Development of [11C]mG3N001 as a PET imaging ligand for metabotropic glutamate receptor 3 (mGluR3)

1041 Objectives: Metabotropic glutamate receptors (mGluRs) have emerged as new drug targets with great potential for treatment of a range of central nervous system (CNS) disorders. Numerous studies indicate that mGluR3 is a key receptor involved in glial−neuronal communication, and inhibition of mGluR3 has been studied for the treatment of cognitive disorders, schizophrenia, depression, and Alzheimer’s disease. However, a practical difficulty has been the dearth of compounds that discriminate between mGluR2 and mGluR3 because they share great sequence homology. There is a need to develop a PET ligand for imaging mGluR3. Herein we describe the synthesis and evaluation of [11C]mG3N001, a new PET ligand based on a selective mGluR3 negative allosteric modulator (NAM). Methods: Compound mG3N001 was selected for PET probe development as it was the first class of potent mGluR3-selective NAM, which showed great selectivity against other mGluRs (>50 fold). The cold compound mG3N001 was prepared based on the reported procedure. The synthesis of [11C]mG3N001 was carried out by demethylation of mG3N001 by 1.5 equivalent of BBr3 in dry DCM under nitrogen to give the phenolic precursor. [11CH3I] was trapped in a TRACERlab FX-M synthesizer reactor (General Electric) preloaded with a solution of the phenolic precursor (1.0 mg) in dry DMF (300ul) with K2CO3 (10.0 mg). The solution was stirred at 100 ℃ for 3 min, and water (0.7 ml) was added. The reaction mixture was purified by reverse phase semi-preparative HPLC (Agilent Eclipse XDB-18 column, 5µm, 9.4×250 mm, 5.0 mL/min, 70% H2O+TFA(0.1%)/30%ACN+TFA(0.1%)), and the desired fractions were collected. Dynamic volumetric PET imaging studies on male Sprague−Dawley rats were acquired with a PET-CT scanner for 60 min (Triumph-II, Tri-Foil Imaging, Northridge, CA). PET data were reconstructed using a 3D-MLEM method resulting in a full width at half-maximum resolution of 1 mm. Time-activity curves (TACs) were exported as decay-corrected activity per unit volume. Results: [11C]mG3N001 was radiolabeled in 20.7±5.2% non-decay corrected radiochemical yield at the end of synthesis (30 min; n = 3). [11C]mG3N001 was formulated into 10% ethanolic saline solution before injection with excellent chemical and radiochemical purities (>99%). The PET imaging studies of [11C]mG3N001 demonstrated good brain-penetration ability and a widespread regional distribution with the highest radioactivity in hypothalamus, followed by olfactory bulb and thalamus, which is in line with the mGluR3 distribution in the brain. Time-activity curves showed reversible binding with ten minutes accumulation to reach peak activity followed by slow washout to 42% in 50 min. Conclusions: Our preliminary PET imaging studies demonstrated that [11C]mG3N001 has good brain penetration which may serve as a promising PET probe for neuroimaging of mGluR3. As far as we know, this is the first mGluR3-selective PET imaging probe. Acknowledgements: The studies were supported by NIH/NIA K25AG061282 and NIH/NINDS R01NS100164 for generously providing funding for this research.