Syntheses and pharmacological characterization of novel thiazole derivatives as potential mGluR5 PET ligands

Molecular Insights into Metabotropic Glutamate Receptor Allosteric Modulation

The metabotropic glutamate (mGlu) receptors are a group of eight family C G protein-coupled receptors that are expressed throughout the central nervous system (CNS) and periphery. Within the CNS the different subtypes are found in neurons, both pre- and/or postsynaptically, where they mediate modulatory roles and in glial cells. The mGlu receptor family provides attractive targets for numerous psychiatric and neurologic disorders, with the majority of discovery programs focused on targeting allosteric sites, with allosteric ligands now available for all mGlu receptor subtypes. However, the development of allosteric ligands remains challenging. Biased modulation, probe dependence, and molecular switches all contribute to the complex molecular pharmacology exhibited by mGlu receptor allosteric ligands. In recent years we have made significant progress in our understanding of this molecular complexity coupled with an increased understanding of the structural basis of mGlu allosteric modulation.

Preclinical evaluation and test-retest studies of [(18)F]PSS232, a novel radioligand for targeting metabotropic glutamate receptor 5 (mGlu5)

PURPOSE

A novel, (18)F-labelled metabotropic glutamate receptor subtype 5 (mGlu5) derivative of [(11)C]ABP688 ([(11)C]1), [(18)F]PSS232 ([(18)F] ]5), was evaluated in vitro and in vivo for its potential as a PET agent and was used in test-retest reliability studies

METHODS

The radiosynthesis of [(18)F]5 was accomplished via a one-step reaction using a mesylate precursor. In vitro stability was determined in PBS and plasma, and with liver microsomal enzymes. Metabolite studies were performed using rat brain extracts, blood and urine. In vitro autoradiography was performed on horizontal slices of rat brain using 1 and 8, antagonists for mGlu5 and mGlu1, respectively. Small-animal PET, biodistribution, and test-retest studies were performed in Wistar rats. In vivo, dose-dependent displacement studies were performed using 6 and blocking studies with 7.

RESULTS

[(18)F]5 was obtained in decay-corrected maximal radiochemical yield of 37 % with a specific activity of 80 - 400 GBq/μmol. Treatment with rat and human microsomal enzymes in vitro for 60 min resulted in 20 % and 4 % of hydrophilic radiometabolites, respectively. No hydrophilic decomposition products or radiometabolites were found in PBS or plasma. In vitro autoradiography on rat brain slices showed a heterogeneous distribution consistent with the known distribution of mGlu5 with high binding to hippocampal and cortical regions, and negligible radioactivity in the cerebellum. Similar distribution of radioactivity was found in PET images. Under displacement conditions with 6, reduced [(18)F]5 binding was found in all brain regions except the cerebellum. 7 reduced binding in the striatum by 84 % on average. Test-retest studies were reproducible with a variability ranging from 6.8 % to 8.2 %. An extended single-dose toxicity study in Wistar rats showed no compound-related adverse effects.

CONCLUSION

The new mGlu5 radiotracer, [(18)F]5, showed specific and selective in vitro and in vivo properties and is a promising radioligand for PET imaging of mGlu5 in humans.

Pharmacology of metabotropic glutamate receptor allosteric modulators: structural basis and therapeutic potential for CNS disorders

The metabotropic glutamate receptors (mGlus) mediate a neuromodulatory role throughout the brain for the major excitatory neurotransmitter, glutamate. Seven of the eight mGlu subtypes are expressed within the CNS and are attractive targets for a variety of psychiatric and neurological disorders including anxiety, depression, schizophrenia, Parkinson's disease, and Fragile X syndrome. Allosteric modulation of these class C 7-transmembrane spanning receptors represents a novel approach to facilitate development of mGlu subtype-selective probes and therapeutics. Allosteric modulators that interact with sites topographically distinct from the endogenous ligand-binding site offer a number of advantages over their competitive counterparts. In particular for CNS therapeutics, allosteric modulators have the potential to maintain the spatial and temporal aspects of endogenous neurotransmission. The past 15 years have seen the discovery of numerous subtype-selective allosteric modulators for the majority of the mGlu family members, including positive, negative, and neutral allosteric modulators, with a number of mGlu allosteric modulators now in clinical trials.

Approaches for probing allosteric interactions at 7 transmembrane spanning receptors

In recent years, allosteric modulation of 7 transmembrane spanning receptors (7TMRs) has become a highly productive and exciting field of receptor pharmacology and drug discovery efforts. Positive and negative allosteric modulators (PAMs and NAMs, respectively) present a number of pharmacological and therapeutic advantages over conventional orthosteric ligands, including improved receptor-subtype selectivity, a lower propensity to induce receptor desensitization, the preservation of endogenous temporal and spatial activation of receptors, greater chemical flexibility for optimization of drug metabolism and pharmacokinetic parameters, and saturability of effect at target receptors, thus improving safety concerns and risk of overdose. Additionally, the relatively new concept of allosteric modulator-mediated receptor signal bias opens up a number of intriguing possibilities for PAMs, NAMs, and allosteric agonists, including the potential to selectively activate therapeutically beneficial signaling cascades, which could yield a superior tissue selectivity and side effect profile of allosteric modulators. However, there are a number of considerations and caveats that must be addressed when screening for and characterizing the properties of 7TMR allosteric modulators. Mode of pharmacology, methodology used to monitor receptor activity, detection of appropriate downstream analytes, selection of orthosteric probe, and assay time-course must all be considered when implementing any high-throughput screening campaign or when characterizing the properties of active compounds. Yet compared to conventional agonist/antagonist drug discovery programs, these elements of assay design are often a great deal more complicated when working with 7TMRs allosteric modulators. Moreover, for classical pharmacological methodologies and analyses, like radioligand binding and the assessment of compound affinity, the properties of allosteric modulators yield data that are more nuanced than orthosteric ligand-receptor interactions. In this review, we discuss the current methodologies being used to identify and characterize allosteric modulators, lending insight into the approaches that have been most successful in accurately and robustly identifying hit compounds. New label-free technologies capable of detecting phenotypic cellular changes in response to receptor activation are powerful tools well suited for assessing subtle or potentially masked cellular responses to allosteric modulation of 7TMRs. Allosteric modulator-induced receptor signal bias and the assay systems available to probe the various downstream signaling outcomes of receptor activation are also discussed.

Synthesis and evaluation of novel α-fluorinated (E)-3-((6-methylpyridin-2-yl)ethynyl)cyclohex-2-enone-O-methyl oxime (ABP688) derivatives as metabotropic glutamate receptor subtype 5 PET radiotracers

In the search for an optimal fluorine-18-labeled positron emission tomography (PET) radiotracer for imaging metabotropic glutamate receptor subtype 5 (mGluR5), we have prepared a series of five α-fluorinated derivatives based on the ABP688 structural manifold by application of a two-step enolization/NFSI α-fluorination method. Their binding affinities were evaluated in vitro, and the most promising candidate (Z)-16 exhibited a K(i) of 5.7 nM and a clogP value of 2.3. The synthesis of the precursor tosylate (E)-22 revealed a preference for the (E)-configurational isomer (K(i) = 31.2 nM), and successful radiosynthesis afforded (E)-[(18)F]-16 which was used as a model PET tracer to establish plasma and PBS stability. (E)-[(18)F]-16 (K(d) = 70 nM) exhibited excellent specificity for mGluR5 in autoradiographic studies on horizontal rat brain slices in vitro.

[Nuclear medicine imaging in patients with Parkinson's syndrome: an update]

Nuclear medicine imaging using positron emission tomography (PET) or single photon emission computed tomography (SPECT) has enabled to study not only the metabolism and blood flow in specific brain areas but also the quantification of the function of distinct molecules. With respect to Parkinson's disease PET and later SPECT allowed the number of dopaminergic neurons to be assessed in vivo. These quantifications are relevant to establishing a clinical diagnosis, assessing the progression of the disease or the survival of transplanted dopaminergic neurons. In addition both techniques have markedly contributed to our understanding of the pathophysiology of this disorder. More recently, molecular imaging has been directed towards understanding the pathophysiology of non-motor symptoms in this disorder.