Biological properties of 2′-[¹⁸F]fluoroflumazenil for central benzodiazepine receptor imaging

A novel positron emitting agent, 2′-[¹⁸F]fluoroflumazenil (fluoroethyl 8-fluoro-5-methyl-6-oxo-5,6-dihydro-4H-benzo-[f]imidazo[1,5-a][1,4] diazepine-3-carboxylate, FFMZ), has been reported for benzodiazepine imaging. In the present study, biological properties of [¹⁸F]FFMZ were investigated. Stability tests of [¹⁸F]FFMZ in human and rat sera were performed. Biodistribution was investigated in mice and phosphorimages of brains were obtained from rats. A receptor binding assay was performed using rat brain (mixture of cortex and cerebellum) homogenate. A static positron emission tomography (PET) image was obtained from a normal human volunteer. Although [¹⁸F]FFMZ was stable in human serum, it was rapidly hydrolyzed in rat serum. The hydrolysis was 39%, 63% and 92% at 10, 30 and 60 min, respectively. According to the biodistribution study in mice, somewhat even distribution (between 2∼3% ID/g) was observed in most organs. Intestinal uptake increased up to 6% ID/g at 1 h due to biliary excretion. Bone uptake slowly increased from 1.5% to 3.5% ID/g at 1 h. High uptakes in the cortex, thalamus and cerebellum, which could be completely blocked by coinjection of cold FMZ, were observed by phosphorimaging study using rats. Determination of K_d value and B_max using rat brain tissue was performed by Scatchard plotting and found 1.45±0.26 nM and 1.08±0.03 pmol/mg protein, respectively. The PET image of the normal human volunteer showed high uptake in the following decreasing order: frontal cortex, temporal cortex, occipital cortex, cerebellum, parietal cortex and thalamus. In conclusion, the new FMZ derivative, [¹⁸F]FFMZ appears to be a promising PET agent for central benzodiazepine receptor imaging with a convenient labeling procedure and a specific binding property.