Synthesis and in vitro and in vivo evaluation of [11C]methyl-BIII277CL for imaging the PCP-binding site of the NMDA receptor by pet

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.

PET studies of 18F-memantine in healthy volunteers

Previous studies in mice and PET investigations in a Rhesus monkey showed that the regional uptake of 18F-memantine could be blocked by pharmacological doses of memantine and (+)-MK-801. In the present study, the binding characteristics of 18F-memantine was examined in five healthy volunteers. In humans, 18F-memantine was homogeneously distributed in gray matter i.e. cortex and basal ganglia regions, as well as the cerebellum. No radioactive metabolites were detected in plasma during the time-frame of the PET studies. The uptake of 18F-memantine in receptor-rich regions such as striatum and frontal cortex could be well described by a 1-tissue compartment model. The DV" values of all gray matter regions were similar and ranged from 15 to 20 ml/ml. The white matter showed lower DV" values of 15 +/- 1.4 ml/ml. These results suggest that 18F-memantine distribution in human brain does not reflect the regional NMDA receptor concentration, and therefore, this radioligand is not suitable for the PET imaging of the NMDA receptors.

Synthesis, radiolabelling and biological characterization of (D)-7-iodo-N-(1-phosphonoethyl)-5-aminomethylquinoxaline-2,3-dione, a glycine-binding site antagonist of NMDA receptors

(D)-7-Iodo-N-(1-phosphonoethyl)-5-aminomethylquinoxaline-2,3 -dione (I-PAMQX), is a potent, in vivo active antagonist acting at the glycine binding site of the NMDA receptor complex. Radioiodinated [131I]I-PAMQX was prepared with good yields and high specific activity from its 7-bromo analogue. Biodistribution studies of [131I]I-PAMQX in mice showed a relatively slow clearance from the blood. The uptake of radioactivity was highest in the kidneys, moderate in the heart, lung, liver and bones, and low in the brain.