In vitro and antinociceptive profile of HON0001, an orally active NMDA receptor NR2B subunit antagonist

Abstract

The analgesic activity and side effect liabilities of a novel NR2B antagonist, 7-hydroxy-6-methoxy-2-methyl-1-(2-(4-(trifluoromethyl)phenyl)ethyl)-1,2,3,4-tetrahydroisoquinoline hydrochloride (HON0001) were investigated. HON0001 inhibited [³H]MK-801 binding to rat brain membranes in a biphasic manner, with IC₅₀ values of 54.68 ± 4.96 nM and 46.48 ± 5.85 μM for high- and low-affinity sites, respectively. HON0001 inhibited [³H]ifenprodil binding to membranes of rat cerebral cortex with an IC₅₀ value of 57.01 ± 3.4 nM, consistent with the results obtained for high-affinity sites of [³H]MK-801 binding. HON0001 exhibited no or negligible affinity for other receptors, transporters and ion channels, while HON0001 had a moderate agonistic activity at μ-opioid receptors and affinity for dopamine D₁ receptors. HON0001 exhibited an analgesic effect in carrageenan-induced mechanical hyperalgesia and in the Seltzer model of partial sciatic nerve ligation following oral administration. In contrast, unlike MK-801, HON0001 did not affect spontaneous locomotor activity, rotarod performance and step-through latency in a passive avoidance task even at doses much higher than antinociceptive doses. HON0001 exhibited excellent brain penetration with a brain-to-plasma ratio of 34.5. These findings show that HON0001 is an orally active NR2B antagonist and that it may be useful for treating patients with neuropathic and other conditions without causing the side effects often observed with currently available non-subtype selective NMDA receptor antagonists.

Introduction

Injury to a peripheral tissue or nerve often results in a neuropathic pain condition which is characterized by spontaneous pain, allodynia and hyperalgesia. These neuropathies may be persistent and are problematic because they are often poorly managed by conventional opioid analgesics and non-steroidal anti-inflammatory drugs. Neuropathic pain is not only due to an increase in the sensitivity of primary afferent nociceptors at the site of injury, but also depends on N-methyl-d-aspartate (NMDA) receptor-mediated central changes in synaptic excitability (Liu et al., 1998, Liu and Sandkuhler, 1998).

Recent attention has focused on NMDA receptor antagonists for the treatment of neuropathic pain. Indeed, ketamine, a non-selective NMDA receptor antagonist, has been reported to produce symptomatic relief in a number of neuropathies (Max et al., 1995). However, although NMDA receptor antagonists have been proven to be efficacious for various types of experimental and clinical pain situations, their use as analgesics is limited by serious side effects including psychotomimetic symptoms and effects on learning and memory (Balster and Willetts, 1996, Boyce et al., 1999). One promising approach to obtain safer analgesics includes subtype-selective NMDA receptor antagonism.

The NMDA receptor is a ligand-gated ion channel composed of multiple subunits which are classified into two families: the NR1 subunit, of which there are at least eight splice variants (NR1a–h) and the NR2 subunit which has four subtypes (NR2A–D) each encoded by a separate gene (Mori and Mishina, 1995, Nakanishi, 1992). In rodent and human brain, the NR1 subunit is widely distributed (Nakanishi, 1992), while NR2 subunits display regional heterogeneity in expression. The predominant NR2 subunits in the forebrain are NR2A and NR2B, with NR2C expressed mainly in the cerebellum. NR2D expression is confined to the diencephalons and midbrain (Wenzel et al., 1995).

The heterogeneity of NMDA receptor subtype distribution in the central nervous system (CNS) has encouraged the development of subtype-selective compounds, with the possibility that such compounds may lack the side effects observed with nonselective NMDA receptor antagonists. Accumulating evidence has indicated the pivotal roles of NR2B receptors in pain perception. NR2B is located in the forebrain and laminas I and II of the dorsal horn (Boyce et al., 1999). It has been reported that rapid and prolonged increase in tyrosine phosphorylation of the NR2B subunit, but not the NR2A subunit, in the spinal cord was observed during the development and maintenance of inflammatory hyperalgesia, which may cause long-lasting NMDA receptor activation (Guo et al., 2001). Mice overexpressing the NR2B subunit in the anterior cingulate and insular cortices exhibit enhanced persistent pain and allodynia (Wei et al., 2001). Moreover, ifenprodil, a selective NR2B antagonist, has been shown to have antinociceptive effects at doses that do not impair motor performance (Boyce et al., 1999, Chizh et al., 2001), suggesting that this class of NMDA receptor antagonists may have a wider therapeutic window than nonselective NMDA receptor antagonists. This hypothesis was further supported by the findings that more potent and selective NR2B antagonists such as CP101,606 and Ro 63-1908 have wider therapeutic windows than ifenprodil in suppressing hyperalgesia in animal models of chronic pain and lack CNS side effects (Boyce et al., 1999, Taniguchi et al., 1997).

Recently, we synthesized a new NR2B antagonist, 7-hydroxy-6-methoxy-2-methyl-1-(2-(4-(trifluoromethyl)phenyl)ethyl)-1,2,3,4-tetrahydroisoquinoline hydrochloride (HON0001) based on the structural feature of ifenprodil. Here, we describe the in vitro and in vivo antinociceptive profile of HON0001.