Validation of two fluoro-analogues of N,N-dimethyl-2-(2'-amino-4'-hydroxymethyl-phenylthio)benzylamine as serotonin transporter imaging agents using microPET.

Carbon-11 (C-11) N,N-dimethyl-2-(2'-amino-4'-hydroxymethyl-phenylthio)benzylamine ([(11)C]HOMADAM) has been reported as highly specific and selective positron emission tomography (PET) radiotracer showing fast kinetics for the human brain serotonin transporter (SERT). In our continued effort to develop appropriate PET SERT radioligand that can be labeled with either C-11 or fluorine-18 (F-18), two new C-11 labeled analogues of HOMADAM, [(11)C]-N,N-dimethyl-2-(2'-amino-5'-fluoro-4'-hydroxymethyl-phenylthio)benzylamine ([(11)C]-(2)) and [(11)C]-N,N-dimethyl-2-(2'-amino-4-fluoro-4'-hydroxymethyl-phenylthio)benzylamine ([(11)C]-(3)) have been synthesized and evaluated along the previously reported [(11)C]-N,N-dimethyl-2-(2'-amino-5-fluoro-4'-hydroxymethyl-phenylthio)benzylamine ([(11)C]-(1)). The in vitro competitive binding assays were performed in cells transfected with human SERT (hSERT), human dopamine transporter (hDAT), and human norepinephrine transporter (hNET). [(11)C]-(2) and [(11)C]-(3) were prepared by methylation of their monomethylbenzylamine precursors 13 and 22 with cyclotron produced [(11)C]iodomethane ([(11)C]CH(3)I), respectively. Uptake and kinetics of [(11)C]-(2) and [(11)C]-(3) in the brain regions of interest were determined in anesthetized rhesus monkeys using Concorde microPET P4. 2 and 3 displayed moderate and high affinity for the SERT with Kis (SERT) = 5.45 and 1.10 nM (vs [(3)H]citalopram), respectively. After High Performance Liquid Chromatography (HPLC) purification, [(11)C]-(2) and [(11)C]-(3) were obtained in 23 and 9% radiochemical yield (RCY) and log Ps(7.4) of 1.77 and 1.91, respectively. The microPET images of [(11)C]-(2) and [(11)C]-(3) showed clear localization in the monkey brain regions rich in SERT with midbrain to cerebellum ratios of 1.75 and 3.86 at 85 min post injection, respectively, comparing to 3.40 for [(11)C]-(1), at the same time. [(11)C]-(3) was selected for further examination and showed to be specific to the SERT as displacement with citalopram (a potent SERT ligand) reduced radioactivity in SERT rich regions, such as midbrain, to the cerebellum level. Compound 2, the 5'-fluoro-analogue of HOMADAM, had the lowest brain uptake and target to non-target ratios. Compound 3, the 4-fluoro-analogue of HOMADAM, had good brain uptake and higher midbrain and thalamus to cerebellum ratios than compound 1, the 5-fluoro-analogue of HOMADAM. Although 1 and 3 presented better imaging properties than 2, none of the three candidates was suitable to surpass the binding or distributional qualities of the parent HOMADAM. Alternative fluoro-analogues of HOMADAM will soon be characterized, in future work, as SERT radioimaging agents.