Context-Dependent Pharmacology Exhibited by Negative Allosteric Modulators of Metabotropic Glutamate Receptor 7

Abstract

Phenotypic studies of mice lacking metabotropic glutamate receptor subtype 7 (mGluR7) suggest that antagonists of this receptor may be promising for the treatment of central nervous system disorders such as anxiety and depression. Suzuki et al. (J Pharmacol Exp Ther 323:147-156, 2007) recently reported the in vitro characterization of a novel mGluR7 antagonist called 6-(4-methoxyphenyl)-5-methyl-3-(4-pyridinyl)-isoxazolo[ 4,5-c]pyridin-4(5H)-one (MMPIP), which noncompetitively inhibited the activity of orthosteric and allosteric agonists at mGluR7. We describe that MMPIP acts as a noncompetitive antagonist in calcium mobilization assays in cells coexpressing mGluR7 and the promiscuous G protein G␣ 15 . Assessment of the activity of a small library of MMPIP-derived compounds using this assay reveals that, despite similar potencies, compounds exhibit differences in negative cooperativity for agonist-mediated calcium mobilization. Examination of the inhibitory activity of MMPIP and analogs using endogenous G i/o -coupled assay readouts indicates that the pharmacology of these ligands seems to be context-dependent, and MMPIP exhibits differences in negative cooperativity in certain cellular backgrounds. Electrophysiological studies reveal that, in contrast to the orthosteric antagonist (2S)-2-amino-2-[(1S,2S)-2-carboxyclycloprop-1-yl]-3-(xanth-9-yl) propanoic acid (LY341495), MMPIP is unable to block agonist-mediated responses at the Schaffer collateral-CA1 synapse, a location at which neurotransmission has been shown to be modulated by mGluR7 activity. Thus, MMPIP and related compounds differentially inhibit coupling of mGluR7 in different cellular backgrounds and may not antagonize the coupling of this receptor to native G i/o signaling pathways in all cellular contexts. The pharmacology of this compound represents a striking example of the potential for context-dependent blockade of receptor responses by negative allosteric modulators.The G protein-coupled receptors (GPCRs) play critical roles in regulating a broad range of signaling pathways within virtually every organ system (George et al., 2002) and are the targets of intense discovery efforts for new drug candidates. Despite this concentrated focus, selective ligands do not exist for the majority of these receptors (Howard et al., 2001). Many GPCRs interact with natural ligands based on complex or highly restricted chemical scaffolds (for example, amino acids, peptides, lipids, etc.) that are not well suited to optimization of selective small-molecule reagents. In addition, high conservation of the orthosteric sites of GPCR subfamilies presents challenges in developing high selectivity for a given receptor subtype. For these reasons, increasing interest has been placed on identifying compounds that interact with GPCRs at allosteric, rather than orthosteric, binding

A negative allosteric modulator demonstrates biased antagonism of the follicle stimulating hormone receptor

“…This may be due to its inability to suppress FSH induced estradiol production. Nevertheless, the effect of the FSHR NAM on estradiol production was clearly surprising and resembles a similar effect observed by Niswender et al using MMPIP an mGluR7 NAM (Niswender et al, 2010). In that study, MMPIP effectively blocked L-AP4 mediated calcium mobilization while it was completely ineffective in blocking either L-AP4 induced cAMP accumulation or L-AP4-mediated depression of synaptic transmission at the Schaffer collateral-CA1 synapses.…”

Opportunities and challenges in the discovery of allosteric modulators of GPCRs for treating CNS disorders

“…This includes biased or functionally selective NAMs for mGluR7 (REF. 95), prostaglandin D 2 receptors 96 and substance K (also known as neurokinin 2) receptors 97 . The unique potential of GPCR NAMs to selectively inhibit agonist effects on specific signalling pathways is not shared by orthosteric antagonists and provides an exciting opportunity to develop biased NAMs that target the pathways that are most crucial for achieving a therapeutic effect.…”

“…Inadvertent optimization of allosteric modulators that induce an unrecognized stimulus bias could lead to the discovery of compounds that are potent and have high efficacy in the cell-based assay used to drive chemical optimization, but display an unexplained lack of efficacy in native systems, in vivo models or clinical studies. For instance, early optimization of a series of mGluR7 NAMs using a calcium-fluorescence assay yielded highly selective NAMs with nanomolar potency, that nonetheless failed to block functional responses to mGluR7 activation in the hippocampal formation 95 . This context-dependent NAM activity was initially missed when a single assay for mGluR7 function was used, but became evident when the effects of these mGluR7 NAMs were investigated in other cell-based assays and native tissues.…”

Blocking Metabotropic Glutamate Receptor Subtype 7 (mGlu7) via the Venus Flytrap Domain (VFTD) Inhibits Amygdala Plasticity, Stress, and Anxiety-related Behavior

“…Increased lipophilicity of compounds may be associated with undesirable off-target effects. Furthermore, single and/or different allosteric ligands do not necessarily affect all downstream signaling pathways of a given GPCR (30).…”