Development of a new radioligand, N-(5-fluoro-2-phenoxyphenyl)-N-(2-[18F]fluoroethyl-5-methoxybenzyl)acetamide, for PET imaging of peripheral benzodiazepine receptor in primate brain

To develop a positron emission tomography (PET) ligand for imaging the 'peripheral benzodiazepine receptor' (PBR) in brain and elucidating the relationship between PBR and brain diseases, four analogues (4-7) of N-(2,5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)acetamide (2) were synthesized and evaluated as ligands for PBR. Of these compounds, fluoromethyl (4) and fluoroethyl (5) analogues had similar or higher affinities for PBR than the parent compound 2 (K-i = 0.16 nM for PBR in rat brain sections). Iodomethyl analogue 6 displayed a moderate affinity, whereas tosyloxyethyl analogue 7 had weak affinity. Radiolabeling was performed for the fluoroalkyl analogues 4 and 5 using fluorine-18 (F-18, beta(+); 96.7%, T-1/2 = 109.8 min). Ligands [F-18]4 and [F-18]5 were respectively synthesized by the alkylation of desmethyl precursor 3 with [F-18]fluoromethyl iodide ([F-18]8) and 2-[F-18]fluoroethyl bromide ([F-18]9). The distribution patterns of [F-18]4 and [F-18]5 in mice were consistent with the known distribution of PBR. However, compared with [F-18]5, [F-18]4 displayed a high uptake in the bone of mice. The PET image of [F-18]4 for monkey brain also showed significant radioactivity in the bone, suggesting that this ligand was unstable for in vivo defluorination and was not a useful PET ligand. Ligand [F-18]5 displayed a high uptake in monkey brain especially in the occipital cortex, a region with richer PBR than the other regions in the brain. The radioactivity level of [F-18]5 in monkey brain was 1.5 times higher than that of [C-11]2, and 6 times higher than that of (R)-(1-(2-chlorophenyl)-N-[C-11]methyl,N-(1-methylpropyl)isoquinoline ([C-11]1). Moreover, the in vivo binding of [F-18]5 was significantly inhibited by PBR-selective 2 or 1, indicating that the binding of [F-18]5 in the monkey brain was mainly due to PBR. Metabolite analysis revealed that [F-18]4 was rapidly metabolized by defluorination to [F-18]F- in the plasma and brain of mice, whereas [F-18]5 was metabolized by debenzylation to a polar product [F-18]13 only in the plasma. No radioactive metabolite of [F-18]5 was detected in the mouse brain. The biological data indicate that [F-18]5 is a useful PET ligand for PBR and is currently used for imaging PBR in human brain.