High-affinity partial agonists of the vanilloid receptor

Acute Effect of Exposure to Extreme Heat (100 ± 3 °C) on Lower Limb Maximal Resistance Strength

The aim of this study was to evaluate the acute effect of a single dry sauna bath lasting twelve minutes on the indirect determination of the one maximum repetition (1RM) leg press among trained and untrained participants. Thirty young men participated in the study, a trained group (TG; n = 15; age: 20.97 ± 0.44 years) and an untrained group (UG; n = 15; age: 21.03 ± 0.11 years). Subjects in the TG had performed resistance training for at least two years before the beginning of the experiment. All participants performed two indirect tests of their one maximum repetition leg press on two different days, with a rest period of three weeks between tests. Additionally, anthropometric, body composition, blood pressure, body temperature, and rated perceived exertion were evaluated. On the second testing day, all of the participants took a dry sauna bath lasting 12 min immediately before performing the leg press test. In the second evaluation (pre-heating in the sauna), the UG experienced increases in absolute RM (178.48 ± 56.66 to 217.60 ± 59.18 kg; p < 0.05; R = 0.798), relative RM (2.65 ± 0.61 to 3.24 ± 0.58 kg·g body mass^-1; p < 0.05; R = 0.798), and muscular RM (5.64 ± 1.20 to 6.77 ± 1.14 kg·kg muscle mass^-1; p < 0.05; R = 0.797). The TG also increased their values on the second day in absolute RM (284.96 ± 62.41 to 314.92 ± 1.04 kg; p < 0.01; R = 0.886), in relative RM (3.61 ± 0.88 to 3.99 ± 1.85 kg*kg body mass^-1; p < 0.01; R = 0.886), and muscular RM (7.83 ± 1.69 to 8.69 ± 1.85 kg·kg muscle mass^-1; p < 0.01; R = 0.854). A passive, extreme-heat sauna bath lasting 12 min taken immediately before a relative maximum repetition test seems to provoke clear positive responses for the development of strength.

Combination of a Rapidly Penetrating Agonist and a Slowly Penetrating Antagonist Affords Agonist Action of Limited Duration at the Cellular Level

The capsaicin receptor TRPV1 (transient receptor potential vanilloid 1) has been an object of intense interest for pharmacological development on account of its critical role in nociception. In the course of structure activity analysis, it has become apparent that TRPV1 ligands may vary dramatically in the rates at which they interact with TRPV1, presumably reflecting differences in their abilities to penetrate into the cell. Using a fast penetrating agonist together with an excess of a slower penetrating antagonist, we find that we can induce an agonist response of limited duration and, moreover, the duration of the agonist response remains largely independent of the absolute dose of agonist, as long as the ratio of antagonist to agonist is held constant. This general approach for limiting agonist duration under conditions in which absolute agonist dose is variable should have more general applicability.

2-Sulfonamidopyridine C-region analogs of 2-(3-fluoro-4-methylsulfonamidophenyl)propanamides as potent TRPV1 antagonists

A series of 2-sulfonamidopyridine C-region derivatives of 2-(3-fluoro-4-methylsulfonamidophenyl)propanamide were investigated as hTRPV1 ligands. Systematic modification on the 2-sulfonamido group provided highly potent TRPV1 antagonists. The N-benzyl phenylsulfonamide derivatives 12 and 23 in particular showed higher affinities than that of lead compound 1. Compound 12 exhibited strong analgesic activity in the formalin pain model. Docking analysis of its chiral S-form 12S in our hTRPV1 homology model indicated that its high affinity might arise from additional hydrophobic interactions not present in lead compound 1S.

Structure activity relationships of benzyl C-region analogs of 2-(3-fluoro-4-methylsulfonamidophenyl)propanamides as potent TRPV1 antagonists

A series of 2-substituted 4-(trifluoromethyl)benzyl C-region analogs of 2-(3-fluoro-4-methylsulfonamidophenyl)propanamides were investigated for hTRPV1 antagonism. The analysis indicated that the phenyl C-region derivatives exhibited better antagonism than those of the corresponding pyridine surrogates for most of the series examined. Among the phenyl C-region derivatives, the two best compounds 43 and 44S antagonized capsaicin selectively relative to their antagonism of other activators and showed excellent potencies with K(i(CAP))=0.3 nM. These two compounds blocked capsaicin-induced hypothermia, consistent with TRPV1 as their site of action, and they demonstrated promising analgesic activities in a neuropathic pain model without hyperthermia. The docking study of 44S in our hTRPV1 homology model indicated that its binding mode was similar with that of its pyridine surrogate in the A- and B-regions but displayed a flipped configuration in the C-region.

α-Substituted 2-(3-fluoro-4-methylsulfonamidophenyl)acetamides as potent TRPV1 antagonists

A series of α-substituted acetamide derivatives of previously reported 2-(3-fluoro-4-methylsulfonamidophenyl)propanamide leads (1, 2) were investigated for antagonism of hTRPV1 activation by capsaicin. Compound 34, which possesses an α-m-tolyl substituent, showed highly potent and selective antagonism of capsaicin with Ki(CAP)=0.1 nM. It thus reflected a 3-fold improvement in potency over parent 1. Docking analysis using our homology model indicated that the high potency of 34 might be attributed to a specific hydrophobic interaction of the m-tolyl group with the receptor.

Performance in the heat-physiological factors of importance for hyperthermia-induced fatigue

This article presents a historical overview and an up-to-date review of hyperthermia-induced fatigue during exercise in the heat. Exercise in the heat is associated with a thermoregulatory burden which mediates cardiovascular challenges and influence the cerebral function, increase the pulmonary ventilation, and alter muscle metabolism; which all potentially may contribute to fatigue and impair the ability to sustain power output during aerobic exercise. For maximal intensity exercise, the performance impairment is clearly influenced by cardiovascular limitations to simultaneously support thermoregulation and oxygen delivery to the active skeletal muscle. In contrast, during submaximal intensity exercise at a fixed intensity, muscle blood flow and oxygen consumption remain unchanged and the potential influence from cardiovascular stressing and/or high skin temperature is not related to decreased oxygen delivery to the skeletal muscles. Regardless, performance is markedly deteriorated and exercise-induced hyperthermia is associated with central fatigue as indicated by impaired ability to sustain maximal muscle activation during sustained contractions. The central fatigue appears to be influenced by neurotransmitter activity of the dopaminergic system, but inhibitory signals from thermoreceptors arising secondary to the elevated core, muscle and skin temperatures and augmented afferent feedback from the increased ventilation and the cardiovascular stressing (perhaps baroreceptor sensing of blood pressure stability) and metabolic alterations within the skeletal muscles are likely all factors of importance for afferent feedback to mediate hyperthermia-induced fatigue during submaximal intensity exercise. Taking all the potential factors into account, we propose an integrative model that may help understanding the interplay among factors, but also acknowledging that the influence from a given factor depends on the exercise hyperthermia situation.

Altered pharmacology of native rodent spinal cord TRPV1 after phosphorylation

BACKGROUND AND PURPOSE

Evidence suggests that phosphorylation of TRPV1 is an important component underlying its aberrant activation in pathological pain states. To date, the detailed pharmacology of diverse TRPV1 receptor agonists and antagonists has yet to be reported for native TRPV1 under phosphorylating conditions. Our goal was to optimize a relatively high-throughput methodology to allow pharmacological characterization of the native TRPV1 receptor using a spinal cord neuropeptide release assay under naive and phosphorylating states.

EXPERIMENTAL APPROACH

Herein, we describe characterization of rodent TRPV1 by measurement of CGRP release from acutely isolated lumbar (L1-L6) spinal cord using a 96-well technique that combines use of native, adult tissue with quantitation of CGRP release by ELISA.

KEY RESULTS

We have studied a diverse panel of TRPV1 agonists and antagonists under basal and phosphorylating conditions. We show that TRPV1-mediated CGRP release is evoked, in a temperature-dependent manner, by a PKC activator, phorbol 12,13-dibutyrate (PDBu); and that treatment with PDBu increases the potency and efficacy of known TRPV1 chemical agonists, in an agonist-specific manner. We also show that the pharmacological profile of diverse TRPV1 antagonists is dependent on whether the stimulus is PDBu or capsaicin. Of note, HPPB was identified as an antagonist of capsaicin-evoked, but a potentiator of PDBu-evoked, CGRP release.

CONCLUSIONS AND IMPLICATIONS

Our findings indicate that both TRPV1 agonist and antagonist profiles can be differentially altered by PKC activation. These findings may offer new insights for targeting TRPV1 in pain states.

Exaggerated Pressor Response in Relation to Attenuated Muscle Temperature Response during Contraction in Ischemic Heart Failure

It is known that muscle temperature (T_m) increases with exercise. The purpose of this study was to examine if contraction-induced increase in T_m was altered in rats with heart failure (HF) induced by chronic myocardial infraction (MI) as compared with healthy control animals. A temperature probe was inserted in the triceps surae muscle to continuously measure T_m throughout experiments. Static muscle contraction was induced by electrical stimulation of the sciatic nerve for 1 min. As baseline T_m was 34°C, contraction increased temperature by 1.6 ± 0.18°C in nine health control rats and by 1.0 ± 0.15°C in 10 MI rats (P < 0.05 vs. control). Note that there were no differences in developed muscle tension and muscle weight between the two groups. In addition, muscle contraction increased mean arterial pressure by 23 ± 3 mmHg in control rats and by 31 ± 3 mmHg in MI rats (P < 0.05 vs. control). A regression analysis further shows that there is an inverse liner relationship between the pressor response and static contraction-induced increase in T_m. Our data suggest that T_m increase evoked by contraction is impaired in MI rats. The abnormal alteration in T_m likely modifies the reflex cardiovascular responses in MI via mechanisms of temperature-sensitive receptors on muscle afferent nerves.

Structure-activity relationships of vanilloid receptor agonists for arteriolar TRPV1

BACKGROUND AND PURPOSE

The transient receptor potential vanilloid 1 (TRPV1) plays a role in the activation of sensory neurons by various painful stimuli and is a therapeutic target. However, functional TRPV1 that affect microvascular diameter are also expressed in peripheral arteries and we attempted to characterize this receptor.

EXPERIMENTAL APPROACH

Sensory TRPV1 activation was measured in rats by use of an eye wiping assay. Arteriolar TRPV1-mediated smooth muscle specific responses (arteriolar diameter, changes in intracellular Ca(2+)) were determined in isolated, pressurized skeletal muscle arterioles obtained from the rat and wild-type or TRPV1(-/-) mice and in canine isolated smooth muscle cells. The vascular pharmacology of the TRPV1 agonists (potency, efficacy, kinetics of action and receptor desensitization) was determined in rat isolated skeletal muscle arteries.

KEY RESULTS

Capsaicin evoked a constrictor response in isolated arteries similar to that mediated by noradrenaline, this was absent in arteries from TRPV1 knockout mice and competitively inhibited by TRPV1 antagonist AMG9810. Capsaicin increased intracellular Ca(2+) in the arteriolar wall and in isolated smooth muscle cells. The TRPV1 agonists evoked similar vascular constrictions (MSK-195 and JYL-79) or were without effect (resiniferatoxin and JYL-273), although all increased the number of responses (sensory activation) in the eye wiping assay. Maximal doses of all agonists induced complete desensitization (tachyphylaxis) of arteriolar TRPV1 (with the exception of capsaicin). Responses to the partial agonist JYL-1511 suggested 10% TRPV1 activation is sufficient to evoke vascular tachyphylaxis without sensory activation.

CONCLUSIONS AND IMPLICATIONS

Arteriolar TRPV1 have different pharmacological properties from those located on sensory neurons in the rat.

2-(4-Methylsulfonylaminophenyl) propanamide TRPV1 antagonists: Structure-activity relationships in the B and C-regions

On the basis of the previous lead N-4-t-butylbenzyl 2-(3-fluoro-4-methylsulfonylaminophenyl) propanamide (3) as a potent TRPV1 antagonist, structure-activity relationships for the B (propanamide part) and C-region (4-t-butylbenzyl part) have been investigated for rTRPV1 in CHO cells. The B-region was modified with dimethyl, cyclopropyl and reverse amides and then the C-region was replaced with 4-substituted phenyl, aryl alkyl and diaryl alkyl derivatives. Among them, compound 50 showed high binding affinity with K(i)=21.5nM, which was twofold more potent than 3 and compound 54 exhibited potent antagonism with K(i(ant))=8.0nM comparable to 3.

N-4-t-Butylbenzyl 2-(4-methylsulfonylaminophenyl) propanamide TRPV1 antagonists: Structure-activity relationships in the A-region

Structure-activity relationships for the A-region in a series of N-4-t-butylbenzyl 2-(4-methylsulfonylaminophenyl) propanamides as TRPV1 antagonists have been investigated. Among them, the 3-fluoro analogue 54 showed high binding affinity and potent antagonism for both rTRPV1 and hTRPV1 in CHO cells. Its stereospecific activity was demonstrated with marked selectivity for the (S)-configuration (54S versus 54R). A docking study of 54S with our hTRPV1 homology model highlighted crucial hydrogen bonds between the ligand and the receptor contributing to its potency.

TRPV1 activation is not an all-or-none event: TRPV1 partial agonism/antagonism and its regulatory modulation

TRPV1 has emerged as a promising therapeutic target for pain as well as a broad range of other conditions such as asthma or urge incontinence. The identification of resiniferatoxin as an ultrapotent ligand partially able to dissect the acute activation of TRPV1 from subsequent desensitization and the subsequent intense efforts in medicinal chemistry have revealed that TRPV1 affords a dramatic landscape of opportunities for pharmacological manipulation. While agonism and antagonism have represented the primary directions for drug development, the pharmacological complexity of TRPV1 affords additional opportunities. Partial agonism/partial antagonism, its modulation by signaling pathways, variable desensitization, and slow kinetics of action can all be exploited through drug design.

Conformationally constrained analogues of N'-(4-tert-butylbenzyl)-N-(4-methylsulfonylaminobenzyl)thiourea as TRPV1 antagonists

A series of bicyclic analogues having indan and tetrahydronaphthalene templates in the A-region were designed as conformationally constrained analogues of our previously reported potent TRPV1 antagonists (1, 3). The activities for rat TRPV1 of the conformationally restricted analogues were moderately or markedly diminished, particularly in the case of the tetrahydronaphthalene analogues. The analysis indicated that steric constraints at the benzylic position in the bicyclic analogues may be an important factor for their unfavorable interaction with the receptor.

The functional regulation of TRPV1 and its role in pain sensitization

Transient receptor potential V1 (TRPV1) is specifically expressed in the nociceptive receptors and can detect a variety of noxious stimuli, thus potentiating pain sensitization. While peripheral delivery of capsaicin causes the desensitization of sensory neurons, thus alleviating pain. Therefore capsaicin is used in the clinical treatment of various types of pain; however, these treatments will bring many side effects, such as a strong burning pain in the early stages of treatment which hampers the further use of capsaicin. Thus, the studies of the functional regulation of TRPV1 are mainly focused on two aspects: to develop more potent analogues of capsaicin with less side effects; or to elucidate the mechanisms of TRPV1 in pain sensitivity, especially of that TRPV1 as a target of various protein kinases such as PKD1 and Cdk5 is involved pain hypersensitivity. Thus we would summarize the progress of these two aspects in this mini review.

Differential modulation of agonist and antagonist structure activity relations for rat TRPV1 by cyclosporin A and other protein phosphatase inhibitors

The transient receptor potential V1 channel (vanilloid receptor, TRPV1) represents a promising therapeutic target for inflammatory pain and other conditions involving C-fiber sensory afferent neurons. Sensitivity of TRPV1 is known to be subject to modulation by numerous signaling pathways, in particular by phosphorylation, and we wished to determine whether TRPV1 structure activity relations could be differentially affected. We demonstrate here that the structure activity relations of TRPV1, as determined by (45)Ca(2) uptake, were substantially altered by treatment of the cells with cyclosporin A, an inhibitor of protein phosphatase 2B. Whereas the potency of resiniferatoxin for stimulation of (45)Ca(2) was not altered by cyclosporin A treatment, the potencies of some other agonists were increased up to 8-fold. Among the antagonists examined, potencies were reduced to a lesser extent, ranging from 1- to 2.5-fold. Finally, the efficacy of partial agonists was increased. In contrast to cyclosporin A, okadaic acid, an inhibitor of protein phosphatases 1 and 2A, had little effect on agonist potencies, and calyculin A, an inhibitor of protein phosphatases 1 and 2A but with somewhat different selectivity from that of okadaic acid, caused changes in structure activity relations distinct from those induced by cyclosporin A. Because phosphatase activity differentially modulates the structure activity relations of TRPV1 agonists and antagonists, our findings predict that it may be possible to design agonists and antagonists selective for TRPV1 in a specific regulatory environment. A further implication is that it may be desirable to tailor screening approaches for drug discovery to reflect the desired regulatory state of the targeted TRPV1.

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).

The vanilloid receptor TRPV1: 10 years from channel cloning to antagonist proof-of-concept

The clinical use of TRPV1 (transient receptor potential vanilloid subfamily, member 1; also known as VR1) antagonists is based on the concept that endogenous agonists acting on TRPV1 might provide a major contribution to certain pain conditions. Indeed, a number of small-molecule TRPV1 antagonists are already undergoing Phase I/II clinical trials for the indications of chronic inflammatory pain and migraine. Moreover, animal models suggest a therapeutic value for TRPV1 antagonists in the treatment of other types of pain, including pain from cancer. We argue that TRPV1 antagonists alone or in conjunction with other analgesics will improve the quality of life of people with migraine, chronic intractable pain secondary to cancer, AIDS or diabetes. Moreover, emerging data indicate that TRPV1 antagonists could also be useful in treating disorders other than pain, such as urinary urge incontinence, chronic cough and irritable bowel syndrome. The lack of effective drugs for treating many of these conditions highlights the need for further investigation into the therapeutic potential of TRPV1 antagonists.

The vanilloid receptor TRPV1 is tonically activated in vivo and involved in body temperature regulation

The vanilloid receptor TRPV1 (transient receptor potential vanilloid 1) is a cation channel that serves as a polymodal detector of pain-producing stimuli such as capsaicin, protons (pH <5.7), and heat. TRPV1 antagonists block pain behaviors in rodent models of inflammatory, neuropathic, and cancer pain, suggesting their utility as analgesics. Here, we report that TRPV1 antagonists representing various chemotypes cause an increase in body temperature (hyperthermia), identifying a potential issue for their clinical development. Peripheral restriction of antagonists did not eliminate hyperthermia, suggesting that the site of action is predominantly outside of the blood-brain barrier. Antagonists that are ineffective against proton activation also caused hyperthermia, indicating that blocking capsaicin and heat activation of TRPV1 is sufficient to produce hyperthermia. All TRPV1 antagonists evaluated here caused hyperthermia, suggesting that TRPV1 is tonically activated in vivo and that TRPV1 antagonism and hyperthermia are not separable. TRPV1 antagonists caused hyperthermia in multiple species (rats, dogs, and monkeys), demonstrating that TRPV1 function in thermoregulation is conserved from rodents to primates. Together, these results indicate that tonic TRPV1 activation regulates body temperature.

TRPV1: a therapeutic target for novel analgesic drugs?

The vanilloid receptor TRPV1 is now recognized as a molecular integrator of painful stimuli ranging from noxious heat to endovanilloids in inflammation. Pharmacological blockade of TRPV1 represents a new strategy in pain relief. TRPV1 antagonists are expected to prevent pain by silencing receptors where pain is generated rather than stopping the propagation of pain, as most-traditional pain killers do. This hypothesis has already being tested in the clinic by administering small molecule TRPV1 antagonists (e.g. GlaxoSmithKline SB-705498) for migraine and dental pain. Paradoxically, in some murine models of chronic pain, TRPV1-deficient mice exhibit more pain-related behavior than their wild-type littermates, indicating that the understanding of TRPV1 in pain is still incomplete. Moreover, there is mounting evidence to suggest the existence of functional TRPV1 both in the brain and in various non-neuronal tissues. The biological role of these receptors remains elusive, but their tissue distribution clearly indicates that they are involved in many more functions than just pain perception. Here, we review the potential therapeutic indications and adverse effects of TRPV1 antagonists.

Kinetics of penetration influence the apparent potency of vanilloids on TRPV1

Evidence that the ligand binding site of TRPV1 lies on the inner face of the plasma membrane and that much of the TRPV1 itself is localized to internal membranes suggests that the rate of ligand entry into the cell may be an important determinant of the kinetics of ligand action. In this study, we synthesized a BODIPY TR-labeled fluorescent capsaicin analog (CHK-884) so that we could directly measure ligand entry. We report that CHK-884 penetrated only slowly into Chinese hamster ovary (CHO) cells expressing rat TRPV1, with a t1/2 of 30 +/- 4 min, and localized in the endoplasmic reticulum and Golgi. Although CHK-884 was only weakly potent for TRPV1 binding (Ki = 6400 +/- 230 nM), it was appreciably more potent when assayed by intracellular calcium imaging and was 3.2-fold more potent with a 1-h incubation time (37 nM) than with a 5-min incubation time. Olvanil, a highly lipophilic vanilloid, yielded an EC50 of 4.3 nM upon intracellular calcium imaging with an incubation time of 1 h, compared with an EC50 value of 29.5 nM for calcium imaging assayed at 5 min. Likewise, the antagonist 5-iodo-resiniferatoxin (5-iodo-RTX) displayed a Ki of 4.2 pM if incubated with CHO-TRPV1 cells for 2 h before addition of capsaicin compared with 1.5 nM if added simultaneously. We conclude that some vanilloids may have slow kinetics of uptake; this slow uptake may affect assessment of structure activity relations and may represent a significant factor for vanilloid drug design.

Phosphorylation-dependent desensitization by anandamide of vanilloid receptor-1 (TRPV1) function in rat skeletal muscle arterioles and in Chinese hamster ovary cells expressing TRPV1

It has been proposed that activation of vanilloid receptor-1 (TRPV1) affects the vasotone of resistance arteries. One of the endogenous activators of TRPV1 is anandamide. The effects of anandamide on TRPV1 responsiveness were tested on isolated, pressurized (80 mm Hg) skeletal muscle (m. gracilis) arterioles (179 +/- 33 microm in diameter). We found that the TRPV1 agonist capsaicin (1 microM) elicited a substantial constriction in isolated arterioles (51 +/- 12%). In contrast, anandamide (0-100 microM) did not affect arteriolar diameter significantly (3 +/- 5%). Isolated vessels were also preincubated with anandamide (30 microM for 20 min). This anandamide pretreatment completely blocked capsaicin-induced arteriolar constriction (response decreased to 1 +/- 0.6%), and this inhibition was reversed by a protein phosphatase-2B inhibitor (cyclosporin-A; 100 nM, 5 min) treatment (constriction, 31 +/- 1%). An exogenous TRPV1-expressing cell line [Chinese hamster ovary (CHO)-TRPV1] was used to specifically evaluate TRPV1-mediated effects of anandamide. The efficacy of anandamide in this system, as determined by 45Ca2+ uptake, was 65 +/- 8% of that of capsaicin. Upon treatment of the cells with cyclosporin-A or the protein kinase C activator phorbol 12-myristate 13-acetate (PMA), anandamide was transformed to a full agonist. Anandamide treatment caused an acute desensitization in these cells as measured by intracellular Ca2+ imaging. Application of cyclosporin-A or PMA reversed this desensitization. Our data suggest that anandamide may cause a complete (albeit phosphorylation-dependent) desensitization of TRPV1 in skeletal muscle arterioles and in CHO-TRPV1 cells, which apparently transforms the ligand-gated TRPV1 into a phosphorylation-gated channel. This property of anandamide may provide a new therapeutic strategy to manipulate TRPV1 activity.

New Treatments for Neuropathic Pain

Existing treatments for neuropathic pain deliver inadequate pain relief, unacceptable side effects, or both. The unmet medical need for more effective treatment is driving a large volume of research to discover new drugs. Most existing treatments are drugs introduced to treat other pain conditions or other medical conditions, such as antidepressants and anticonvulsants, which were found empirically to be effective for neuropathic pain. Only recently have drug discovery efforts have become mechanistically driven, addressing targets identified by a molecular neurobiological approach to the pathophysiology of neuropathic states.

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.

5-Iodoresiniferatoxin evokes hypothermia in mice and is a partial transient receptor potential vanilloid 1 agonist in vitro

Transient receptor potential vanilloid 1 (TRPV1) is a capsaicin- and heat-gated ion channel required for normal in vivo responses to these painful stimuli. However, growing evidence suggests that TRPV1 also participates in thermoregulation. Therefore, we examined the effects of a selective TRPV1 antagonist, 5-iodoresiniferatoxin (I-RTX), on mouse body temperature. Surprisingly, s.c. administration of I-RTX (0.1-1 micromol/kg) evoked a hypothermic response similar to that evoked by capsaicin (9.8 micromol/kg) in naive wild-type mice, but not in mice pretreated with resiniferatoxin, a potent TRPV1 agonist, or in naive TRPV1-null mice. In response to I-RTX in vitro, HEK293 cells expressing rat TRPV1 exhibited increases in intracellular Ca(2+) (biphasic, EC(50) = 56.7 nM and 9.9 microM) that depended on Ca(2+) influx and outwardly rectifying, capsazepine-sensitive currents that were smaller than those evoked by 1 microM capsaicin. Thus, I-RTX induces TRPV1-dependent hypothermia in vivo and is a partial TRPV1 agonist in vitro.

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.

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.

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.

Design of a high-affinity competitive antagonist of the vanilloid receptor selective for the calcium entry-linked receptor population

We describe the synthesis and characterization of N-(4-chlorobenzyl) -N'-(4-hydroxy-3-iodo-5-methoxybenzyl)thiourea (IBTU), a novel antagonist of the vanilloid receptor 1 (TRPV1 or VR1). IBTU competitively inhibited 45Ca2+ uptake into CHO cells heterologously expressing rat TRPV1, whether induced by capsaicin or resiniferatoxin (Ki = 99 +/- 23 and 93 +/- 34 nM, respectively). IBTU was thus somewhat more potent (5-fold) than capsazepine. In contrast to its antagonism of vanilloid-induced calcium uptake, IBTU (30 microM) inhibited [3H]resiniferatoxin binding to TRPV1 by less than 10%. We hypothesize that these dramatically distinct potencies reflect different fractions of TRPV1 in this system: namely, a minor plasma membrane fraction controlling 45Ca2+ uptake, and the predominant intracellular fraction that dominates the [3H]resiniferatoxin binding measurements. Intracellular Ca2+ imaging supports this explanation. IBTU antagonized the elevation in intracellular Ca2+ in response to 50 nM capsaicin with an IC50 of 106 +/- 35 nM. Likewise, 600 nM IBTU was able to antagonize the elevation in intracellular Ca2+ in response to 100 pM resiniferatoxin in the presence of normal (1.8 mM) extracellular Ca2+, where the increase in intracellular calcium reflects calcium influx. In contrast, in the absence of extracellular Ca2+, where in this system resiniferatoxin induces a modest increase in calcium from intracellular stores, IBTU was unable to block the response to resiniferatoxin, although the TRPV1 antagonist 5-iodoresiniferatoxin was able to do so. In summary, IBTU is a novel, potent TRPV1 antagonist with marked selectivity between subpopulations of TRPV1 and may permit the function of these distinct pools to be explored and potentially exploited.

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.