Add time:07/25/2019         Source:sciencedirect.com

Four nitro- and fluorobenzamides (1–4) have been synthesized in good yields from nitro- and fluoro-substituted benzoyl chloride with 4-amino-1-benzylpiperidine. In vitro studies showed that these compounds have high affinities to σ receptors. N-(N-Benzylpiperidin-4-yl)-2-fluorobenzamide (3), in particular, bound to σ receptors with high affinity (Ki = 3.4 nM, guinea pig brain membranes) and high selectivity (σ-2σ-1 = 120). It was, therefore, labeled with 18F and evaluated as a σ receptor radioligand. N-(N-Benzylpiperidin-4-yl)-2-[18F]fluorobenzamide (3a) was synthesized in one step by nucleophilic substitution of the 2-nitro precursor (1) with [18F]fluoride in DMSO at 140 °C for 20 min followed by purification with HPLC in 4–10% yield (decay corrected). The synthesis time was 90 min and the specific activity was 0.4-1.0 Ci/μmol. Tissue distribution in mice revealed that the uptakes of 3a in the brain, heart, liver, lungs, spleen, kidneys and small intestine were high, and the radioactivity in these organs remained constant from 60 to 120 min post-injection. The radioactivity in the bone did not significantly increase, suggesting in vivo defluorination may not be the major route of metabolism of 3a in mice. Blocking studies with haloperidol in rats indicated that the uptake of compound 3a in the rat brain was selective to haloperidol-sensitive σ sites. These results suggest that compound 3a is a potent σ receptor radioligand and may be a potential ligand for PET imaging of σ receptors in humans.

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