Species differences in mGluR5 binding sites in mammalian central nervous system determined using in vitro binding with [18F]F-PEB

Binding of [F-18]3-fluoro-5-[(pyridin-3-yl)ethynyl]benzonitrile ([F-18]F-PEB) was evaluated in membranes and tissue sections prepared from rat, rhesus and human brain. Saturation equilibrium binding experiments with frozen brain cortex and caudate-putamen membranes of young adult rhesus and human and with cortex and striatum from rat yielded data indicative of specific high-affinity binding (K-D=0.1-0.15 nM, n >= 3) to a saturable site previously shown to be metabotropic glutamate receptor 5 (mGluR5; Patel S, Ndubizu 0, Hamill T, Chaudhary A, Bums HD, Hargreaves RJ, Gibson RE. Screening cascade and development of potential positron emission tomography radiotracers for mGluR5: in vitro and in vivo characterization. Mol Imaging Biol 2005;7:314-323). High-affinity binding of [F-18]F-PEB was also detected in cerebellum membranes from rat, rhesus and human. The density of binding sites (B-max) measured using [F-18]F-PEB followed the rank order cortex similar to caudate-putamen/striatum > cerebellum for all three species, with the cerebellum B-max being significantly lower than that observed in the other regions. Receptor autoradiography studies in tissue sections confirmed that the regional distribution of [F-18]F-PEB in mammalian central nervous system is consistent with that of mGluR5 and that a small but specific mGluR5 signal is observed in rhesus and human cerebellum. A small and quantifiable specific signal could also be observed in rat cerebellum using this radiotracer. Immumohistochemical analysis in brain sections revealed a rank order of staining in rhesus and human brain of cortex similar to caudate-putamen > cerebellurn. Rat brain immunohistochemistry followed the same rank order, although the staining in the cerebellum was significantly lower. Using a no-wash wipe assay, the development of a specific signal within 20 min of incubation of tissue brain sections (> 60% in the cortex and striatum; 36-49% in the cerebellum) from all three species confirmed previous in vivo data from rat and rhesus monkey that [F-18]PEB is likely to provide a useful in vivo signal using positron emission tomography (PET). This study provides the first quantitative demonstration and direct comparison of a PET tracer candidate identifying mGluR5 binding sites in mammalian cerebellum, which subsequently raises questions in terms of using the cerebellum as a null tissue in PET imaging studies in the laboratory and the clinic. (c) 2007 Elsevier Inc. All rights reserved.