intramolecular I-N charge-transfer (CT) complex that forms to different extent, depending on the flexibility of the scaffold. Under this condition, the molecules fold and apparently attain the correct conformation for competing substance P binding to the NK1 receptor, lending plausibility to the role of dipolar charged, spatially close aromatic moieties as topochemical elements in the mechanism of action of substance P antagonists. The excited-behavior parallels that in the ground state, as the quenching of the singlet state at 340 nm is found to be linearly dependent on the biological activity, too. Upon decreasing solvent polarity (methanol vs dioxane) the emissions of the dipolar state at around 370 nm disappears, while exciplex emission in the range of 400-500 nm occurs. This transition from charge-separated to exciplex-like states by lowering the dielectric constant of the medium very likely reflects a change in the structural features of the intramolecular I-N stacked complex, from a twisted or an asymmetrically overlapped conformation of the indolyl and naphthyl rings to a face-to-face geometry. Implications of the rigidity of the molecules, arising from the formation of the intramolecular CT complex, on the ellipticity are briefly discussed.

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.

Pharmacology of the peptidomimetic, MEN 11149, a new potent, selective and orally effective tachykinin NK1 receptor antagonist

In this study we investigated the pharmacological properties of MEN 11149, 2-(2-naphthyl)-1-N-[(1R,2S)-2-N-[1(H)indol-3-ylcarbonyl]aminocy clohexanecarbonyl]-1-[N'-methyl-N'-(4-methylphenylacetyl)]di aminoethane, a novel partially retro-inverse pseudo peptide antagonist of tachykinin NK1 receptors. MEN 11149 potently inhibits the binding of [3H]substance P to tachykinin NK1 sites in IM9 cells (pKi = 8.5 +/- 0.1). The compound is highly specific for the human tachykinin NK1 receptors, since it has negligible effects (pKi < 6) on the binding of specific ligands to tachykinin NK2, NK3 receptors and a battery of central and peripheral receptors or ion channels. The tachykinin NK1 receptor antagonism of MEN 11149 appears to be insurmountable since, in saturation binding experiments, both K(D) and Bmax are significantly affected by incubation with the compound (1-30 nM). In isolated guinea-pig ileum, MEN 11149 (0.1-100 nM) shifts to the right in a non-parallel way the substance P methyl ester-induced cumulative concentration-response curve with progressive inhibition of the maximal response (pK(B) = 9.6 +/- 0.1). When tested for reversibility at 5 nM in the same preparation, the compound displays a slow dissociation rate compared to the fast dissociation rate with 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) at 5 nM. In the same preparation, MEN 11149 (10 microM) did not affect the cumulative concentration-response curve to acetylcholine. In vivo, MEN 11149 dose dependently antagonizes [Sar9,Met(O2)11]substance P-induced bronchoconstriction in anaesthetized guinea-pigs (ID50 = 83 +/- 31 nmol/kg i.v.). The duration of the effect exceeds 3 h. MEN 11149 does not affect the bronchoconstriction induced by neurokinin A. The compound dose dependently inhibits [Sar9,Met(O2)11]substance P-induced plasma protein extravasation in guinea-pig bronchi whether administered intravenously (ID50 = 0.22 +/- 0.02 micromol/kg) or orally (ID50 = 0.97 +/- 0.21 micromol/kg). These results demonstrate that MEN 11149 is a potent, highly selective and orally effective insurmountable antagonist of tachykinin NK1 receptors with a long duration of action.

The tachykinin NK1 receptor. Part I: ligands and mechanisms of cellular activation

The tachykinin NK1 receptor is widely expressed in the mammalian central and peripheral nervous system. Powerful pharmacological tools (agonists and antagonists) are now available to elucidate the physiological role of NK1 receptors at these levels, as well as to understand their role in diseases and establish the possible therapeutic usefulness of NK1 receptor antagonists for treatment of human diseases. The structure-activity studies that have led to the development of potent peptide and non-peptide ligands for the tachykinin NK1 receptor are here reviewed. Among the peptide agonists and antagonists, linear and cyclic sequences have been developed. The non peptide antagonists belong to different chemical classes, i.e. steroids, perhydroisoindolones, quinuclidines, piperidines and tryptophane derivatives. The first non peptide antagonists for NK1 receptors have been obtained by random screening of chemical compounds large collections. The resulting leads were optimized with 'classic' structure activity approaches, aiming at identifying 'common' motifs for interaction with the receptor by ligands of different chemical classes. The results derived from the recent application of molecular biology techniques were useful to drive the design of new ligands toward a precise structural definition of ligand-receptor bi-molecular interactions. Studies on mutant receptors have established that the sites of interaction of peptide agonists and non peptide antagonists with the tachykinin NK1 receptor are largely non overlapping. Moreover, data obtained from mutagenesis of the NK1 receptor further indicate that some amino acid residues in the NK1 receptor sequence are critical for determining the binding affinity of some but not all ligands. Therefore, different antagonists discovered from random screening may not possess common points of interaction or common structural and conformational characteristics for their interaction with the tachykinin NK1 receptor. The tachykinin NK1 receptor couples with G-proteins to determine its biological effects in target cells. Several G-proteins both sensitive (Go, Gi) and insensitive (Gq, G11) to pertussis toxin can mediate the action of NK1 receptors. Moreover, several second messanger signalling systems (elevation of intracellular calcium, stimulation of phosphoinositol turnover, arachidonic acid mobilization, cAMP accumulation) have to be activated following NK1 receptor signalling. Also a direct modulation of certain ion channels at membrane level has been proposed. The NK1 receptor undergoes prompt and significant tachyphylaxis upon exposure to the agonist: this has been shown to be linked with receptor internalization which also occurs physiologically when the NK1 receptor is stimulated by endogenous tachykinins.