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

BACKGROUND: There 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. RESULTS: Systemic injection of A-967079 (30 micromol/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. CONCLUSIONS: Blockade 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.

Discovery of TRPV1 antagonist ABT-116.

The synthesis and SAR of a series of indazole TRPV1 antagonists leading to the discovery of 21 (ABT-116) is described. Biological studies demonstrated potent in vitro and in vivo activity for 21, as well as suitable physicochemical and pharmacokinetic properties for advancement to clinical development for pain management.

Synthesis and biological evaluation of 5-substituted and 4,5-disubstituted-2-arylamino oxazole TRPV1 antagonists.

The synthesis and structure-activity relationships of a series of 5-monosubstituted and 4,5-disubstituted 2-arylaminooxazoles as novel antagonists of the transient receptor potential vanilloid 1 (TRPV1) receptor are described. The 7-hydroxy group of the tetrahydronaphthyl moiety on the 2-amino substituent of the oxazole ring was important for obtaining excellent in vitro potency at the human TRPV1 receptor, while a variety of alkyl and phenyl substituents at the 4- and 5-positions of the oxazole ring were well tolerated and yielded potent TRPV1 antagonists. Despite excellent in vitro potency, the 5-monosubstituted compounds suffered from poor pharmacokinetics. It was found that 4,5-disubstitution on the oxazole ring was critical to the improvement of the overall pharmacokinetic profile of these analogues, which led to the discovery of compound (R)-27, a novel TRPV1 antagonist with good oral activity in preclinical animal models of pain.

In vitro structure-activity relationship and in vivo characterization of 1-(aryl)-3-(4-(amino)benzyl)urea transient receptor potential vanilloid 1 antagonists.

The synthesis and structure-activity relationship of 1-(aryl)-3-(4-(amino)benzyl)urea transient receptor potential vanilloid 1 (TRPV1) antagonists are described. A variety of cyclic amine substituents are well tolerated at the 4-position of the benzyl group on compounds containing either an isoquinoline or indazole heterocyclic core. These compounds are potent antagonists of capsaicin activation of the TRPV1 receptor in vitro. Analogues, such as compound 45, have been identified that have good in vivo activity in animal models of pain. Further optimization of 45 resulted in compound 58 with substantially improved microsome stability and oral bioavailability, as well as in vivo activity.

Novel transient receptor potential vanilloid 1 receptor antagonists for the treatment of pain: structure-activity relationships for ureas with quinoline, isoquinoline, quinazoline, phthalazine, quinoxaline, and cinnoline moieties.

Novel transient receptor potential vanilloid 1 (TRPV1) receptor antagonists with various bicyclic heteroaromatic pharmacophores were synthesized, and their in vitro activity in blocking capsaicin activation of TRPV1 was assessed. On the basis of the contribution of these pharmacophores to the in vitro potency, they were ranked in the order of 5-isoquinoline > 8-quinoline = 8-quinazoline > 8-isoquinoline > or = cinnoline approximately phthalazine approximately quinoxaline approximately 5-quinoline. The 5-isoquinoline-containing compound 14a (hTRPV1 IC50 = 4 nM) exhibited 46% oral bioavailability and in vivo activity in animal models of visceral and inflammatory pain. Pharmacokinetic and pharmacological properties of 14a are substantial improvements over the profile of the high-throughput screening hit 1 (hTRPV1 IC50 = 22 nM), which was not efficacious in animal pain models and was not orally bioavailable.

5,6,7-trisubstituted 4-aminopyrido[2,3-d]pyrimidines as novel inhibitors of adenosine kinase.

The synthesis and structure-activity relationship of a series of 5,6,7-trisubstituted 4-aminopyrido[2,3-d]pyrimidines as novel nonnucleoside adenosine kinase inhibitors is described. A variety of alkyl, aryl, and heteroaryl substituents were found to be tolerated at the C5, C6, and C7 positions of the pyridopyrimidine core. These studies have led to the identification of analogues that are potent inhibitors of adenosine kinase with in vivo analgesic activity.

Discovery of (R)-1-(7-chloro-2,2-bis(fluoromethyl)chroman-4-yl)-3-(3-methylisoquinolin-5-yl)urea (A-1165442): a temperature-neutral transient receptor potential vanilloid-1 (TRPV1) antagonist with analgesic efficacy.

The synthesis and characterization of a series of selective, orally bioavailable 1-(chroman-4-yl)urea TRPV1 antagonists is described. Whereas first-generation antagonists that inhibit all modes of TRPV1 activation can elicit hyperthermia, the compounds disclosed herein do not elevate core body temperature in preclinical models and only partially block acid activation of TRPV1. Advancing the SAR of this series led to the eventual identification of (R)-1-(7-chloro-2,2-bis(fluoromethyl)chroman-4-yl)-3-(3-methylisoquinolin-5-yl)urea (A-1165442, 52), an analogue that possesses excellent pharmacological selectivity, has a favorable pharmacokinetic profile, and demonstrates good efficacy against osteoarthritis pain in rodents.

An automated electrophysiological assay for differentiating Ca(v)2.2 inhibitors based on state dependence and kinetics.

Ca(V)2.2 (N-type) calcium channels are key regulators of neurotransmission. Evidence from knockout animals and localization studies suggest that Ca(V)2.2 channels play a critical role in nociceptive transmission. Additionally, ziconotide, a selective peptide inhibitor of Ca(V)2.2 channels, is clinically used to treat refractory pain. However, the use of ziconotide is limited by its low therapeutic index, which is believed, at least in part, to be a consequence of ziconotide inhibiting Ca(V)2.2 channels regardless of the channel state. Subsequent efforts have focused on the discovery of state-dependent inhibitors that preferentially bind to the inactivated state of Ca(V)2.2 channels in order to achieve an improved safety profile relative to ziconotide. Much less attention has been paid to understanding the binding kinetics of these state-dependent inhibitors. Here, we describe a novel electrophysiology-based assay on an automated patch platform designed to differentiate Ca(V)2.2 inhibitors based on their combined state dependence and kinetics. More specifically, this assay assesses inactivated state block, closed state block, and monitors the kinetics of recovery from block when channels move between states. Additionally, a use-dependent assay is described that uses a train of depolarizing pulses to drive channels to a similar level of inactivation for comparison. This use-dependent protocol also provides information on the kinetics of block development. Data are provided to show how these assays can be utilized to screen for kinetic diversity within and across chemical classes.

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

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 (IC(50): 51 nmol/L, electrophysiology, 67 nmol/L, Ca(2+) assay) and rat TRPA1 (IC(50): 101 nmol/L, electrophysiology, 289 nmol/L, Ca(2+) 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 (ED(50): 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.

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.

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 micromol/kg, reduced spinal glutamate and CGRP release, while a single sub-efficacious dose of A-995662 (25 micromol/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.

Alpha-methylation at benzylic fragment of N-aryl-N'-benzyl ureas provides TRPV1 antagonists with better pharmacokinetic properties and higher efficacy in inflammatory pain model.

SAR studies for N-aryl-N'-benzyl urea class of TRPV1 antagonists have been extended to cover alpha-benzyl alkylation. Alkylated compounds showed weaker in vitro potencies in blocking capsaicin activation of TRPV1 receptor, but possessed improved pharmacokinetic properties. Further structural manipulations that included replacement of isoquinoline core with indazole and isolation of single enantiomer led to TRPV1 antagonists like (R)-16a with superior pharmacokinetic properties and greater potency in animal model of inflammatory pain.

P2X receptor ligands and pain.

P2X receptors belong to a superfamily of ligand-gated ion channels that conduct the influx of Ca(2+), Na(+) and K(+) cations following activation by extracellular nucleotides such as ATP. Molecular cloning studies have identified seven subunits, namely P2X(1-7), that share approximately 40 - 50% identity in amino acid sequences within the subfamily. Using gene-silencing, pharmacological and electrophysiological approaches, recent studies have revealed roles for P2X(2), P2X(3), P2X(4) and P2X(7) receptors in nociceptive signalling. Homomeric P2X(3) and heteromeric P2X(2/3) receptors are highly localised in the peripheral sensory afferent neurons that conduct nociceptive sensory information to the spinal chord and brain. The discovery of A-317491, a selective and potent non-nucleotide P2X(3) antagonist, provided a pharmacological tool to determine the site and mode of action of P2X(3)-containing receptors in different pain behaviours, including neuropathic, inflammatory and visceral pain. Other P2X receptors (P2X(4) and P2X(7)) that are predominantly expressed in microglia, macrophages and cells of immune origin can trigger the release of cytokines, such as IL-1-beta and TNF-alpha. Genetic disruption of P2X(4) and P2X(7) signalling has been demonstrated to reduce inflammatory and neuropathic pain, suggesting that these two receptors might serve as integrators of neuroinflammation and pain. This article provides an overview of recent scientific literature and patents focusing on P2X(3), P2X(4) and P2X(7) receptors, and the identification of small molecule ligands for the potential treatment of neuropathic and inflammatory pain.

Structure-activity studies of a novel series of 5,6-fused heteroaromatic ureas as TRPV1 antagonists.

Novel 5,6-fused heteroaromatic ureas were synthesized and evaluated for their activity as TRPV1 antagonists. It was found that 4-aminoindoles and indazoles are the preferential cores for the attachment of ureas. Bulky electron-withdrawing groups in the para-position of the aromatic ring of the urea substituents imparted the best in vitro potency at TRPV1. The most potent derivatives were assessed in in vivo inflammatory and neuropathic pain models. Compound 46, containing the indazole core and a 3,4-dichlorophenyl group appended to it via a urea linker, demonstrated in vivo analgesic activity upon oral administration. This derivative also showed selectivity versus other receptors in the CEREP screen and exhibited acceptable cardiovascular safety at levels exceeding the therapeutic dose.

Synthesis and biological evaluation of 6,7-disubstituted 4-aminopyrido[2,3-d]pyrimidines as adenosine kinase inhibitors.

The synthesis and structure-activity relationship of a series of 6,7-disubstituted 4-aminopyrido[2,3-d]pyrimidines as novel non-nucleoside adenosine kinase inhibitors is described. A variety of substituents, primarily aryl, at the C6 and C7 positions of the pyridopyrimidine core were found to yield analogues that are potent inhibitors of adenosine kinase. In contrast to the 5,7-disubstituted and 5,6,7-trisubstituted pyridopyrimidine series, these analogues exhibited only modest potency to inhibit AK in intact cells.

Adenosine kinase inhibitors: polar 7-substitutent of pyridopyrimidine derivatives improving their locomotor selectivity.

We have discovered that polar 7-substituents of pyridopyrimidine derivatives affect not only whole cell AK inhibitory potency, but also selectivity in causing locomotor side effects in vivo animal models. We have identified compound, 1o, which has potent whole cell AK inhibitory potency, analgesic activity and minimal reduction of locomotor activity.