Michael E. Kort

A-803467, a potent and selective Nav1.8 sodium channel blocker, attenuates neuropathic and inflammatory pain in the rat.

Activation of tetrodotoxin-resistant sodium channels contributes to action potential electrogenesis in neurons. Antisense oligonucleotide studies directed against Nav1.8 have shown that this channel contributes to experimental inflammatory and neuropathic pain. We report here the discovery of A-803467, a sodium channel blocker that potently blocks tetrodotoxin-resistant currents (IC50 = 140 nM) and the generation of spontaneous and electrically evoked action potentials in vitro in rat dorsal root ganglion neurons. In recombinant cell lines, A-803467 potently blocked human Nav1.8 (IC50 = 8 nM) and was >100-fold selective vs. human Nav1.2, Nav1.3, Nav1.5, and Nav1.7 (IC50 values ≥1 μM). A-803467 (20 mg/kg, i.v.) blocked mechanically evoked firing of wide dynamic range neurons in the rat spinal dorsal horn. A-803467 also dose-dependently reduced mechanical allodynia in a variety of rat pain models including: spinal nerve ligation (ED50 = 47 mg/kg, i.p.), sciatic nerve injury (ED50 = 85 mg/kg, i.p.), capsaicin-induced secondary mechanical allodynia (ED50 ≈ 100 mg/kg, i.p.), and thermal hyperalgesia after intraplantar complete Freunds adjuvant injection (ED50 = 41 mg/kg, i.p.). A-803467 was inactive against formalin-induced nociception and acute thermal and postoperative pain. These data demonstrate that acute and selective pharmacological blockade of Nav1.8 sodium channels in vivo produces significant antinociception in animal models of neuropathic and inflammatory pain.

Selective blockade of TRPA1 channel attenuates pathological pain without altering noxious cold sensation or body temperature regulation

&NA; Despite the increasing interest in TRPA1 channel as a pain target, its role in cold sensation and body temperature regulation is not clear; the efficacy and particularly side effects resulting from channel blockade remain poorly understood. Here we use a potent, selective, and bioavailable antagonist to address these issues. A‐967079 potently blocks human (IC50: 51 nmol/L, electrophysiology, 67 nmol/L, Ca2+ assay) and rat TRPA1 (IC50: 101 nmol/L, electrophysiology, 289 nmol/L, Ca2+ assay). It is >1000‐fold selective over other TRP channels, and is >150‐fold selective over 75 other ion channels, enzymes, and G‐protein‐coupled receptors. Oral dosing of A‐967079 produces robust drug exposure in rodents, and exhibits analgesic efficacy in allyl isothiocyanate‐induced nocifensive response and osteoarthritic pain in rats (ED50: 23.2 mg/kg, p.o.). A‐967079 attenuates cold allodynia produced by nerve injury but does not alter noxious cold sensation in naive animals, suggesting distinct roles of TRPA1 in physiological and pathological states. Unlike TRPV1 antagonists, A‐967079 does not alter body temperature. It also does not produce locomotor or cardiovascular side effects. Collectively, these data provide novel insights into TRPA1 function and suggest that the selective TRPA1 blockade may present a viable strategy for alleviating pain without untoward side effects. Selective blockade of TRPA1 channel attenuates pathological pain without altering noxious cold sensation or body temperature regulation.

TRPA1 modulation of spontaneous and mechanically evoked firing of spinal neurons in uninjured, osteoarthritic, and inflamed rats

BackgroundThere is growing evidence supporting a role for TRPA1 receptors in the neurotransmission of peripheral mechanical stimulation. In order to enhance understanding of TRPA1 contributions to mechanotransmission, we examined the effects a selective TRPA1 receptor antagonist, A-967079, on spinal neuronal activity following peripheral mechanical stimulation in uninjured, CFA-inflamed, and osteoarthritc (OA) rats.ResultsSystemic injection of A-967079 (30 μmol/kg, i.v.) decreased the responses of wide dynamic range (WDR), and nociceptive specific (NS) neurons following noxious pinch stimulation of the ipsilateral hind paw in uninjured and CFA-inflamed rats. Similarly, A-967079 reduced the responses of WDR neurons to high-intensity mechanical stimulation (300 g von Frey hair) of the knee joint in both OA and OA-sham rats. WDR neuronal responses to low-intensity mechanical stimulation (10 g von Frey hair) were also reduced by A-967079 administration to CFA-inflamed rats, but no effect was observed in uninjured rats. Additionally, the spontaneous activity of WDR neurons was decreased after A-967079 injection in CFA-inflamed rats but was unaltered in uninjured, OA, and OA-sham animals.ConclusionsBlockade of TRPA1 receptors disrupts transmission of high-intensity mechanical stimulation to the spinal cord in both uninjured and injured rats indicating that TRPA1 receptors have an important role in noxious mechanosensation in both normal and pathological conditions. TRPA1 receptors also contribute to the transmission of low-intensity mechanical stimulation, and to the modulation of spontaneous WDR firing, but only after an inflammatory injury.

Analgesic potential of TRPV1 antagonists

The discovery of TRPV1 antagonists as a new class of analgesic agents for the treatment of chronic pathological pain has been pursued aggressively across the pharmaceutical industry. This effort has led to the identification of several TRPV1 antagonists that have entered clinical trials, including ABT-102 (Abbott), SB-705498 (GSK), AMG-517 (Amgen), MK2295 (Merck/Neurogen), and GRC-6211 (Lilly/Glenmark). Using the published structures for ABT-102, SB-705498, AMG-517, and lead compounds representing six additional TRPV1 antagonist chemotypes, a pharmacophore model that describes the common structural features found in potent TRPV1 antagonists was established. The TRPV1 antagonist pharmacophore fits within the pore region of a TRPV1 receptor homology model, with critical hydrogen bond interactions proposed between the TRPV1 antagonist pharmacophore and Tyr 667 on helix six. In spite of the putative common binding site for all TRPV1 antagonists included in this particular TRPV1 pharmacophore, these ligands have demonstrated that they can still offer distinct pharmacological profiles, likely due to differences in their pharmacokinetic profiles. This is highlighted by differences in temperature elevation observed when comparing the clinical candidates ABT-102 and AMG-517.

Molecular Determinants of Species-Specific Activation or Blockade of TRPA1 Channels

TRPA1 is an excitatory, nonselective cation channel implicated in somatosensory function, pain, and neurogenic inflammation. Through covalent modification of cysteine and lysine residues, TRPA1 can be activated by electrophilic compounds, including active ingredients of pungent natural products (e.g., allyl isothiocyanate), environmental irritants (e.g., acrolein), and endogenous ligands (4-hydroxynonenal). However, how covalent modification leads to channel opening is not understood. Here, we report that electrophilic, thioaminal-containing compounds [e.g., CMP1 (4-methyl-N-[2,2,2-trichloro-1-(4-nitro-phenylsulfanyl)-ethyl]-benzamide)] covalently modify cysteine residues but produce striking species-specific effects [i.e., activation of rat TRPA1 (rTRPA1) and blockade of human TRPA1 (hTRPA1) activation by reactive and nonreactive agonists]. Through characterizing rTRPA1 and hTRPA1 chimeric channels and point mutations, we identified several residues in the upper portion of the S6 transmembrane domains as critical determinants of the opposite channel gating: Ala-946 and Met-949 of rTRPA1 determine channel activation, whereas equivalent residues of hTRPA1 (Ser-943 and Ile-946) determine channel block. Furthermore, side-chain replacements at these critical residues profoundly affect channel function. Therefore, our findings reveal a molecular basis of species-specific channel gating and provide novel insights into how TRPA1 respond to stimuli.

Discovery and biological evaluation of 5-aryl-2-furfuramides, potent and selective blockers of the Nav1.8 sodium channel with efficacy in models of neuropathic and inflammatory pain.

Nav1.8 (also known as PN3) is a tetrodotoxin-resistant (TTx-r) voltage-gated sodium channel (VGSC) that is highly expressed on small diameter sensory neurons and has been implicated in the pathophysiology of inflammatory and neuropathic pain. Recent studies using an Nav1.8 antisense oligonucleotide in an animal model of chronic pain indicated that selective blockade of Nav1.8 was analgesic and could provide effective analgesia with a reduction in the adverse events associated with nonselective VGSC blocking therapeutic agents. Herein, we describe the preparation and characterization of a series of 5-substituted 2-furfuramides, which are potent, voltage-dependent blockers (IC50 < 10 nM) of the human Nav1.8 channel. Selected derivatives, such as 7 and 27, also blocked TTx-r sodium currents in rat dorsal root ganglia (DRG) neurons with comparable potency and displayed >100-fold selectivity versus human sodium (Nav1.2, Nav1.5, Nav1.7) and human ether-a-go-go (hERG) channels. Following systemic administration, compounds 7 and 27 dose-dependently reduced neuropathic and inflammatory pain in experimental rodent models.

Transient receptor potential vanilloid-1 antagonists: a survey of recent patent literature

Importance of the field: Transient receptor potential vanilloid-1 (TRPV1, vanilloid receptor-1) is a nonspecific cation channel that can be activated by multiple endogenous stimuli and by capsaicin, the active ingredient in chili peppers. TRPV1 is expressed predominantly on sensory neurons where it is proposed to serve as a key nodal point in pain transmission pathways. Pharmacological blockade of TRPV1 represents a compelling strategy for the treatment of a variety of disease states, particularly those requiring chronic pain management. Area covered in the review: This review summarizes patent literature and progress in defining the utility of small molecule TRPV1 antagonists during 2008 – 2009. What the reader will gain: Representative compounds and key characterization data comprising multiple chemical series are highlighted. Take home message: The continued profusion of reports, in both the primary and patent literature, attests to the sustained interest in the TRPV1 class of therapeutics. Although a number of compounds have now been brought forward for human clinical trials, the therapeutic utility of TRPV1 antagonists is yet to be validated unequivocally.

A-995662 [(R)-8-(4-methyl-5-(4-(trifluoromethyl)phenyl)oxazol-2-ylamino)-1,2,3,4-tetrahydronaphthalen-2-ol], a novel, selective TRPV1 receptor antagonist, reduces spinal release of glutamate and CGRP in a rat knee joint pain model.

&NA; The TRPV1 antagonist A‐995662 demonstrates analgesic efficacy in monoiodoacetate‐induced osteoarthritic (OA) pain in rat, and repeated dosing results in increased in vivo potency and a prolonged duration of action. To identify possible mechanism(s) underlying these observations, release of neuropeptides and the neurotransmitter glutamate from isolated spinal cord was measured. In OA rats, basal release of glutamate, bradykinin and calcitonin gene‐related peptide (CGRP) was significantly elevated compared to naïve levels, whereas substance P (SP) levels were not changed. In vitro studies showed that capsaicin‐evoked TRPV1‐dependent CGRP release was 54.7 ± 7.7% higher in OA, relative to levels measured for naïve rats, suggesting that TRPV1 activity was higher under OA conditions. The efficacy of A‐995662 in OA corresponded with its ability to inhibit glutamate and CGRP release from the spinal cord. A single, fully efficacious dose of A‐995662, 100 &mgr;mol/kg, reduced spinal glutamate and CGRP release, while a single sub‐efficacious dose of A‐995662 (25 &mgr;mol/kg) was ineffective. Multiple dosing with A‐995662 increased the potency and duration of efficacy in OA rats. Changes in efficacy did not correlate with plasma concentrations of A‐995662, but were accompanied with reductions in spinal glutamate release. These findings suggest that repeated dosing of TRPV1 antagonists enhances therapeutic potency and duration of action against OA pain, at least in part, by the sustained reduction in release of glutamate and CGRP from the spinal cord.

Transient Receptor Potential Channel Ankyrin-1 Is Not a Cold Sensor for Autonomic Thermoregulation in Rodents

The rodent transient receptor potential ankyrin-1 (TRPA1) channel has been hypothesized to serve as a temperature sensor for thermoregulation in the cold. We tested this hypothesis by using deletion of the Trpa1 gene in mice and pharmacological blockade of the TRPA1 channel in rats. In both Trpa1−/− and Trpa1+/+ mice, severe cold exposure (8°C) resulted in decreases of skin and deep body temperatures to ∼8°C and 13°C, respectively, both temperatures being below the reported 17°C threshold temperature for TRPA1 activation. Under these conditions, Trpa1−/− mice had the same dynamics of body temperature as Trpa1+/+ mice and showed no weakness in the tail skin vasoconstriction response or thermogenic response to cold. In rats, the effects of pharmacological blockade were studied by using two chemically unrelated TRPA1 antagonists: the highly potent and selective compound A967079, which had been characterized earlier, and the relatively new compound 43 ((4R)-1,2,3,4-tetrahydro-4-[3-(3-methoxypropoxy)phenyl]-2-thioxo-5H-indeno[1,2-d]pyrimidin-5-one), which we further characterized in the present study and found to be highly potent (IC50 against cold of ∼8 nm) and selective. Intragastric administration of either antagonist at 30 mg/kg before severe (3°C) cold exposure did not affect the thermoregulatory responses (deep body and tail skin temperatures) of rats, even though plasma concentrations of both antagonists well exceeded their IC50 value at the end of the experiment. In the same experimental setup, blocking the melastatin-8 (TRPM8) channel with AMG2850 (30 mg/kg) attenuated cold-defense mechanisms and led to hypothermia. We conclude that TRPA1 channels do not drive autonomic thermoregulatory responses to cold in rodents.

TRPV1 antagonists: clinical setbacks and prospects for future development.

Abstract Clinical data demonstrating target-mediated elevation of core body temperature in humans have been a major setback in the development of TRPV1 antagonists as novel therapeutics for the treatment of chronic pain. However, multiple recent reports of TRPV1 antagonists that do not affect core body temperature in preclinical species suggest a potential path forward for the field. Stimulus-specific agents have been identified that fully block TRPV1 activation by capsaicin, do not effectively block activation of TRPV1 by acidic (