Fluorine-18 labeled diphenyl sulfide derivatives for imaging serotonin transporter (SERT) in the brain.

OBJECTIVES: Serotonin transporters (SERT) play an important role in controlling serotonin concentration in the synaptic cleft and in managing postsynaptic signal transduction. Inhibitors of SERT binding are well known as selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine, sertraline, paroxetine, and escitalopram, that are commonly prescribed antidepressants. Positron emission tomography (PET) and single photon emission tomography (SPECT) imaging agents targeting SERT may be useful for studying its function and providing a tool for monitoring drug treatment. METHODS: A series of novel 18F-labeled diphenyl sulfide derivatives were prepared and tested for their binding affinity. Among them, 2-((2-((dimethylamino)-methyl)-4-(2-(2-fluoroethoxy)ethoxy)phenyl)thio)aniline, 1, which showed excellent binding toward serotonin transporter (SERT) in the brain (Ki = 0.09 nM), was selected for further evaluation. An active OTs intermediate, 7, was treated with [18F]F-/K222 to provide [18F]1 in one step and in high radiochemical yields. This new SERT targeting agent was evaluated in rats by biodistribution studies and animal PET imaging studies. RESULTS: The radiolabeling reaction led to the desired [18F]1. After HPLC purification no-carrier-added [18F]1 was obtained (radiochemical yield, 23-47% (n = 10,); radiochemical purity >99%; molar activity, 15-28 GBq/µmol). Biodistribution studies with [18F]1 showed good brain uptake (1.04% dose/g at 2 min post-injection), high uptake into the hypothalamus (1.55% dose/g at 30 min), and a high target-to-non-target (hypothalamus to cerebellum) ratio of 6.1 at 120 min post-injection. A PET imaging study in normal rats showed excellent uptake in the midbrain and thalamus regions known to be rich in SERT binding sites at 60 min after iv injection. Chasing experiment with escitalopram (iv, 2 mg/kg) in a rat at 60 min after iv injection caused a noticeable reduction in the regional radioactivity and the target-to-non-target ratio, suggesting binding by [18F]1 was highly specific and reversible for SERT binding sites in the brain. CONCLUSIONS: A novel diphenyl sulfide derivative, [18F]1 for SERT imaging was successfully prepared and evaluated. Results suggest that this new chemical entity is targeting SERT binding sites in the brain, and it is a suitable candidate for future commercial development.

[(18)F](2S,4S)-4-(3-Fluoropropyl)glutamine as a tumor imaging agent.

Although the growth and proliferation of most tumors is fueled by glucose, some tumors are more likely to metabolize glutamine. In particular, tumor cells with the upregulated c-Myc gene are generally reprogrammed to utilize glutamine. We have developed new 3-fluoropropyl analogs of glutamine, namely [(18)F](2S,4R)- and [(18)F](2S,4S)-4-(3-fluoropropyl)glutamine, 3 and 4, to be used as probes for studying glutamine metabolism in these tumor cells. Optically pure isomers labeled with (18)F and (19)F (2S,4S) and (2S,4R)-4-(3-fluoropropyl)glutamine were synthesized via different routes and isolated in high radiochemical purity (≥95%). Cell uptake studies of both isomers showed that they were taken up efficiently by 9L tumor cells with a steady increase over a time frame of 120 min. At 120 min, their uptake was approximately two times higher than that of l-[(3)H]glutamine ([(3)H]Gln). These in vitro cell uptake studies suggested that the new probes are potential tumor imaging agents. Yet, the lower chemical yield of the precursor for 3, as well as the low radiochemical yield for 3, limits the availability of [(18)F](2S,4R)-4-(3-fluoropropyl)glutamine, 3. We, therefore, focused on [(18)F](2S,4S)-4-(3-fluoropropyl)glutamine, 4. The in vitro cell uptake studies suggested that the new probe, [(18)F](2S,4S)-4-(3-fluoropropyl)glutamine, 4, is most sensitive to the LAT transport system, followed by System N and ASC transporters. A dual-isotope experiment using l-[(3)H]glutamine and the new probe showed that the uptake of [(3)H]Gln into 9L cells was highly associated with macromolecules (>90%), whereas the [(18)F](2S,4S)-4-(3-fluoropropyl)glutamine, 4, was not (<10%). This suggests a different mechanism of retention. In vivo PET imaging studies demonstrated tumor-specific uptake in rats bearing 9L xenographs with an excellent tumor to muscle ratio (maximum of ∼8 at 40 min). [(18)F](2S,4S)-4-(3-fluoropropyl)glutamine, 4, may be useful for testing tumors that may metabolize glutamine related amino acids.

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

INTRODUCTION: In 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 [(18)F]FPBM ([(18)F]2-(2'-((dimethylamino)methyl)-4'-(3-fluoropropoxy)phenylthio)benzenamine, 1). METHOD: To improve and automate the radiolabeling of [(18)F]FPBM, 1, an intermediate, [(18)F]3-fluoropropyltosylate, [(18)F]4, was prepared first, and then it was reacted with the phenol precursor (4-(2-aminophenylthio)-3-((dimethylamino)methyl)phenol, 3) to afford [(18)F]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. RESULTS: This two-step radiolabeling reaction successfully produced the desired [(18)F]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 [(18)F]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 [(18)F]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). CONCLUSION: An improved radiolabeling method by a SPE purification for preparation of [(18)F]FPBM, 1, was developed. The results suggest that it is feasible to use this labeling method to prepare [(18)F]FPBM, 1, without affecting in vivo SERT binding.