Development of fluorine-18-labeled 5-HT1A antagonists

We have synthesized five fluorinated derivatives of WAY 100635, N-{2-[4-(2-methoxyphenyl)piperazino]ethyl)-N-(2-pyridyl)cyclohexanecarboxamide (4a), using various acids in place of the cyclohexanecarboxylic acid (CHCA, 2a) in the reaction scheme. The five acids are 4-fluorobenzoic acid (FB, 2b), 4-fluoro-3-methylbenzoic acid (MeFB, 2c), trans-4-fluorocyclohexanecarboxylic acid (FC, 2d), 4-(fluoromethyl)benzoic acid (FMeB, 2e), and 3-nitro-4-(fluoromethyl)benzoic acid (NFMeB, 2f) (see Scheme 1). These compounds were radiolabeled with fluorine-18, and their biological properties were evaluated in rats and compared with those of [C-11]carbonyl WAY 100635 ([carbonyl-C-11]4a), [Carbonyl-C-11]4a cleared the brain with a biological half-life averaging 41 min. The metabolite-corrected blood radioactivity had a half-life of 29 min. [F-18]FCWAY ([F-18]4d) gave half-lives and intercepts comparable to [carbonyl-C-11]4a in the brain, but the blood clearance was faster. [F-18]FBWAY ([F-18]4b) showed an early rapid net efflux from the whole brain, clearing with a biological half-life of 35 min. The metabolite-corrected blood half-life was 41 min. The comparable whole brain and blood half-lives for Me[F-18]FBWAY ([F-18]4c) were 16 and 18 min, respectively. For each compound, the corresponding carboxylic acid was identified as a major metabolite in blood. Fluoride was also found after injection of [F-18]4d. However, for all compounds there was a good correlation (R > 0.97) between the differential uptake ratio (DUR, (%ID/g) x body weight (g)/100) in individual rat brain regions at 30 min after injection and the concentration of receptors as determined by in vitro quantitative autoradiography in rat. Specific binding ratios [region of interest (ROI)/ cerebellum-1] in control studies for cortex (Ctx) and hippocampus (H) were higher for [carbonyl-C-11]4a and [F-18]4d compared to [F-18]4b and [F-18]4c. [F-18]4d has similar pharmacokinetic properties and comparable specific binding ratios to [carbonyl-11C]4a. Fifty nanomoles of 4a blocked only 30% of the specific binding of [F-18]4d, while complete blockade was obtained from co-injection of 200 nmol of 4a (H/Cb-1 from 17.2 to 0.6). [F-18]4b and [F-18]4c showed lower specific binding ratios than [carbonyl-C-11]4a and [F-18]4d. [F-18]4c was superior to [F-18]4b since its specific binding was more readily blocked by 4a. These studies suggest that [F-18]4c should be a useful compound to assess dynamic changes in serotonin levels while [F-18]4d, with its high contrast and F-18 label, should provide better statistics and quantification for static measurement of 5-HT1A receptor distribution.