Pharmacological characterisation of MDI-222, a novel AMPA receptor positive allosteric modulator with an improved safety profile.

PURPOSE: There is considerable interest in positive allosteric modulators (PAMs) of the α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) subtype of ionotropic glutamate receptors as therapeutic agents for a range of cognitive and mood disorders. However, the challenge is to increase AMPA receptor (AMPAR) function sufficient to enhance cognitive function but not to the extent that there are mechanism-related pro-convulsant or convulsant side effects. In this present study, we report the preclinical pharmacology data for MDI-222, an AMPAR PAM which enhances cognition but has a much reduced side-effect (i.e. convulsant) liability relative to other molecules of this mechanism. METHODS: The pharmacological effects of MDI-222 were characterised in in vitro and in vivo preclinical electrophysiology, efficacy (cognition), side-effect (pro-convulsant/convulsant), tolerability and toxicity assays. RESULTS: We demonstrate that MDI-222 is an AMPAR PAM, since it enhanced AMPAR function in vitro at human (hGluA1-4) and rat (rGluA2) homomeric receptors, and potentiated hetero-oligomeric AMPARs in rat neurons. MDI-222 enhanced electrically evoked AMPAR-mediated synaptic transmission in the anaesthetised rat at 10 mg/kg (administered intravenously) and did not significantly lower the seizure threshold in the pro-convulsant maximal electroshock threshold test (MEST) at any dose tested up to a maximum of 30 mg/kg (administered by oral gavage (p.o.)). MDI-222 reversed a delay-induced deficit in novel object recognition (NOR) in rats with a minimum effective dose (MED) of 0.3 mg/kg (p.o.) following acute administration, which was reduced to 0.1 mg/kg following sub-chronic administration, and improved passive avoidance performance in scopolamine-impaired rats with a MED of 10 mg/kg p.o. On the other hand, MDI-222 was not pro-convulsant in the MEST, resulting in a therapeutic window between plasma concentrations that enhanced cognitive performance and those associated with mechanism-related side effects of ⩾1000-fold. Unfortunately, despite the excellent preclinical profile of this compound, further development had to be halted due to non-mechanism-related issues. CONCLUSIONS: We conclude that MDI-222 is an AMPAR PAM which enhances cognitive performance in rats and has a significantly improved safety profile in preclinical species.

Electrophysiological characterisation of the actions of kynurenic acid at ligand-gated ion channels.

To better understand the effects of the tryptophan metabolite kynurenic acid (kynA) in the brain, we characterised its actions at five ligand-gated ion channels: NMDA, AMPA, GABA(A), glycine and alpha7 nicotinic acetylcholine receptors. Using whole-cell patch-clamp recordings, we found that kynA was a more potent antagonist at human NR1a/NR2A compared with NR1a/NR2B receptors (IC(50): 158 muM and 681 muM, respectively; in 30 muM glycine). KynA inhibited AMPA-evoked currents to a similar degree in cultured hippocampal neurons and a human GluR2(flip/unedited) cell line (IC(50): 433 and 596 muM, respectively) and at higher concentrations, kynA also inhibited the strychnine-sensitive glycine receptor ( approximately 35% inhibition by 3 mM kynA). Interestingly, kynA inhibited the peak amplitude (IC(50): 2.9 mM for 10 muM GABA) and slowed the decay kinetics of GABA-evoked currents in cultured neurons. In contrast, we found that kynA (1-3 mM) had no effect on ACh-evoked, methyllycaconitine (MLA)-sensitive currents in a human alpha7 nicotinic receptor (nAChR) cell line, rat hippocampal neurons in primary culture or CA1 stratum radiatum interneurons in rat brain slices. However, DMSO (>1%) did inhibit alpha7 nAChR-mediated currents. In conclusion, kynA is an antagonist at NMDA, AMPA and glycine receptors and a modulator of GABA(A) receptors, but we find no evidence for any effect of kynA at the alpha7 nAChR.

Conservation of functional and pharmacological properties in the distantly related temperature sensors TRVP1 and TRPM8.

Members of the transient receptor potential (TRP) superfamily of ion channels have now been defined as molecular transducers capable of reproducing the spectrum of temperature sensation exhibited by mammals. Because of their pivotal role in sensory transduction, many of these channels represent good targets for drug discovery. With a view to gaining further insight into the functional and pharmacological properties of these channels, we have used the whole-cell patch-clamp technique to study the human cold-sensitive menthol receptor transient receptor potential melastatin 8 (TRPM8) and compared its behavior with that of its distant relative, the heat-sensitive capsaicin-gated transient receptor potential vanilloid 1 (TRPV1). It is remarkable to find that TRPM8, in addition to its behavior as an outwardly rectifying, nonselective cation channel, shares many functional and pharmacological properties with TRPV1. TRPM8 exhibits prominent time- and voltage-dependent behavior, a property that may underlie the conserved rectification or gating mechanisms exhibited by these channels. We also show that TRPM8 is modulated by ethanol but unlike TRPV1 is insensitive to extracellular acidification. There is also significant overlap in the antagonist pharmacology of these channels with many TRPV1 antagonists such as capsazepine, N-(4-tertiarybutylphenyl)-4-(3-chloropyridin-2-yl) tetrahydropyrazine-1(2H)-carboxamide (BCTC), (2R)-4-(3-chloro-2-pyridinyl)-2-methyl-N-[4-(trifluoromethyl)phenyl]-1-piperazinecarboxamide (CTPC), and N-(2-bromophenyl)-N'-{2-[ethyl(3-methylphenyl)amino]ethyl}-urea (SB-452533) exhibiting similar activity at TRPM8. Overall, the degree of pharmacological overlap between TRPV1 and TRPM8 has implications for the interpretation of studies conducted with these ligands to date and highlights a clear challenge for the design of selective TRP channel antagonists. Our finding that N-(3-methoxyphenyl)-4-chlorocinnamide (SB-366791), at least, represents an apparently selective antagonist for TRPV1 suggests that this goal is attainable.