Synthesis and evaluation of phorboid 20-homovanillates: discovery of a class of ligands binding to the vanilloid (capsaicin) receptor with different degrees of cooperativity

Phorbol Diesters and 12-Deoxy-16-hydroxyphorbol 13,16-Diesters Induce TGFα Release and Adult Mouse Neurogenesis

A small library of phorbol 12,13-diesters bearing low lipophilicity ester chains was prepared as potential neurogenic agents in the adult brain. They were also used in a targeted UHPLC-HRMS screening of the latex of Euphorbia resinifera. Two new 12-deoxy-16-hydroxyphorbol 13,16-diesters were isolated, and their structures were deduced using two-dimensional NMR spectroscopy and NOE experiments. The ability of natural and synthetic compounds to stimulate transforming growth factor alpha (TFGα) release, to increase neural progenitor cell proliferation, and to stimulate neurogenesis was evaluated. All compounds that facilitated TGFα release promoted neural progenitor cell proliferation. The presence of two acyloxy moieties on the tigliane skeleton led to higher levels of activity, which decreased when a free hydroxyl group was at C-12. Remarkably, the compound bearing isobutyryloxy groups was the most potent on the TGFα assay and at inducing neural progenitor cell proliferation in vitro, also leading to enhanced neurogenesis in vivo when administered intranasally to mice.

Biological Evaluation of Natural and Synthesized Homovanillic Acid Esters as Inhibitors of Intestinal Fatty Acid Uptake in Differentiated Caco-2 Cells

With raising prevalence of obesity, the regulation of human body fat is increasingly relevant. The modulation of fatty acid uptake by enterocytes represents a promising target for body weight maintenance. Recent results demonstrated that the trigeminal active compounds capsaicin, nonivamide, and trans-pellitorine dose-dependently reduce fatty acid uptake in differentiated Caco-2 cells as a model for the intestinal barrier. However, non-pungent alternatives have not been investigated and structural determinants for the modulation of intestinal fatty acid uptake have not been identified so far. Thus, based on the previous results, we synthesized 23 homovanillic acid esters in addition to the naturally occurring capsiate and screened them for their potential to reduce intestinal fatty acid uptake using the fluorescent fatty acid analog Bodipy-C12 in differentiated Caco‑2 cells as an enterocyte model. Whereas pre-incubation with 100 µM capsiate did not change fatty acid uptake by Caco-2 enterocytes, a maximum inhibition of −47% was reached using 100 µM 1‑methylpentyl-2-(4-hydroxy-3-methoxy-phenyl)acetate. Structural analysis of the 24 structural analogues tested in the present study revealed that a branched fatty acid side chain, independent of the chain length, is one of the most important structural motifs associated with inhibition of fatty acid uptake in Caco-2 enterocytes. The results of the present study may serve as an important basis for designing potent dietary inhibitors of fatty acid uptake.

Antiviral Activity of Diterpene Esters on Chikungunya Virus and HIV Replication

Recently, new daphnane, tigliane, and jatrophane diterpenoids have been isolated from various Euphorbiaceae species, of which some have been shown to be potent inhibitors of chikungunya virus (CHIKV) replication. To further explore this type of compound, the antiviral activity of a series of 29 commercially available natural diterpenoids was evaluated. Phorbol-12,13-didecanoate (11) proved to be the most potent inhibitor, with an EC50 value of 6.0 ± 0.9 nM and a selectivity index (SI) of 686, which is in line with the previously reported anti-CHIKV potency for the structurally related 12-O-tetradecanoylphorbol-13-acetate (13). Most of the other compounds exhibited low to moderate activity, including an ingenane-type diterpene ester, compound 28, with an EC50 value of 1.2 ± 0.1 μM and SI = 6.4. Diterpene compounds are known also to inhibit HIV replication, so the antiviral activities of compounds 1-29 were evaluated also against HIV-1 and HIV-2. Tigliane- (4β-hydroxyphorbol analogues 10, 11, 13, 15, 16, and 18) and ingenane-type (27 and 28) diterpene esters were shown to inhibit HIV replication in vitro at the nanomolar level. A Pearson analysis performed with the anti-CHIKV and anti-HIV data sets demonstrated a linear relationship, which supported the hypothesis made that PKC may be an important target in CHIKV replication.

The carbonate analogues of 5'-halogenated resiniferatoxin as TRPV1 ligands

A series of carbonate analogues of 5'-halogenated RTX have been investigated in order to examine the effect of the carbonate group as a linker and the role of halogens in the reversal of activity from agonism to antagonism for rat and human TRPV1 heterologously expressed in Chinese hamster ovary cells. The carbonate analogues showed similar activities to the corresponding RTX derivatives in rat TRPV1 but lower potency in human TRPV1. 5-Halogenation converted the agonists to partial agonists or full antagonists and the extent of antagonism reflected the order of I>Br>Cl>F, with a somewhat greater extent of antagonism for the derivatives of the 4-amino RTX surrogates compared to the corresponding derivatives of RTX itself. The carbonate analogues of I-RTX (60) and 5-bromo-4-amino-RTX (66) were potent and full antagonists with Ki(ant)=2.23 and 2.46 nM, respectively, for rat TRPV1, which were ca. 5-fold more potent than I-RTX (2) under our conditions. The conformational analysis of the I-RTX-carbonate (60) indicated that its bent conformation was similar to that of I-RTX, consistent with compound 60 and I-RTX showing comparable potent antagonism.

Dissecting the pharmacophore of curcumin. Which structural element is critical for which action?

The dietary phenolic curcumin (1a) is the archetypal network pharmacological agent, but is characterized by an ill-defined pharmacophore. Nevertheless, structure-activity studies of 1a have mainly focused on a single biological end-point and on a single structural element, the aliphatic bis-enoyl moiety. The comparative investigation of more than one end-point of curcumin and the modification of its aromatic region have been largely overlooked. To address these issues, we have investigated the effect of aromatic C-prenylation in the three archetypal structural types of curcuminoids, namely, curcumin itself (1a), its truncated analogue 2a (C5-curcumin), and (as the reduced isoamyl version) the tetrahydro derivative 3a, comparatively evaluating reactivity with thiols and activity in biochemical (inhibition of NF-κB, HIV-1-Tat transactivation, Nrf2 activation) and phenotypic (anti-HIV action) assays sensitive, to a various extent, to thia-Michael addition. Prenylation, a validated maneuver for bioactivity modulation in plant phenolics, had no effect on Michael reactivity, but was detrimental for all biological end-points investigated, dissecting thiol trapping from activity, while hydrogenation attenuated, but did not completely abrogate, the activity of 1a. The C5-curcuminoid 2a outperformed the natural product in all end-points investigated and was identified as a novel high-potency anti-HIV lead in a cellular model of HIV infection. Taken together, these observations show that Michael reactivity is a critical element of the curcumin pharmacophore, but also reveal a surprising sensitivity of bioactivity to C-prenylation of the vanillyl moiety.

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.

Receptor activity and conformational analysis of 5'-halogenated resiniferatoxin analogs as TRPV1 ligands

A series of 5'-halogenated resiniferatoxin analogs have been investigated in order to examine the effect of halogenation in the A-region on their binding and the functional pattern of agonism/antagonism for rat TRPV1 heterologously expressed in Chinese hamster ovary cells. Halogenation at the 5-position in the A-region of RTX and of 4-amino RTX shifted the agonism of parent compounds toward antagonism. The extent of antagonism was greater as the size of the halogen increased (I > Br > Cl > F) while the binding affinities were similar, as previously observed for our potent agonists. In this series, 5-bromo-4-amino RTX (39) showed very potent antagonism with K(i) (ant) = 2.81 nM, which was thus 4.5-fold more potent than 5'-iodo RTX, previously reported as a potent TRPV1 antagonist. Molecular modeling analyses with selected agonists and the corresponding halogenated antagonists revealed a striking conformational difference. The 3-methoxy of the A-region in the agonists remained free to interact with the receptor whereas in the case of the antagonists, the compounds assumed a bent conformation, permitting the 3-methoxy to instead form an internal hydrogen bond with the C4-hydroxyl of the diterpene.

Synthesis and biological evaluation of 12-aminoacylphorboids

Spurred by the paradoxical anti-inflammatory activity of some aminoacylphorbol derivatives, the naturally occurring and epimeric N,N-dimethylvalinoyl-4alpha-4-deoxyphorbol derivatives 3b and 3d have been prepared from 4alpha-4-deoxyphorbol (3e), a byproduct of the isolation of phorbol from Croton oil and a phorboid polyol so far largely overlooked in terms of biological activity. The configuration of the side chain stereocenter was confirmed for both natural products, and to investigate the side chain structure-activity relationships within this class of compounds, their corresponding N,N-dimethylglycinate (3g) and nor (3h) and di-nor derivatives (3i, 3j) were also prepared. By using a PKC-sensitive model of HIV-1 latency (activation of HIV- gene expression in Jurkat-LAT-GFP cells), it was found that both 3b and 3d can activate PKC-dependent responses, while a series of experiments with isoform-specific PKC inhibitors showed that these compounds target PKCalpha and -delta. Both N,N-dimethylation and the presence of side chain alpha-substitution were critical for activity. Selective PKC binding, rather than COX inhibition, might explain the paradoxical anti-inflammatory activity of extracts containing aminoacylphorboids in the mouse ear edema assay.

Structure-activity relationships of the ultrapotent vanilloid resiniferatoxin (RTX): The side chain benzylic methylene

The side chain benzylic methylene is a critical element for the vanilloid activity of resiniferatoxin (2a, RTX), and introduction of branching, oxygen functions, or isosteric substitution at this center proved detrimental, with a decrease of potency of 2-3 orders of magnitude compared to the natural product. Conversely, only a modest erosion of activity was observed upon alpha-methylation and alpha-methylenation of the side chain. Surprisingly, introduction of an iodine atom in the guaiacyl moiety of the oxygen isoster 2h led to an unexpected and remarkable (>1000-fold) increase of potency, affording 2i, a compound that outperforms RTX in terms of vanilloid agonism and represents the first one-digit picomolar ligand of a TRP channel discovered to date.

The endoplasmic reticulum of dorsal root ganglion neurons contains functional TRPV1 channels

Transient receptor potential vanilloid type 1 (TRPV1) is a plasma membrane Ca(2+) channel involved in transduction of painful stimuli. Dorsal root ganglion (DRG) neurons express ectopic but functional TRPV1 channels in the endoplasmic reticulum (ER) (TRPV1(ER)). We have studied the properties of TRPV1(ER) in DRG neurons and HEK293T cells expressing TRPV1. Activation of TRPV1(ER) with capsaicin or other vanilloids produced an increase of cytosolic Ca(2+) due to Ca(2+) release from the ER. The decrease of [Ca(2+)](ER) was directly revealed by an ER-targeted aequorin Ca(2+) probe, expressed in DRG neurons using a herpes amplicon virus. The sensitivity of TRPV1(ER) to capsaicin was smaller than the sensitivity of the plasma membrane TRPV1 channels. The low affinity of TRPV1(ER) was not related to protein kinase A- or C-mediated phosphorylations, but it was due to inactivation by cytosolic Ca(2+) because the sensitivity to capsaicin was increased by loading the cells with the Ca(2+) chelator BAPTA. Decreasing [Ca(2+)](ER) did not affect the sensitivity of TRPV1(ER) to capsaicin. Disruption of the TRPV1 calmodulin-binding domains at either the C terminus (Delta35AA) or the N terminus (K155A) increased 10-fold the affinity of TRPV1(ER) for capsaicin, suggesting that calmodulin is involved in the inactivation. The lack of TRPV1 sensitizers, such as phosphatidylinositol 4,5-bisphosphate, in the ER could contribute to decrease the affinity for capsaicin. The low sensitivity of TRPV1(ER) to agonists may be critical for neuron health, because otherwise Ca(2+) depletion of ER could lead to ER stress, unfolding protein response, and cell death.

Non-vanillyl resiniferatoxin analogues as potent and metabolically stable transient receptor potential vanilloid 1 agonists

A series of non-vanillyl resiniferatoxin analogues, having 4-methylsulfonylaminophenyl and fluorophenyl moieties as vanillyl surrogates, have been investigated as ligands for rat TRPV1 heterologously expressed in Chinese hamster ovary cells. Although lacking the metabolically problematic 4-hydroxy substituent on the A-region phenyl ring, the compounds retained substantial agonist potency. Indeed, the 3-methoxy-4-methylsulfonylaminophenyl analog (1) was modestly (2.5-fold) more potent than RTX, with an EC(50)=0.106 nM. Further, it resembled RTX in its kinetics and pattern of stimulation of the levels of intracellular calcium in individual cells, as revealed by imaging. Compound 1 displayed modestly enhanced in vitro stability in rat liver microsomes and in plasma, suggesting that it might be a pharmacokinetically more favorable surrogate of resiniferatoxin. Molecular modeling analyses with selected analogues provide evidence that the conformational differences could affect their binding affinities, especially for the ester versus amide at the B-region.

Perineural resiniferatoxin selectively inhibits inflammatory hyperalgesia

Resiniferatoxin (RTX) is an ultrapotent capsaicin analog that binds to the transient receptor potential channel, vanilloid subfamily member 1 (TRPV1). There is a large body of evidence supporting a role for TRPV1 in noxious-mediated and inflammatory hyperalgesic responses. In this study, we evaluated low, graded, doses of perineural RTX as a method for regional pain control. We hypothesized that this approach can provide long-term, but reversible, blockade of a portion of nociceptive afferent fibers within peripheral nerves when given at a site remote from the neuronal perikarya in the dorsal root ganglia. Following perineural RTX application to the sciatic nerve, we demonstrated a significant inhibition of inflammatory nociception that was dose- and time-dependent. At the same time, treated animals maintained normal proprioceptive sensations and motor control, and other nociceptive responses were largely unaffected. Using a range of mechanical and thermal algesic tests, we found that the most sensitive measure following perineural RTX administration was inhibition of inflammatory hyperalgesia. Recovery studies showed that physiologic sensory function could return as early as two weeks post-RTX treatment, however, immunohistochemical examination of the DRG revealed a partial, but significant reduction in the number of the TRPV1-positive neurons. We propose that this method could represent a beneficial treatment for a range of chronic pain problems, including neuropathic and inflammatory pain not responding to other therapies.

Natural and synthetic cage compounds incorporating the 2,9,10-trioxatricyclo[4.3.1.03,8]decane type moiety

The 2,9,10-trioxatricyclo[4.3.1.0(3,8)]decane moiety is a tetracyclic cage-like orthoester incorporated in the structure of a series of daphnane derivatives such as resiniferatoxin (RTX), kirkinine, synaptolepis factors, huratoxin etc., exhibiting various biological activities. The approaches to the preparation of cage-like orthoesters starting from partially acylated or unmodified 1,2,4-trihydroxycyclohexane moieties built onto natural as well as synthetic compounds are discussed. Orthoester derivatives of RTX analogs, Ceverathrum alkaloids, myoinositol and pyranoses are included. Stereochemical requirements to the formation of the orthoester unit are discussed. The biological activity of different compounds containing the cage-like orthoester structural fragment is given. The literature is reviewed till 2004.

Peripherally induced resiniferatoxin analgesia

Selective blockade of nociceptive pathways represents a mechanism-based approach that has attracted a large variety of pharmacological and molecular investigations. A potential site for selective intervention is the primary afferent nociceptive nerve terminal. Binding of resiniferatoxin (RTX) to the vanilloid-1 receptor (VR1) stimulates and then inactivates heat and vanilloid-responsive nerve endings involved in heat and inflammatory pain signaling which can progress to localized degeneration of the peripheral ending followed by regeneration. Application of RTX directly to peripheral nerve endings produces a long term, reversible attenuation of nociceptive transmission. Heat hyperalgesia and mechanical allodynia were assessed prior to injection of RTX into the hindpaw (baseline) and at acute (minutes-hours) and more chronic (days-weeks) times after injection. Acutely, an inverse dose-to-pain response (guarding, licking) for RTX (0.0625-2.0 microg) occurs, followed by selective attenuation of peripheral pain transmission. Thermal nociception was decreased in a concentration-dependent fashion and lasted up to 21 days, without impairing motor function. Administration of RTX blocked both inflammation-induced hyperalgesia and spinal c-Fos induction. The results demonstrate the efficacy and therapeutic potential of reversible, peripheral C-fiber 'inactivation' for intermediate duration pain control.

Low pH potentiates both capsaicin binding and channel gating of VR1 receptors

Capsaicin ion channels are highly expressed in peripheral nervous terminals and involved in pain and thermal sensations. One characteristic of the cloned VR1 receptor is its multimodal responses to various types of noxious stimuli. The channel is independently activated by capsaicin and related vanilloids at submicromolar range, by heat above 40 degrees C, and by protons at pH below 6.5. Furthermore, simultaneous applications of two or more stimuli lead to cross sensitization of the receptor, with an apparent increase in the sensitivity to any individual stimulus when applied alone. We studied here the mechanism underlying such cross-sensitization; in particular, between capsaicin and pH, two prototypical stimuli for the channel. By analyzing single-channel currents recorded from excised-patches expressing single recombinant VR1 receptors, we examined the effect of pH on burst properties of capsaicin activation at low concentrations and the effect on gating kinetics at high concentrations. Our results indicate that pH has dual effects on both capsaicin binding and channel gating. Lowering pH enhances the apparent binding affinity of capsaicin, promotes the occurrences of long openings and short closures, and stabilizes at least one of the open conformations of the channel. Our data also demonstrate that capsaicin binding and protonation of the receptor interact allosterically, where the effect of one can be offset by the effect of the other. These results provide important basis to further understand the nature of the activation pathways of the channel evoked by different stimuli as well as the general mechanism underling the cross-sensitization of pain.

Respiratory actions of vanilloid receptor agonists in the nucleus of the solitary tract: comparison of resiniferatoxin with non-pungent agents and anandamide

1. Activation of vanilloid receptors on sensory nerve terminals in the commissural nucleus of the solitary tract (cNTS) of rats with capsaicin, produces respiratory slowing. In this study, we used microinjection techniques employing pungent and non-pungent vanilloids to further characterize vanilloid receptors in the cNTS. 2. Microinjection of the pungent vanilloid, resiniferatoxin (RTX), into the cNTS of urethane-anaesthetized rats, dose-dependently reduced respiratory rate without affecting tidal volume. RTX was 20 fold more potent at slowing respiration ( approximately ED(50), 100 pmol) than capsaicin ( approximately ED(50), 2 nmol). Doses of RTX greater than 100 pmol caused either irregular (dyspnoeic) breathing or terminal apnoea (>250 pmol). The respiratory slowing response to RTX (75 pmol), was dose-dependently attenuated by injecting RTX (but not vehicle) into the same site 60 min earlier. 3. The non-pungent phorbol derivative of RTX, phorbol 12-phenylacetete 13-acetate 20-homovanillate (PPAHV, 0.1-1 nmol), also slowed respiration (ED(50), approximately 1 nmol) and almost abolished response to RTX (75 pmol) injected into the same site 60 min later. 4. In contrast to RTX, PPAHV and capsaicin, the putative endogenous vanilloid receptor agonist, arachidonyl ethanolamide (AEA), and non-pungent capsaicin derivative, olvanil, had no direct effect on respiration. However, both AEA and olvanil dose-dependently reduced the respiratory response to injection of RTX (75 pmol) 60 min later into the same site (EC(50)s, for AEA and olvanil, approximately 2 and 0.2 nmol, respectively). 5. These studies suggest that both pungent and non-pungent vanilloids interact with vanilloid receptors in the cNTS. However, whereas RTX and PPAHV activate and subsequently desensitize vanilloid receptors on sensory nerve terminals in the cNTS, olvanil and AEA fail to activate despite readily desensitizing responses to RTX in this region.

High affinity antagonists of the vanilloid receptor

The vanilloid receptor VR1 has attracted great interest as a sensory transducer for capsaicin, protons, and heat, and as a therapeutic target. Here we characterize two novel VR1 antagonists, KJM429 [N-(4-tert-butylbenzyl)-N'-[4-(methylsulfonylamino)benzyl]thiourea] and JYL1421 [N-(4-tert-butylbenzyl)-N'-[3-fluoro-4-(methylsulfonylamino)benzyl]thiourea], with enhanced activity compared with capsazepine on rat VR1 expressed in Chinese hamster ovary (CHO) cells. JYL1421, the more potent of the two novel antagonists, inhibited [(3)H]resiniferatoxin binding to rVR1 with an affinity of 53.5 +/- 6.5 nM and antagonized capsaicin-induced calcium uptake with an EC(50) of 9.2 +/- 1.6 nM, reflecting 25- and 60-fold greater potencies than capsazepine. Both JYL1421 and KJM429 antagonized RTX as well as capsaicin and their mechanism was competitive. The responses to JYL1421 and KJM429 differed for calcium uptake by rVR1 induced by heat or pH. JYL1421 antagonized the response to both pH 6.0 and 5.5, whereas KJM429 antagonized at pH 6.0 but was an agonist at lower pH (<5.5). For heat, JYL1421 fully antagonized and KJM429 partially antagonized. Capsazepine showed only weak antagonism for both pH and heat. Responses of rVR1 to different activators could thus be differentially affected by different ligands. In cultured dorsal root ganglion neurons, JYL1421 and KJM429 likewise behaved as antagonists for capsaicin, confirming that the antagonism is not limited to heterologous expression systems. Finally, JYL1421 and KJM429 had little or no effect on ATP-induced calcium uptake in CHO cells lacking rVR1, unlike capsazepine. We conclude that JYL1421 is a competitive antagonist of rVR1, blocking response to all three of the agonists (capsaicin, heat, and protons) with enhanced potency relative to capsazepine.

3-D-QSAR analysis of N-(3-acyloxy-2-benzylpropyl)-N'-dihydroxytetrahydrobenzazepine and tetrahydroisoquinoline and N-(3-acyloxy-2-benzylpropyl)-N'-(4-hydroxy-3-methoxybenzyl) thioureas analogues as potent vanilloid receptor ligands

3-D-Quantitative structure--activity relationships of N-(3-acyloxy-2-benzylpropyl)-N'-dihydroxytetrahydro-benzazepine and tetrahydroisoquinoline and N-(3-acyloxy-2-benzylpropyl)-N'-(4-hydroxy-3-methoxybenzyl) thiourea analogues as potent vanilloid receptor ligands were investigated using the CoMFA and the COMSIA methods. The best CoMFA model obtained in this study from 29 substituted thiourea analogues is a two-component model with the following statistics. R(2)((cv))=0.407 and RMSE((cv))=0.532 for the cross-validation, and R(2)=0.705 and RMSE=0.375 for the fitted. The best COMSIA model obtained from the same 29 compounds is a two-component model with the following statistics: R(2)((cv))=0.336 and RMSE((cv))=0.563 for the cross-validation, and R(2)=0.693 and RMSE=0.382 for the fitted.

Phenolic modification as an approach to improve the pharmacology of the 3-acyloxy-2-benzylpropyl homovanillic amides and thioureas, a promising class of vanilloid receptor agonists and analgesics

In order to improve the analgesic activity and pharmacokinetics of thioureas 2 and 3, which we previously developed as potent vanilloid receptor (VR) agonists, we prepared and characterized phenolic modifications of them and of their amide surrogates (7, 8). The aminoethyl analogue of the amide template 13 was a potent analgesic with an EC50=0.96 microg/kg in the AA-induced writhing test and with better in vivo stability than the parent phenol.

Pharmacological differences between the human and rat vanilloid receptor 1 (VR1)

Vanilloid receptors (VR1) were cloned from human and rat dorsal root ganglion libraries and expressed in Xenopus oocytes or Chinese Hamster Ovary (CHO) cells. Both rat and human VR1 formed ligand gated channels that were activated by capsaicin with similar EC(50) values. Capsaicin had a lower potency on both channels, when measured electrophysiologically in oocytes compared to CHO cells (oocytes: rat=1.90+/-0.20 microM; human=1.90+/-0.30 microM: CHO cells: rat=0.20+/-0.06 microM; human=0.19+/-0.08 microM). In CHO cell lines co-expressing either rat or human VR1 and the calcium sensitive, luminescent protein, aequorin, the EC(50) values for capsaicin-induced responses were similar in both cell lines (rat=0.35+/-0.06 microM, human=0.53+/-0.03 microM). The threshold for activation by acidic solutions was lower for human VR1 channels than that for rat VR1 (EC(50) pH 5.49+/-0.04 and pH 5.78+/-0.09, respectively). The threshold for heat activation was identical (42 degrees C) for rat and human VR1. PPAHV was an agonist at rat VR1 (EC(50) between 3 and 10 microM) but was virtually inactive at the human VR1 (EC(50)>10 microM). Capsazepine and ruthenium red were both more potent at blocking the capsaicin response of human VR1 than rat VR1. Capsazepine blocked the human but not the rat VR1 response to low pH. Capsazepine was also more effective at inhibiting the noxious heat response of human than of rat VR1.

N-(3-Acyloxy-2-benzylpropyl)-N'-(4-hydroxy-3-methoxybenzyl) thiourea derivatives as potent vanilloid receptor agonists and analgesics

A series of N-(3-acyloxy-2-benzylpropyl)-N'-(4-hydroxy-3-methoxybenzyl) thiourea derivatives were investigated as vanilloid receptor ligands in an effort to discover a novel class of analgesics. The proposed pharmacophore model of resiniferatoxin. which includes the C20 homovanillic moiety, the C3-carbonyl and the orthoester phenyl ring as key pharmacophoric groups, was utilized as a guide for drug design. The compounds were synthesized after several steps from diethylmalonate and evaluated in vitro in a receptor binding assay and in a capsaicin-activated channel assay. Additional evaluation of analgesic activity, anti-inflammatory activity and pungency was conducted in animal models by the writhing test, the ear edema assay, and the eye-wiping test, respectively. Among the new compounds, 23 and 28 were found to be the most potent receptor agonists of the series with Ki values of 19 nM and 11 nM, respectively. Their strong in vitro potencies were also reflected by an excellent analgesic profile in animal tests with ED50 values of 0.5 microg kg for 23 and 1.0 microg/kg for 28. Relative to capsaicin these compounds appear to be ca. 600 and 300 times more potent. Both 23 and 28 were found to be less pungent than capsaicin based on the eye-wiping test. However, the compounds did not show significant anti-inflammatory activity. A molecular modeling study comparing the energy-minimized structures of resiniferatoxin and 35 demonstrated a good correlation in the spatial disposition of the corresponding key pharmacophores. The thioureas described in this investigation, which were designed as simplified resiniferatoxin surrogates, represent a novel class of potent vanilloid receptor agonists endowed with potent analgesic activity and reduced pungency.

Functional and desensitizing effects of the novel synthetic vanilloid-like agent 12-phenylacetate 13-acetate 20-homovanillate (PPAHV) in the perfused rat hindlimb

1. In the present study, the effects of the novel vanilloid agonist, 12-phenylacetate 13-acetate 20-homovanillate (PPAHV), on oxygen consumption (VO(2)) and vascular resistance (perfusion pressure, PP) were investigated in the constant flow, perfused rat hindlimb. The acute desensitizing properties of this novel synthetic agent were also examined. 2. Maximum stimulation of VO(2) was produced by 0.2 microM PPAHV (delta VO(2), 0.83+/-0.06 micromol g(-1) h(-1)) and was accompanied by mild vasoconstriction (increase in PP; 8.0+/-1.1 mmHg). The highest concentration of PPAHV tested (2 microM) caused inhibition of VO(2) (delta VO(2), -2.73+/-0.51 micromol g(-1) h(-1)) and strong vasoconstriction (delta PP, 42.0+/-1.2 mmHg). 3. Capsazepine (10 microM) caused a parallel shift to the right of both VO(2) and PP concentration-response curves for PPAHV (pK(b)=5.00), indicative of competitive binding to vanilloid receptors. 4. The stimulation of VO(2) produced by 0.2 microM PPAHV decreased, but was not completely abolished, after repeated infusion of PPAHV (change in VO(2), first infusion, 0.66+/-0.18 micromol g(-1) h(-1); sixth infusion, 0.29+/-0. 08 micromol g(-1) h(-1), P<0.05), an acute tachyphylactic response not previously seen with the repeated infusion of other vanilloid analogues. Conversely, the PP response to repeated PPAHV infusion increased (delta PP, first infusion, 5.8+/-0.7 mmHg; sixth infusion, 9.0+/-0.6 mmHg, P<0.05). 5. In conclusion, PPAHV produces vasoconstriction and a biphasic effect on VO(2) in the perfused rat hindlimb very similar to that induced by naturally occurring vanilloids. Both effects are blocked by the competitive antagonist capsazepine. Since, the metabolic response to low concentrations of PPAHV (stimulation of VO(2)) undergoes tachyphylaxis, the present data suggest that PPAHV desensitizes putative vanilloid receptors in the hindlimb.

Kirkinine, a new daphnane orthoester with potent neurotrophic activity from Synaptolepis kirkii

The bioassay-guided fractionation of a dichloromethane extract from the roots of Synaptolepis kirkii using neuronal viability as a model allowed the isolation of the new daphnane orthoester kirkinine (1a) as a powerful neurotrophic constituent.

Phorboid 20-homovanillates induce apoptosis through a VR1-independent mechanism

BACKGROUND

Vanilloids, such as capsaicin and resiniferatoxin (RTX), are recognized at the cell surface by vanilloid receptor type 1 (VR1), which has recently been cloned. VR1 mediates the effects of capsaicin and RTX in VR1-expressing cells, but vanilloids can induce apoptosis through a pathway not mediated by VR1. Phorboid 20-homovanillates can be used to investigate cell death induced by vanilloids.

RESULTS

12,13-Diacylphorbol-20 homovanillates were prepared by the sequential esterification of the natural polyol. Phorbol 12-phenylacetate 13-acetate 20-homovanillate (PPAHV) induced apoptosis in Jurkat cells to the same extent as RTX. Apoptosis was preceded by an increase in intracellular reactive oxygen species and by the loss of mitochondrial transmembrane potential. PPAHV-induced apoptosis was mediated by a pathway involving caspase-3 activation and was initiated at the S phase of the cell cycle. The cell-death pathway triggered by VR1 activation was studied in 293T cells transfected with the cloned rat vanilloid receptor. In this system, capsaicin and PPAHV induced cell death by an apparent necrotic mechanism, which was selectively inhibited by the competitive vanilloid receptor antagonist capsazepine. Interestingly, phorbol-12, 13-bisnonanoate-20-homovanillate, an analogue of PPAHV, induced cell death in VR1-transfected cells but could not trigger apoptosis in the Jurkat cell line.

CONCLUSIONS

Vanilloids can induce cell death through different signalling pathways. The cell death induced in a VR1-independent manner has the hallmark of apoptosis, whereas the cell death mediated by vanilloids binding to VR1 is seemingly necrotic. Phorboid homovanillates that have antitumour and anti-inflammatory activities but lack the undesirable side effects of the natural vanilloids could be developed as potential drugs.

Characterization using FLIPR of rat vanilloid receptor (rVR1) pharmacology

The vanilloid receptor (VR1) is a ligand-gated ion channel, which plays an important role in nociceptive processing. Therefore, a pharmacological characterization of the recently cloned rat VR1 (rVR1) was undertaken. HEK293 cells stable expressing rVR1 (rVR1-HEK293) were loaded with Fluo-3AM and then incubated at 25 degrees C for 30 min with or without various antagonists or signal transduction modifying agents. Then intracellular calcium concentrations ([Ca(2+)](i)) were monitored using FLIPR, before and after the addition of various agonists. The rank order of potency of agonists (resiniferatoxin (RTX)>capsaicin>olvanil>PPAHV) was as expected, and all were full agonists. The potencies of capsaicin and olvanil, but not RTX or PPAHV, were enhanced at pH 6.4 (pEC(50) values of 7.47+/-0.06, 7.16+/-0.06, 8.19+/-0.06 and 6.02+/-0.03 respectively at pH 7.4 vs 7.71+/-0.05, 7.58+/-0.14, 8.10+/-0.05 and 6.04+/-0.08 at pH 6.4). Capsazepine, isovelleral and ruthenium red all inhibited the capsaicin (100 nM)-induced Ca(2+) response in rVR1-HEK293 cells, with pK(B) values of 7.52+/-0.08, 6.92+/-0.11 and 8.09+/-0.12 respectively (n=6 each). The response to RTX and olvanil were also inhibited by these compounds. None displayed any agonist-like activity. The removal of extracellular Ca(2+) abolished, whilst inhibition of protein kinase C with chelerythrine chloride (10 microM) partially (approximately 20%) inhibited, the capsaicin (10 microM)-induced Ca(2+) response. However, tetrodotoxin (3 microM), nimodipine (10 microM), omega-GVIA conotoxin (1 microM), thapsigargin (1 microM), U73122 (3 microM) or H-89 (3 microM) had no effect on the capsaicin (100 nM)-induced response. In conclusion, the recombinant rVR1 stably expressed in HEK293 cells acts as a ligand-gated Ca(2+) channel with the appropriate agonist and antagonist pharmacology, and therefore is a suitable model for studying the effects of drugs at this receptor.

3-Acyloxy-2-phenalkylpropyl amides and esters of homovanillic acid as novel vanilloid receptor agonists

A series of 3-acyloxy-2-phenalkylpropyl amides and esters of homovanillic acid were designed and synthesized as vanilloid receptor agonists containing the three principal pharmacophores of resiniferatoxin. Amide analogues 23, 5 and 11 were found to be potent agonists in vanilloid receptor assay both for ligand binding and for activation.

Resiniferatoxin-type phorboid vanilloids display capsaicin-like selectivity at native vanilloid receptors on rat DRG neurons and at the cloned vanilloid receptor VR1

1 Although the cloned rat vanilloid receptor VR1 appears to account for both receptor binding and calcium uptake, the identification of vanilloids selective for one or the other response is of importance because these ligands may induce distinct patterns of biological activities. 2 Phorbol 12,13-didecanoate 20-homovanillate (PDDHV) evoked 45Ca(2+)-uptake by rat dorsal root ganglion neurons (expressing native vanilloid receptors) in culture with an EC50 of 70 nM but inhibited [3H]-resiniferatoxin (RTX) binding to rat dorsal root ganglion membranes with a much lower potency (Ki>10,000 nM). This difference in potencies represents a more than 100 fold selectivity for capsaicin-type pharmacology. 3 45Ca2+ influx by PDDHV was fully inhibited by the competitive vanilloid receptor antagonist capsazepine, consistent with the calcium uptake occurring via vanilloid receptors. 4 PDDHV induced calcium mobilization in CHO cells transfected with the cloned rat vanilloid receptor VR1 with an EC50 of 125 nM and inhibited [3H]-RTX binding to these cells with an estimated Ki of 10,000 nM. By contrast, PDDHV failed to evoke a measurable calcium response in non-transfected CHO cells, confirming its action through VR1. 5 We conclude that PDDHV is two orders of magnitude more potent for inducing calcium uptake than for inhibiting RTX binding at vanilloid receptors, making this novel vanilloid a ligand selective for capsaicin-type pharmacology. These results emphasize the importance of monitoring multiple endpoints for evaluation of vanilloid receptor structure-activity relations. Furthermore, PDDHV now provides a tool to explore the biological correlates of capsaicin-type vanilloid pharmacology.

A non-pungent triprenyl phenol of fungal origin, scutigeral, stimulates rat dorsal root ganglion neurons via interaction at vanilloid receptors

1. A [3H]-resiniferatoxin (RTX) binding assay utilizing rat spinal cord membranes was employed to identify novel vanilloids in a collection of natural products of fungal origin. Of the five active compounds found (scutigeral, acetyl-scutigeral, ovinal, neogrifolin, and methyl-neogrifolin), scutigeral (Ki=19 microM), isolated from the edible mushroom Albatrellus ovinus, was selected for further characterization. 2. Scutigeral induced a dose-dependent 45Ca uptake by rat dorsal root ganglion neurons with an EC50 of 1.6 microM, which was fully inhibited by the competitive vanilloid receptor antagonist capsazepine (IC50=5.2 microM). 3. [3H]-RTX binding isotherms were shifted by scutigeral (10-80 microM) in a competitive manner. The Schild plot of the data had a slope of 0.8 and gave an apparent Kd estimate for scutigeral of 32 microM. 4. Although in the above assays scutigeral mimicked capsaicin, it was not pungent on the human tongue up to a dose of 100 nmol per tongue, nor did it provoke protective wiping movements in the rat (up to 100 microM) upon intraocular instillation. 5. In accord with being non-pungent, scutigeral (5 microM) did not elicit a measurable inward current in isolated rat dorsal root ganglion neurons under voltage-clamp conditions. It did, however, reduce the proportion of neurons (from 61 to 15%) that responded to a subsequent capsaicin (1 microM) challenge. In these neurons, scutigeral both delayed (from 27 to 72 s) and diminished (from 5.0 to 1.9 nA) the maximal current evoked by capsaicin. 6. In conclusion, scutigeral and its congeners form a new chemical class of vanilloids, the triprenyl phenols. Scutigeral promises to be a novel chemical lead for the development of orally active, non-pungent vanilloids.

Maprouneacin, a new daphnane diterpenoid with potent antihyperglycemic activity from Maprounea africana

Bioassay-guided fractionation of the EtOH extract of M. africana, using the in vivo noninsulin-dependent diabetes mellitus db/db mouse model, resulted in the isolation of the new daphnane-type diterpenoid maprouneacin (2). Compound 2 showed potent glucose-lowering properties when given by the oral route.

A non-pungent resiniferatoxin analogue, phorbol 12-phenylacetate 13 acetate 20-homovanillate, reveals vanilloid receptor subtypes on rat trigeminal ganglion neurons

Capsaicin, the vanilloid responsible for the pungent taste of hot peppers, binds to receptors found primarily in polymodal nociceptors. Capsaicin initially stimulates polymodal nociceptors and subsequently inhibits them from responding to a variety of stimuli. This property makes it useful clinically as an analgesic and anti-inflammatory compound. There is mounting, albeit indirect, evidence for the existence of several subtypes of vanilloid receptors. One such piece of evidence comes from studying analogues of capsaicin, such as phorbol 12-phenylacetate 13 acetate 20-homovanillate. This compound binds to (capsaicin) vanilloid receptors on sensory neurons, but unlike capsaicin it is non-pungent and does not produce hypothermia. To determine how sensory neurons respond to phorbol 12-phenylacetate 13 acetate 20-homovanillate, and to compare these responses with those evoked by capsaicin, whole-cell patch-clamp measurements were performed on cultured rat trigeminal ganglion neurons. It was found that 63% of the neurons held at -60 mV were activated by 3 microM, phorbol 12-phenylacetate 13 acetate 20-homovanillate, and 87% of these were also activated by 1 microM capsaicin. In a given neuron, phorbol 12-phenylacetate 13 acetate 20-homovanillate, like capsaicin, could activate kinetically distinct inward currents. The current-voltage curves characterizing phorbol 12-phenylacetate 13 acetate 20-homovanillate responses were asymmetric and had reversal potentials at -5.8 +/- 6.0 mV and 10.4 +/- 4 mV. The averaged dose-response curves for phorbol 12-phenylacetate 13 acetate 20-homovanillate were fit to the Hill equation and had binding constants (K(1/2)s) of 2.73 microM and 0.96 microM and Hill coefficients (ns) of approximately 1 for a rapidly- and slowly-activating current, respectively. These parameters are consistent with those obtained from binding experiments and calcium-influx experiments on sensory nerves. Repeated applications of phorbol 12-phenylacetate 13 acetate 20-homovanillate every 3 min caused a complete reduction in the rapidly-activating currents leaving only a reduced slowly-activating current. This provides strong evidence for the independence of these currents and the existence of subtypes of vanilloid receptors. Additional evidence for the existence of receptor subtypes is that 10 microM capsazepine, a specific and competitive inhibitor of capsaicin-evoked responses, did not inhibit the phorbol 12-phenylacetate 13 acetate 20-homovanillate-induced currents in some neurons and partially inhibited them in other neurons. Thus, there are capsazepine-sensitive and capsazepine-insensitive subtypes of vanilloid receptors. In summary, we have obtained electrophysiological and pharmacological evidence for distinct subtypes of vanilloid receptors.

Conformational studies on resiniferatoxin (RTX), an ultrapotent vanilloid agonist

In polar solution, NOE studies show a pronounced clustering of the aromatic moieties (9,13,14-phenylacetate orthoester and 20-homovanillate) of the ultrapotent vanilloid agonist resiniferatoxin (RTX). This clustering is absent in nonpolar solution. Low energy clustered structures from molecular dynamics simulations account for the observed NOEs. These results suggest that the phenylorthoacetate moiety can assist the attainment of specific alignments between the terpenoid core and the vanillyl moiety, possibly preorganizing them for ideal receptor binding.

Euphorbium: modern research on its active principle, resiniferatoxin, revives an ancient medicine

Resiniferatoxin, an ultrapotent capsaicin analog present in the latex of Euphorbia resinifera, interacts at a specific membrane recognition site (referred to as the vanilloid receptor), expressed by primary sensory neurons mediating pain perception as well as neurogenic inflammation. Desensitization to resiniferatoxin is a promising approach to mitigate neuropathic pain and other pathological conditions in which sensory neuropeptides released from capsaicin-sensitive neurons play a crucial role. Clinical trials to evaluate the potential of topical resiniferatoxin treatment to relieve pain associated with diabetic polyneuropathy and postherpetic neuralgia are in progress. Though resiniferatoxin was isolated only two decades ago, the dried latex of Euphorbia resinifera, called Euphorbium, has been in medicinal use since the time of recorded history. This review highlights the most important events in the history of this ancient medicine, from the first written record of the therapeutic potential of Euphorbium (at the time of the reign of the Roman Emperor Augustus) to the identification of its active principle as resiniferatoxin in 1975. A brief overview of the enormous contribution of resiniferatoxin to our current understanding of the anatomical localization, function, and pharmacology of vanilloid receptors is provided. Lastly, the mechanisms are summarized by which capsaicin and resiniferatoxin, despite sharing receptors, may have dissimilar biological actions.

Vanilloid receptors: new insights enhance potential as a therapeutic target

Compounds related to capsaicin and its ultrapotent analog, resiniferatoxin (RTX), collectively referred to as vanilloids, interact at a specific membrane recognition site (vanilloid receptor), expressed almost exclusively by primary sensory neurons involved in nociception and neurogenic inflammation. Desensitization to vanilloids is a promising therapeutic approach to mitigate neuropathic pain and pathological conditions (e.g. vasomotor rhinitis) in which neuropeptides released from primary sensory neurons play a major role. Capsaicin-containing preparations are already commercially available for these purposes. The use of capsaicin, however, is severely limited by its irritancy, and the synthesis of novel vanilloids with an improved pungency/desensitization ratio is an on-going objective. This review highlights the emerging evidence that the vanilloid receptor is not a single receptor but a family of receptors, and that these receptors recognize not simply RTX and capsaicin structural analogs but are broader in their ligand-binding selectivity. We further focus on ligand-induced messenger plasticity, a recently discovered mechanism underlying the analgesic actions of vanilloids. Lastly, we give a brief overview of the current clinical uses of vanilloids and their future therapeutic potential. The possibility is raised that vanilloid receptor subtype-specific drugs may be synthesized, devoid of the undesirable side-effects of capsaicin.