Interaction of Novel Positive Allosteric Modulators of Metabotropic Glutamate Receptor 5 with the Negative Allosteric Antagonist Site Is Required for Potentiation of Receptor Responses

Abstract

Exciting advances have been made in the discovery of selective positive allosteric modulators of the metabotropic glutamate receptor (mGluR) mGluR5. These compounds may provide a novel approach that could be useful in the treatment of certain central nervous system disorders. However, because of their low potencies, previously described mGluR5 potentiators are not useful for functional studies in native preparations. In addition, binding sites at which these compounds act have not been identified. It has been suggested that two allosteric potentiators, 3,3Ј-difluorobenzaldazine and 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB), act by binding to the same allosteric site as the negative allosteric modulators of mGluR5 such as 2-methyl-6-(phenylethynyl)pyridine (MPEP). However, another mGluR5 potentiator, N- {4-chloro-2-[(1,3-dioxo-1,3-dihydro-2H-isoindol-2-yl)methyl]phenyl}-2-hydroxybenzamide, does not bind to this site, bringing this hypothesis into question. We have synthesized a series of CDPPB analogs and report that these compounds bind to the MPEP site with affinities that are closely related to their potencies as mGluR5 potentiators. Furthermore, allosteric potentiation is antagonized by a neutral ligand at the MPEP site and reduced by a mutation of mGluR5 that eliminates MPEP binding. Together, these data suggest that interaction with the MPEP site is important for allosteric potentiation of mGluR5 by CDPPB and related compounds. In addition, whole-cell patch-clamp studies in midbrain slices reveal that a highly potent analog of CDPPB, 4-nitro-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (VU-29), selectively potentiates mGluR5 but not mGluR1-mediated responses in midbrain neurons, whereas a previously identified allosteric potentiator of mGluR1 has the opposite effect.Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. In addition to eliciting fast excitatory synaptic responses, glutamate has important neuromodulatory effects by the activation of G protein-coupled receptors (GPCRs) termed metabotropic glutamate receptors (mGluRs). The mGluRs play important roles in a broad range of central nervous system functions and have potential as novel targets for the development of new therapeutic agents for a number of neurological and psychiatric disorders, including Parkinson's disease (Marino

Progress toward Positive Allosteric Modulators of the Metabotropic Glutamate Receptor Subtype 5 (mGlu5)

“…Importantly, CDPPB and VU-29 were shown to potentiate normal mGlu 5 responses in native subthalamic nucleus (STN) brain slices where mGlu 5 is expressed and shown to contribute to neuronal depolarization. 48 In contrast to mGlu 5 responses in the STN in brain regions where mGlu 5 is expressed but functionally incapable of depolarization, including the substantia nigra pars reticulata (SNr neurons), VU-29 and related mGlu 5 potentiators did not enhance agonistinduced depolarizations. VU-29 has also been shown to enhance both hippocampal LTD and LTP synaptic plasticity, which are critical forms of synaptic plasticity important for learning and memory formation.…”

“…Further SAR studies were conducted as a means to improve the potency and physicochemical properties of CDPPB with the goal to alleviate the need for the use of toxic vehicle formulations previously required for systemic dosing of CDPPB. 47,48 Two subsequent compounds from the pyrazol-5-yl-benzamide series partially succeeded in fulfilling this goal with 10-fold improvements in potency and MPEP site affinity. These included 4-nitro-N-(1-(2-fluorophenyl)-3-phenyl-1H-pyrazol-5-yl)benzamide (VU-1545, 6, Table 1) 47 and the related des-fluoro derivative VU-29 (7, Table 1).…”

“…These included 4-nitro-N-(1-(2-fluorophenyl)-3-phenyl-1H-pyrazol-5-yl)benzamide (VU-1545, 6, Table 1) 47 and the related des-fluoro derivative VU-29 (7, Table 1). 48 Although neither VU-1545 nor VU-29 proved to have utility in vivo due to poor physicochemical properties inherent to the scaffold (cLogP > 4.5), a number of important in vitro relationships were established with these advanced CDPPB analogues, including confirmation for MPEP site interaction as a requirement for functional activity demonstrating a consistent 15À20-fold cooperativity driven leftward shift in potency relative to affinity. Importantly, CDPPB and VU-29 were shown to potentiate normal mGlu 5 responses in native subthalamic nucleus (STN) brain slices where mGlu 5 is expressed and shown to contribute to neuronal depolarization.…”

Metabotropic Glutamate Receptors: Potential Drug Targets for Psychiatric Disorders

“…mGlu5 receptor positive allosteric modulators began to be developed after 2000. Three different structural types of mGlu5 receptor positive allosteric modulators were reported: DFB, CDPPB, and CPPHA [152][153][154]. These modu-lators do not affect ligand binding to the orthosteric glutamate binding site, but potentiate its response to glutamate.…”

Metabotropic glutamate receptor 5 binding and protein expression in schizophrenia and following antipsychotic drug treatment

“…Specifically, mGluR5 PAMs improve performance in sucrose preference, recognition memory, set-shifting, Y-maze, Morris water maze, inhibitory avoidance learning and conditioned aversion tasks whilst also reversing hyperlocomotive and PPI deficits in phencyclidine, apomorphine and/or amphetamine treated rodents, models routinely used to test antipsychotic drug potential (see Vinson and Conn, 2012). Most mGluR5 PAMs interact with the MPEP allosteric binding site (Chen et al, 2007), which was the target of the radioligand used in the present study to characterize mGluR5 binding. As we report that this specific allosteric binding site and mGluR5 protein levels are unaltered in schizophrenia pathology, this suggests that the binding potential of mGluR5 PAMs will not be affected in patients with schizophrenia.…”