A Novel Nonpeptide Antagonist of the Kinin B1Receptor: Effects at the Rabbit Receptor

Synthetic route towards 1,2,3,4-tetrahydroquinoxaline/piperidine combined tricyclic ring system

The synthetic route toward novel tricyclic, nitrogen-containing system is disclosed. Three novel compounds possessing structural features of 1,2,3,4-tetrahydroquinoxaline and decahydropyrido[3,4-b]pyrazine are synthesized starting from readily available precursors in six or seven steps, of which the last three or four steps respectively are diastereoselective. Key reaction steps include N-acylation, Hofmann rearrangement and ring-closing Buchwald? Hartwig reaction. Compounds trans-8, cis-12 and trans-12 are synthesized in order to prove that this novel, tricyclic system can be functionalized with various groups. Synthetic significance of this heterocyclic system lies in the possibility for the orthogonal functionalization of three different amino groups, allowing fine structural tuning.

New insights into the stereochemical requirements of the bradykinin B1 receptor antagonists binding

Bradykinin (BK) is a nonapeptide involved in several pathophysiological conditions including among others, septic and haemorrhagic shock, anaphylaxis, arthritis, rhinitis, asthma, inflammatory bowel disease. Accordingly, BK antagonists have long been sought after for therapeutic intervention. Action of BK is mediated through two different G-protein coupled receptors known as B1 and B2. Although there are several B1 antagonists reported in literature, their pharmacological profile is not yet optimal so that new molecules need to be discovered. In the present work we have constructed an atomistic model of the B1 receptor and docked diverse available non-peptide antagonists in order to get a deeper insight into the structure-activity relationships involving binding to this receptor. The model was constructed by homology modeling using the chemokine CXC4 and bovine rhodopsin receptors as template. The model was further refined using molecular dynamics for 600ns with the protein embedded in a POPC bilayer. From the refinement process we obtained an average structure that was used for docking studies using the Glide software. Antagonists selected for the docking studies include Compound 11, Compound 12, Chroman28, SSR240612, NPV-SAA164 and PS020990. The results of the docking study underline the role of specific receptor residues in ligand binding. The results of this study permitted to define a pharmacophore that describes the stereochemical requirements of antagonist binding, and can be used for the discovery of new compounds.

The isolated human umbilical vein as a bioassay for kinin-generating proteases: An in vitro model for therapeutic angioedema agents

AIMS

The isolated human umbilical vein is a robust contractile bioassay for ligands of the bradykinin (BK) B2 receptor (B2R), also extendable to B1 receptor (B1R) pharmacology. We hypothesized that, as a freshly isolated vessel, it also contains traces of plasma proteins that may confer responses to exogenous proteases via the formation of kinins.

MAIN METHODS

Rings of human umbilical veins were mounted in organ baths containing Krebs buffer maintained at 37°C and purified proteases were introduced in the bathing fluid along with additional drugs/proteins that permit mechanistic analysis of effects.

KEY FINDINGS

The previously described contractile response to human recombinant tissue kallikrein (KLK-1, 1-10nM) is not influenced by metabolic inhibitors, suggesting its dependence on a preexisting reservoir of low molecular weight-kininogen (LK). Active plasma kallikrein (apK, ≤5nM) was inactive in fresh tissues, unless high molecular weight-kininogen (HK, 39-197nM) replenishment was applied. The effects of KLK-1 and HK+apK are abolished by pretreating tissues with icatibant, but not with tranexamic acid. C1-esterase inhibitor inhibited only HK+apK. Purified plasmin and neutrophil proteinase-3 produced small contractions in the presence of HK only, and tissue plasminogen activator, none. B1R stimulation was pharmacologically evidenced in response to KLK-1 if LK was supplied.

SIGNIFICANCE

The pharmacology of KLK-1 and HK+apK in the human isolated umbilical vein is essentially based on the activity of locally generated kinins and this assay models the inhibitory action of some therapeutic agents active in angioedema states. Proteases that indirectly generate kinins have little activity in the system.

Biotechnological Fluorescent Ligands of the Bradykinin B1 Receptor: Protein Ligands for a Peptide Receptor

The bradykinin (BK) B1 receptor (B1R) is a peculiar G protein coupled receptor that is strongly regulated to the point of being inducible in immunopathology. Limited clinical evidence suggests that its expression in peripheral blood mononuclear cells is a biomarker of active inflammatory states. In an effort to develop a novel imaging/diagnostic tool, we report the rational design and testing of a fusion protein that is a ligand of the human B1R but not likely to label peptidases. This ligand is composed of a fluorescent protein (FP) (enhanced green FP [EGFP] or mCherry) prolonged at its N-terminus by a spacer peptide and a classical peptide agonist or antagonist (des-Arg9-BK, [Leu8]des-Arg9-BK, respectively). The design of the spacer-ligand joint peptide was validated by a competition assay for [3H]Lys-des-Arg9-BK binding to the human B1R applied to 4 synthetic peptides of 18 or 19 residues. The labeling of B1R-expressing cells with EGFP or mCherry fused with 7 of such peptides was performed in parallel (microscopy). Both assays indicated that the best design was FP-(Asn-Gly)n-Lys-des-Arg9-BK; n = 15 was superior to n = 5, suggesting benefits from minimizing steric hindrance between the FP and the receptor. Cell labeling concerned mostly plasma membranes and was inhibited by a B1R antagonist. EGFP-(Asn-Gly)15-Lys-des-Arg9-BK competed for the binding of [3H]Lys-des-Arg9-BK to human recombinant B1R, being only 10-fold less potent than the unlabeled form of Lys-des-Arg9-BK to do so. The fusion protein did not label HEK 293a cells expressing recombinant human BK B2 receptors or angiotensin converting enzyme. This study identifies a modular C-terminal sequence that can be adapted to protein cargoes, conferring high affinity for the BK B1R, with possible applications in diagnostic cytofluorometry, histology and drug delivery (e.g., in oncology).

Lack of direct interaction between enalaprilat and the kinin B1 receptors

It has been recently proposed that the second extracellular loop of the human bradykinin (BK) B1 receptor (B1R) contains a conserved HExxH motif also present in peptidases possessing a Zn2+ prosthetic group, such as angiotensin converting enzyme (ACE), and that ACE inhibitors directly activate B1R signaling in endothelial cells. However, the binding of ACE inhibitors to the B1Rs has never been directly evaluated. Information about binding of a radiolabeled inhibitor to natural or recombinant ACE in intact cells (physiologic ionic composition) was also collected. We used the tritiated form of an ACE inhibitor previously proposed to activate the B1R, enalaprilat, to address these questions using recombinant human B1Rs and naturally expressed or recombinant ACE. [3H]Lys-des-Arg9-BK bound to the human recombinant B1Rs with high affinity (KD 0.35 nM) in HEK 293a cells. [3H]Enalaprilat (0.25-10 nM) did not bind to cells expressing recombinant human B1R, but bound with a subnanomolar affinity to recombinant ACE or to naturally expressed ACE in human umbilical vein endothelial cells. The radioligand was further validated using a binding competition assay that involved unlabeled ACE inhibitors or their prodrug forms in endothelial cells. Membranes of HEK 293a cells that expressed B1Rs did not hydrolyze hippuryl-glycylglycine (an ACE substrate). Enalaprilat did not stimulate calcium signaling in HEK 293a cells that expressed B1Rs. A typical ACE inhibitor did not bind to nor stimulate the human B1Rs; nevertheless, several other indirect mechanisms could connect ACE inhibition to B1R stimulation in vivo.

Does zaltoprofen antagonize the bradykinin receptors?

Zaltoprofen is a nonsteroidal antiinflammatory drug that has been proposed to inhibit with some selectivity the nociception mediated by the bradykinin (BK) B(2) receptor. In order to test the predictive power of this claim, we applied the drug to vascular smooth muscle assays previously found useful to characterize B(2) receptor antagonists (contractility, human isolated umbilical vein) or B(1) receptor antagonists (contraction, rabbit aorta; relaxation, rabbit mesenteric artery). Zaltoprofen (up to 30 microM) failed to antagonize BK or des-Arg(9)-BK-induced contraction in the umbilical vein and aorta, respectively. The drug (1 microM) abated des-Arg(9)-BK-induced, prostaglandin-mediated relaxation of the precontracted mesenteric artery, consistent with its known activity as a cyclooxygenase (COX) inhibitor. However, zaltoprofen (10 microM) did not inhibit kinin-stimulated phospholipase A(2) activity in HEK 293 cells expressing recombinant forms of the rabbit B(1) or B(2) receptors. Nonpeptide antagonists of either receptor subtype were active in this respect. The results do not support that zaltoprofen, a COX inhibitor, antagonizes kinin receptors or influences their signaling with selectivity in the tested systems.

Neuropeptide and kinin antagonists

Neuropeptides and kinins are important messengers in the nervous system and--on the basis of their anatomical localisation and the effects produced when the substances themselves are administered, to animals or to human subjects-a significant number of them have been suggested to have a role in pain and inflammation. Experiments in gene deletion (knock-out or null mutant) mice and parallel experiments with pharmacological receptor antagonists in a variety of species have strengthened the evidence that a number of peptides, notably substance P and calcitonin gene-related peptide (CGRP), and the kinins have a pathophysiological role in nociception. Clinical studies with non-peptide pharmacological antagonists are now in progress to determine if blocking the action of these peptides might have utility in the treatment of pain.

Role of Nuclear Factor-κB and Protein Kinase C Signaling in the Expression of the Kinin B1Receptor in Human Vascular Smooth Muscle Cells

Kinin B1 receptor expression was characterized in human umbilical artery smooth muscle cells to further elucidate the function and specificity of three previously proposed pathways [nuclear factor-kappaB (NF-kappaB), protein kinase C, and agonist autoregulation] that regulate this inducible G protein-coupled receptor. Radioligand binding assays, real-time reverser transcription/polymerase chain reaction with an optional actinomycin D treatment period, and NF-kappaB immunofluorescence were primarily employed in these primary cell cultures. Various stimulatory compounds that increase receptor mRNA stability only (human and bovine sera, cycloheximide) or that stimulate NF-kappaB nuclear translocation and both mRNA concentration and stability [interleukin (IL)-1beta, phorbol 12-myristate 13-acetate (PMA)] all increased the density of binding sites for the tritiated B1 receptor agonist [3H]Lys-des-Arg9-bradykinin (without change in receptor affinity) in cell-based assays. Small interfering RNA assays indicated that NF-kappaB p65 is necessary for the effective expression of the cell surface B1 receptor under basal or IL-1beta, fetal bovine serum (FBS), or PMA stimulation conditions. Dexamethasone cotreatment reproduced these effects. IL-1beta-, FBS-, or PMA-induced stabilization of B1 receptor mRNA was inhibited by the addition of the protein kinase C inhibitor 3-[1-[3-(dimethylamino)propyl]-1H-indol-3-yl]-4-(1H-indol-3-yl)-1H-pyrrole-2,5-dione monohydrochloride (GF-109203x), which also diminished the Bmax under FBS or PMA treatment. Lys-des-Arg9-bradykinin had little effect on NF-kappaB activation, the Bmax, or receptor mRNA abundance or stability. Both NF-kappaB and protein kinase C signaling are required for the effective expression of the kinin B1 receptor in human umbilical artery smooth muscle cells.

Antagonist, partial agonist and antiproliferative actions of B-9870 (CU201) as a function of the expression and density of the bradykinin B1 and B2 receptors

BACKGROUND AND PURPOSE

A bradykinin (BK) B2 receptor (B2R) antagonist, B-9870 (CU201), has been proposed to behave as a 'biased agonist' at B2Rs and to exert anti-neoplasic effects. It was unclear whether these effects were determined by the activation of B2Rs by the drug. B-9870 was evaluated for antagonism or stimulation of several responses mediated by the rabbit B2R or B1 receptor (B1R); its anti-proliferative activity was also characterized.

EXPERIMENTAL APPROACH AND KEY RESULTS

B-9870 was an insurmountable B2R antagonist in the rabbit jugular vein contractility assay, but a partial agonist in HEK 293 cells expressing the rabbit B2R or a green fluorescent protein (GFP) conjugate of the latter (ERK1/2 phosphorylation, [Ca2+]i, [3H]-arachidonate release, endocytosis). The agonist-like effects of B-9870 were inhibited by the B2R antagonist LF 16.0687 and absent in untransfected cells. In addition, B-9870 was a surmontable antagonist of the rabbit B1R in the aorta contractility assay, and blocked Lys-des-Arg9-BK-induced ERK1/2 phosphorylation in HEK 293 cells expressing a fluorescent B1R conjugate. B-9870 inhibited the growth of MDA-MB-231 cells. The latter effect was not influenced by B1R or B2R antagonists and was not apoptotic. MDA-MB-231 cells expressed a small population of B2Rs but no B1Rs; they responded to BK (small calcium transients) and B-9870 behaved as an antagonist.

CONCLUSION AND IMPLICATIONS

B-9870 is a dual B1R and B2R antagonist with confirmed stimulating effects at the B2R in high expression systems only. Its cell type-specific anti-proliferative effect occurs at a high concentration, independently from kinin receptors and apoptosis.

Targeting the bradykinin B1 receptor to reduce pain

The bradykinin B1 receptor is an inducible G-protein-coupled receptor. It is induced or upregulated at the site of inflammation or injury. A large body of preclinical data supports the development of B1 antagonists as novel therapeutics for the treatment of pain and inflammation. The necessary in vitro and in vivo drug discovery tools are currently available to evaluate novel B1 antagonists. Two major classes of small-molecule B1 antagonists, arylsulfonamide-based and biphenyl-based B1 antagonists, have been disclosed in the last few years.

Inhibition of human and rabbit arterial smooth muscle cell migration mediated by the kinin B1 receptor: role of receptor density and released mediators

Bradykinin (BK)-related peptides are suspected to negatively influence diverse functions in vascular smooth muscle cells (SMCs), notably via stimulation of the inducible B1 receptor (B1R), and have been shown to inhibit the migration of rat SMCs. The present study had several objectives: (i) to test whether B1R mediates the inhibition of migration of arterial SMCs from additional species (the human and the rabbit); (ii) whether B1R density influences this action and whether autocrine NO or prostanoid release modulate it; and (iii) the possible signaling interaction between the B1R and phosphatidylinositol-3 kinase (PI-3K) has been addressed. The peptidase resistant B1R agonist Sar-[D-Phe8]des-Arg9-BK (10 nmol/L – 1 μmol/L) was an inhibitor of migration in human or rabbit arterial SMCs in a wound closure assay, more effectively if the medium composition allowed a high B1R expression (20% fetal bovine serum (FBS) + interleukin-1β (IL-1β) in human SMCs, 10% FBS in rabbit cells). The effect of the B1R agonist on motility was abrogated by a B1R antagonist, B-9858, but not by the B2R antagonist Hoe 140; a peptidase-resistant B2R agonist, [Phe8Ψ(CH2-NH)-Arg9]BK, had a marginal or no effect on migration. Sar-[D-Phe8]des-Arg9-BK (1 μmol/L) did not significantly influence SMC proliferation. The B1R-mediated inhibition of SMC migration was not affected by pharmacological inhibition of the nitric oxide synthases or cyclooxygenases-1 or -2, but was correlated to an inhibition of PI-3K in both types of SMCs. The inhibition of SMC migration mediated by the kinin B1R is likely independent from NO or prostanoid release, applicable to several species, and correlated to receptor density and the inhibition of PI-3K.

Dual antagonists of the bradykinin B1 and B2 receptors based on a postulated common pharmacophore from existing non-peptide antagonists

We have recently drawn attention to the fact that most non-peptide antagonists of the kinin B1 receptor reported so far are structurally related, possessing the core motif phenyl-SO2-NR-(spacer(2-4))-CO-NRR. This is found in compound A (N-[2-[4-(4,5-dihydro-1H-imidazol-2- yl)phenyl]ethyl] - 2- [(2R)-1-(2-napthylsulfonyl)-3-oxo-1,2,3,4-tetrahydroquinoxalin-2-yl]acetamide), a very potent and selective B1 receptor antagonist. A subset of specific bradykinin B2 receptor antagonists (LF16-0687, bradyzide and derivatives) possesses a similar 'scaffold' (phenyl-SO2-NR-CRR-CO-NRR). We investigated whether simple molecules mimicking the postulated pharmacophores could be identified in two public chemical databases. Receptor binding to B1 and B2 receptors expressed by rabbit cultured smooth-muscle cells was confirmed for some of these newly identified agents, with a loss of receptor subtype selectivity. For instance, compound 3[2-(3-oxo-1-(toluene-4-sulfonyl)-1,2,3,4-4H-quinoxalin-2-yl)-N-phenyl-acetamide] exhibits IC50 values of 2.13 and 126 microM in the radioligand competition assays for B1 and B2 receptors, respectively, and a pA2 of 6.27 at the rabbit B1 receptor in a functional test (Lys-des-Arg9-bradykinin-induced contractility of the isolated aorta). Compound 5 (a close analog of compound 3) is a more balanced dual antagonist of low potency (IC50 values of 30 and 117 microM, respectively). As predicted, compounds modeled on a postulated pharmacophore common to some non-peptide B1 or B2 receptor antagonists exhibit measurable binding with decreased receptor subtype selectivity. Dual B1/B(2) receptor antagonists are of possible therapeutic interest and should be developed.

The kallikrein-kinin system: current and future pharmacological targets

The kallikrein-kinin system is an endogenous metabolic cascade, triggering of which results in the release of vasoactive kinins (bradykinin-related peptides). This complex system includes the precursors of kinins known as kininogens and mainly tissue and plasma kallikreins. The pharmacologically active kinins, which are often considered as either proinflammatory or cardioprotective, are implicated in many physiological and pathological processes. The interest of the various components of this multi-protein system is explained in part by the multiplicity of its pharmacological activities, mediated not only by kinins and their receptors, but also by their precursors and their activators and the metallopeptidases and the antiproteases that limit their activities. The regulation of this system by serpins and the wide distribution of the different constituents add to the complexity of this system, as well as its multiple relationships with other important metabolic pathways such as the renin-angiotensin, coagulation, or complement pathways. The purpose of this review is to summarize the main properties of this kallikrein-kinin system and to address the multiple pharmacological interventions that modulate the functions of this system, restraining its proinflammatory effects or potentiating its cardiovascular properties.

Molecular identification and pharmacological profile of the bovine kinin B1 receptor

To support the study of kinin pharmacology in bovine models of cultured endothelial cells (ECs), the Bovine Genome Project was searched for a B1receptor (B1R) gene ortholog (BDKRB1). A contig complementary to an intronless coding nucleotide sequence was found. The sequence was amplified from bovine EC DNA, further cloned into pcDNA3 and expressed in COS-1 cells. The bovine B1R sequence was confirmed and extended. A putative Zn2+-binding motif HEXXH is not present (replaced by HDAWP). The receptor binds [3H]Lys-des-Arg9-bradykinin in a saturable manner (Kd0.36 nM) and exhibits a pharmacological profile similar to that of human B1R.

A Nonpeptide Antagonist Reveals a Highly Glycosylated State of the Rabbit Kinin B1Receptor

The inducible kinin B1 receptor is emerging as an attractive therapeutic target for the treatment of pain and inflammation. Although many studies described its regulation at the transcriptional level, little is known about the maturation of the B1 receptor. Using two human embryonic kidney (HEK) 293 cell lines stably expressing rabbit B1 receptors tagged with the yellow fluorescent protein at the C terminus (B1R-YFP) or the N-terminal myc epitope (myc-B1R), we showed that receptors are mainly retained in a perinuclear compartment and detectable as low-glycosylated species under control conditions. Interference with the ubiquitin-proteasome pathway function (proteasome inhibitors, coexpression with dominant-negative ubiquitin) blocked B1 receptor degradation and amplified its intracellular accumulation. A potent nonpeptide antagonist specifically increased the abundance of highly glycosylated B1R-YFP forms at the cell surface (accessible to chymotrypsin digestion in intact cells); this compound augmented low-glycosylated receptors in brefeldin A-treated cells, supporting the hypothesis that it reaches a newly synthesized receptor in the endoplasmic reticulum. Cell-impermeant peptide or low-affinity nonpeptide B1 receptor antagonists failed to influence the level of highly glycosylated receptors. Chemical chaperones stabilized all B1R-YFP species and up-regulated endogenous B1 receptors expressed at the surface of rabbit smooth muscle cells. Although myc-B1Rs behaved similarly to B1R-YFP in most aspects, antibody-based detection assays failed to reveal highly glycosylated species of this construct. Taken together, these results show that B1 receptors overexpressed in HEK 293 cells are degraded by the proteasome. Furthermore, a pharmacological chaperone highlights the existence of a highly N-glycosylated form of the rabbit kinin B1 receptor at the cell surface.

Endogenous Aminopeptidase N Decreases the Potency of Peptide Agonists and Antagonists of the Kinin B1Receptors in the Rabbit Aorta

The B(1) receptor for kinins is selectively stimulated by bradykinin-related fragments lacking the C-terminal arginine, des-arginine(9)-bradykinin (des-Arg(9)-BK), and Lys-des-Arg(9)-BK. The latter peptide is the optimal agonist at the human and rabbit receptor. The B(1) receptor is inducible as a function of inflammatory conditions in the vasculature. We studied the effect of endogenously expressed peptidases on the potency of ligands of this receptor in an established bioassay, the rabbit aorta contractility. The potency measured for agonists (EC(50)) or antagonists (pA(2) scale) in this assay was compared with the affinity of each agent determined using [(3)H]Lys-des-Arg(9)-BK binding competition in cultured aortic smooth muscle cells and with the competition K(i) for the hydrolysis of the aminopeptidase chromogenic substrate L-Ala-p-nitroanilide by smooth muscle cell membranes. The contractile potency of the agonist Lys-des-Arg(9)-BK is decreased by in situ metabolism, and aminopeptidase N mediates most of the distortion (inhibited by amastatin but not efficiently by puromycin). At the other end of the spectrum, the fully protected agonist Sar-[D-Phe(8)]des-Arg(9)-BK is not significantly potentiated by peptidase inhibitors. A similar distortion of apparent potency was observed for some peptide antagonists used in the contractility assay, B-10350 (Lys-Lys-[Hyp(3), Igl(5), d-Tic(7), CpG(8)]des-Arg(9)-BK) and Lys-[Leu(8)]des-Arg(9)-BK being intensely potentiated by amastatin treatment and effective L-Ala-p-nitroanilide competitors. N-Protected peptide antagonists or a nonpeptide antagonist of the B(1) receptor were not potentiated by amastatin. The coexpression of aminopeptidase N and the kinin B(1) receptor in rabbit arterial tissue is of interest for the inactivation of the high-affinity agonist Lys-des-Arg(9)-BK and for the design of hydrosoluble antagonist drugs.