Trisubstituted 1,3,5-Triazines as Histamine H4 Receptor Antagonists with Promising Activity In Vivo

Pain is a very unpleasant experience that makes life extremely uncomfortable. The histamine H₄ receptor (H₄R) is a promising target for the treatment of inflammatory and immune diseases, as well as pain. H₄R ligands have demonstrated analgesic effects in a variety of pain models, including inflammatory pain. Continuing the search for active H₄R ligands among the alkyl derivatives of 1,3,5-triazine, we obtained 19 new compounds in two series: acyclic (I) and aliphatic (II). In vitro pharmacological evaluation showed their variable affinity for H₄R. The majority of compounds showed a moderate affinity for this receptor (K_i > 100 nM), while all compounds tested in ß-arrestin and cAMP assays showed antagonistic activity. The most promising, compound 6, (4-(cyclopentylmethyl)-6-(4-methylpiperazin-1-yl)-1,3,5-triazin-2-amine; K_i = 63 nM) was selected for further in vitro evaluation: blood-brain barrier permeability (PAMPA assay; P_e = 12.26 × 10^-6 cm/s) and toxicity tests (HepG2 and SH-5YSY cells; no toxicity up to 50 µM). Next, compound 6 tested in vivo in a carrageenan-induced inflammatory pain model showed anti-inflammatory and analgesic effects (strongest at 50 mg/kg i.p.). Furthermore, in a histamine- and chloroquine-induced pruritus model, compound 6 at a dose of 25 mg/kg i.p. and 50 mg/kg i.p., respectively, reduced the number of scratch bouts. Thus, compound 6 is a promising ligand for further studies.

Histamine H4 Receptor Expression in Triple-negative Breast Cancer: An Exploratory Study

Triple-negative breast cancer (TNBC) is an aggressive breast cancer subtype. There are neither universally accepted prognostic markers nor molecular targets related to TNBC. The histamine H4 receptor (H4R) has been characterized in TNBC experimental models, demonstrating its critical role in tumor development and progression. In this study, H4R expression was compared in breast cancer subtypes and correlated with clinical features using The Cancer Genome Atlas data (Pan-Cancer Atlas). The H4R status was further evaluated by immunohistochemistry in 30 TNBC human samples in relation to clinicopathological parameters. Results indicate that H4R was downregulated in basal-like/TNBC compared with luminal A and normal breast-like tumors. The higher expression of H4R was associated with improved progression-free and overall survival outcomes in basal-like/TNBC. H4R immunoreactivity was detected in about 70% of tumors, and its expression was positively correlated with the levels in the histologically normal peritumoral tissue. High H4R expression in peritumoral tissue correlated with reduced number of lymph node involvement and unifocal TNBC, while it was associated with increased patient survival. In conclusion, the H4R might represent a potential prognostic biomarker in TNBC. Further studies in large cohorts are needed to better understand the significance of H4R in breast cancer biology.

Specific Engineered G Protein Coupling to Histamine Receptors Revealed from Cellular Assay Experiments and Accelerated Molecular Dynamics Simulations

G protein-coupled receptors (GPCRs) are targets of extracellular stimuli and hence occupy a key position in drug discovery. By specific and not yet fully elucidated coupling profiles with α subunits of distinct G protein families, they regulate cellular responses. The histamine H2 and H4 receptors (H2R and H4R) are prominent members of Gs- and Gi-coupled GPCRs. Nevertheless, promiscuous G protein and selective Gi signaling have been reported for the H2R and H4R, respectively, the molecular mechanism of which remained unclear. Using a combination of cellular experimental assays and Gaussian accelerated molecular dynamics (GaMD) simulations, we investigated the coupling profiles of the H2R and H4R to engineered mini-G proteins (mG). We obtained coupling profiles of the mGs, mGsi, or mGsq proteins to the H2R and H4R from the mini-G protein recruitment assays using HEK293T cells. Compared to H2R-mGs expressing cells, histamine responses were weaker (pEC50, Emax) for H2R-mGsi and -mGsq. By contrast, the H4R selectively bound to mGsi. Similarly, in all-atom GaMD simulations, we observed a preferential binding of H2R to mGs and H4R to mGsi revealed by the structural flexibility and free energy landscapes of the complexes. Although the mG α5 helices were consistently located within the HR binding cavity, alternative binding orientations were detected in the complexes. Due to the specific residue interactions, all mG α5 helices of the H2R complexes adopted the Gs-like orientation toward the receptor transmembrane (TM) 6 domain, whereas in H4R complexes, only mGsi was in the Gi-like orientation toward TM2, which was in agreement with Gs- and Gi-coupled GPCRs structures resolved by X-ray/cryo-EM. These cellular and molecular insights support (patho)physiological profiles of the histamine receptors, especially the hitherto little studied H2R function in the brain, as well as of the pharmacological potential of H4R selective drugs.

TRPV1 and TRPA1 Channels Are Both Involved Downstream of Histamine-Induced Itch

Two histamine receptor subtypes (HR), namely H1R and H4R, are involved in the transmission of histamine-induced itch as key components. Although exact downstream signaling mechanisms are still elusive, transient receptor potential (TRP) ion channels play important roles in the sensation of histaminergic and non-histaminergic itch. The aim of this study was to investigate the involvement of TRPV1 and TRPA1 channels in the transmission of histaminergic itch. The potential of TRPV1 and TRPA1 inhibitors to modulate H1R- and H4R-induced signal transmission was tested in a scratching assay in mice in vivo as well as via Ca²⁺ imaging of murine sensory dorsal root ganglia (DRG) neurons in vitro. TRPV1 inhibition led to a reduction of H1R- and H4R- induced itch, whereas TRPA1 inhibition reduced H4R- but not H1R-induced itch. TRPV1 and TRPA1 inhibition resulted in a reduced Ca²⁺ influx into sensory neurons in vitro. In conclusion, these results indicate that both channels, TRPV1 and TRPA1, are involved in the transmission of histamine-induced pruritus.

Combined Treatment With H1 and H4 Receptor Antagonists Improves Th2 Inflammatory Responses in the Nasal Mucosa of Allergic Rhinitis Rats

BACKGROUND

Histamine H1 receptor (H1R) antagonists are the first-line drugs for the treatment of allergic rhinitis (AR) at present. Emerging evidence supports an important role of histamine H4 receptor (H4R) in allergic diseases. However, information regarding the effects of combined treatment with H1 and H4 receptor antagonists in AR is limited.

OBJECTIVES

We aimed to assess the effects of combined treatment with H1R and H4R antagonists on Th2 inflammatory responses in the nasal mucosa of AR rats.

METHODS

Sprague Dawley rats were sensitized with ovalbumin and treated with H1R antagonist desloratadine or/and H4R antagonist JNJ7777120. Western blotting was used to assay the phenotypic markers of mature dendritic cells in the nasal mucosa, including major histocompatibility complex class II (MHC-II) and co-stimulatory molecules CD80, CD86 and OX40 ligand (OX40L). Th2 inflammatory cytokines including interleukin-4, 5 and 13 in nasal lavage fluids were determined by using enzyme-linked immunoassay.

RESULTS

The treatment with desloratadine alone down-regulated the CD86 expression, and decreased the production of Th2 cytokines, but had no impact on the expression of MHC-II, CD80 and OX40L. The administration of NJ7777120 alone reduced the levels of CD86, OX40L and Th2 cytokines, whereas MHC-II and CD80 expression was unaffected. The combination of desloratadine and JNJ7777120 showed more significant synergistic therapeutic effects than monotherapy.

CONCLUSION

H4R antagonist acted synergistically with H1R antagonist to reduce Th2 inflammatory responses by down-regulating CD86 and OX40L expression in the nasal mucosa of AR rats. The combination with H1R and H4R antagonists might be a new strategy for AR treatment.

Enigmatic Histamine Receptor H4 for Potential Treatment of Multiple Inflammatory, Autoimmune, and Related Diseases

The histamine H4 receptor, belonging to the family of G-protein coupled receptors, is an increasingly attractive drug target. It plays an indispensable role in many cellular pathways, and numerous H4R ligands are being studied for the treatment of several inflammatory, allergic, and autoimmune disorders, including pulmonary fibrosis. Activation of H4R is involved in cytokine production and mediates mast cell activation and eosinophil chemotaxis. The importance of this receptor has also been shown in inflammatory models: peritonitis, respiratory tract inflammation, colitis, osteoarthritis, and rheumatoid arthritis. Recent studies suggest that H4R acts as a modulator in cancer, neuropathic pain, vestibular disorders, and type-2 diabetes, however, its role is still not fully understood.

Stimulation of histamine H4 receptor participates in cold-induced browning of subcutaneous white adipose tissue

Although many studies have shown that histamine and its signaling regulate energy homeostasis through the central nervous system, their roles in adipose tissues remain poorly understood. Here, we identified that the histamine H4 receptor (HrH4) was highly expressed in adipocytes at a level higher than that of the other three receptors (i.e., HrH1, HrH2, and HrH3). The HrH4 expression in adipocytes responded to cold through thermogenesis and lipolysis, supported by results from both mouse and cell models. When HrH4 expression was knocked down in the subcutaneous white adipose tissue (scWAT), browning and lipolysis effects triggered by cold were ablated, and the oxygen consumption was also lowered both at the normal and cold conditions. Moreover, mice exhibited browned scWAT, accelerated metabolic rates, and tolerance to hypothermia when 4-methylhistamine (4MH), a selective HrH4 agonist, was adjacently injected to the scWAT. Consistent with these findings, 4MH also triggered the browning and lipolytic effects in cultured C3H10T1/2 adipocytes. Mechanically, we demonstrated that p38/MAPK and ERK/MAPK pathways were involved in these processes. In conclusion, our findings have uncovered an effective role of HrH4 in adipose tissue browning.

Pathophysiological Role of Histamine H4 Receptor in Cancer: Therapeutic Implications

Cancer is a leading cause of death in both developed and developing countries. Although advances in cancer research lead to improved anti-neoplastic therapies, they continue to have unfavorable outcomes, including poor response and severe toxicity. Thus, the challenge for the new therapeutic approaches is to increase anti-tumor efficacy by targeting different molecules encompassed in the tumor and its microenvironment, as well as their specific interactions. The histamine H4 receptor (H4R) is the last discovered histamine receptor subtype and it modulates important immune functions in innate and in adaptive immune responses. Several ligands have been developed and some of them are being used in clinical trials for immune disorders with promising results. When searched in The Cancer Genome Atlas (TCGA) database, human H4R gene was found to be expressed in bladder cancer, kidney cancer, breast cancer, gastrointestinal cancers, lung cancer, endometrial cancer, and skin cancer. In the present work, we aimed to briefly summarize current knowledge in H4R's pharmacology and in the clinical use of H4R ligands before focusing on recent data reporting the expression of H4R and its pathophysiological role in cancer, representing a potential molecular target for cancer therapeutics. H4R gene and protein expression in different types of cancers compared with normal tissue as well as its relationship with patient prognosis in terms of survival will be described.

Human β-Defensin 2 Expression in Oral Epithelium: Potential Therapeutic Targets in Oral Lichen Planus

Human β-defensin 2 (hBD-2) is a potent antimicrobial peptide that participates in defense against invading bacteria. We recently showed that bacterial components and histamine, through histamine H4 receptor (H4R), are involved in the pathogenesis of the potentially malignant lesion, oral lichen planus (OLP). However, the underlying mechanisms remain unknown. We, therefore, investigated the role of hBD2–histamine crosstalk signaling in promoting OLP pathology. Biopsies from OLP and oral tongue squamous cell carcinoma (OTSCC) patients, and healthy controls were used. Two OTSCC cell lines and normal human oral keratinocytes (HOKs) were used. HBD-2 and other targets were mapped by immunostaining and analyzed by ImageJ2 software. The highly sensitive droplet-digital PCR technology and qRT-PCR were utilized to study the clinically derived and in vitro samples, respectively. H4R was challenged with the specific agonist HST-10 and inverse agonist ST-1007. HBD-2 was highly induced in OLP lesions. In contrast, hBD2 expression was attenuated in OTSCC tissues, while very low levels of hBD-2 messenger RNA (mRNA) were observed in OTSCC cells. Together with tumor necrosis factor-α (TNF-α), histamine upregulated hBD-2 mRNA expression in HOKs. Activation of H4R seems to modulate the expression of epithelial hBD-2. These findings suggest the involvement of hBD-2 in the pathogenesis of OLP and may, thus, be harnessed for therapeutic interventions in OLP.

In vivo Evidence for Partial Activation of Eosinophils via the Histamine H4-Receptor: Adoptive Transfer Experiments Using Eosinophils From H4R-/- and H4R+/+ Mice

Our previous in vitro studies revealed that histamine via histamine the H₄-receptors (H₄R), as compared to other stimuli, such as eotaxin or formylpeptides, rather partially activates eosinophilic granulocytes (eosinophils). In order to evaluate the H₄R-mediated activation of eosinophils in vivo, we employed dextran sodium sulfate (DSS)-induced colitis in mice, closely resembling human ulcerative colitis (UC), which is largely characterized by a local eosinophilic infiltration of the colon. IL-5-deficient BALB/c mice served as a model with reduced endogenous numbers of eosinophils, in which wild-type (H₄R^+/+) or H₄R-deficient (H₄R^−/−) eosinophils were adoptively transferred during the course of DSS-induced colitis. During the 1-week observation period, transfer of eosinophils transiently reversed the acute clinical colitis-like phenotype (body weight loss, perianal bleeding, soft stool consistency) resulting from IL-5-deficiency. This reversion was significantly more pronounced upon transfer of eosinophils from H₄R^+/+ mice as compared to those from H₄R^−/− mice. Already at the end of the observation period, the clinical effects of the transfer of H₄R^+/+ and H₄R^−/− eosinophils became similar, as were the results of the histological examination of the cola and the analyses of cytokine production in cola and in re-stimulated lymph node cells performed at this time. Thus, analyzing clinical and pathological parameters representative of colitis in this model, we demonstrate that as well as in vitro, also in vivo histamine via the H₄R only partially activates eosinophils.

Human basophil chemotaxis and activation are regulated via the histamine H4 receptor

BACKGROUND

IgE-mediated cross-linking of FcεRI results in the release of mediators stored in basophil granules, such as histamine and proteases, and in the de novo synthesis of sulfidoleukotrienes.

OBJECTIVE

In this study, we investigated the role of the histamine receptors, in particular that of the histamine H4 receptor (H4R), in modulating human basophil function.

METHODS

The mRNA expression of the histamine receptors was measured by real-time PCR. Migration of basophils was assessed using the modified Boyden chamber technique. The expression levels of CD63 and CD203c on the cell surface and the sulfidoleukotriene release were determined by flow cytometry and ELISA, respectively.

RESULTS

We could show that highly purified basophils express the H1R, H2R, and H4R but not the H3R mRNA. Human basophils expressed higher H4R mRNA levels as compared to the expression levels of the H1R (P < 0.01). Histamine and the H4R agonist ST-1006 initiated active migration of basophils (P < 0.001). A significant reduction in FcεRI cross-linking-mediated surface expression of CD63 and CD203c was observed on basophils after pre-incubation with histamine or the specific H4R agonist ST-1006 (P < 0.01). The synthesis and release of sulfidoleukotrienes from basophils after activation with different stimuli, by FcεRI cross-linking or by stimulation with hymenoptera venom allergens, were significantly reduced by histamine or the H4R agonist ST-1006 (P < 0.05-0.001).

CONCLUSION

These data imply that the H4R regulates IgE-dependent processes in human basophils and provides a novel function of the H4R preventing an overwhelming immune reaction by engagement of a negative feedback loop.

Histamine H4 receptor as a new therapeutic target for choroidal neovascularization in age-related macular degeneration

BACKGROUND AND PURPOSE

The present treatment for choroidal neovascularization (CNV) associated with age-related macular degeneration (AMD) is not sufficient. Hence, we examined the therapeutic efficacy of reducing histamine H4 receptor expression on CNV in mice.

EXPERIMENTAL APPROACH

H4 receptor expression was examined in CNVs from patients with AMD. In mice, laser photocoagulation was performed in the retina to induce experimental CNV (laser CNV). Protein and mRNA expression levels were determined and CNV volume measured in wild-type and Hrh4(-/-) mice with laser CNV. The effects of JNJ7777120, an H4 receptor antagonist, administered intravitreously, on CNV volume and pathological vessel leakage were determined in mice with laser CNV and controls. Fundus imaging, retinal histology and electroretinography were performed on eyes injected with JNJ7777120 to evaluate retinal toxicity.

KEY RESULTS

Human H4 receptors were only confirmed in CNV samples from AMD patients and not in the other subretinal tissues. Mouse H4 receptors were expressed in retinal pigment epithelium only after inducing laser CNV in wild-type mice, and were co-localized with the macrophage marker F4/80. Laser CNV volume was reduced in Hrh4(-/-) mice compared with that in wild-type mice, and JNJ7777120 suppressed laser-induced CNV volume and pathological CNV leakage in wild-type mice. Also eyes injected with JNJ7777120 did not show retinal degeneration.

CONCLUSIONS AND IMPLICATIONS

H4 receptors are expressed in macrophages that accumulate around CNVs. Suppressing H4 receptor expression prevented the pathological vessel leakage without showing retinal toxicity, indicating that the H4 receptor has potential as a novel therapeutic target in AMD.

Histamine H4 receptor knockout mice display reduced inflammation in a chronic model of atopic dermatitis

BACKGROUND

The histamine H4 receptor (H4R) was brought into focus as a new therapeutic target for the treatment of allergic disorders such as atopic dermatitis (AD). H4R antagonists have already been tested in several animal models of AD, but these studies have yielded conflicting results.

MATERIAL AND METHODS

The development of ovalbumin-induced AD-like skin lesions was analysed in H4R(-/-) mice and in H4R antagonist (JNJ28307474)-treated mice.

RESULTS

H4R(-/-) mice showed a clear amelioration of the skin lesions, with a diminished influx of inflammatory cells and a reduced epidermal hyperproliferation at lesional skin sites. H4R(-/-) mice had a reduced amount of ovalbumin-specific IgE, a reduced number of splenocytes and lymph node cells with a decreased number of CD4+ T cells. The H4R modulated the cytokine secretion of CD4+ T cells and splenocytes and altered the cellular profile in the lymph nodes. The anti-inflammatory effect could only partially be mimicked by JNJ28307474 and only when the H4R antagonist was given during sensitization and challenge and not when JNJ28307474 was only given during the provocation phase of the allergic reaction.

CONCLUSION

The H4R modulates inflammation in a chronic allergic dermatitis setting. However, results of this study indicate that it is necessary to block the H4R during ontogeny and development of the allergic inflammation.

Histamine H4 receptor in oral lichen planus

OBJECTIVES

Oral lichen planus (OLP) is an autoimmune disease characterized by a band-like T-cell infiltrate below the apoptotic epithelial cells and degenerated basement membrane. We tested the hypothesis that the high-affinity histamine H4 receptors (H4 Rs) are downregulated in OLP by high histamine concentrations and proinflammatory T-cell cytokines.

MATERIALS AND METHODS

Immunohistochemistry and immunofluorescence staining, image analysis and quantitative real-time polymerase chain reaction of tissue samples and cytokine-stimulated cultured SCC-25 and primary human oral keratinocytes.

RESULTS

H4 R immunoreactivity was weak in OLP and characterized by mast cell (MC) hyperplasia and degranulation. In contrast to controls, H4 R immunostaining and MC counts were negatively correlated in OLP (P = 0.003). H4 R agonist at nanomolar levels led to a rapid internalization of H4 Rs, whereas high histamine concentration and interferon-γ decreased HRH4 -gene transcripts.

CONCLUSION

Healthy oral epithelial cells are equipped with H4 R, which displays a uniform staining pattern in a MC-independent fashion. In contrast, in OLP, increased numbers of activated MCs associate with increasing loss of epithelial H4 R. Cell culture experiments suggest a rapid H4 R stimulation-dependent receptor internalization and a slow cytokine-driven decrease in H4 R synthesis. H4 R may be involved in the maintenance of healthy oral mucosa. In OLP, this maintenance might be impaired by MC degranulation and inflammatory cytokines.

BODIPY(®) FL histamine as a new modality for quantitative detection of histamine receptor upregulation upon IgE sensitization in murine bone marrow-derived mast cells

Flow cytometry is one of the most widely used methods for the qualitative and quantitative analysis of cell surface expressed proteins by making use of fluorescent specific antibodies. Lacking an antibody validated for flow cytometry, an alternative approach for labeling cell surface receptors is the use of fluorescently tagged ligands. In this study, histamine H4 receptor transfected Chinese hamster ovary cells and murine bone marrow-derived mast cells (mBMMCs) were selected for studying the possibility of staining individual histamine receptors using BODIPY(®) FL histamine and selective antagonists. Flow cytometric measurements and supporting calculations showed that BODIPY FL histamine is suitable tool for quantitating cell surface histamine receptors. The binding, and competitive inhibition of this fluorescent ligand were characterized, which were in good agreement with a semi-empirical model constructed from fundamental protein-binding relationships. Using this method it was shown for the first time that even though mature mBMMCs express H2R and H4R to the same extent, immunoglobulin E sensitization results in H4R upregulation only, while the surface expression of H2R remains unchanged.

Detailed analysis of biased histamine H₄ receptor signalling by JNJ 7777120 analogues

BACKGROUND AND PURPOSE

The histamine H₄ receptor, originally thought to signal merely through Gαi proteins, has recently been shown to also recruit and signal via β-arrestin2. Following the discovery that the reference antagonist indolecarboxamide JNJ 7777120 appears to be a partial agonist in β-arrestin2 recruitment, we have identified additional biased hH₄R ligands that preferentially couple to Gαi or β-arrestin2 proteins. In this study, we explored ligand and receptor regions that are important for biased hH₄R signalling.

EXPERIMENTAL APPROACH

We evaluated a series of 48 indolecarboxamides with subtle structural differences for their ability to induce hH₄R-mediated Gαi protein signalling or β-arrestin2 recruitment. Subsequently, a Fingerprints for Ligands and Proteins three-dimensional quantitative structure-activity relationship analysis correlated intrinsic activity values with structural ligand requirements. Moreover, a hH₄R homology model was used to identify receptor regions important for biased hH₄R signalling.

KEY RESULTS

One indolecarboxamide (75) with a nitro substituent on position R7 of the aromatic ring displayed an equal preference for the Gαi and β-arrestin2 pathway and was classified as unbiased hH₄R ligand. The other 47 indolecarboxamides were β-arrestin2-biased agonists. Intrinsic activities of the unbiased as well as β-arrestin2-biased indolecarboxamides to induce β-arrestin2 recruitment could be correlated with different ligand features and hH₄R regions.

CONCLUSION AND IMPLICATIONS

Small structural modifications resulted in diverse intrinsic activities for unbiased (75) and β-arrestin2-biased indolecarboxamides. Analysis of ligand and receptor features revealed efficacy hotspots responsible for biased-β-arrestin2 recruitment. This knowledge is useful for the design of hH₄R ligands with biased intrinsic activities and aids our understanding of the mechanism of H₄R activation.

Profiling of histamine H4 receptor agonists in native human monocytes

BACKGROUND AND PURPOSE

Since the identification of the histamine H₄ receptor, several ligands activating this receptor have been described and more compounds are in development. These ligands are well characterized in pharmacological assays, including radioligand competition binding studies, GTPγS and GTPase assays. In most cases, these experiments are performed in transfected cell lines, expressing unnaturally high levels of target receptors and G-protein signalling components. In this study we investigated the specific properties of H₄ receptor ligands in native cells.

EXPERIMENTAL APPROACH

Histamine and five different H₄ receptor agonists - 4-methylhistamine, UR-PI376, clobenpropit, VUF8430 and ST-1006 - were characterized in freshly isolated human monocytes. The ligands (10 nM-10 μM) were tested as inhibitors of IL-12p70 secretion from human monocytes and the effects of the H₂ receptor antagonist ranitidine and the H₄ receptor antagonist JNJ7777120 on their action was investigated.

KEY RESULTS

Histamine and all the tested agonists reduced IL-12p70 secretion into monocyte supernatants by 40-70%. The potencies varied with pEC50 values ranging from 5.7 to 6.9, depending on the agonist used. All potencies were lower than those determined in the original investigations of the compounds. Pretreatment of monocytes with H₂ or H₄ receptor antagonists showed that some H₄ receptor ligands also had low activity at the H₂ receptor.

CONCLUSIONS AND IMPLICATIONS

Our study demonstrates discrepancies between the potencies obtained from assays in transfected cell lines and assays in native human cells, indicating the importance of evaluating H₄ receptor ligands in native cells.

Design and pharmacological characterization of VUF14480, a covalent partial agonist that interacts with cysteine 98(3.36) of the human histamine H₄ receptor

BACKGROUND AND PURPOSE

The recently proposed binding mode of 2-aminopyrimidines to the human (h) histamine H₄ receptor suggests that the 2-amino group of these ligands interacts with glutamic acid residue E182(5.46) in the transmembrane (TM) helix 5 of this receptor. Interestingly, substituents at the 2-position of this pyrimidine are also in close proximity to the cysteine residue C98(3.36) in TM3. We hypothesized that an ethenyl group at this position will form a covalent bond with C98(3.36) by functioning as a Michael acceptor. A covalent pyrimidine analogue will not only prove this proposed binding mode, but will also provide a valuable tool for H4 receptor research.

EXPERIMENTAL APPROACH

We designed and synthesized VUF14480, and pharmacologically characterized this compound in hH4 receptor radioligand binding, G protein activation and β-arrestin2 recruitment experiments. The ability of VUF14480 to act as a covalent binder was assessed both chemically and pharmacologically.

KEY RESULTS

VUF14480 was shown to be a partial agonist of hH4 receptor-mediated G protein signalling and β-arrestin2 recruitment. VUF14480 bound covalently to the hH₄ receptor with submicromolar affinity. Serine substitution of C98(3.36) prevented this covalent interaction.

CONCLUSION AND IMPLICATIONS

VUF14480 is thought to bind covalently to the hH₄ receptor-C98(3.36) residue and partially induce hH₄ receptor-mediated G protein activation and β-arrestin2 recruitment. Moreover, these observations confirm our previously proposed binding mode of 2-aminopyrimidines. VUF14480 will be a useful tool to stabilize the receptor into an active confirmation and further investigate the structure of the active hH₄ receptor.