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.

Antihyperalgesic effects of (R,E)-N-(2-hydroxy-2,3-dihydro-1H-inden-4-yl)-3-(2-(piperidin-1-yl)-4-(trifluoromethyl)phenyl)-acrylamide (AMG8562), a novel transient receptor potential vanilloid type 1 modulator that does not cause hyperthermia in rats.

Antagonists of the vanilloid receptor TRPV1 (transient receptor potential vanilloid type 1) have been reported to produce antihyperalgesic effects in animal models of pain. These antagonists, however, also caused concomitant hyperthermia in rodents, dogs, monkeys, and humans. Antagonist-induced hyperthermia was not observed in TRPV1 knockout mice, suggesting that the hyperthermic effect is exclusively mediated through TRPV1. Since antagonist-induced hyperthermia is considered a hurdle for developing TRPV1 antagonists as therapeutics, we investigated the possibility of eliminating hyperthermia while maintaining antihyperalgesia. Here, we report four potent and selective TRPV1 modulators with unique in vitro pharmacology profiles (profiles A through D) and their respective effects on body temperature. We found that profile C modulator, (R,E)-N-(2-hydroxy-2,3-dihydro-1H-inden-4-yl)-3-(2-(piperidin-1-yl)-4-(trifluoromethyl)phenyl)acrylamide (AMG8562), blocks capsaicin activation of TRPV1, does not affect heat activation of TRPV1, potentiates pH 5 activation of TRPV1 in vitro, and does not cause hyperthermia in vivo in rats. We further profiled AMG8562 in an on-target (agonist) challenge model, rodent pain models, and tested for its side effects. We show that AMG8562 significantly blocks capsaicin-induced flinching behavior, produces statistically significant efficacy in complete Freund's adjuvant- and skin incision-induced thermal hyperalgesia, and acetic acid-induced writhing models, with no profound effects on locomotor activity. Based on the data shown here, we conclude that it is feasible to modulate TRPV1 in a manner that does not cause hyperthermia while maintaining efficacy in rodent pain models.

Design and synthesis of peripherally restricted transient receptor potential vanilloid 1 (TRPV1) antagonists.

Transient receptor potential vanilloid 1 (TRPV1) channel antagonists may have clinical utility for the treatment of chronic nociceptive and neuropathic pain. We recently advanced a TRPV1 antagonist, 3 (AMG 517), into clinical trials as a new therapy for the treatment of pain. However, in addition to the desired analgesic effects, this TRPV1 antagonist significantly increased body core temperature following oral administration in rodents. Here, we report one of our approaches to eliminate or minimize the on-target hyperthermic effect observed with this and other TRPV1 antagonists. Through modifications of our clinical candidate, 3 a series of potent and peripherally restricted TRPV1 antagonists have been prepared. These analogues demonstrated on-target coverage in vivo but caused increases in body core temperature, suggesting that peripheral restriction was not sufficient to separate antagonism mediated antihyperalgesia from hyperthermia. Furthermore, these studies demonstrate that the site of action for TRPV1 blockade elicited hyperthermia is outside the blood-brain barrier.

4-Aminopyrimidine tetrahydronaphthols: a series of novel vanilloid receptor-1 antagonists with improved solubility properties.

8-(6-(4-(Trifluoromethyl)phenyl)pyrimidin-4-ylamino)-1,2,3,4-tetrahydronaphthalen-2-ol (4) and analogs (5-10) were shown to be potent inhibitors of human and rat TRPV1 in vitro with increased solubility over our previous series. Synthesis, SAR, and improvements in metabolic stability and absorption of these compounds are described herein.

Substituted aryl pyrimidines as potent and soluble TRPV1 antagonists.

Clinical candidate AMG 517 (1) is a potent antagonist toward multiple modes of activation of TRPV1; however, it suffers from poor solubility. Analogs with various substituents at the R region of 3 were prepared to improve the solubility while maintaining the potent TRPV1 activity of 1. Compounds were identified that maintained potency, had good pharmacokinetic properties, and improved solubility relative to 1.

Species-specific pharmacology of Trichloro(sulfanyl)ethyl benzamides as transient receptor potential ankyrin 1 (TRPA1) antagonists.

Agonists of TRPA1 such as mustard oil and its key component AITC cause pain and neurogenic inflammation in humans and pain behavior in rodents. TRPA1 is activated by numerous reactive compounds making it a sensor for reactive compounds in the body. Failure of AITC, formalin and other reactive compounds to trigger pain behavior in TRPA1 knockout mice, as well as the ability of TRPA1 antisense to alleviate cold hyperalgesia after spinal nerve ligation, suggest that TRPA1 is a potential target for novel analgesic agents. Here, we have characterized CHO cells expressing human and rat TRPA1 driven by an inducible promoter. As reported previously, both human and rat TRPA1 are activated by AITC and inhibited by ruthenium red. We have also characterized noxious cold response of these cell lines and show that noxious cold activates both human and rat TRPA1. Further, we have used CHO cells expressing human TRPA1 to screen a small molecule compound library and discovered that 'trichloro(sulfanyl)ethyl benzamides' (AMG2504, AMG5445, AMG7160 and AMG9090) act as potent antagonists of human TRPA1 activated by AITC and noxious cold. However, trichloro(sulfanyl)ethyl benzamides' (TCEB compounds) displayed differential pharmacology at rat TRPA1. AMG2504 and AMG7160 marginally inhibited rat TRPA1 activation by AITC, whereas AMG5445 and AMG9090 acted as partial agonists. In summary, we conclude that both human and rat TRPA1 channels show similar AITC and noxious cold activation profiles, but TCEB compounds display species-specific differential pharmacology at TRPA1.

Novel vanilloid receptor-1 antagonists: 1. Conformationally restricted analogues of trans-cinnamides.

The vanilloid receptor-1 (VR1 or TRPV1) is a member of the transient receptor potential (TRP) family of ion channels and plays a role as an integrator of multiple pain-producing stimuli. From a high-throughput screening assay, measuring calcium uptake in TRPV1-expressing cells, we identified an N-aryl trans-cinnamide (AMG9810, compound 9) that acts as a potent TRPV1 antagonist. We have demonstrated the antihyperalgesic properties of 9 in vivo and have also reported the discovery of novel, orally bioavailable cinnamides derived from this lead. Herein, we expand our investigations and describe the synthesis and biological evaluation of a series of conformationally constrained analogues of the s-cis conformer of compound 9. These investigations resulted in the identification of 4-amino- and 4-oxopyrimidine cores as suitable isosteric replacements for the trans-acrylamide moiety. The best examples from this series, pyrimidines 79 and 74, were orally bioavailable and exhibited potent antagonism of both rat (IC50 = 4.5 and 0.6 nM, respectively) and human TRPV1 (IC50 = 7.4 and 3.7 nM, respectively). In addition, compound 74 was shown to be efficacious at blocking a TRPV1-mediated physiological response in vivo in the capsaicin-induced hypothermia model in rats; however, it was ineffective at preventing thermal hyperalgesia induced by complete Freund's adjuvant in rats.

Novel vanilloid receptor-1 antagonists: 2. Structure-activity relationships of 4-oxopyrimidines leading to the selection of a clinical candidate.

A series of novel 4-oxopyrimidine TRPV1 antagonists was evaluated in assays measuring the blockade of capsaicin or acid-induced influx of calcium into CHO cells expressing TRPV1. The investigation of the structure-activity relationships in the heterocyclic A-region revealed the optimum pharmacophoric elements required for activity in this series and resulted in the identification of subnanomolar TRPV1 antagonists. The most potent of these antagonists were thoroughly profiled in pharmacokinetic assays. Optimization of the heterocyclic A-region led to the design and synthesis of 23, a compound that potently blocked multiple modes of TRPV1 activation. Compound 23 was shown to be effective in a rodent "on-target" biochemical challenge model (capsaicin-induced flinch, ED50 = 0.33 mg/kg p.o.) and was antihyperalgesic in a model of inflammatory pain (CFA-induced thermal hyperalgesia, MED = 0.83 mg/kg, p.o.). Based on its in vivo efficacy and pharmacokinetic profile, compound 23 (N-{4-[6-(4-trifluoromethyl-phenyl)-pyrimidin-4-yloxy]-benzothiazol-2-yl}-acetamide; AMG 517) was selected for further evaluation in human clinical trials.

Novel vanilloid receptor-1 antagonists: 3. The identification of a second-generation clinical candidate with improved physicochemical and pharmacokinetic properties.

Based on the previously reported clinical candidate, AMG 517 (compound 1), a series of related piperazinylpyrimidine analogues were synthesized and evaluated as antagonists of the vanilloid 1 receptor (VR1 or TRPV1). Optimization of in vitro potency and physicochemical and pharmacokinetic properties led to the discovery of (R)-N-(4-(6-(4-(1-(4-fluorophenyl)ethyl)piperazin-1-yl)pyrimidin-4-yloxy)benzo[d]thiazol-2-yl)acetamide (16p), a potent TRPV1 antagonist [rTRPV1(CAP) IC50 = 3.7 nM] with excellent aqueous solubility (>or=200 microg/mL in 0.01 N HCl) and a reduced half-life (rat t1/2 = 3.8 h, dog t1/2 = 2.7 h, monkey t1/2 = 3.2 h) as compared to AMG 517. In addition, compound 16p was shown to be efficacious at blocking a TRPV1-mediated physiological response in vivo (ED50 = 1.9 mg/kg, p.o. in the capsaicin-induced flinch model in rats) and was also effective at reducing thermal hyperalgesia induced by complete Freund's adjuvant in rats (MED = 1 mg/kg, p.o). Based on its improved overall profile, compound 16p (AMG 628) was selected as a second-generation candidate for further evaluation in human clinical trials as a potential new treatment for chronic pain.

Trisubstituted pyrimidines as transient receptor potential vanilloid 1 (TRPV1) antagonists with improved solubility.

A series of trisubstituted pyrimidines were synthesized to improve aqueous solubility of our first TRPV1 clinical candidate (1; AMG 517), while maintaining potent TRPV1 inhibitory activity. Structure-activity and structure-solubility studies led to the identification of compound 26. The aqueous solubility of 26 (>or=200microg/mL, 0.01 HCl; 6.7microg/mL, phosphate buffered saline (PBS); 150microg/mL, fasted-state simulated intestinal fluid (SIF)) was significantly improved over 1. In addition, compound 26 was found to be orally bioavailable (rat F(oral)=24%) and had potent TRPV1 antagonist activity (capsaicin IC(50)=1.5nM) comparable to that of 1.

Design of potent, orally available antagonists of the transient receptor potential vanilloid 1. Structure-activity relationships of 2-piperazin-1-yl-1H-benzimidazoles.

The vanilloid receptor-1 (VR1 or TRPV1) is a membrane-bound, nonselective cation channel that is predominantly expressed by peripheral neurons sensing painful stimuli. TRPV1 antagonists produce antihyperalgesic effects in animal models of inflammatory and neuropathic pain. Herein, we describe the synthesis and the structure-activity relationships of a series of 2-(4-pyridin-2-ylpiperazin-1-yl)-1H-benzo[d]imidazoles as novel TRPV1 antagonists. Compound 46ad was among the most potent analogues in this series. This compound was orally bioavailable in rats and was efficacious in blocking capsaicin-induced flinch in rats in a dose-dependent manner. Compound 46ad also reversed thermal hyperalgesia in a model of inflammatory pain, which was induced by complete Freund's adjuvant (CFA).

Discovery of potent, orally available vanilloid receptor-1 antagonists. Structure-activity relationship of N-aryl cinnamides.

The vanilloid receptor-1 (TRPV1 or VR1) is a member of the transient receptor potential (TRP) family of ion channels and plays a role in regulating the function of sensory nerves. A growing body of evidence demonstrates the therapeutic potential of TRPV1 modulators, particularly in the management of pain. As a result of our screening efforts, we identified (E)-3-(4-tert-butylphenyl)-N-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)acrylamide (1), an antagonist that blocks the capsaicin-induced and pH-induced uptake of (45)Ca(2+) in TRPV1-expressing Chinese hamster ovary cells with IC(50) values of 17 +/- 5 and 150 +/- 80 nM, respectively. In this report, we describe the synthesis and structure-activity relationship of a series of N-aryl cinnamides, the most potent of which (49a and 49b) exhibit good oral bioavailability in rats (F(oral) = 39% and 17%, respectively).

KTP/532 YAG laser treatment for allergic rhinitis.

BACKGROUND: The purpose of this study was to investigate the efficacy of the KTP/532 YAG laser to reduce nasal congestion and discharge in patients with allergic rhinitis. METHODS: Forty-eight patients with symptoms of allergic rhinitis were treated with the KTP/532 laser. All had positive skin tests for common allergens. Treatments were provided on an ambulatory basis in one to three sessions under local anesthesia with lidocaine nose spray. Outcome was determined by daily symptom reports and regular endoscopy examination and interviews for 12 months. RESULTS: Treatment was very well tolerated. There were no major side effects. At examination after 1 year, nasal obstruction was improved in 69% and nasal discharge in 40% of cases. CONCLUSION: The KTP/532 YAG laser is effective for the treatment of nasal obstruction and discharge. Comparison with other techniques showed it to be the most effective in reducing nasal discharge. It can be done as an office procedure and does not damage the nasal mucous membrane. The KTP/532 YAG laser is effective as an additional treatment for patients refractory to medication.

Structure-activity relationship (SAR) investigations of substituted imidazole analogs as TRPV1 antagonists.

A novel series of 4,5-biarylimidazoles as TRPV1 antagonists were designed based on the previously reported 4,6-disubstituted benzimidazole series. The analogs were evaluated for their ability to block capsaicin- or acid-induced calcium influx in TRPV1-expressing CHO cells. These studies led to the identification of a highly potent and orally bioavailable TRPV1 antagonist, imidazole 33.

Repeated administration of vanilloid receptor TRPV1 antagonists attenuates hyperthermia elicited by TRPV1 blockade.

Capsaicin, the active ingredient in some pain-relieving creams, is an agonist of a nonselective cation channel known as the transient receptor potential vanilloid type 1 (TRPV1). The pain-relieving mechanism of capsaicin includes desensitization of the channel, suggesting that TRPV1 antagonism may be a viable pain therapy approach. In agreement with the above notion, several TRPV1 antagonists have been reported to act as antihyperalgesics. Here, we report the in vitro and in vivo characterization of a novel and selective TRPV1 antagonist, N-(4-[6-(4-trifluoromethyl-phenyl)-pyrimidin-4-yloxy]-benzothiazol-2-yl)-acetamide I (AMG 517), and compare its pharmacology with that of a closely related analog, tert-butyl-2-(6-([2-(acetylamino)-1,3-benzothiazol-4-yl]oxy)pyrimidin-4-yl)-5-(trifluoromethyl)phenylcarbamate (AMG8163). Both AMG 517 and AMG8163 potently and completely antagonized capsaicin, proton, and heat activation of TRPV1 in vitro and blocked capsaicin-induced flinch in rats in vivo. To support initial clinical investigations, AMG 517 was evaluated in a comprehensive panel of toxicology studies that included in vivo assessments in rodents, dogs, and monkeys. The toxicology studies indicated that AMG 517 was generally well tolerated; however, transient increases in body temperature (hyperthermia) were observed in all species after AMG 517 dosing. To further investigate this effect, we tested and showed that the antipyretic, acetaminophen, suppressed the hyperthermia caused by TRPV1 blockade. We also showed that repeated administration of TRPV1 antagonists attenuated the hyperthermia response, whereas the efficacy in capsaicin-induced flinch model was maintained. In conclusion, these studies suggest that the transient hyperthermia elicited by TRPV1 blockade may be manageable in the development of TRPV1 antagonists as therapeutic agents. However, the impact of TRPV1 antagonist-induced hyperthermia on their clinical utility is still unknown.

A polyclonal antibody to the prepore loop of transient receptor potential vanilloid type 1 blocks channel activation.

Transient receptor potential vanilloid type 1 (TRPV1) can be activated by multiple chemical and physical stimuli such as capsaicin, anandamide, protons, and heat. Capsaicin interacts with the binding pocket constituted by transmembrane regions 3 and 4, whereas protons act through residues in the prepore loop of TRPV1. Here, we report on characterization of polyclonal and monoclonal antibodies to the prepore loop of TRPV1. A rabbit anti-rat TRPV1 polyclonal antibody (Ab-156H) acted as a full antagonist of proton activation (IC(50) values for pH 5 and 5.5 were 364.68 +/- 29.78 and 28.31 +/- 6.30 nM, respectively) and as a partial antagonist of capsaicin, heat, and pH 6 potentiated chemical ligand (anandamide and capsaicin) activation (50-79% inhibition). Ab-156H antagonism of TRPV1 is not affected by the conformation of the capsaicin-binding pocket because it is equally potent at wild-type (capsaicin-sensitive) rat TRPV1 and its T550I mutant (capsaicin-insensitive). With the goal of generating monoclonal antagonist antibodies to the prepore region of human TRPV1, we used a recently developed rabbit immunization protocol. Although rabbit polyclonal antiserum blocked human TRPV1 activation, rabbit monoclonal antibodies (identified on the basis of selective binding to Chinese hamster ovary cells expressing human TRPV1) did not block activation by either capsaicin or protons. Thus, rabbit polyclonal antibodies against rat and human TRPV1 prepore region seem to partially lock or stabilize the channel in the closed state, whereas rabbit anti-human TRPV1 monoclonal antibodies bind to the prepore region but do not lock or stabilize the channel conformation.

Proton activation does not alter antagonist interaction with the capsaicin-binding pocket of TRPV1.

Vanilloid receptor 1 (TRPV1) is activated by chemical ligands (e.g., capsaicin and protons) and heat. In this study, we show that (2E)-3-[2-piperidin-1-yl-6-(trifluoromethyl)pyridin-3-yl]-N-quinolin-7-ylacrylamide (AMG6880), 5-chloro-6-[(3R)-3-methyl-4-[6-(trifluoromethyl)-4-(3,4,5-trifluorophenyl)-1H-benzimidazol-2-yl]piperazin-1-yl]pyridin-3-yl)methanol (AMG7472), and N-(4-tertiarybutylphenyl)-4-(3-chloropyridin-2-yl)tetrahydropyrazine-1(2H)-carboxamide (BCTC) are potent antagonists of rat TRPV1 activation by either capsaicin or protons (pH 5) (defined here as group A antagonists), whereas (2E)-3-(6-tert-butyl-2-methylpyridin-3-yl)-N-(1H-indol-6-yl)acrylamide (AMG0610), capsazepine, and (2E)-3-(4-chlorophenyl)-N-(3-methoxyphenyl)acrylamide (SB-366791) are antagonists of capsaicin, but not proton, activation (defined here as group B antagonists). By using capsaicin-sensitive and insensitive rabbit TRPV1 channels, we show that antagonists require the same critical molecular determinants located in the transmembrane domain 3/4 region to block both capsaicin and proton activation, suggesting the presence of a single binding pocket. To determine whether the differential pharmacology is a result of proton activation-induced conformational changes in the capsaicin-binding pocket that alter group B antagonist affinities, we have developed a functional antagonist competition assay. We hypothesized that if group B antagonists bind at the same or an overlapping binding pocket of TRPV1 as group A antagonists, and proton activation does not alter the binding pocket, then group B antagonists should compete with and prevent group A antagonism of TRPV1 activation by protons. Indeed, we found that each of the group B antagonists competed with and prevented BCTC, AMG6880 or AMG7472 antagonism of rat TRPV1 activation by protons with pA2 values similar to those for blocking capsaicin, indicating that proton activation does not alter the conformation of the TRPV1 capsaicin-binding pocket. In conclusion, group A antagonists seem to lock the channel conformation in the closed state, blocking both capsaicin and proton activation.

Synthesis and evaluation of thiazole carboxamides as vanilloid receptor 1 (TRPV1) antagonists.

A thiazole derivative, 2-(2,6-dichlorobenzyl)-N-(4-isopropylphenyl) thiazole-4-carboxamide (1), was identified as a TRPV1 antagonist. We synthesized various thiazole analogs and evaluated them for their ability to block capsaicin- or acid-induced calcium influx in TRPV1-expressing CHO cells. The IC(50) values of the most potent antagonists were ca. 0.050microM in these assays.

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.