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Key Words: "C-M-MPEP, mGluR5, PET imaging.Binding of glutamate at the mGluR5, which belongs to class I of the mGluRs, results in a G-protein mediated activation of phospholipase C (PLC) in dendrites and cell body neurons in most of brain areas. An excessive activation of these glutamate receptors is thought to be implicated in a variety of neurological diseases such as epilepsy, focal and global ischemia, pain and neurodegenerative diseases. In order to map the mGluR5 in the brain and to evaluate its possible influence in the above-mentioned disorders by PET, M-MPEP (2-methyl-6-(3-methoxyphenyl) ethynyl-pyridine), a new and highly potent (KD = 3.5 nM) non-competitive antagonist for the mGluR5 was radiolabelled with carbon-1 1. The radiosynthesis of "C-M-MPEP was accomplished by the 0-methylation of the appropriate desmethyl precursor (hydroxy-MPEP) using "C-methyl iodide. "C-M-MPEP was purified by reversed-phase HPLC using a semi-preparative C-18 p-Bondapak column and a solvent system consisting of 0.1% H3P04/MeOH (70:30) as mobile phase. A total synthesis time of 50 minutes was required and the radiochemical yield ranged from 10-20%. The product was formulated in a solution containing Polysorbatum (0.1%) and 0.9% saline and radiochemical purity was >99%. Specific radioactivities ranged from 30-60GBq/mmol (800-16OOCilmmol). Using the shake flask method, the log P value for "C-M-MPEP was determined in phosphate buffer (pH=7.4)/octanol and amounted to 1.3. Biodistribution studies were carried out in rats. For the brain regions examined (striatum, cortex, hippocampus and cerebellum), radioactivity uptake ranged from 0.018-0.030 % I.D./ g organ at 20 minutes postinjection (pi.). In order to determine the pharmacokinetics of "C-M-MPEP uptake in the brain, PET studies in rats were also performed. The PET-study indicated an initial rapid uptake of radioactivity into the rat brain. The highest uptake was observed at 7 min p.i. followed by a fast clearance. Blockade studies by co-injection of "C-M-MPEP and non-labelled M-MPEP (lmg/kg) showed an increased uptake of radioactivity in the rat brain (Figure 1). Further iiz vivo J. Labelled C p d Radiopharm 44, Suppl. 1 (2001)

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A new benzomorphane derivative, [11C]methyl-BIII277CL, was evaluated as a potential radiotracer for visualizing the PCP-binding site of the N-methyl-D-aspartate (NMDA) receptor by positron emission tomography (PET). Methyl-BIII277CL was prepared by reacting the desmethyl compound (BIII277CL) with dimethylsulfate. The pharmacological profile of methyl-BIII277CL was determined by in vitro receptor-screening assays. At a concentration of 100 nM, methyl-BIII277CL showed a significant interaction with the PCP-binding site of the NMDA receptor (79% inhibition of specific binding) and the sigma-binding site (46% inhibition). In displacement assays using mice cortical membranes, methyl-BIII277CL displayed a high affinity at the PCP-binding site of the NMDA receptor (Ki = 49 +/- 14 nmol/L) and a 130-fold lower interaction with the sigma1-binding site (Ki = 6.35 +/- 0.26 micromol/L). For saturation experiments and in vivo studies, methyl-BIII277CL was radiolabeled with 11C at the O-position of the desmethyl precursor (BIII277CL) using [11C]methyliodide with a specific activity of 35-70 GBq/micromol at the end of synthesis (EOS). In saturation assays using rat whole brain membranes [11C]methyl-BIII277CL showed a Kd of 6 +/- 1 nmol/L and a Bmax of 670 +/- 154 fmol/mg protein. Biodistribution and PET studies in rats and pigs, however, indicated a lack of specific binding and unfavorable pharmacokinetics. Kinetic modeling using the 1-tissue compartment model demonstrated for [11C]methyl-BIII277CL a low distribution volume (Dv = 0.98 mL/mL(tissue)) and very high values for the kinetic parameters K1 and k2 (K1 = 0.36 mL/mL(tissue)/min and k2 = 0.37min(-1)) in pig cortex. [11C]methyl-BIII277CL, due to the lack of specificity in vivo, may not be a candidate for imaging the PCP-binding site of the NMDA receptor.

The N-methyl-D-aspartate (NMDA), a subtype of glutamate receptors, is a ligandgated ion channel, which contributes to excitatory neurotransmission throughout the central nervous system (CNS). It plays a crucial role in brain development, synaptic plasticity and the pathophysiology of major neurological disorders, such as stroke, traumatic brain injury, epilepsy, Parkinson's and Huntington's disease (1). A large number of NMDA antagonists have thus 'been labelled with either "C or I8F as potential agents for imaging the PCP (phencyclidine)-binding site of the NMDA receptor complex. However, to date none of these radioligands has proved to be useful. As part of our programme on the development of radioligands for the glutamatergic NMDA-receptor complex, we have synthesized and evaluated the potential of Methyl-BIII277CL (2, Fig. 1) as an imaging agent for the PCP-binding site of the NMDA-receptor complex: The corresponding desmethyl compound, BIII277CL a, Fig.1) has previously been described and is a potent NMDA antagonist with high. selectivity for the PCP binding site (2-4). Figure 1: Synthesis of Methyl-BlII277CL (2). Methyl-BlII277CL(2) was prepared by reacting the desmethyl compound I with dimethylsulfate. The chemical yield of 2 was greater 90%. In vitro receptor screening assays were carried out on more than 20 CNS receptors. Whereas 2 at a concentration of 0.1pM failed to alter the binding of radioligands to almost all the examined CNS receptors, substantial inhibitory effects (80% inhibition) were observed at the PCP-binding site. To further characterize its in v i m binding affinity