Thrombin receptors as drug discovery targets

Design, synthesis and biological evaluation of non-peptide PAR1 thrombin receptor antagonists based on small bifunctional templates: arginine and phenylalanine side chain groups are keys for receptor activity

In the present study, we report the synthesis and biological evaluation of a series of new non-peptide PAR(1) mimetic receptor antagonists, based on conformational analysis of the S(42)FLLR(46) tethered ligand (TL) sequence of PAR(1). These compounds incorporate the key pharmacophore groups in the TL sequence, guanidyl, amino and phenyl, which are essential for triggering receptor activity. Compounds 5 and 15 (50-100 microM) inhibited both TFLLR-amide (10 microM) and thrombin-mediated (0.5 and 1 U/ml; 5 and 10 microM) calcium signaling in a cultured human HEK cell assay.

Synthesis and pharmacological evaluation of peptide-mimetic protease-activated receptor-1 antagonists containing novel heterocyclic scaffolds

Protease-activated receptor-1 (PAR-1) is a G-coupled receptor activated by alpha-thrombin and other proteases. In this paper we describe the synthesis and the pharmacological evaluation of novel peptide-mimetic antagonists (compounds 1-16) characterized by the presence of new heterocyclic nuclei such as 2-methyl-indole (5- and 6-substituted) and 1,4-benzodiazepine moiety. The new derivatives, tested in order to evaluate their antagonist potency by using human platelet aggregation induced by PAR-1AP, resulted in some cases (compounds 1 and 4) more potent than the reference. The compounds, tested on aortic rings, confirmed the results obtained in the aggregation assay.

Discovery of a nonpeptidic small molecule antagonist of the human platelet thrombin receptor (PAR-1)

The synthesis and biological evaluation of a series of nonpeptidic small molecule antagonists of the human platelet thrombin receptor (PAR-1) are described. Optimization of the 5-amino-3-arylisoxazole lead resulted in an approximate 100-fold increase in potency. The most potent of these compounds (54) inhibits platelet activation with IC(50)s of 90 nM against the thrombin receptor agonist peptide (TRAP) and 510 nM against thrombin as the agonist. Further, antagonist 54 fully blocks platelet aggregation stimulated by 1 nM thrombin for 10 min.

Potent, low molecular weight thrombin receptor antagonists

Several benzimidazole derivatives have been identified as potent thrombin receptor (PAR-1) antagonists as represented by compound 1h, which showed an IC(50) of 33 nM.

Discovery and initial structure-activity relationships of trisubstituted ureas as thrombin receptor (PAR-1) antagonists

Thrombin is the most potent agonist of platelet activation, and its effects are predominantly mediated by platelet thrombin receptors. Therefore, antagonists of the thrombin receptor have potential utility for the treatment of thrombotic disorders. Screening of combinatorial libraries revealed 2 to be a potent antagonist of the thrombin receptor. Modifications of this structure produced 11k, which inhibits thrombin receptor stimulated secretion and aggregation of platelets.

Thrombin receptor (PAR-1) antagonists. Solid-phase synthesis of indole-based peptide mimetics by anchoring to a secondary amide

A novel, 10-step, solid-phase method, based on a secondary amide linker, was developed to construct a diverse library of indole-based SFLLR peptide mimetics as thrombin receptor (protease-activated receptor 1, PAR-1) antagonists. The key steps include stepwise reductive alkylation, urea formation, and Mannich reaction. Screening of the library led to a quick development of the SAR and the significant improvement of PAR-1 activity.

Inhibition of cellular action of thrombin by N3-cyclopropyl-7-[[4-(1-methylethyl)phenyl]methyl]-7H-pyrrolo[3, 2-f]quinazoline-1,3-diamine (SCH 79797), a nonpeptide thrombin receptor antagonist

A growing body of evidence suggests an important contribution of the cellular actions of thrombin to thrombosis and restenosis following angioplasty. Recently we reported on SCH 79797 (N3-cyclopropyl-7-¿[4-(1-methylethyl)phenyl]methyl¿-7H-pyrrolo[3, 2-f]quinazoline-1,3-diamine) and its analogs as new potent, nonpeptide thrombin receptor antagonists. This study further characterizes the biochemical and pharmacological actions of pyrroloquinazoline inhibitors of protease activated receptor-1 (PAR-1) in human platelets and coronary artery smooth muscle cells (hCASMC). SCH 79797 and its N-methyl analog (SCH 203099) inhibited binding of a high-affinity thrombin receptor-activating peptide ([(3)H]haTRAP, Ala-Phe(p-F)-Arg-ChA-HArg-[(3)H]Tyr-NH(2)) to PAR-1 with IC(50) values of 70 and 45 nM, respectively. SCH 79797 inhibited [(3)H]haTRAP binding in a competitive manner. SCH 79797 and SCH 203099 inhibited alpha-thrombin- and haTRAP-induced aggregation of human platelets, but did not inhibit human platelet aggregation induced by the tethered ligand agonist for protease-activated receptor-4 (PAR-4), gamma-thrombin, ADP, or collagen. SCH 203099 inhibited surface expression of P-selectin induced by haTRAP and thrombin, and it did not increase P-selectin expression or prevent thrombin cleavage of the receptor. Thrombin and TFLLRNPNDK-NH(2) (TK), a PAR-1-selective agonist, produced transient increases in cytosolic free Ca(2+) concentration ([Ca(2+)](i)) in hCASMC. This increase in [Ca(2+)](i) was inhibited effectively by SCH 79797. However, the Ca(2+) transients induced by SLIGKV-NH(2,) a PAR-2-selective agonist, were not inhibited by SCH 79797. Thrombin- and TK-stimulated [(3)H]thymidine incorporation also was inhibited completely by SCH 79797. The results of this study demonstrate that SCH 79797 and SCH 203099 are potent, selective antagonists of PAR-1 in human platelets and hCASMC. These data also suggest that the thrombin stimulation of Ca(2+) transients and mitogenesis in hCASMC is mediated primarily through activation of PAR-1.

Thrombin receptor-activating peptides (TRAPs): investigation of bioactive conformations via structure-activity, spectroscopic, and computational studies

The thrombin receptor (PAR-1) is an unusual transmembrane G-protein coupled receptor in that it is activated by serine protease cleavage of its extracellular N-terminus to expose an agonist peptide ligand, which is tethered to the receptor itself. Synthetic peptides containing the agonist motif, such as SFLLRN for human PAR-1, are capable of causing full receptor activation. We have probed the possible bioactive conformations of thrombin receptor-activating peptides (TRAPs) by systematic introduction of certain conformational perturbations, involving alpha-methyl, ester psi(COO), and reduced-amide psi(CH2N) scans, into the minimum-essential agonist sequence (SFLLR) to probe the importance of the backbone conformation and amide NH hydrogen bonding. We performed extensive conformational searches of representative pentapeptides to derive families of putative bioactive structures. In addition, we employed 1H NMR and circular dichroism (CD) to characterize the conformational disposition of certain pentapeptide analogues experimentally. Activation of platelet aggregation by our pentapeptide analogues afforded a structure-function correlation for PAR-1 agonist activity. This correlation was assisted by PAR-1 receptor binding data, which gauged the affinity of peptide ligands for the thrombin receptor independent of a functional cellular response derived from receptor activation (i.e. a pure molecular recognition event). Series of alanine-, proline-, and N-methyl-scan peptides were also evaluated for comparison. Along with the known structural features for PAR-1 agonist peptides, our work adds to the understanding of peptide topography relative to platelet functional activity and PAR-1 binding. The absolute requirement of a positively charged N-terminus for strong agonist activity was contradicted by the N-terminal hydroxyl peptide psi(HO)S-FLLR-NH2. The amide nitrogen between residues 1 and 2 was found to be a determinant of receptor recognition and the carbonyl groups along the backbone may be involved in hydrogen bonding with the receptor. Position 3 (P3) of TRAP-5 is known to tolerate a wide variety of side chains, but we also found that the amide nitrogen at this position can be substituted by an oxygen, as in SF-psi(COO)-LLR-NH2, without diminishing activity. However, this peptide bond is sensitive to conformational changes in that SFPLR-NH2 was active, whereas SF-NMeL-LR-NH2 was not. Additionally, we found that position 3 does not tolerate rigid spacers, such as 3-aminocyclohexane-1-carboxylic acid and 2-aminocycloalkane-1-carboxylic acid, as analogues 1A, 1B, 2A, 2B, 3, 4, 5A and 5B lack agonist activity. On the basis of our results, we suggest that an extended structure of the agonist peptide is principally responsible for receptor recognition (i.e. binding) and that hydrophobic contact may occur between the side chains of the second (Phe) and fourth (Leu) residues (i.e. P2-P4 interaction).

Design, synthesis, and biological characterization of a peptide-mimetic antagonist for a tethered-ligand receptor

Protease-activated receptors (PARs) represent a unique family of seven-transmembrane G protein-coupled receptors, which are enzymatically cleaved to expose a truncated extracellular N terminus that acts as a tethered activating ligand. PAR-1 is cleaved and activated by the serine protease alpha-thrombin, is expressed in various tissues (e.g., platelets and vascular cells), and is involved in cellular responses associated with hemostasis, proliferation, and tissue injury. We have discovered a series of potent peptide-mimetic antagonists of PAR-1, exemplified by RWJ-56110. Spatial relationships between important functional groups of the PAR-1 agonist peptide epitope SFLLRN were employed to design and synthesize candidate ligands with appropriate groups attached to a rigid molecular scaffold. Prototype RWJ-53052 was identified and optimized via solid-phase parallel synthesis of chemical libraries. RWJ-56110 emerged as a potent, selective PAR-1 antagonist, devoid of PAR-1 agonist and thrombin inhibitory activity. It binds to PAR-1, interferes with PAR-1 calcium mobilization and cellular function (platelet aggregation; cell proliferation), and has no effect on PAR-2, PAR-3, or PAR-4. By flow cytometry, RWJ-56110 was confirmed as a direct inhibitor of PAR-1 activation and internalization, without affecting N-terminal cleavage. At high concentrations of alpha-thrombin, RWJ-56110 fully blocked activation responses in human vascular cells, albeit not in human platelets; whereas, at high concentrations of SFLLRN-NH(2), RWJ-56110 blocked activation responses in both cell types. Thus, thrombin activates human platelets independently of PAR-1, i.e., through PAR-4, which we confirmed by PCR analysis. Selective PAR-1 antagonists, such as RWJ-56110, should serve as useful tools to study PARs and may have therapeutic potential for treating thrombosis and restenosis.

Heterocycle-peptide hybrid compounds. Aminotriazole-containing agonists of the thrombin receptor (PAR-1)

The thrombin receptor PAR-1 is activated by alpha-thrombin to stimulate cells, including platelets, through the tethered-ligand sequence SFLLRN. We have discovered a novel series of heterocycle-peptide hybrids comprised of a tripeptide segment, such as Cha-Arg-Phe, and an N-terminal heterocyclic group, many of which behave as full PAR-1 agonists. Certain compounds with an aminotriazole group, such as 4 and 16, are nearly as potent as SFLLRN-NH2 in inducing platelet aggregation. Also, some arylethenoyl "N-capped" compounds, such as 52 and 57, exhibit mixed PAR-1 agonist-antagonist activity.

Macrocyclic hexapeptide analogues of the thrombin receptor (PAR-1) activation motif SFLLRN

The thrombin receptor (PAR-1) is activated by alpha-thrombin to stimulate various cell types, including platelets, through the tethered-ligand sequence SFLLRN. Macrocyclic peptide analogues of SFLLRN were synthesized and evaluated in vitro. In general, the compounds were much less potent in inducing platelet aggregation relative to SFLLRN-NH2 and did not act as antagonists of alpha-thrombin. Derivative 3c was the most potent macrocycle in activating PAR-1, with an EC50 of 24 microM.

Thrombin receptor (PAR-1) antagonists. Heterocycle-based peptidomimetics of the SFLLR agonist motif

The thrombin receptor (PAR-1) is activated by alpha-thrombin to stimulate various cell types, including platelets, through the tethered-ligand sequence SFLLRN. A series of azole-based carboxamides, designed after SFLLR, were synthesized and evaluated in vitro. The compounds inhibited platelet aggregation induced by SFLLRN-NH2 or alpha-thrombin, and blocked the binding of [3H]-S-(p-F-Phe)-Har-L-Har-KY-NH2 to a CHRF membrane preparation of PAR-1. Oxazole 30 bound to PAR-1 with an IC50 of 1.6 microM, and gave IC50 values of 25 microM and 6.6 microM against alpha-thrombin- and SFLLRN-NH2-induced platelet aggregation, respectively.

Pharmacological characterization of protease-activated receptor (PAR-1) in rat astrocytes

The proteolytic action of thrombin on its receptor (protease-activated receptor-1 or PAR-1) results in a conformational change in which the new N-terminal sequence auto-activates the receptor. Peptide analogs of this N-terminal sequence (TRAPs) are able to mimic the effect of thrombin and an extensive search has led to the definition of the structural requirement for the agonist and antagonist activity on thrombin receptors in several peripheral systems. Thrombin plays an important role in central and peripheral nervous system development and PAR-1 is present in neurons and astrocytes. We have now characterized thrombin receptors pharmacologically in cultured rat astrocytes by using [3H]thymidine incorporation and reversal of stellation induced by Bt2cAMP as end-points. Thrombin increased [3H]thymidine incorporation into DNA with an EC50 of 1 nM and induced a complete reversion of cell stellation. The effects of thrombin on [3H]thymidine incorporation were mimicked by TRAP-14 (EC50 = 3 microM) and a peptide containing non-natural amino acids Ala-Phe(p-F)-Arg-Cha-HArg-Tyr-NH2 (A6Y; EC50 = 0.8 microM). Similarly, these two peptides reversed Bt2cAMP-induced stellation. The effect of thrombin, TRAP-14 and A6Y on [3H]thymidine incorporation into DNA was significantly prevented by L9R, a 9-amino-acid peptide (Leu-Val-Arg-D-Cys-Gly-Lys-His-Ser-Arg; IC50 = 180 microM against thrombin and TRAP-14 and 800 microM against A6Y) previously described as an antagonist in human platelet aggregation. L9R antagonized also thrombin effects on astrocyte morphology. These results demonstrate that rat astrocytes express PAR-1 receptors which are pharmacologically similar to those previously characterized in human platelets.

Dual effects of thrombin and a 14-amino acid peptide agonist of the thrombin receptor on septal cholinergic neurons

We have compared the effects of thrombin and of the 14-amino acid peptide agonist (TRAP-14) of the thrombin protease activated receptor (PAR) on cholinergic neurons in pure cultures of rat septal neurons and in co-cultures of septal neurons and glial cells. In pure septal cultures, low concentrations of thrombin (up to 10 nM) did not affect choline acetyltransferase (ChAT) activity, a marker of cholinergic neurons, or 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) reduction, an index of cell viability. However, 100 nM thrombin decreased ChAT activity and MTT reduction by 44 and 17%, respectively. In co-cultures, a low concentration of thrombin (1 nM) increased ChAT activity (+75%), whereas a high concentration (100 nM) decreased it (-83%). At this high concentration, thrombin was neurotoxic, as indicated by a large decrease in MTT reduction (-80%). Thrombin effects on ChAT activity were mimicked by TRAP-14 both in pure septal cultures (no effect at 0.1 microM and -63% at 100 microM) and in co-cultures (+25% at 0.1 microM and -28% at 100 microM). In contrast, this peptide did not affect MTT reduction. These dual effects of thrombin and TRAP-14 on ChAT activity in co-cultures, were also observed on pure cultures of septal cells supplied with NGF. The activation and inhibition by TRAP-14 of the expression of ChAT activity in septal neuron/glial cell cultures were inhibited by a 9-amino acid peptide antagonist of thrombin PAR. Thus, the effects of thrombin on cholinergic neurons seem to be mainly mediated by thrombin PAR and glial cells seem to play a major role in these thrombin actions.

Azetidin-2-one derivatives as inhibitors of thrombin

A series of 3-(3-guanidinopropyl)-azetidin-2-one derivatives was prepared and evaluated as inhibitors of cleavage of synthetic substrates in vitro by the serine proteases thrombin, trypsin and plasmin. The N-unsubstituted, 4-phenethyl derivative 9a demonstrated weak inhibition of these enzymes but acetylation of the beta-lactam N atom afforded 9b, an effective, time-dependent inhibitor of thrombin and a potent inhibitor of plasmin. Variation of the 4-position of the beta-lactam ring was examined in conjunction with different N-substituents to provide a series of potent, time-dependent inhibitors of thrombin. A C-4 substituent was essential for good inhibitory properties and, in general, polar C-4 substituents enhanced the selectivity of inhibition for thrombin compared to plasmin. A trans relationship between the C-4 and C-3 substituents was found to be superior to a cis disposition whilst homologation of the guanidinopropyl side chain to that of a guanidinobutyl moiety reduced activity. Several compounds were effective inhibitors of thrombin-induced clot formation in human plasma in vitro but activity in this assay did not correlate well with inhibition of thrombin-induced cleavage of a synthetic substrate, presumably a consequence of inherent chemical instability and degradation in plasma.