Protease activated receptor 2 (PAR2) modulators: a patent review (2010-2015)

Protease activated receptor 2 deficiency retards progression of abdominal aortic aneurysms by modulating phenotypic transformation of vascular smooth muscle cells via ERK signaling

Abdominal aortic aneurysm (AAA) is characterized by localized structural deterioration of the aortic wall, leading to progressive dilatation and rupture. Protease activated receptor 2 (PAR2) dependent signaling has been implicated in the pathophysiology of atherosclerosis through the regulation of smooth muscle cell function. However, its role in AAA remains unclear. This study investigates the function and potential mechanism of PAR2 in AAA progression. Angiotensin II (Ang II) and β-aminopropionitrile (BAPN) were administered to wild type (WT) mice to induce AAA. Increased PAR2 expression was observed in the aneurysmal tissues of these mice and in Ang II-treated vascular smooth muscle cells (VSMCs). We demonstrated that PAR2 deficiency markedly inhibited aorta dilatation and vascular remodeling in the AAA model relative to WT mice. Immunohistochemical staining showed significant upregulation of contractile markers and a reduction in synthetic markers in PAR2 knockout mice. Consistent with in vivo results, PAR2 knockdown diminished the effects of Ang II on VSMCs phenotypic switching, resulting in reduced proliferation and migration. Conversely, a PAR2 agonist (SLIGRL) induced the opposite effect, which was partially mitigated by pretreatment with an extracellular signal-regulated kinase (ERK) inhibitor (PD98059). This study suggests that PAR2 deficiency restrains aortic expansion and mitigates adverse vascular remodeling in AAA models, mediated in part by the ERK signaling pathway, indicating that PAR2 could be a potential therapeutic target for mitigating AAA development or progression.

Protease-Activated Receptor 2 in inflammatory skin disease: current evidence and future perspectives

Protease-activated receptor-2 (PAR2) is a class-A G protein-coupled receptor (GPCR) activated by serine proteases and is expressed by multiple tissues, including the skin. PAR2 is involved in the skin inflammatory response, promoting Th2 inflammation, delaying skin barrier repair, and affecting the differentiation of keratinocytes. It also participates in the transmission of itch and pain sensations in the skin. Increasing evidence indicates that PAR2 plays an important role in the pathogenesis of inflammatory skin diseases such as acne vulgaris, rosacea, psoriasis, and atopic dermatitis. Additional focus will be placed on potential targeted therapies based on PAR2. The Goal of this review is to outline the emerging effects of PAR2 activation in inflammatory skin disease and highlight the promise of PAR2 modulators.

Discovery of Protease-activated receptor 2 antagonists derived from phenylalanine for the treatment of breast cancer

Protease-activated receptor 2 (PAR2) has garnered attention as a potential therapeutic target in breast cancer. PAR2 is implicated in the activation of extracellular signal-regulated kinase 1/2 (ERK 1/2) via G protein and beta-arrestin pathways, contributing to the proliferation and metastasis of breast cancer cells. Despite the recognized role of PAR2 in breast cancer progression, clinically effective PAR2 antagonists remain elusive. To address this unmet clinical need, we synthesized and evaluated a series of novel compounds that target the orthosteric site of PAR2. Using in silico docking simulations, we identified compound 9a, an optimized derivative of compound 1a ((S)-N-(1-(benzylamino)-1-oxo-3-phenylpropan-2-yl)benzamide), which exhibited enhanced PAR2 antagonistic activity. Subsequent molecular dynamics simulations comparing 9a with the partial agonist 9d revealed that variations in ligand-induced conformational changes and interactions dictated whether the compound acted as an antagonist or agonist of PAR2. The results of this study suggest that further development of 9a could contribute to the advancement of PAR2 antagonists as potential therapeutic agents for breast cancer.

1-Piperidine Propionic Acid as an Allosteric Inhibitor of Protease Activated Receptor-2

In the last decades, studies on the inflammatory signaling pathways in multiple pathological contexts have revealed new targets for novel therapies. Among the family of G-protein-coupled Proteases Activated Receptors, PAR2 was identified as a driver of the inflammatory cascade in many pathologies, ranging from autoimmune disease to cancer metastasis. For this reason, many efforts have been focused on the development of potential antagonists of PAR2 activity. This work focuses on a small molecule, 1-Piperidine Propionic Acid (1-PPA), previously described to be active against inflammatory processes, but whose target is still unknown. Stabilization effects observed by cellular thermal shift assay coupled to in-silico investigations, including molecular docking and molecular dynamics simulations, suggested that 1-PPA binds PAR2 in an allosteric pocket of the receptor inactive conformation. Functional studies revealed the antagonist effects on MAPKs signaling and on platelet aggregation, processes mediated by PAR family members, including PAR2. Since the allosteric pocket binding 1-PPA is highly conserved in all the members of the PAR family, the evidence reported here suggests that 1-PPA could represent a promising new small molecule targeting PARs with antagonistic activity.

Allosteric Regulation of G-Protein-Coupled Receptors: From Diversity of Molecular Mechanisms to Multiple Allosteric Sites and Their Ligands

Allosteric regulation is critical for the functioning of G protein-coupled receptors (GPCRs) and their signaling pathways. Endogenous allosteric regulators of GPCRs are simple ions, various biomolecules, and protein components of GPCR signaling (G proteins and β-arrestins). The stability and functional activity of GPCR complexes is also due to multicenter allosteric interactions between protomers. The complexity of allosteric effects caused by numerous regulators differing in structure, availability, and mechanisms of action predetermines the multiplicity and different topology of allosteric sites in GPCRs. These sites can be localized in extracellular loops; inside the transmembrane tunnel and in its upper and lower vestibules; in cytoplasmic loops; and on the outer, membrane-contacting surface of the transmembrane domain. They are involved in the regulation of basal and orthosteric agonist-stimulated receptor activity, biased agonism, GPCR-complex formation, and endocytosis. They are targets for a large number of synthetic allosteric regulators and modulators, including those constructed using molecular docking. The review is devoted to the principles and mechanisms of GPCRs allosteric regulation, the multiplicity of allosteric sites and their topology, and the endogenous and synthetic allosteric regulators, including autoantibodies and pepducins. The allosteric regulation of chemokine receptors, proteinase-activated receptors, thyroid-stimulating and luteinizing hormone receptors, and beta-adrenergic receptors are described in more detail.

Proteinase-activated receptor-2 antagonist C391 inhibits Alternaria-induced airway epithelial signalling and asthma indicators in acute exposure mouse models

BACKGROUND AND PURPOSE

Despite the availability of a variety of treatment options, many asthma patients have poorly controlled disease with frequent exacerbations. Proteinase-activated receptor-2 (PAR2) has been identified in preclinical animal models as important to asthma initiation and progression following allergen exposure. Proteinase activation of PAR2 raises intracellular Ca²⁺ , inducing MAPK and β-arrestin signalling in the airway, leading to inflammatory and protective effects. We have developed C391, a potent PAR2 antagonist effective in blocking peptidomimetic- and trypsin-induced PAR2 signalling in vitro as well as reducing inflammatory PAR2-associated pain in vivo. We hypothesized that PAR2 antagonism by C391 would attenuate allergen-induced acutely expressed asthma indicators in murine models.

EXPERIMENTAL APPROACH

We evaluated the ability of C391 to alter Alternaria alternata-induced PAR2 signalling pathways in vitro using a human airway epithelial cell line that naturally expresses PAR2 (16HBE14o-) and a transfected embryonic cell line (HEK 293). We next evaluated the ability for C391 to reduce A. alternata-induced acutely expressed asthma indicators in vivo in two murine strains.

KEY RESULTS

C391 blocked A. alternata-induced, PAR2-dependent Ca²⁺ and MAPK signalling in 16HBE14o- cells, as well as β-arrestin recruitment in HEK 293 cells. C391 effectively attenuated A. alternata-induced inflammation, mucus production, mucus cell hyperplasia and airway hyperresponsiveness in acute allergen-challenged murine models.

CONCLUSIONS AND IMPLICATIONS

To our best knowledge, this is the first demonstration of pharmacological intervention of PAR2 to reduce allergen-induced asthma indicators in vivo. These data support further development of PAR2 antagonists as potential first-in-class allergic asthma drugs.

Protective immune potential of multiple antigenic peptide (MAP) constructs comprising peptides that are shared by several cysteine peptidases against Schistosoma mansoni infection in mice

In vaccine trials, Schistosoma mansoni cathepsin B1 (SmCB1), helminth cathepsins of the L family (e.g., SmCL3), and papain consistently induce highly significant reductions in challenge worm burden and egg viability, but generated no additive protective effects when used in combination. The protective capacity of the cysteine peptidases is associated with modest (SmCB1) and poor (cathepsins L) production of cytokines and antibodies, essentially of the type 2 axis, and is only marginally reduced upon use of proteolytically inactive enzymes. In this work, peptides shared by SmCB1, cathepsins of the L family, papain and other allergens were selected, synthesized as tetrabranched multiple antigen peptide constructs (MAP-1 and MAP-2), and used in two independent experiments to immunize outbred mice, in parallel with papain. The two peptides elicited significant (P <  0.05) reduction in challenge worm burden when compared to unimmunized mice, albeit lower than that achieved by papain. Protection was associated with modest serum type 2 cytokines and antibody levels in MAP-, and papain-immunized mice. Immunization with papain also elicited a reduction in parasite egg load, viability, and granuloma numbers in liver and intestine. MAP-1 and MAP-2 immunogens displayed some opposite effects- MAP-1 leading to higher egg numbers with poor vitality, whereas MAP-2 immunization yielded fewer eggs. Cysteine peptidase thus appear to carry peptides that elicit opposing outcomes, highlighting the difficulty of reaching fully fledged protection, unless a vaccine is based on carefully selected peptides and combined with an effective adjuvant.

Punicalagin Regulates Signaling Pathways in Inflammation-Associated Chronic Diseases

Inflammation is a complex biological defense system associated with a series of chronic diseases such as cancer, arthritis, diabetes, cardiovascular and neurodegenerative diseases. The extracts of pomegranate fruit and peel have been reported to possess health-beneficial properties in inflammation-associated chronic diseases. Punicalagin is considered to be the major active component of pomegranate extracts. In this review we have focused on recent studies into the therapeutic effects of punicalagin on inflammation-associated chronic diseases and the regulatory roles in NF-κB, MAPK, IL-6/JAK/STAT3 and PI3K/Akt/mTOR signaling pathways. We have concluded that punicalagin may be a promising therapeutic compound in preventing and treating inflammation-associated chronic diseases, although further clinical studies are required.

Shuttling of cellular proteins between the plasma membrane and nucleus (Review)

Recently accumulated evidence has indicated that the nucleomembrane shuttling of cellular proteins is common, which provides new insight into the subcellular translocation and biological functions of proteins synthesized in the cytoplasm. The present study aimed to clarify the trafficking of proteins between the plasma membrane and nucleus. These proteins primarily consist of transmembrane receptors, membrane adaptor proteins, adhesive proteins, signal proteins and nuclear proteins, which contribute to proliferation, apoptosis, chemoresistance, adhesion, migration and gene expression. The proteins frequently undergo cross-talk, such as the interaction of transmembrane proteins with signal proteins. The transmembrane proteins undergo endocytosis, infusion into organelles or proteolysis into soluble forms for import into the nucleus, while nuclear proteins interact with membrane proteins or act as receptors. The nucleocytosolic translocation involves export or import through nuclear membrane pores by importin or exportin. Nuclear proteins generally interact with other transcription factors, and then binding to the promoter for gene expression, while membrane proteins are responsible for signal initiation by binding to other membrane and/or adaptor proteins. Protein translocation occurs in a cell-specific manner and is closely linked to cellular biological events. The present review aimed to improve understanding of cytosolic protein shuttling between the plasma membrane and nucleus and the associated signaling pathways.

G Protein-Coupled Receptors in Osteoarthritis: A Novel Perspective on Pathogenesis and Treatment

G protein-coupled receptors (GPCRs) are transmembrane receptor proteins that trigger numerous intracellular signaling pathways in response to the extracellular stimuli. The GPCRs superfamily contains enormous structural and functional diversity and mediates extensive biological processes. Until now, critical roles have been established in many diseases, including osteoarthritis (OA). Existing studies have shown that GPCRs play an important role in some OA-related pathogenesis, such as cartilage matrix degradation, synovitis, subchondral bone remodeling, and osteophyte formation. However, current pharmacological treatments are mostly symptomatic and there is a paucity of disease-modifying OA drugs so far. Targeting GPCRs is capable of inhibiting cartilage matrix degradation and synovitis and up-regulating cartilage matrix synthesis, providing a new therapeutic strategy for OA. In this review, we have comprehensively summarized the structures, biofunctions, and the novel roles of GPCRs in the pathogenesis and treatment of OA, which is expected to lay the foundation for the development of novel therapeutics against OA. Even though targeting GPCRs may ameliorate OA progression, many GPCRs-related therapeutic strategies are still in the pre-clinical stage and require further investigation.

The PAR2 inhibitor I-287 selectively targets Gαq and Gα12/13 signaling and has anti-inflammatory effects

Protease-activated receptor-2 (PAR2) is involved in inflammatory responses and pain, therefore representing a promising therapeutic target for the treatment of immune-mediated inflammatory diseases. However, as for other GPCRs, PAR2 can activate multiple signaling pathways and those involved in inflammatory responses remain poorly defined. Here, we describe a new selective and potent PAR2 inhibitor (I-287) that shows functional selectivity by acting as a negative allosteric regulator on Gα_q and Gα_12/13 activity and their downstream effectors, while having no effect on G_i/o signaling and βarrestin2 engagement. Such selective inhibition of only a subset of the pathways engaged by PAR2 was found to be sufficient to block inflammation in vivo. In addition to unraveling the PAR2 signaling pathways involved in the pro-inflammatory response, our study opens the path toward the development of new functionally selective drugs with reduced liabilities that could arise from blocking all the signaling activities controlled by the receptor.

Avet et al. characterize I-287, an inhibitor to protease-activated receptor 2 using BRET-assays. They find that I-287 selectively inhibits Gα_q and Gα_12/13 without affecting the activation of G_i/o or the recruitment of βarrestin2 and that it blocks inflammation in vitro and in vivo.

Punicalagin Ameliorates Lupus Nephritis via Inhibition of PAR2

Lupus nephritis (LN) is the most frequent phenotype in patients with systemic lupus erythematosus (SLE) and has a high rate of progression to end-stage renal disease, in spite of intensive treatment and maintenance therapies. Recent evidence suggests that protease-activated receptor-2 (PAR2) is a therapeutic target for glomerulonephritis. In this study, we performed a cell-based high-throughput screening and identified a novel potent PAR2 antagonist, punicalagin (PCG, a major polyphenol enriched in pomegranate), and evaluated the effects of PCG on LN. The effect of PCG on PAR2 inhibition was observed in the human podocyte cell line and its effect on LN was evaluated in NZB/W F1 mice. In the human podocyte cell line, PCG potently inhibited PAR2 (IC₅₀ = 1.5 ± 0.03 µM) and significantly reduced the PAR2-mediated activation of ERK1/2 and NF-κB signaling pathway. In addition, PCG significantly decreased PAR2-induced increases in ICAM-1 and VCAM-1 as well as in IL-8, IFN-γ, and TNF-α expression. Notably, the intraperitoneal administration of PCG significantly alleviated kidney injury and splenomegaly and reduced proteinuria and renal ICAM-1 and VCAM-1 expression in NZB/W F1 mice. Our results suggest that PCG has beneficial effects on LN via inhibition of PAR2, and PCG is a potential therapeutic agent for LN.

Protease-Activated Receptor-2 Decreased Zonula Occlidens-1 and Claudin-1 Expression and Induced Epithelial Barrier Dysfunction in Allergic Rhinitis

BACKGROUND

Protease-activated receptor-2 (PAR-2)-modulated tight junctions (TJs) have been suggested to be involved in the pathogenesis of chronic inflammatory diseases. However, immunopathogenesis remains to be investigated among patients with allergic rhinitis (AR).

OBJECTIVE

This study sought to investigate the role of PAR-2 in the modulation of epithelial barrier function and the expression of TJs in the nasal mucosa of AR patients.

METHODS

The expression of TJs and PAR-2 of the nasal mucosa in AR patients and control subjects by immunohistochemistry, quantitative real-time polymerase chain reaction (qRT-PCR), and western blotting. In vitro, Primary human nasal epithelial cells (pHNECs) of AR patients were stimulated by Der p1 to analyze the correlation between PAR-2 and TJs expression. Der p1-induced pHNECs were treated with the PAR-2 agonist SLIGRL-NH2 and antagonist FSLLRY-NH2. Fluorescein isothiocyanate-dextran 4 kDa detection was employed as an indicator of epithelial permeability.

RESULTS

Lower expression levels of TJs in the nasal epithelium of AR patients were observed in comparison with that in control subjects. The PAR-2 level was markedly increased following treatment with 1,000 ng/mL of Der p1 for 24 hours in a cellular model of AR. The expression of PAR-2 was increased in Der p1-induced pHNECs of AR patients and correlated inversely with zonula occlidens (ZO)-1 and claudin-1. Treatment with Der p1 further downregulated TJs expression and promoted an increased epithelial permeability in Der p1-induced pHNECs.

CONCLUSIONS

PAR-2 could downregulate the expression of ZO-1 and claudin-1, which is involved in epithelial barrier dysfunction in AR.

Cellular effects of factor VII activating protease (FSAP)

Factor VII activating protease (FSAP) is a circulating serine protease of broad specificity that is likely to be involved in many pathophysiological processes. The activation of the circulating zymogen form of FSAP by histones, released from damaged cells, underlines its roles in regulating host responses to tissue damage and inflammation. Some of the direct cellular effects of FSAP are mediated through protease-activated receptors (PARs). Knock-down of each one of the four PARs in endothelial cells indicated that PAR-1 and -3 are involved in regulating endothelial permeability in response to FSAP. Overexpression of PARs in cell lines led to the conclusion that PAR-2 and -1 were the main receptors for FSAP. Studies with synthetic peptides and receptor mutants demonstrate that FSAP cleaves PAR-1 and -2 at their canonical cleavage site. However, PAR-1 is not activated by FSAP in all cells, which may be related to other, as yet, undefined factors. Inhibition of apoptosis by FSAP is mediated through PAR-1 and was observed in neurons, astrocytes and A549 cells. FSAP also mediates cellular effects by modulating the activity of growth factors, generation of bradykinin, C5a and C3a generation or histone inactivation. These cellular effects need to be further investigated at the in vivo level.

Protease-activated receptor 2 (PAR-2) antagonist AZ3451 as a novel therapeutic agent for osteoarthritis

Osteoarthritis (OA) is a highly prevalent joint disorder blamed for pain and disability in older individuals. It's commonly accepted that inflammation, apoptosis, autophagy and cellular senescence participate in the progress of OA. Protease activated receptor 2 (PAR2), a member of the G-protein coupled receptors, is involved in the regulation of various inflammation diseases. Previous studies have identified PAR2 as a potential therapeutic target for the treatment of OA. Here, we investigated the role of PAR2 antagonist AZ3451 in inflammation response, apoptosis, autophagy and cellular senescence during OA. We confirmed that PAR2 expression was significantly up-regulated in OA articular cartilage tissues as well as in interleukin 1β (IL-1β) stimulated chondrocytes. We demonstrated AZ3451 could prevent the IL-1β-induced inflammation response, cartilage degradation and premature senescence in chondrocytes. Further study showed that AZ3451 attenuated chondrocytes apoptosis by activating autophagy in vitro. The P38/MAPK, NF-κB and PI3K/AKT/mTOR pathways were involved in the protective effect of AZ3451. In vivo, we found that intra-articular injection of AZ3451 could ameliorate the surgery induced cartilage degradation in rat OA model. Our work provided a better understanding of the mechanism of PAR2 in OA, and indicated that PAR2 antagonist AZ3451 might serve as a promising strategy for OA treatment.

Protease activated receptors (PAR)-1 and -2 mediate cellular effects of factor VII activating protease (FSAP)

Factor VII activating protease (FSAP) is a circulating serine protease implicated in thrombosis, atherosclerosis, stroke, and cancer. Using an overexpression strategy, we have systematically investigated the role of protease activated receptors (PAR)-1, -2, -3, and -4 on FSAP-mediated signaling in HEK293T and A549 cells. Cleavage of PAR-reporter constructs and MAPK phosphorylation was used to monitor receptor activation. FSAP cleaved PAR-2 and to a lesser degree PAR-1, but not PAR-3 or PAR-4 in both cell types. Robust MAPK activation in response to FSAP was observed after PAR-2, but not PAR-1 overexpression in HEK293T. Recombinant serine protease domain of wild type FSAP, but not the Marburg I isoform of FSAP, could reproduce the effects of plasma purified FSAP. Canonical cleavage of both PARs was suggested by mass spectrometric analysis of synthetic peptide substrates from the N-terminus of PARs and site directed mutagenesis studies. Surprisingly, knockdown of endogenous PAR-1, but not PAR-2, prevented the apoptosis-inhibitory effect of FSAP, suggesting that PAR1 is nevertheless a direct or indirect target in some cell types. This molecular characterization of PAR-1 and -2 as cellular receptors of FSAP will help to define the actions of FSAP in the context of cancer and vascular biology.

Protease-activated receptor-2 signaling through β-arrestin-2 mediates Alternaria alkaline serine protease-induced airway inflammation

Alternaria alternata is a fungal allergen associated with severe asthma and asthma exacerbations. Similarly to other asthma-associated allergens, Alternaria secretes a serine-like trypsin protease(s) that is thought to act through the G protein-coupled receptor protease-activated receptor-2 (PAR2) to induce asthma symptoms. However, specific mechanisms underlying Alternaria-induced PAR2 activation and signaling remain ill-defined. We sought to determine whether Alternaria-induced PAR2 signaling contributed to asthma symptoms via a PAR2/β-arrestin signaling axis, identify the protease activity responsible for PAR2 signaling, and determine whether protease activity was sufficient for Alternaria-induced asthma symptoms in animal models. We initially used in vitro models to demonstrate Alternaria-induced PAR2/β-arrestin-2 signaling. Alternaria filtrates were then used to sensitize and challenge wild-type, PAR2-/- and β-arrestin-2-/- mice in vivo. Intranasal administration of Alternaria filtrate resulted in a protease-dependent increase of airway inflammation and mucin production in wild-type but not PAR2-/- or β-arrestin-2-/- mice. Protease was isolated from Alternaria preparations, and select in vitro and in vivo experiments were repeated to evaluate sufficiency of the isolated Alternaria protease to induce asthma phenotype. Administration of a single isolated serine protease from Alternaria, Alternaria alkaline serine protease (AASP), was sufficient to fully activate PAR2 signaling and induce β-arrestin-2-/--dependent eosinophil and lymphocyte recruitment in vivo. In conclusion, Alternaria filtrates induce airway inflammation and mucus hyperplasia largely via AASP using the PAR2/β-arrestin signaling axis. Thus, β-arrestin-biased PAR2 antagonists represent novel therapeutic targets for treating aeroallergen-induced asthma.

Neuropathic symptoms of the ocular surface: dryness, pain, and itch

PURPOSE OF REVIEW

This review aims to describe the recent findings on epidemiology, pathophysiology, and management of neuropathic symptoms of the ocular surface, with a focus on potential similarities between sensations of dry eye, pain and itch.

RECENT FINDINGS

A narrative review of the literature was undertaken. Key references from research in dry eye, neuropathic symptoms of the ocular surface, ocular pain and itch, as well as general references on itch and pain neurobiology were included. Recent findings suggest aspects of dry eye, chronic ocular pain and itch symptomatology are driven by neuropathic pain mechanisms involving peripheral and central sensitization processes.

SUMMARY

Ocular dryness, pain, and itch are prevalent complaints with several of shared features. Multiple lines of evidence suggest that peripheral and central neuronal sensitization processes are involved in generating and maintaining ocular sensory symptoms. Research is warranted on the epidemiology of ocular sensations, molecular mechanisms involved in nociception and pruriception in the eye, electrophysiological alterations in animal models of eye conditions, and therapeutic modalities that can alleviate unpleasant ocular sensations.

Synthetic Indolactam V Analogues as Inhibitors of PAR2-Induced Calcium Mobilization in Triple-Negative Breast Cancer Cells

Human proteinase-activated receptor 2 (PAR2), a transmembrane G-protein-coupled receptor (GPCR), is an attractive target for a novel anticancer therapy, as it plays a critical role in cell migration and invasion. Selective PAR2 inhibitors therefore have potential as anti-metastatic drugs. Knowing that the natural product teleocidin A2 is able to inhibit PAR2 in tumor cells, the goal of the present study was to elaborate structure-activity relationships and to identify potent PAR2 inhibitors with lower activity against the adverse target, protein kinase C (PKC). For this purpose, an efficient gram-scale total synthesis of indolactam V (i.e., the parent structure of all teleocidins) was developed, and a library of derivatives was prepared. Some compounds were indeed found to exhibit high potency as PAR2 inhibitors at low nanomolar concentrations with improved selectivity (relative to teleocidin A2). The pseudopeptidic fragment bridging the C3 and C4 positions of the indole core proved to be essential for target binding, whereas activity and target selectivity depends on the substituents at N1 or C7. This study revealed novel derivatives that show high efficacy in PAR2 antagonism combined with increased selectivity.

A Potent Antagonist of Protease-Activated Receptor 2 That Inhibits Multiple Signaling Functions in Human Cancer Cells

Protease-activated receptor 2 (PAR2) is a cell surface protein linked to G-protein dependent and independent intracellular signaling pathways that produce a wide range of physiological responses, including those related to metabolism, inflammation, pain, and cancer. Certain proteases, peptides, and nonpeptides are known to potently activate PAR2. However, no effective potent PAR2 antagonists have been reported yet despite their anticipated therapeutic potential. This study investigates antagonism of key PAR2-dependent signaling properties and functions by the imidazopyridazine compound I-191 (4-(8-(tert-butyl)-6-(4-fluorophenyl)imidazo[1,2-b]pyridazine-2-carbonyl)-3,3-dimethylpiperazin-2-one) in cancer cells. At nanomolar concentrations, I-191 inhibited PAR2 binding of and activation by structurally distinct PAR2 agonists (trypsin, peptide, nonpeptide) in a concentration-dependent manner in cells of the human colon adenocarcinoma grade II cell line (HT29). I-191 potently attenuated multiple PAR2-mediated intracellular signaling pathways leading to Ca²⁺ release, extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation, Ras homologue gene family, member A (RhoA) activation, and inhibition of forskolin-induced cAMP accumulation. The mechanism of action of I-191 was investigated using binding and calcium mobilization studies in HT29 cells where I-191 was shown to be noncompetitive and a negative allosteric modulator of the agonist 2f-LIGRL-NH₂ The compound alone did not activate these PAR2-mediated pathways, even at high micromolar concentrations, indicating no bias in these signaling properties. I-191 also potently inhibited PAR2-mediated downstream functional responses, including expression and secretion of inflammatory cytokines and cell apoptosis and migration, in human colon adenocarcinoma grade II cell line (HT29) and human breast adenocarcinoma cells (MDA-MB-231). These findings indicate that I-191 is a potent PAR2 antagonist that inhibits multiple PAR2-induced signaling pathways and functional responses. I-191 may be a valuable tool for characterizing PAR2 functions in cancer and in other cellular, physiological, and disease settings.