A-887826 is a structurally novel, potent and voltage-dependent Nav1.8 sodium channel blocker that attenuates neuropathic tactile allodynia in rats

doi:10.1016/j.neuropharm.2010.05.009

Neuropharmacology

Volume 59, Issue 3, September 2010, Pages 201-207

https://doi.org/10.1016/j.neuropharm.2010.05.009 Get rights and content

Abstract

Activation of sodium channels is essential to action potential generation and propagation. Recent genetic and pharmacological evidence indicates that activation of Na_v1.8 channels contributes to chronic pain. Herein, we describe the identification of a novel series of structurally related pyridine derivatives as potent Na_v1.8 channel blockers. A-887826 exemplifies this series and potently (IC₅₀ = 11nM) blocked recombinant human Na_v1.8 channels. A-887826 was ∼3 fold less potent to block Na_v1.2, ∼10 fold less potent to block tetrodotoxin-sensitive sodium (TTX-S Na⁺) currents and was >30 fold less potent to block Na_V1.5 channels. A-887826 potently blocked tetrodotoxin-resistant sodium (TTX-R Na⁺) currents (IC₅₀ = 8nM) from small diameter rat dorsal root ganglion (DRG) neurons in a voltage-dependent fashion. A-887826 effectively suppressed evoked action potential firing when DRG neurons were held at depolarized potentials and reversibly suppressed spontaneous firing in small diameter DRG neurons from complete Freund’s adjuvant inflamed rats. Following oral administration, A-887826 significantly attenuated tactile allodynia in a rat neuropathic pain model. Further characterization of TTX-R current block in rat DRG neurons demonstrated that A-887826 (100 nM) shifted the mid-point of voltage-dependent inactivation of TTX-R currents by ∼4 mV without affecting voltage-dependent activation and did not exhibit frequency-dependent inhibition. The present data demonstrate that A-887826 is a structurally novel and potent Na_v1.8 blocker that inhibits rat DRG TTX-R currents in a voltage-, but not frequency-dependent fashion. The ability of this structurally novel Na_v1.8 blocker to effectively reduce tactile allodynia in neuropathic rats further supports the role of Na_v1.8 sodium channels in pathological pain states.

Introduction

Voltage-gated sodium channels play important roles in action potential generation and propagation (Waxman et al., 1999). At least 9 genes have been identified that encode functional sodium channels, namely Na_v1.1–Na_v1.9, and each subtype can be functionally classified as either tetrodotoxin-sensitive (TTX-S) or TTX-resistant (TTX-R) (Catterall et al., 2005). Both TTX-S and TTX-R sodium channels are expressed on primary afferent sensory neurons (Black et al., 2008, Renganathan et al., 2002). Recent pharmacological and genetic data have provided evidence for specific involvement of Na_v1.3, 1.7, 1.8, and 1.9 in the processing of nociceptive information (Dib-Hajj et al., 2009). Of these, Na_v1.8 is a TTX-R sodium channel that is highly localized on primary sensory afferent neurons (Akopian et al., 1999, Djouhri et al., 2003).

Studies using gene ablation, antisense (Porreca et al., 1999, Joshi et al., 2006) or small interfering RNA (siRNA) (Dong et al., 2007) to knockdown expression of Na_v1.8 have demonstrated a significant reduction in mechanical allodynia and hyperalgesia in inflammatory and neuropathic pain models in rats. Suppression of Na_v1.8 expression also produces a reduction in visceral pain in several experimental models (Yoshimura et al., 2001, Laird et al., 2002). The importance of Na_v1.8 channels in pain sensitivity is also supported by recent data showing that activation of Na_v1.8 sodium channels is the primary driver of nociceptor excitability under cold conditions (Zimmermann et al., 2007).

Systemic or spinal delivery of non-selective sodium channel blockers reduces hyperalgesia and allodynia in animal pain models (Veneroni et al., 2003, Brochu et al., 2006, Gaida et al., 2005, Akada et al., 2006, Ekberg et al., 2006). Previously, we reported the discovery of A-803467, a potent, voltage-dependent, and highly selective Na_v1.8 channel blocker that effectively reduces chronic pain in preclinical models. The discovery of A-803467 demonstrated that pharmacological selective sodium channel blockers is achievable and also provided support for the role of Na_v1.8 in chronic pain states (Jarvis et al., 2007).

In the present study, we report the discovery and characterization of A-887826, 5-(4-butoxy-3-chlorophenyl)-N-((2-morpholinopyridin-3-yl)methyl)nicotinamide (Fig. 1A). A-887826 and related pyridine analogs represent structurally novel and highly potent Na_v1.8 sodium channel blockers that are chemically distinct from A-803467. A-887826 potently blocks Na_v1.8 channels in a voltage-, but not frequency-, dependent fashion and is less active at other sodium channels including Na_v1.2 and Na_v1.5. Following systemic administration, A-887826 produced dose-related attenuation of tactile allodynia in an animal model of neuropathic pain.

Section snippets

Animals

Male Sprague–Dawley rats (Charles River, Wilmington, MA) weighing 200–300 grams were utilized for in vivo studies. All procedures involving animal use were carried out in accordance with the Abbott Laboratories Institutional Animal Care and Use Committee. Rats were maintained in a temperature-controlled facility with a 12 h light/dark cycle and had ad libitum access to water and regular rodent chow.

HEK cells expressing human sodium channels

Human embryonic kidney (HEK-293) cells expressing recombinant sodium channels were grown in

Pyridine-based blockers of recombinant human Na_V1.8 Na⁺ currents

Based on the A-803467 furfuramide scaffold (Kort et al., 2008), a series of structurally related 5-aryl nicotinamide derivatives was prepared and evaluated for their ability to block recombinant human Na_v1.8 channels (Table 1). Substitution at the 4-position provided potent Na_v1.8 blocking activity (e.g., compound 1) and incorporation of an adjacent halogen (e.g., 3-chloro, compound 2) was well tolerated. The potency could be further enhanced by increasing steric bulk (e.g. compounds 3–6).

Discussion

The present data demonstrate that A-887826 potently blocks both recombinant human and native rat Na_v1.8 sodium channels in a voltage-dependent fashion. A-887826 was more potent in blocking human Nav1.8 channels and native rat TTX-R Na⁺ currents compared to its activity at human Na_v1.2 and Na_v1.5 sodium channels and rat TTX-S Na⁺ currents. Additionally, A-887826 showed little or no activity in assays of a large array of other channels and receptors. A-887826 represents a structurally novel Na_v

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A-887826 is a structurally novel, potent and voltage-dependent Nav1.8 sodium channel blocker that attenuates neuropathic tactile allodynia in rats

Abstract

Introduction

Section snippets

Animals

HEK cells expressing human sodium channels

Pyridine-based blockers of recombinant human NaV1.8 Na+ currents

Discussion

Eur. J. Pharmacol.

J. Biol. Chem.

Neuropharmacology

Neuroscience

Neuropharmacology

Pain

Brain Res. Mol. Brain Res.

Eur. J. Pharmacol.

Pain

J. Biol. Chem.

Urology

Brain Res.

The role of sodium channels in chronic inflammatory and neuropathic pain

J. Pain

The tetrodotoxin-resistant sodium channel SNS has a specialized function in pain pathways

Nat. Neurosci.

Multiple sodium channel isoforms and mitogen-activated protein kinases are present in painful human neuromas

Ann. Neurol.

Effect of drugs used for neuropathic pain management on tetrodotoxin-resistant Na(+) currents in rat sensory neurons

Anesthesiology

Block of peripheral nerve sodium channels selectively inhibits features of neuropathic pain in rats

Mol. Pharmacol.

International Union of Pharmacology. XLVII. Nomenclature and structure-function relationships of voltage-gated sodium channels

Pharmacol. Rev.

Voltage-gated sodium channels: therapeutic targets for pain

Pain Med.

A-887826 is a structurally novel, potent and voltage-dependent Na_v1.8 sodium channel blocker that attenuates neuropathic tactile allodynia in rats

Pyridine-based blockers of recombinant human Na_V1.8 Na⁺ currents