Pharmacology of ACEA-1416: a potent systemically active NMDA receptor glycine site antagonist

Abstract

Excitatory amino acid receptor antagonists show potential for the treatment of ischemic stroke and head trauma. In search of novel antagonists, a series of alkyl- and alkoxyl-substituted 1,4-dihydro-2,3-quinoxalinediones were synthesized and assayed for inhibition of glutamate receptors. We report on the pharmacological characterization of one such compound, 7-chloro-6-methyl-5-nitro-1,4-dihydro-2,3-quinoxalinedione (ACEA-1416). Electrophysiological assays showed that ACEA-1416 is a potent antagonist of rat brain NMDA receptors expressed in Xenopus oocytes, and NMDA receptors expressed by cultured rat cortical neurons. Antagonism is via competitive inhibition at glycine co-agonist sites ( K b = 7.9 nM in oocytes, K b = 11 nM in neurons). ACEA-1416 also antagonizes AMPA receptors, though potency is considerably lower ( K b = 3.5 μM in oocytes, K b = 1.6 μM in neurons). Oocyte assays indicated that ACEA-1416 is weak or inactive as an antagonist at NMDA receptor glutamate binding sites ( K b > 5.9 μM) and metabotropic glutamate receptors ( K b > 57 μM). Many NMDA receptor glycine site antagonists show poor penetration of the blood-brain barrier. Systemic bioavailability of ACEA-1416 was assessed by measuring the ability of the compound to protect against electroshock-induced seizures in mice. Protective effects of ACEA-1416 had rapid onset following i.v. administration. Peak efficacy was at ∼ 2 min and the biological half-time of protection was ∼ 60 min. The ED 50 measured at peak efficacy was ∼ 1.5 mg/kg. Our results show that ACEA-1416 is a high potency systemically active NMDA receptor glycine site antagonist and a moderate potency AMPA receptor antagonist. Separate studies indicate that ACEA-1416 is efficacious as a neuroprotectant in a rat model of focal cerebral ischemia. Taken together, our results suggest that ACEA-1416 has potential for clinical development as a neuroprotectant.