Synthesis and evaluation of [18F] labeled benzamides: high affinity sigma receptor ligands for PET imaging.

We have synthesized and characterized four new fluorinated halobenzamides as sigma receptor ligands for use with positron emission tomography (PET). All the compounds were found to have high sigma-1 affinities (Ki = 0.38-0.98 nM), and the 4-fluoro-substituted benzamides were found to be more potent sigma-2 ligands (Ki = 3.77-4.02 nM) than their corresponding 2-fluoro analogs (Ki = 20.3-22.8 nM). The [18F] radiochemical syntheses of two of the analogs gave overall yields between 3-10% (EOS), radiochemical purities > 99%, and specific activities between 800-1200 Ci/mmol (29.6-44.4 TBq/mmol). Rat biodistribution and blocking experiments were performed with 2-[18F](N-fluorobenzylpiperidin-4yl)-4-iodobenzamide, the analog with the best Ki value for sigma-1 sites (0.38 nM). Results of these experiments demonstrate specific uptake of the compound in tissues believed to contain sigma receptors, such as lungs, kidneys, heart, brain, and spleen and indicate its potential as a candidate for use in PET imaging of tissues containing these receptors.