An improved preparation of [18F]FPBM: A potential serotonin transporter (SERT) imaging agent

IntroductionIn vivo positron emission tomography (PET) imaging of the serotonin transporter (SERT) is a valuable tool in drug development and in monitoring brain diseases with altered serotonergic function. We have developed a two-step labeling reaction for the preparation of the high serotonin affinity ligand [¹⁸F]FPBM ([¹⁸F]2-(2′-((dimethylamino)methyl)-4′-(3-fluoropropoxy)phenylthio)benzenamine, 1).MethodTo improve and automate the radiolabeling of [¹⁸F]FPBM, 1, an intermediate, [¹⁸F]3-fluoropropyltosylate, [¹⁸F]4, was prepared first, and then it was reacted with the phenol precursor (4-(2-aminophenylthio)-3-((dimethylamino)methyl)phenol, 3) to afford [¹⁸F]FPBM, 1. To optimize the labeling, this O-alkylation reaction was evaluated under different temperatures, using different bases and varying amounts of precursor 3. The desired product was obtained after a solid phase extraction (SPE) purification.ResultsThis two-step radiolabeling reaction successfully produced the desired [¹⁸F]FPBM, 1, with an excellent radiochemical purity (> 95%, n = 8). Radiochemical yields were between 31% and 39% (decay corrected, total time of labeling: 70 min, n = 8). The SPE purification cannot completely remove pseudo-carriers in the final dose of [¹⁸F]FPBM, 1. The concentrations of major pseudo-carriers were measured by UV-HPLC (476–676, 68–95 and 50–71 μg for precursor 3, O-hydroxypropyl and O-allyloxy derivatives, 5 and 6, respectively). To investigate the potential inhibition of SERT binding of these pseudo-carriers, we performed in vitro competition experiments evaluated by autoradiography. Known amounts of ‘standard’ FPBM, 1, of the pseudo-carriers, 5 and 6, were added to the HPLC-purified [¹⁸F]1 dose. The inhibition of ‘standard’ FPBM, 1, binding to the SERT binding sites, using monkey brain sections, were measured (EC₅₀ = 13, 46, 7.1 and 8.3 nM, respectively for 1, precursor 3, O-hydroxypropyl and O-allyloxy derivative of 3).ConclusionAn improved radiolabeling method by a SPE purification for preparation of [¹⁸F]FPBM, 1, was developed. The results suggest that it is feasible to use this labeling method to prepare [¹⁸F]FPBM, 1, without affecting in vivo SERT binding.