Relationship between In Vivo Receptor Occupancy and Efficacy of Metabotropic Glutamate Receptor Subtype 5 Allosteric Modulators with Different In Vitro Binding Profiles

Abstract

Allosteric modulators of the metabotropic glutamate receptor subtype 5 (mGlu 5 ) have exciting potential as therapeutic agents for multiple brain disorders. Translational studies with mGlu 5 modulators have relied on mGlu 5 allosteric site positron emission tomography (PET) radioligands to assess receptor occupancy in the brain. However, recent structural and modeling studies suggest that closely related mGlu 5 allosteric modulators can bind to overlapping but not identical sites, which could complicate interpretation of in vivo occupancy data, even when PET ligands and drug leads are developed from the same chemical scaffold. We now report that systemic administration of the novel mGlu 5 positive allosteric modulator VU0092273 displaced the structurally related mGlu 5 PET ligand, [ 18 F]FPEB, with measures of in vivo occupancy that closely aligned with its in vivo efficacy. In contrast, a close analog of VU0092273 and [ 18 F]FPEB, VU0360172, provided robust efficacy in rodent models in the absence of detectable occupancy. Furthermore, a structurally unrelated mGlu 5 negative allosteric modulator, VU0409106, displayed measures of in vivo occupancy that correlated well with behavioral effects, despite the fact that VU0409106 is structurally unrelated to [18 F]FPEB. Interestingly, all three compounds inhibit radioligand binding to the prototypical MPEP/FPEB allosteric site in vitro. However, VU0092273 and VU0409106 bind to this site in a fully competitive manner, whereas the interaction of VU0360172 is noncompetitive. Thus, while close structural similarity between PET ligands and drug leads does not circumvent issues associated with differential binding to a given target, detailed molecular pharmacology analysis accurately predicts utility of ligand pairs for in vivo occupancy studies.

Development of Novel, CNS Penetrant Positive Allosteric Modulators for the Metabotropic Glutamate Receptor Subtype 1 (mGlu1), Based on an N-(3-Chloro-4-(1,3-dioxoisoindolin-2-yl)phenyl)-3-methylfuran-2-carboxamide Scaffold, That Potentiate Wild Type and Mutant mGlu1 Receptors Found in Schizophrenics

“…5–8,27,28 We also assessed selectivity of 17d , 17e , and 17g versus the other remaining six mGlu receptors in our standard fold-shift assay paradigm at a 10 μ M concentration. 30,31,33,36 17d and 17g were selective against mGlu 2,3,5,6,7,8 , inducing less than a ~2-fold shift of the glutamate CRC at 10 μ M. 17e was selective versus mGlu 2,3,8 (less than ~2-fold shift of glutamate CRC at 10 μ M, a modest 2.7-fold shift at mGlu 5 , and larger shifts at mGlu 6 (6.7) and mGlu 7 (7.0). Thus, the mGlu 1 PAMs are suitable for in vivo studies.…”

Metabotropic glutamate receptor 3 activation is required for long-term depression in medial prefrontal cortex and fear extinction

“…However, it is also possible that in vivo efficacy requires high mGlu 3 receptor occupancy, a property that has been reported for mGlu 1 and mGlu 5 NAMs (33,34). In contrast, mGlu receptor positive allosteric modulators (PAMs) can produce full efficacy with relatively low occupancy in the CNS because of the contributions of both affinity and cooperativity to PAM potency at a receptor (35,36). Unfortunately, there are currently no selective mGlu 3 radioligands that would allow us to measure receptor occupancy of VU0477950.…”

Iodonium Ylide–Mediated Radiofluorination of 18F-FPEB and Validation for Human Use

“…18 F-3-Fluoro-5-[(pyridin-3-yl)ethynyl] benzonitrile ( 18 F-FPEB) is a metabotropic glutamate receptor subtype 5 (mGlu 5 ) antagonist used in preclinical (9, 10) and clinical PET neuroimaging research (11, 12). Radiosynthesis of 18 F-FPEB and several structurally related radiotracers for mGlu 5 has been challenging because nucleophilic aromatic substitution by 18 F-fluoride is not a favored reaction due to the absence of electron-withdrawing groups located ortho or para to the labeling site.…”