[3H]-Methoxymethyl-MTEP and [3H]-Methoxy-PEPy: potent and selective radioligands for the metabotropic glutamate subtype 5 (mGlu5) receptor

doi:10.1016/S0960-894X(02)00997-6

Bioorganic & Medicinal Chemistry Letters

Volume 13, Issue 3, 10 February 2003, Pages 351-354

https://doi.org/10.1016/S0960-894X(02)00997-6 Get rights and content

Abstract

The design, synthesis, and characterization of two potent, non-competitive radioligands, [³H]-methoxymethyl-MTEP and [³H]-methoxy-PEPy, that are selective for the mGlu5 receptor are described.

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Introduction

Metabotropic glutamate (mGlu) receptors are a family of G-protein coupled receptors in the mammalian nervous system that are activated by l-glutamate.¹ Group I mGlu receptors (mGlu1 and mGlu5) are primarily localized at the periphery of the postsynaptic membranes of synapses in many brain regions, including the hippocampus, thalamic nuclei, cerebellar cortex, and spinal cord. Stimulation of mGlu1 and mGlu5 leads to phosphoinositide hydrolysis and elevation of intracellular Ca²⁺ levels ([Ca²⁺]_i) via G-protein coupling to phospholipase C. Excessive activation of mGlu5 receptors has been implicated in several disease states including pain,² anxiety and depression,3, 4, 5, 6, 7, 8 and drug addiction or withdrawal.⁹ Research in these laboratories has therefore focused on the discovery of potent and selective mGlu5 receptor antagonists as potential therapeutic agents.

Recently we reported that exploration of SAR around the prototypical non-competitive mGlu5 receptor antagonist MPEP (1) led to the discovery of 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP; 3) a potent and highly selective mGlu5 receptor antagonist active in animal models of anxiety.¹⁰ To accomplish the goal of discovering compounds such as MTEP the design and synthesis of highly potent and selective radiolabeled mGlu5 receptor antagonists was required, which allowed the development of robust in vitro and in vivo binding assays.11, 12 The recent disclosure by researchers at Novartis of a selective mGlu5 receptor radioligand [³H]-2-[(3-methoxyphenyl)ethynyl]-6-methylpyridine ([³H]-M-MPEP; 2),¹³ a derivative of MPEP (1), prompted the present communication reporting our parallel efforts to develop potent and highly selective tritiated mGlu5 receptor antagonists. This work resulted in the discovery of [³H]-3-(methoxymethyl)-5-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine ([³H]-methoxymethyl-MTEP; 1, Scheme 1), and [³H]-3-methoxy-5-(pyridin-2-ylethynyl)pyridine ([³H]-methoxy-PEPy; 1, Scheme 2). The synthesis and in vitro characterization of these pharmacological tools and described herein.

Section snippets

Chemistry

The development of a viable radioligand typically requires the optimization of two parameters: (a) high specific binding (i.e., high affinity for the receptor in question), and (b) low non-specific binding to other endogenous binding sites. Non-specific binding is generally more pronounced for highly lipophilic molecules. We therefore sought to identify selective mGlu5 receptor antagonists with excellent in vitro potency at the mGlu5 receptor and low LogD (low lipophilicity). Thus, in addition

Pharmacological Characterization

Both [³H]-1, Scheme 1 and [³H]-1, Scheme 2 showed high specific binding to rat brain membranes, as defined with 1 (10 μM) as cold displacer, and the specific binding was greater than 90% of total binding.11, 12 Analysis of rat brain binding of [³H]-1, Scheme 1 or [³H]-5 revealed a single binding site that was saturable and of high affinity with K_d=20 nM for [³H]-4 and K_d=3.4 nM for [³H]-1, Scheme 2. Association experiments demonstrated that both [³H]-1, Scheme 1 and [³H]-5 binding reached

Conclusion

Two potent and selective tritiated mGlu5 receptor antagonists ([³H]-1, Scheme 1 and [³H]-1, Scheme 2) were designed, synthesized and the in vitro pharmacology evaluated in binding assays. These radioligands have enabled the development of in vitro binding assays using either rat brain tissue or cells expressing recombinant hmGlu5 receptors. The use of [³H]-1, Scheme 1 and [³H]-1, Scheme 2 in an in vivo receptor occupancy assay in rodents which has been critical to mGlu5 receptor antagonist

Acknowledgements

The authors thank Bill Bray, Janice Chung, and Frank Tang for expert technical assistance.

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[3H]-Methoxymethyl-MTEP and [3H]-Methoxy-PEPy: potent and selective radioligands for the metabotropic glutamate subtype 5 (mGlu5) receptor

Abstract

Introduction

Section snippets

Chemistry

Pharmacological Characterization

Conclusion

Acknowledgements

Pharmaco., Biochem. Behav.

Eur. J. Pharmacol.

Neuropharmacology

Bioorg. Med. Chem. Lett.

Drug Discov. Today: HTS Suppl.

Neuropharmacology

Current drug targets: CNS Neurol. Disord.

Current drug targets: CNS Neurol. Disord.

[³H]-Methoxymethyl-MTEP and [³H]-Methoxy-PEPy: potent and selective radioligands for the metabotropic glutamate subtype 5 (mGlu5) receptor