Development of fluorine-18-labeled 5-HT1A antagonists

J Med Chem. 1999 May 6;42(9):1576-86. doi: 10.1021/jm980456f.

Abstract

We have synthesized five fluorinated derivatives of WAY 100635, N-{2-[4-(2-methoxyphenyl)piperazino]ethyl}-N-(2-pyridyl)cyclohe xaneca rboxamide (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 [11C]carbonyl WAY 100635 ([carbonyl-11C]4a). [Carbonyl-11C]4a cleared the brain with a biological half-life averaging 41 min. The metabolite-corrected blood radioactivity had a half-life of 29 min. [18F]FCWAY ([18F]4d) gave half-lives and intercepts comparable to [carbonyl-11C]4a in the brain, but the blood clearance was faster. [18F]FBWAY ([18F]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[18F]FBWAY ([18F]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 [18F]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-11C]4a and [18F]4d compared to [18F]4b and [18F]4c. [18F]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 [18F]4d, while complete blockade was obtained from co-injection of 200 nmol of 4a (H/Cb-1 from 17.2 to 0.6). [18F]4b and [18F]4c showed lower specific binding ratios than [carbonyl-11C]4a and [18F]4d. [18F]4c was superior to [18F]4b since its specific binding was more readily blocked by 4a. These studies suggest that [18F]4c should be a useful compound to assess dynamic changes in serotonin levels while [18F]4d, with its high contrast and F-18 label, should provide better statistics and quantification for static measurement of 5-HT1A receptor distribution.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Benzamides / chemical synthesis*
  • Benzamides / chemistry
  • Benzamides / metabolism
  • Benzamides / pharmacokinetics
  • Brain / metabolism
  • Fluorine Radioisotopes*
  • Isotope Labeling
  • Ligands
  • Piperazines / chemistry
  • Piperazines / pharmacokinetics
  • Pyridines / chemical synthesis*
  • Pyridines / chemistry
  • Pyridines / metabolism
  • Pyridines / pharmacokinetics
  • Rats
  • Receptors, Serotonin / drug effects*
  • Receptors, Serotonin, 5-HT1
  • Serotonin Antagonists / chemical synthesis*
  • Serotonin Antagonists / chemistry
  • Serotonin Antagonists / metabolism
  • Serotonin Antagonists / pharmacokinetics
  • Structure-Activity Relationship
  • Tissue Distribution

Substances

  • Benzamides
  • Fluorine Radioisotopes
  • Ligands
  • Piperazines
  • Pyridines
  • Receptors, Serotonin
  • Receptors, Serotonin, 5-HT1
  • Serotonin Antagonists
  • N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl)cyclohexanecarboxamide