Improved protocol for the radiosynthesis of [18F]FTC-146: A potent and selective sigma-1 receptor radioligand

Abstract

[¹⁸F]FTC-146 was introduced as a very potent and selective sigma-1 receptor radioligand, which has shown promising application as an imaging agent for neuropathic pain with positron emission tomography. In line with a multi-laboratory project on animal welfare, we chose this radioligand to investigate its potential for detecting neuropathic pain and tissue damage in tumor-bearing animals. However, the radiochemical yield (RCY) of around 4–7% was not satisfactory to us, and efforts were made to improve it. Herein, we describe an improved approach for the radiosynthesis of [¹⁸F]FTC-146 resulting in a RCY, which is sevenfold higher than that previously reported. A tosylate precursor was synthesized and radio-fluorination experiments were performed via aliphatic nucleophilic substitution reactions using either K[¹⁸F]F-Kryptofix®222 (K_2.2.2)-carbonate system or tetra-n-butylammonium [¹⁸F]fluoride ([¹⁸F]TBAF). Several parameters affecting the radiolabeling reaction such as solvent, ¹⁸F-fluorination agent with the corresponding amount of base, labeling time, and temperature were investigated. Best labeling reaction conditions were found to be [¹⁸F]TBAF and acetonitrile as solvent at 100°C. The new protocol was then translated to an automated procedure using a FX2 N synthesis module. Finally, the radiotracer reproducibly obtained with RCYs of 41.7 ± 4.4% in high radiochemical purity (>98%) and molar activities up to 171 GBq/μmol.