GB2346883A - GluR5 receptor binding assay - Google Patents

Abstract

A method of assaying the binding activity of a compound to native or recombinant GluR5 receptors, comprising treating a sample containing the GluR5 receptors with a chemical compound, adding labelled ATPA, and assaying the binding activity by measurement of the amount of compound displaced by the ATPA. The method could be used for the screening of compounds for use in the treatment of psychiatric disorders by measurement of the binding affinity of the compound to the GluR5 receptors of the hippocampus.

Description

GLUTAMATE RECEPTOR MODULATORS AND THE TREATMENT OF PSYCHIATRIC DISORDERS This invention relates to a method of treating psychiatric disorders including cognitive disorders and assays for compounds having such activity.

It is well known that excitatory neurotransmission in the mammalian central nervous system is primarily mediated by the amino acid, L-glutamate, acting on ionotropic and metabotropic receptors. Glutamate can act at three types of ionotropic glutamate receptors, (R, S)-2-amino-3- (3-hydroxy-5-methylisoxazol-4-yl) propanoate (AMPA), kainate (KA) and N-methyl-D-aspartate (NMDA) receptors (Hollman and Heinemann, 1994, Annu.

Rev. Neurosci. 17; 31-108).

It is well established that the hippocampus is important in learning and memory (Squire, 1992, Psychol. Rev. 99; 195-231) and it is considered that such cognitive functions are mediated by plastic changes in glutamatergic transmission within the hippocampus involving AMPA, NMDA and metabotropic receptor activation (Bliss and Collingridge, 1993, Nature, 361, 31-39). An example of such a plastic change is long term potentiation which can be demonstrated using standard electrophysiological methods in vivo, and in vitro in hippocampal slices.

Recently, it has been reported that kainate modulates neurotransmitter release in the hippocampus (Chittajullu et al., 1995, Nature 379,78-81), but it remains unclear which receptors underlie this modulating effect of kainate.

We have now discovered that compounds having activity at one of the kainate receptor subtypes, namely GluR5, modulate synaptic transmission within the hippocampus.

Such compounds thus have potential for altering cognitive functions and are therefore indicated for the treatment of cognitive disorders. Compounds with selective activity at GluR5 receptors were not previously described (see Fletcher and Lodge, 1996, Pharmacol. Ther. 70; 65-89).

Thus the invention provides a method for treating a cognitive disorder by administration of a compound that modulates the GluR5 receptor in the hippocampus. The invention further provides the use of a compound that modulates the GluR5 receptor in the hippocampus for the manufacture of a medicament for the treatment of a cognitive disorder.

Activity of compounds acting at the kainate receptor, GluR5, is determined by radiolabelled ligand binding studies at the cloned and expressed human GluR5 receptor (Korczak et al., 1994, Recept. Channels 3; 41-49), by whole cell voltage clamp electrophysiological recordings of functional activity at the human GluR5 receptor (Korczak et al., 1994, Recept. Channels 3; 41-49) and by whole cell voltage clamp electrophysiological recordings of currents in acutely isolated rat dorsal root ganglion neurons (Bleakman et al., 1996, Mol. Pharmacol. 49; 581-585). The selectivity of compounds acting at GluR5 receptors is determined by measurement of activity at other AMPA and kainate receptors including receptorligand binding studies and whole-cell voltage clamp electrophysiological recordings of functional activity at human GluRl, GluR2, GluR3 and GluR4 receptors (Fletcher et al., 1995, Recept. Channels 3; 21-31), receptor-ligand binding studies and whole-cell voltage clamp electrophysiological recordings of functional activity at human GluR6 receptors (Hoo et al., Recept.

Channels 2; 327-338) and whole-cell voltage clamp electrophysiological recordings of functional activity at AMPA receptors in acutely isolated cerebellar Purkinje neurons (Bleakman et al., 1996, Mol.

Pharmacol. 49; 581-585) and other tissues expressing AMPA receptors (Fletcher and Lodge, 1996, Pharmacol. Ther. 70; 65-89).

One compound having activity at the GluR5 receptor is ATPA (2-amino-3- (3-hydroxy-5-tert-butylisoxazol-4- yl) propanoic acid), and the invention includes a method of treating a cognitive disorder by administering ATPA, or a pharmaceutically-acceptable salt thereof.

ATPA is a known compound (Lauridsen et al., 1985; J. Med. Chem. 28: 668-672) and was hitherto regarded as a selective AMPA receptor agonist (Krogsgaard-Larsen et al., 1996, Eur. J. Med. Chem. 31: 515-537). We have discovered that ATPA is a potent GluR5 ligand with nanomolar activity on human GluR5 in binding studies, and is more than 1000-fold less potent on other human AMPA and kainate receptors (see Table 1 below).

Furthermore, in electrophysiological studies we have discovered that ATPA is a potent GluR5 agonist with micromolar activity on human GluR5 and rat DRG neurones and is 100-fold less potent on other human AMPA and kainate receptors (see Table 2 below).

TABLE 1 ATPA selectivity profile in binding studies Cell lines (HEK293 cells) stably transfected with human GluR receptors were employed. Displacement of 3H AMPA by increasing concentrations of ATPA was used on GluR1-4- expressing cells and 3H kainate (KA) on GluR5,6,7 KA2-expressing cells. Estimated activity (Ki) in nM was as follows.

GluR1 GluR2 GluR4 GluR5 GluR6 GluR7 KA2 17590 38616 15747 3. 1 > 1mM 14319 38832 TABLE 2 ATPA selectivity profile in electrochysioloqical studies Functional studies were carried out on HEK293 cells stably transfected with human GluR receptors and on acutely isolated dorsal root ganglion neurons (DRG) using patch-clamp technology (Bleakman et al., 1996, Mol. Pharmacol., 49, 581-585). EC50 values (p. M) for ATPA were estimated for GluR1-4 vs 100 AM AMPA, GluR5 vs 100 RM KA, GluR6 vs 1 FM KA, and DRG vs 30 gM KA, with the following results:

GluRl GluR2 GluR3 GluR4 GluR5 GluR6 DRG > 300 > 300 > 300 > 300 40. 7 > 300 0.56+0.01 Thus, ATPA is highly selective at the GluR5 receptor and preferred compounds useful in the present invention have a binding activity at the GluR5 receptor of at least 10-fold that at any of the remaining glutamate receptors.

The newly discovered properties of ATPA make it a useful research tool for identifying physiological processes mediated by GluR5 and for the investigation of chemical compounds having potential activity at the GluR5 receptor. The invention, therefore, includes the use of ATPA, or a salt thereof, in biological tests which enable the discovery of the activity of chemical compounds at GluR5 receptors. Such biological tests include ligand binding displacement studies, activity in in vitro and in vivo preparations and localization of GluR5 receptors in tissue samples. Thus, the invention provides a method of assaying the binding activity of a chemical compound to recombinant or native GluR5 receptors, which comprises treating a sample such as a fraction of a suspension or homogenate of tissue containing recombinant or native GluR5 receptors, with a measured quantity of the chemical compound, adding a measured quantity of labelled ATPA, or a salt thereof, and assaying the binding activity by measurement of the amount of ATPA bound and chemical compound displaced.

The ATPA reagent can be labelled in any conventional way as, for example, by radiolabelling or pigment or dye, and can be assayed by techniques such as scintillation counting, or other means. Preferably the concentration of ATPA employed in the assay is 100 nM or less.

In the absence of labelled ATPA, binding studies for screening compounds for GluR5 activity may be performed by assays in which ATPA-displaceable binding of another labelled compound with high affinity for the GluR5 receptor is measured. Suitable labelled compounds for this assay include glutamate and kainate. Such assays may be best performed in the presence of compounds that block the binding to other glutamate receptors. A compound active at the GluR5 receptors modulates the remaining ATPA-sensitive binding.

In a further aspect of the invention there is provided a method of screening a compound which comprises measuring the binding affinity of the compound to GluR5 receptors in the hippocampus or other tissue expressing GluR5 receptors, by reference to ATPA, and selecting the compound according to its binding affinity.

Other screening assays include functional tests for GluR5 activity including standard electrophysiological and calcium flux assays on recombinant native GluR5 receptors, in which ATPA has known activity.

As mentioned above, ATPA modulates synaptic transmission in the hippocampus. In particular, in electrophysiological tests similar to those described in I Chittajulu et al., 1995, Nature 379,78-81, ATPAOLgm) ( reduces synaptic inhibition onto CA1 pyramidal neurones.

This activity indicates that GluR5 receptors are important in facilitating excitatory synaptic transmission in the hippocampus. It is further believed that such selective modulation of GluR5 is likely to show benefits in the treatment of cognitive function.

Thus the preferred method of the invention is one in which the compound administered shows/selectivity for the GluR5 receptor.

A preferred method of the invention is one for treating cognitive disorders such as memory and learning disorders, dementia and amnestic disorders.

It is also well known that the hippocampus is involved in many other physiological and pathological functions (Kato, N. (ed) 1996, The Hippocampus: Functions and Clinical Relevance. Elsevier, Amsterdam). Importantly the hippocampus is involved in convulsive disorders (Dingledine et al., 1990 TIPS 11,334-338) and is subject to neurodegeneration as a result of ischaemic, hypoxic and hypoglycaemic episodes (Meldrum and Garthwaite, 1990 TIPS 11; 379-387). It is also well known that GluR5 receptors are distributed in other parts of the brain (Bettler et al., 1990, Nueron 5; 583-595). The invention, therefore, includes the use of modulators of GluR5 function in the treatment of neurological and psychiatric disorders in which the hippocampus or other brain region is implicated and in particular in convulsive disorders and neurodegenerative diseases.

It is further known that kainate receptors are located on dorsal root fibres and dorsal root ganglion neurones.

ATPA is a potent agonist on these neurones (Table 2).

These neurones conduct nociceptive information into the spinal cord. The invention therefore includes the use of modulators of GluR5 function in the treatment of pain, and particularly of severe, chronic, intractable or neuropathic pain.

It will be understood that the amount of the active compound administered to the patient will be determined by a physician, in the light of the relevant circumstances, including the condition to be treated, the chosen route of administration, the actual compound administered, the age, weight, and response of the individual patient, and the severity of the patient's symptoms. For example, dosages per day normally fall within the range of 0.1 to 50 mg/kg of body weight, and in the treatment of adult humans the range is usually from 1 to 15 mg/kg/day. These dosage ranges are not intended to limit the scope of the invention in any way, and in some instances dosage levels below the lower limit of the aforesaid range may be more than adequate, while in other cases still larger doses may be employed without causing any harmful side effect, provided that such larger doses are first divided into several smaller doses for administration throughout the day.

Claims (2)

1. CLAIMS 1. A method of assaying the binding activity of a chemical compound to recombinant or native GluR5 receptors, which comprises treating a sample containing recombinant or native GluR5 receptors with a measured quantity of the chemical compound, adding a measured quantity of labelled ATPA, or a salt thereof, and assaying the binding activity by measurement of the amount of ATPA bound and chemical compound displaced.
2. 2. A method of screening a compound for use in treating psychiatric disorders, which comprises measuring the binding affinity of the compound to GluR5 receptors in the hippocampus or other tissue, by reference to ATPA, and selecting the compound according to its binding affinity.