Acea Pharmaceuticals, Inc.

Previous studies indicate that haloperidol, a therapeutically useful antipsychotic drug, inhibits neuronal N-methyl-D-aspartate (NMDA) responses and has neuroprotective effects against NMDA-induced brain injury. To further characterize this inhibition, we used electrical recordings to assay the effects of haloperidol on four diheteromeric subunit combinations of cloned rat NMDA receptors expressed in Xenopus laevis oocytes: NR1A coexpressed with NR2A, NR2B, NR2C, or NR2D. Haloperidol selectively blocks NR1A/2B subunit combinations (IC50 = approximately 3 microM; maximum inhibition, approximately 85%), whereas the other subunit combinations are > or = 100-fold less sensitive (IC50 = >300 microM). Inhibition of NR1A/2B receptors is insurmountable with respect to glutamate and glycine and does not exhibit voltage dependence. The splice variant combinations NR1B/2B and NR1e/2B are also blocked by haloperidol. In oocytes from some frogs, 30-100 microM haloperidol induces potentiation of NR1A/2A receptor responses. NMDA responses in E16-17 rat cortical neurons cultured for < or = 10 days are inhibited by haloperidol at the same potency and to the extent as NR1/2B receptors (IC50 = approximately 2 microM; maximum inhibition, approximately 80%). In contrast, cells cultured for longer periods show a wide range of sensitivity. This change in pharmacology coincides with a developmental switch in subunit expression; from NR1 expressed with NR2B to NR1 coexpressed with NR2A and NR2B. Inhibition of macroscopic neuronal NMDA responses is mechanistically similar to inhibition of NR1A/2B receptors. Single-channel recordings from neurons show that antagonism is associated with a decrease in the frequency of channel openings and a shortening of mean channel open time. Collectively, our experiments indicate that haloperidol selectively inhibits NMDA receptors comprised of NR1 and NR2B subunits. Inhibition is consistent with action at a noncompetitive allosteric site that is distinct from the glutamate-, glycine-, and phencyclidine-binding sites and is probably mechanistically related to the atypical antagonist ifenprodil. Our results suggest that haloperidol can be used as a tool for investigating NMDA receptor subunit composition and can serve as a structural lead for designing novel subtype-selective NMDA receptor ligands.

5-Chloro-7-trifluoromethyl-1,4-dihydro-2,3-quinoxalinedione (ACEA-1011) has analgesic properties in animal models of tonic pain. To investigate the mechanisms underlying this effect we used electrical recording techniques to characterize the in vitro pharmacology of ACEA-1011 at mammalian glutamate receptors. Two preparations were used: Xenopus oocytes expressing rat brain receptors and cultured rat cortical neurons. Results showed that ACEA-1011 is a competitive antagonist at NMDA receptor glycine sites. Apparent antagonist affinities (Kb values) were 0.4 to 0.8 microM in oocytes and approximately 0.6 microM in neurons. IC50 values for ACEA-1011 against four binary subunit combinations of cloned rat NMDA receptors (NR1A/NR2A, 2B, 2C or 2D) ranged from 0.4 to 8 microM (1 microM glycine). The 20-fold variation in sensitivity was due to a combination of subunit-dependent differences in glycine and antagonist affinities; EC50 values for glycine ranged between 0.08 to 0.8 microM and Kb values for ACEA-1011 between 0.2 to 0.8 microM. In addition, ACEA-1011 inhibited AMPA-preferring non-NMDA receptors by competitive antagonism at glutamate binding sites. Kb values were 4 to 9 microM in oocytes and 9 to 10 microM in neurons. The ED50 for ACEA-1011 in a mouse maximum electroshock-induced seizure model was approximately 12 mg/kg i.v.. Our results indicate that ACEA-1011 is a systemically active broad selectivity ionotropic glutamate receptor antagonist.

N-methyl-D-aspartate (NMDA) receptor antagonists show therapeutic potential as neuroprotectants, analgesics, and anticonvulsants. In this context, we used electrical recording techniques to study the in vitro pharmacology of two novel quinoxalinediones, i.e., ACEA-1021 and ACEA-1031 (5-nitro-6,7- dichloro- and 5-nitro-6,7-dibromo-1,4-dihydro-2,3-quinoxalinedione, respectively). Assays with NMDA receptors expressed by rat brain poly(A)+ RNA in Xenopus oocytes and with NMDA receptors in cultured rat cortical neurons indicated that ACEA-1021 and ACEA-1031 are potent competitive antagonists at NMDA receptor glycine sites. Apparent dissociation constants (Kb values) for ACEA-1021 and ACEA-1031 ranged between 6 and 8 nM for oocyte assays and between 5 and 7 nM for neuronal assays. Cloned NMDA receptors expressed in oocytes showed up to 50-fold variation in sensitivity, depending upon subunit composition. For example, using fixed agonist concentrations (10 microM glycine and 100 microM glutamate) IC50 values for ACEA-1021 with four binary combinations were as follows: NMDA receptor (NR)1A/2A, 29 nM; NR1A/2B, 300 nM; NR1A/2C, 120 nM; NR1A/2D, 1500 nM. Measurement of EC50 for glycine and calculation of Kb for the inhibitors indicated that differences in IC50 values are due to subunit-dependent variations in glycine affinity (EC50 ranged between approximately 0.1 and 1 microM) combined with variations in affinity of the antagonists themselves (Kb of approximately 2-13 nM). In addition to the strong antagonism of NMDA receptors, ACEA-1021 and ACEA-1031 were also moderately potent competitive inhibitors of non-NMDA receptors activated either by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid or by kainate. Antagonist affinities were similar whether measured with receptors expressed by rat brain poly(A)+ RNA in oocytes (Kb of 1-2 microM) or with cultured neurons (Kb of 1.5-3.3 microM). Our results suggest that the in vivo neuro-protective actions of ACEA-1021 and ACEA-1031 are predominantly due to inhibition at NMDA receptor glycine sites, although additional inhibition at non-NMDA receptors may play an ancillary role.