Utilization of an intramolecular hydrogen bond to increase the CNS penetration of an NK(1) receptor antagonist.

This paper describes the synthesis and physical and biological effects of introducing different substituents at the alpha-position of the tryptophan containing neurokinin-1 receptor antagonist [(R)-2-(1H-indol-3-yl)-1-methyl-1-((S)-1-phenyl-ethylcarbamoyl)-ethyl]-carbamic acid benzofuran-2-ylmethyl ester (CI 1021). The described compounds all exhibit less than 5 nM binding affinities for the human neurokinin-1 receptor and selectivity over the tachykinin NK(2) and NK(3) receptor subtypes. Application of variable temperature nuclear magnetic resonance spectroscopy studies of the amide and urethane protons was utilized to determine the existence of an intramolecular hydrogen bond. This intramolecular hydrogen bond increases the apparent lipophilicity to allow increased central nervous system penetration and pharmacological activity (gerbil foot tap test) in the case of the highest affinity compound [(S)-1-dimethylaminomethyl-2-(1H-indol-3-yl)-1-((S)-1-phenyl-ethylcarbamoyl)-ethyl]-carbamic acid benzofuran-2-ylmethyl ester (PD 174424) over those analogues that could not form an intramolecular hydrogen bond.

Structure-based design in drug discovery--the application of a peptoid drug design strategy for the development of non-peptide neuropeptide receptor ligands.

Over the last decade the increasing availability of metabolically- stable non-peptide antagonists targeted at neuropeptide receptors has led directly to a more thorough understanding of the role of neuropeptides in mammalian physiology. By far the majority of these non-peptide neuropeptide receptor antagonists thus far disclosed have been developed from leads identified from broad screening of company compound files or natural product collections, and may thus bear little obvious structural resemblance to the endogenous peptide ligand. This review will focus on an alternative structure-based approach to non-peptide neuropeptide receptor ligand design, referred to as the 'peptoid' drug design strategy, in which an appreciation of the structure of the neuropeptide is the key to the success of this approach. The development and current clinical progress of peptoid cholecystokinin and tachykinin receptor ligands that have thus far resulted from this process will be highlighted and used to exemplify the importance of this novel approach.

'Targeted' molecular diversity: design and development of non-peptide antagonists for cholecystokinin and tachykinin receptors.

A drug design strategy to non-peptide small molecule antagonists of neuropeptides is described that targets the molecular diversity which exists in the 'privileged' data set of the physico-chemical properties represented by the side-chains of the 20 genetically encoded amino acids. The strategy is exemplified by the design of a selective and high affinity cholecystokinin CCK-A antagonist PD 140548, CCK-B antagonist CI-988 (formerly PD 134308) tachykinin NK-1 antagonist PD 154075 and NK-2 antagonist Cam-2291. The NK-3 antagonists, PD 157672 and the non-peptide PD 161182, were developed from an information-rich dipeptide library constructed from 256 N-protected dipeptides and 64 hydrophobic biased dipeptides.

Use of a dipeptide chemical library in the development of non-peptide tachykinin NK3 receptor selective antagonists.

The use of a dipeptide library as the source of a micromolar chemical lead compound for the human tachykinin NK3 receptor is described. The screening of a dipeptide library through a cloned human NK3 receptor binding assay resulted in the identification of Boc(S)Phe(S)PheNH2 (1), which has subsequently been developed, following a 'peptoid' design strategy, into a series of high-affinity NK3 receptor selective antagonists. The structure-activity relationship of the C-terminal portion of this dipeptide lead was first explored and led to the identification of the urea derivative Boc(S)Phe(R)alphaMePheNH(CH2)7NHCONH2 (41, PD157672). This modified dipeptide has a Ke of 7 nM in blocking senktide-induced increases in intracellular calcium levels in human NK3 receptors stably expressed in CHO cells. Subsequent optimization of the N-terminal BocPhe group and the alphaMePhe residue side chain of 41 led to the identification of [S-(R*,S*)]-[2-(2,3-difluorophenyl)-1-methyl-1-[(7-ureidoheptyl)ca r bamoyl]ethyl]carbamic acid 2-methyl-1-phenylpropyl ester (60, PD161182), a non-peptide NK3 receptor selective antagonist. Compound 60 blocks the senktide-evoked increases in intracellular calcium levels in cloned human NK3 receptors stably expressed in CHO cells with Ke of 0.9 nM.

Two classes of structurally different antagonists display similar species preference for the human tachykinin neurokinin3 receptor.

Two classes of structurally different tachykinin neurokinin3 (NK3) antagonists were used to evaluate species difference in antagonist binding between human and rat NK3 receptors. In competition binding experiments with [125I-MePhe7]NKB as radioligand, PD 154740, PD 157672, SR 48968, and SR 142801 displayed lower Ki values for the human NK3 receptor (40 +/- 4, 12 +/- 1,350 +/- 50, and 0.40 +/- 0.05 nM, respectively) than for the rat NK3 receptor (2450 +/- 130, 288 +/- 25, > 10,000, and 11.0 +/- 0.5 nM, respectively). Data from in vitro functional assay showed similar species preference as observed with the competition binding assay. It was shown previously that substitution of only two amino acid residues in the rat receptor to their human counterparts could change the species selectivity of SR 48968, a weak NK3 antagonist. In the double-substituted rat mutant, all three antagonists (PD 154740, PD 157672, and SR 142801) displayed Ki values (76 +/- 8, 16 +/- 2, and 0.50 +/- 0.05 nM, respectively) very similar to the Ki values for the wild-type human NK3 receptor. Thus, in addition to their previously reported effects on SR 48968, these two amino acid residues are responsible for the species selectivity of these three additional NK3 antagonists. Because PD 154740 and PD 157672 are very different structurally from SR 48968 and SR 142801, our results indicate that the two identified residues may be involved in adopting a receptor conformation that favors the binding of NK3 antagonists that display species preference for the human NK3 receptor.

Rational design of high affinity tachykinin NK1 receptor antagonists.

The rational design of a non-peptide tachykinin NK1 receptor antagonist, [(2-benzofuran)-CH2OCO]-(R)-alpha-MeTrp-(S)-NHCH(CH3)P h (28, PD 154075) is described. Compound 28 has a Ki = 9 and 0.35 nM for the NK1 receptor binding site in guinea-pig cerebral cortex membranes and human IM9, cells respectively (using [125I] Bolton-Hunter-SP as the radioligand). It is a potent antagonist in vitro where it antagonises the contractions mediated by SPOMe in the guinea-pig ileum (KB = 0.3 nM). Compound 28 is active in vivo in the guinea-pig plasma extravasation model, where it is able to block the SPOMe-induced protein plasma extravasation (monitored by Evans Blue) in the bladder with an ID50 of 0.02 mg kg-1 iv.