N-(3-acyloxy-2-benzylpropyl)-N'-[4-(methylsulfonylamino)benzyl]thiourea analogues: novel potent and high affinity antagonists and partial antagonists of the vanilloid receptor

Dorsal root ganglion neurons innervating skeletal muscle respond to physiological combinations of protons, ATP, and lactate mediated by ASIC, P2X, and TRPV1

The adequate stimuli and molecular receptors for muscle metaboreceptors and nociceptors are still under investigation. We used calcium imaging of cultured primary sensory dorsal root ganglion (DRG) neurons from C57Bl/6 mice to determine candidates for metabolites that could be the adequate stimuli and receptors that could detect these stimuli. Retrograde DiI labeling determined that some of these neurons innervated skeletal muscle. We found that combinations of protons, ATP, and lactate were much more effective than individually applied compounds for activating rapid calcium increases in muscle-innervating dorsal root ganglion neurons. Antagonists for P2X, ASIC, and TRPV1 receptors suggested that these three receptors act together to detect protons, ATP, and lactate when presented together in physiologically relevant concentrations. Two populations of muscle-innervating DRG neurons were found. One responded to low metabolite levels (likely nonnoxious) and used ASIC3, P2X5, and TRPV1 as molecular receptors to detect these metabolites. The other responded to high levels of metabolites (likely noxious) and used ASIC3, P2X4, and TRPV1 as their molecular receptors. We conclude that a combination of ASIC, P2X5 and/or P2X4, and TRPV1 are the molecular receptors used to detect metabolites by muscle-innervating sensory neurons. We further conclude that the adequate stimuli for muscle metaboreceptors and nociceptors are combinations of protons, ATP, and lactate.

Heteroaryl β-tetralin ureas as novel antagonists of human TRPV1

We report on a series of alpha-substituted-beta-tetralin-derived and related phenethyl-based isoquinolinyl and hydroxynaphthyl ureas as potent antagonists of the human TRPV1 receptor. The synthesis and Structure-activity relationships (SAR) of the series are described.

α-Substituted N-(4-tert-butylbenzyl)-N′-[4-(methylsulfonylamino)benzyl]thiourea analogues as potent and stereospecific TRPV1 antagonists

A series of alpha-substituted N-(4-tert-butylbenzyl)-N'-[4-(methylsulfonylamino)benzyl]thiourea analogues have been investigated as TRPV1 receptor antagonists. alpha-Methyl substituted analogues showed potent and stereospecific antagonism to the action of capsaicin on rat TRPV1 heterologously expressed in Chinese hamster ovary cells. In particular, compounds 14 and 18, which possess the R-configuration, exhibited excellent potencies (respectively, K(i)=41 and 39.2 nM and K(i(ant))=4.5 and 37 nM).

Synthesis and Pharmacological Activity of Urea and Thiourea Derivatives of 4-Azatricyclo[5.2.2.02,6]undec-8-ene-3,5-dione

A series of nineteen new thiourea and urea derivatives of 10-isopropyl-8-methyl-4-azatricyclo[5.2.2.0(2,6)]undec-8-ene-3,5-dione, 1-isopropyl-7-methyl-4-azatricyclo[5.2.2.0(2,6)]undec-8-ene-3,5-dione and 1,7,8,9,10-pentamethyl-4-azatricyclo[5.2.1.0(2,6)]dec-8-ene-3,5-dione have been prepared and studied by (1)H-NMR. The compound k1a (1-(1,7,8,9,10-pentamethyl-3,5-dioxo-4-aza-tricyclo[5.2.1.0(2,6)]dec-8-en-4-yl)-3-phenyl-urea) was tested for pharmacological activity on animal central nervous system (CNS). The activities of synthesized compounds were evaluated for their cytotoxicity and anti-HIV-1 activity in MT-4 cells. Antimicrobial activity of the newly obtained derivatives was tested against some Gram-positive and Gram-negative bacteria and fungi of the Candida species.

Halogenation of 4-hydroxy-3-methoxybenzyl thiourea TRPV1 agonists showed enhanced antagonism to capsaicin

Selected potent TRPV1 agonists (1-6) have been modified by 5- or 6-halogenation on the aromatic A-region to analyze their effects on potency and efficacy (agonism versus antagonism). The halogenation caused enhanced functional antagonism at TRPV1 compared to the corresponding prototype agonists. The analysis of SAR indicated that the antagonism was enhanced as the size of the halogen increased (I>Br>Cl) and when the 6-position was halogenated. Compounds 23c and 31b were found to be potent full antagonists with K(i) (as functional antagonist)=23.1 and 30.3 nM in rTRPV1/CHO system, respectively.

The TRPV1 receptor and nociception

The capsaicin receptor TRPV1 is an emerging target for the treatment of pain with a unique expression profile in peripheral nociceptors and the ability to show polymodal activation, TRPV1 is an important integrator of responses to inflammatory mediators. Sensitization of TRPV1 during chronic pain is believed to contribute to the transduction of noxious signaling for normally innocuous stimuli and consequently the search for novel TRPV1 therapeutics is intense. The current understanding of the physiological role the receptor, as well as the potential therapeutic utility and emerging liabilities of TRPV1 modulators are discussed.

Inflammatory pain: the cellular basis of heat hyperalgesia

Injury or inflammation release a range of inflammatory mediators that increase the sensitivity of sensory neurons to noxious thermal or mechanical stimuli. The heat- and capsaicin-gated channel TRPV1, which is an important detector of multiple noxious stimuli, plays a critical role in the development of thermal hyperalgesia induced by a wide range of inflammatory mediators. We review here recent findings on the molecular mechanisms of sensitisation of TRPV1 by inflammatory mediators, including bradykinin, ATP, NGF and prostaglandins. We describe the signalling pathways believed to be involved in the potentiation of TRPV1, and our current understanding of how inflammatory mediators couple to these pathways.

Pain behaviors produced by capsaicin: influence of inflammatory mediators and nerve injury

The present study was undertaken to characterize spontaneous (ie, nonevoked) pain behaviors (flinching, biting/licking) produced by local injections of capsaicin into the rat hindpaw as a model of chemogenic pain, and to determine effects of inflammatory mediators and nerve injury on such behaviors. Capsaicin antagonists are a potential class of novel topical analgesics, and this model may be of value for preclinical screening of novel compounds. Local injections of capsaicin (0.1-30 microg) into the hindpaw produced flinching and biting/licking behaviors over 5 min, and these were reduced by capsazepine, a competitive antagonist for capsaicin at the TRPV1 receptor. Coadministration of noradrenaline (NA), prostaglandin E(2) (PGE(2)), and 5-hydroxytryptamine (5-HT) augmented capsaicin-evoked responses primarily by extending the duration of behaviors. Partial sciatic nerve ligation decreased flinching produced by capsaicin alone, by capsaicin in combination with each of NA, PGE(2), and 5-HT, and by formalin. Tibial nerve injury also reduced capsaicin-evoked flinching, and responses to formalin, but spinal nerve ligation did not affect either. These results indicate that (1) spontaneous pain behaviors occur as a result of TRPV1 receptor activation with a different time course than evoked responses, (2) inflammatory mediators augment capsaicin-evoked pain behaviors, and (3) various forms of nerve injury produce different effects on capsaicin-evoked pain behaviors.

PERSPECTIVE

The VR1 receptor is a potential target for development of novel topical analgesics. This study characterized pain behaviors produced by local injections of capsaicin in the presence of inflammatory mediators and following various forms of nerve injury. Results are of interest for the preclinical screening of novel VR1 receptor antagonists.

Old Molecules for New Receptors: Trp(Nps) Dipeptide Derivatives as Vanilloid TRPV1 Channel Blockers

The transient receptor potential vanilloid member 1 (TRPV1), an integrator of multiple pain-producing stimuli, is regarded nowadays as an important biological target for the discovery of novel analgesics. Here, we describe the first experimental evidence for the behavior of an old family of analgesic dipeptides, namely Xaa-Trp(Nps) and Trp(Nps)-Xaa (Xaa=Lys, Arg) derivatives, as potent TRPV1 channel blockers. We also report the synthesis and biological investigation of a series of new conformationally restricted Trp(Nps)-dipeptide derivatives with improved TRPV1/NMDA selectivity. Compound 15 b, which incorporates an N-terminal 2S-azetidine-derived Arg residue, was the most selective compound in this series. Collectively, a new family of TRPV1 channel blockers emerged from our results, although further modifications are required to fine-tune the potency/selectivity/toxicity balance.

The transient receptor potential vanilloid 1: role in airway inflammation and disease

The transient receptor potential vanilloid 1 (TRPV1) is an excitatory cation channel, rather selectively expressed in a subpopulation of nociceptive, primary sensory neurons that promote neurogenic inflammation via neuropeptide release. TRPV1 is activated by noxious temperature, low extracellular pH and diverse lipid derivatives, and is uniquely sensitive to vanilloid molecules, including capsaicin. TRPV1 expression and sensitivity is highly regulated by diverse G protein-coupled and tyrosine kinase receptors. Other exogenous or endogenous chemical agents, including reactive oxygen species, ethanol and hydrogen sulphide sensitize/activate TRPV1. In the airways, TRPV1 agonists cause cough, bronchoconstriction, microvascular leakage, hyperreactivity and hypersecretion. Patients with asthma and chronic obstructive pulmonary disease are more sensitive to the tussive effect of TRPV1 agonists and TRPV1 activation may contribute to respiratory symptoms caused by acidic media present in the airways during asthma exacerbation, gastroesophageal reflux induced asthma or in other conditions. TRPV1 antagonists may be useful in the treatment of these diseases.

Novel compounds that interact with both leukotriene B4 receptors and vanilloid TRPV1 receptors

The aim of this study was to investigate the interaction of a series of novel compounds with leukotriene B(4) receptors (BLT) and vanilloid receptor (TRPV1). First, we characterized leukotriene B(4) (LTB(4)) ethanolamide. In guinea pig isolated lung parenchyma, LTB(4) ethanolamide antagonized the contractile action of LTB(4) with an apparent K(B) value of 7.28 nM. Using a Boyden chamber assay, we demonstrated that this compound stimulated human neutrophil migration in a similar manner to LTB(4) but with lower efficacy. In rat TRPV1 (rTRPV1)-expressing Chinese hamster ovary (CHO) cells and dorsal root ganglion (DRG) neurons, LTB(4) and LTB(4) ethanolamide acted as low-efficacy agonists, increasing intracellular calcium concentration ([Ca(2+)](i)) in a capsazepine-sensitive manner. These results prompted us to hypothesize that a molecule may possess pharmacophores such that it is capable of dual antagonism of BLT and TRPV1 receptors. Two novel compounds, N-[2-fluoro-4-[3-(11 hydroxyheptadec-8-enyl)-thioureiomethyl]-phenyl]-methanesulfonamide (O-3367) and N-[4-[3-(11 hydroxyheptadec-8-enyl)-thioureio-methyl]-phenyl]-methanesulfonamide (O-3383), were synthesized. In human neutrophils, both compounds acted as antagonists, significantly attenuating the BLT receptor-mediated ability of LTB(4) to induce migration, with pIC(50) values of 7.22 +/- 0.17 and 5.95 +/- 0.16, respectively. In rTRPV1-expressing CHO cells, they caused a significant rightward shift in the log concentration-response curve for the TRPV1 receptor agonist capsaicin (3-methoxy-4-hydroxy)benzyl-8-methyl-6-nonenamide). In DRG neurons O-3367 significantly attenuated the capsaicin-induced increases in [Ca(2+)](i) with a pIC(50) value of 5.94 +/- 0.004. O-3367 and O-3383 represent novel structural templates for generating compounds possessing dual antagonism at BLT and TRPV1 receptors. In view of the crucial role of both TRPV1 and BLT receptors in the pathophysiology of inflammatory conditions, such compounds may betoken a novel class of highly effective therapeutics.

Gastrointestinal pain in functional bowel disorders: sensory neurons as novel drug targets

Functional bowel disorders (FBDs) are defined by symptoms of gastrointestinal (GI) dysfunction, discomfort and pain in the absence of a demonstrable organic cause. Since the prevalence of FBDs, particularly functional dyspepsia and irritable bowel syndrome, can be as high as 20%, FBDs represent a significant burden in terms of direct healthcare and productivity costs. There is emerging evidence that the discomfort and pain experienced by many FBD patients is due to persistent hypersensitivity of primary afferent neurons, which may develop in response to infection, inflammation or other insults. This concept identifies vagal and spinal sensory neurons as important targets for novel therapies of GI hyperalgesia. Sensory neuron-specific targets can be grouped into three categories: receptors and sensors at the peripheral nerve terminals, ion channels relevant to nerve excitability and conduction and transmitter receptors. Particular therapeutic potential is attributed to targets that are selectively expressed by afferent neurons, such as the transient receptor potential channel TRPV1, acid-sensing ion channels and tetrodotoxin-resistant Na + channels.

Analysis of structure–activity relationships for the ‘A-region’ of N-(4-t-butylbenzyl)-N′-[4-(methylsulfonylamino)benzyl]thiourea analogues as TRPV1 antagonists

The structure-activity relationships for the 'A-region' of N-(4-t-butylbenzyl)-N'-[4-(methylsulfonylamino)benzyl]thiourea analogues have been investigated as TRPV1 receptor antagonists. The 2-halogen analogues showed enhanced antagonism compared to the prototype antagonist.

Analysis of structure–activity relationships for the ‘B-region’ of N-(4-t-butylbenzyl)-N′-[4-(methylsulfonylamino)benzyl]-thiourea analogues as TRPV1 antagonists

The structure-activity relationships for the 'B-region' of N-(4-t-butylbenzyl)-N'-[4-(methylsulfonylamino)benzyl]thiourea analogues have been investigated as TRPV1 receptor antagonists. A docking model of potent antagonist 2 with the sensor region of TRPV1 is proposed.

AMG 9810 [(E)-3-(4-t-butylphenyl)-N-(2,3-dihydrobenzo[b][1,4] dioxin-6-yl)acrylamide], a novel vanilloid receptor 1 (TRPV1) antagonist with antihyperalgesic properties

The vanilloid receptor 1 (VR1 or TRPV1) is a membrane-bound, nonselective cation channel expressed by peripheral sensory neurons. TRPV1 antagonists produce antihyperalgesic effects in animal models of inflammatory and neuropathic pain. Here, we describe the in vitro and in vivo pharmacology of a novel TRPV1 antagonist, AMG 9810, (E)-3-(4-t-butylphenyl)-N-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)acrylamide. AMG 9810 is a competitive antagonist of capsaicin activation (IC50 value for human TRPV1, 24.5 +/- 15.7 nM; rat TRPV1, 85.6 +/- 39.4 nM) and blocks all known modes of TRPV1 activation, including protons (IC50 value for rat TRPV1, 294 +/- 192 nM; human TRPV1, 92.7 +/- 72.8 nM), heat (IC50 value for rat TRPV1, 21 +/- 17 nM; human TRPV1, 15.8 +/- 10.8 nM), and endogenous ligands, such as anandamide, N-arachidonyl dopamine, and oleoyldopamine. AMG 9810 blocks capsaicin-evoked depolarization and calcitonin gene-related peptide release in cultures of rat dorsal root ganglion primary neurons. Screening of AMG 9810 against a panel of G protein-coupled receptors and ion channels indicated selectivity toward TRPV1. In vivo, AMG 9810 is effective at preventing capsaicin-induced eye wiping in a dose-dependent manner, and it reverses thermal and mechanical hyperalgesia in a model of inflammatory pain induced by intraplantar injection of complete Freund's adjuvant. At effective doses, AMG 9810 did not show any significant effects on motor function, as measured by open field locomotor activity and motor coordination tests. AMG 9810 is the first cinnamide TRPV1 antagonist reported to block capsaicin-induced eye wiping behavior and reverse hyperalgesia in an animal model of inflammatory pain.

Biarylcarboxybenzamide derivatives as potent vanilloid receptor (VR1) antagonistic ligands

Seventeen biarylcarboxybenzamide derivatives were prepared for the study of their agonistic/antagonistic activities to the vanilloid receptor (VR1) in rat DRG neurons. The replacement of the piperazine moiety of the lead compound 1 with phenyl ring showed quite enhanced antagonistic activity. Among the prepared derivatives, N-(4-tert-butylphenyl)-4-pyridine-2-yl-benzamide (2, IC(50)=31 nM) and N-(4-tert-butylphenyl)-4-(3-methylpyridine-2-yl)benzamide (3g, IC(50)=31 nM), showed 5-fold higher antagonistic activity than 1 in (45)Ca(2+)-influx assay.

Calcium-dependent and independent mechanisms of capsaicin receptor (TRPV1)-mediated cytokine production and cell death in human bronchial epithelial cells

Activation of the capsaicin receptor (VR1 or TRPV1) in bronchial epithelial cells by capsaicinoids and other vanilloids promotes pro-inflammatory cytokine production and cell death. The purpose of this study was to investigate the role of TRPV1-mediated calcium flux from extracellular sources as an initiator of these responses and to define additional cellular pathways that control cell death. TRPV1 antagonists and reduction of calcium concentrations in treatment solutions attenuated calcium flux, induction of interleukin-6 and 8 gene expression, and IL-6 secretion by cells treated with capsaicin or resiniferatoxin. Most TRPV1 antagonists also attenuated cell death, but the relative potency and extent of protection did not directly correlate with inhibition of total calcium flux. Treatment solutions with reduced calcium content or chelators had no effect on cytotoxicity. Inhibitors of arachidonic acid metabolism and cyclo-oxygenases also prevented cell death indicating that TRPV1 agonists disrupted basal arachidonic acid metabolism and altered cyclo-oxygenase function via a TRPV1-dependent mechanism in order to produce toxicity. These data confirm previous results demonstrating calcium flux through TRPV1 acts as a trigger for cytokine production by vanilloids, and provides new mechanistic insights on mechanisms of cell death produced by TRPV1 agonists in respiratory epithelial cells.

Synthesis and evaluation of pyridazinylpiperazines as vanilloid receptor 1 antagonists

A structurally biased chemical library of pyridazinylpiperazine analogs was prepared in an effort to improve the pharmaceutical and pharmacological profile of the lead compound N-(4-tertiarybutylphenyl)-4-(3-chloropyridin-2-yl)tetrahydropyrazine-1(2H)-carboxamide (BCTC). The library was evaluated for VR1 antagonist activity in capsaicin-induced (CAP) and pH5.5-induced (pH) FLIPR assays in a human VR1-expressing HEK293 cell line. The most potent VR1 antagonists were found to have IC(50) values in the range of 9-200nM with improved pharmaceutical and pharmacological profiles versus the lead BCTC. These compounds represent possible second-generation BCTC analogs.

Structure-activity relationships of simplified resiniferatoxin analogues with potent VR1 agonism elucidates an active conformation of RTX for VR1 binding

We previously described a series of N-(3-acyloxy-2-benzylpropyl) homovanillate and N'-(4-hydroxy-3-methoxybenzyl) thiourea derivatives that were potent VR1 agonists with high-affinities and excellent analgesic profiles. The design of these simplified RTX analogues was based on our RTX-derived pharmacophore model which incorporates the 4-hydroxy-3-methoxyphenyl (A-region), C(20)-ester (B-region), orthophenyl (C1-region) and C(3)-keto (C2-region) groups of RTX. For the purpose of optimizing the spatial arrangement of the four principal pharmacophores on the lead agonists (1-4), we have modified the distances in the parent C-region, 3-acyloxy-2-benzylpropyl groups, by lengthening or shortening one carbon to vary the distances between the pharmacophores. We find that two of the amides, 4 and 19, possess EC(50) values <1 nM for induction of calcium influx in the VR1-CHO cells. As observed previously, the structure-activity relations for inhibition of RTX binding to VR1 and for induction of calcium uptake were distinct, presumably reflecting both intrinsic and methodological factors. In order to find the active conformation of VR1 ligands, the energy-minimized conformations of seven selected agonists were determined and the positions of their four pharmacophores were matched with those of five low energy RTX conformations. The rms values for the overlaps in the pharmacophores were calculated and correlated with the measured binding affinities (K(i)) and calcium influx (EC(50)) values. The binding affinities of the agonists correlated best with the RMS values derived from RTX conformation E (r(2)=0.92), predicting a model of the active conformation of RTX and related vanilloids for binding to VR1. Poorer correlation was obtained between any of the conformations and the EC(50) values for calcium influx.

TRPV1 and the gut: from a tasty receptor for a painful vanilloid to a key player in hyperalgesia

Capsaicin, the pungent ingredient in red pepper, has been used since ancient times as a spice, despite the burning sensation associated with its intake. More than 50 years ago, Nikolaus Jancso discovered that capsaicin can selectively stimulate nociceptive primary afferent neurons. The ensuing research established that the neuropharmacological properties of capsaicin are due to its activation of the transient receptor potential ion channel of the vanilloid type 1 (TRPV1). Expressed by primary afferent neurons innervating the gut and other organs, TRPV1 is gated not only by vanilloids such as capsaicin, but also by noxious heat, acidosis and intracellular lipid mediators such as anandamide and lipoxygenase products. Importantly, TRPV1 can be sensitized by acidosis and activation of various pro-algesic pathways. Upregulation of TRPV1 in inflammatory bowel disease and the beneficial effect of TRPV1 downregulation in functional dyspepsia and irritable bladder make this polymodal nociceptor an attractive target of novel therapies for chronic abdominal pain.

Analysis of structure–activity relationships with the N-(3-acyloxy-2-benzylpropyl)-N′-[4-(methylsulfonylamino)benzyl]thiourea template for vanilloid receptor 1 antagonism

In a continuing effort to elucidate the structure-activity relationships of the lead antagonist N-[2-(3,4-dimethylbenzyl)-3-pivaloyloxypropyl]-N'-[4-(methylsulfonylamino)benzyl]thiourea (1), the distances between the proposed four pharmacophores in 1 have been varied by insertion or deletion of one carbon to optimize their fit to the receptor. In addition, the acyloxy group of the C region was replaced with amide and N-hydroxy amide to identify the pharmacophoric importance of the ester group in the C2 region. The results indicated that the pharmacophoric arrangement of 1 was optimal for receptor binding affinity and antagonism, and the ester of the C2 region was significant for receptor binding. Among the derivatives, compound 19 showed distinct behavior with a 2-fold improvement in antagonism but a 13-fold reduction in binding affinity compared to 1. The partial separation of pharmacophoric requirements of these two assays has been noted before and compound 19 is thus selective for the calcium entry-linked receptor population. The conformational analysis of 1 generated three distinct conformers having different types of hydrophobic interactions, which will be utilized for exploring the active conformation of the VR1 ligand.

Analysis of structure–activity relationships for the ‘B-region’ of N -(3-acyloxy-2-benzylpropyl)- N ′ -[4-(methylsulfonylamino)benzyl]thiourea analogues as vanilloid receptor antagonists: discovery of an N -hydroxythiourea analogue with potent analgesic activity

The structural modifications on the B-region of the potent and high affinity vanilloid receptor (VR1) lead ligand N-(3-acyloxy-2-benzylpropyl)-N(')-[4-(methylsulfonylamino)benzyl]thiourea were investigated by the replacement of the thiourea with diverse isosteric functional groups. Structure-activity analysis indicated that the A-region in this series was the primary factor in determining the agonistic/antagonistic activities regardless of the B-region. The N(C)-hydroxy thiourea analogues (12, 13) showed excellent analgesic activities in the acetic acid writhing assay compared to the parent thiourea analogues.

N-4-Methansulfonamidobenzyl-N′-2-substituted-4-tert-butyl-benzyl thioureas as potent vanilloid receptor antagonistic ligands

A series of N-4-methansulfonamidobenzyl-N'-2-substituted-4-tert-butylbenzyl thioureas were prepared for the study of their agonistic/antagonistic activities to the vanilloid receptor in rat DRG neurons. Their structure-activity relationship reveals that there is a space for another hydrophobic binding interaction around 2-position in 4-tert-butylbenzyl region. Among the prepared derivatives, 6n show the highest antagonistic activity against the vanilloid receptor (IC(50)=15 nM).

Chain-branched 1,3-dibenzylthioureas as vanilloid receptor 1 antagonists

A series of chain-branched 1,3-dibenzylthiourea derivatives were synthesized, and tested their antagonist activity against vanilloid receptor 1. Chain-branching led to a significant change in the mode of action and the potency. (R)-Methyl or ethyl-branched 1,3-dibenzylthiourea derivatives showed the most potent antagonist activity up to the IC(50) value of 0.05 microM which is 10-fold more potent than capsazepine.

Activation and sensitisation of the vanilloid receptor: role in gastrointestinal inflammation and function

The exquisite specific excitatory and desensitising actions of capsaicin on a subpopulation of primary sensory neurons have been instrumental in identifying the roles of these neurons in nociception, reflex responses and neurogenic inflammation. Structure activity studies with capsaicin-like molecules have suggested that a "receptor" should mediate the effects of capsaicin on sensory neurons. The cloning of the vanilloid receptor-1 (VR1) has confirmed this hypothesis. VR1 (TRPV1) belongs to the transient receptor potential (TRP) family of channels, and its activation by various xenobiotics, noxious temperature, extracellular low pH and high concentration of certain lipid derivatives results in cation influx and sensory nerve terminal excitation. TRPV1 may dimerise or form tetramers or heteromers with PLC-gamma and TrkA or even with other TRPs. TRPV1 is markedly upregulated and/or "sensitised" under inflammatory conditions via protein kinase C-epsilon-, cAMP-dependent PK- and PLC-gamma-dependent pathways or by exposure to dietary agents as ethanol. TRPV1 is expressed on sensory neurons distributed in all the regions of the gastrointestinal tract in myenteric ganglia, muscle layer and mucosa. There is evidence of TRPV1 expression also in epithelial cells of the gastrointestinal tract. High expression of TRPV1 has been detected in several inflammatory diseases of the colon and ileum, whereas neuropeptides released upon sensory nerve stimulation triggered by TRPV1 activation seem to play a role in intestinal motility disorders. TRPV1 antagonists, which will soon be available for clinical testing, may undergo scrutiny for the treatment of inflammatory diseases of the gut.

N-4-Substituted-benzyl-N′-tert-butylbenzyl thioureas as vanilloid receptor ligands: investigation on the role of methanesulfonamido group in antagonistic activity

A series of N-4-substituted-benzyl-N'-tert-butylbenzyl thioureas were prepared for the study of their agonistic/antagonistic activities to the vanilloid receptor in rat DRG neurons. Their structure-activity relationship reveals that not only the two oxygens and amide hydrogen of sulfonamido group, but also the optimal size of methyl in methanesulfonamido group play an integral role for the antagonistic activity on vanilloid receptor.

N -[4-(Methylsulfonylamino)benzyl]thiourea analogues as vanilloid receptor antagonists: analysis of structure–activity relationships for the ‘C-Region’

We recently reported that N-(4-t-butylbenzyl)-N'-[4-(methylsulfonylamino)benzyl] thiourea (2) was a high affinity antagonist of the vanilloid receptor with a binding affinity of K(i)=63 nM and an antagonism of K(i)=53.9 nM in rat VR1 heterologously expressed in Chinese hamster ovary (CHO) cells (Mol. Pharmacol. 2002, 62, 947-956). In an effort to further improve binding affinity and antagonistic potency, we have modified the C-region of the lead 4-t-butylbenzyl group with diverse surrogates, such as araalkyl, alkyl, 4-alkynylbenzyl, indanyl, 3,3-diarylpropyl, 4-alkoxybenzyl, 4-substituted piperazine and piperidine. The lipophilic surrogates, arylalkyl and alkyl, conferred modest decreases in binding affinities and antagonistic potencies; the groups having heteroatoms resulted in dramatic decreases. Our findings indicate that 4-t-butylbenzyl is one of the most favorable groups for high receptor binding and potent antagonism to VR1 in this structural series.

7-Hydroxynaphthalen-1-yl-urea and -amide antagonists of human vanilloid receptor 1

A series of structurally simple 7-hydroxynaphthalenyl ureas and amides were discovered to be potent ligands of human vanilloid receptor 1 (VR1). 1-(7-Hydroxynaphthalen-1-yl)-3-(4-trifluoromethylbenzyl)urea 5f exhibited nanomolar binding affinity (K(i)=1.0nM) and upon capsaicin challenge, behaved as a potent functional antagonist (IC(50)=4nM). The synthesis and structure-activity relationships (SARs) for the series are described.