The meta-vinylhalide fluoroalkyl ester nortropanes 1-4 were synthesized as ligands of the serotonin transporter (SERT) for use as positron emission tomography (PET) imaging agents. In vitro competition binding assays demonstrated that 1-4 have a high affinity for the SERT (Ki's = 0.3 - 0.4 nM) and are selective for the SERT over the dopamine and norepinephrine transporters (DAT and NET). MicroPET imaging in anesthetized cynomolgus monkeys with [18F]1-[18F]4 demonstrated that all four tracers behave similarly with peak uptake in the SERT-rich brain regions achieved after 45-55 min followed by a steady washout. An awake monkey study was performed with [18F]1 which demonstrated that the uptake of [18F]1 was not influenced by anesthesia. Chase studies with the SERT ligand 15 displaced [18F]1-[18F]4 but chase studies with the DAT ligand 16 did not displace [18F]1-[18F]4 thus indicating that the tracers were binding specifically to the SERT.

The serotonin 5-HT2C receptor (5-HT2CR) is abundantly expressed throughout the central nervous system, and involved in a variety of neuroendocrine and neurobehavioral processes. The development of a selective radioligand that will enable in vivo imaging and quantification of 5-HT2CR densities represents a significant technological advancement in understanding both the normal function and pathophysiology of the 5-HT2CR. Four 7-halogen-2-phenyl isoindolones (7-F, Cl, Br, I) were synthesized and displayed high affinities for 5-HT2CR and high selectivity over 5-HT2A and 5-HT2B. [11C]7-Chloro-2-[4-methoxy-3-[2-(4-methylpiperidin-1-yl)ethoxy]phenyl]isoindolin-1-one (6) and [11C]7-iodo-2-[4-methoxy-3-[2-(4-methylpiperidin-1-yl)ethoxy]phenyl]isoindolin-1-one (9) were synthesized in high radiochemical yield of 37–44% [n = 10, decay corrected from end of (11C)CH3I synthesis] with high radiochemical purity via O-methylation with [11C]CH3I, respectively. MicroPET imaging studies in male rats with or without 5-HT2C antagonist SB-242084 showed that [11C]6 and [11C]9 display specific bindings to 5-HT2CR in the choroid plexus and hippocampus. In vivo microPET brain imaging studies in rhesus monkeys demonstrated that [11C]6 and [11C]9 exhibit excellent blood-brain barrier penetration. The contrast of bindings to the choroid plexus and hippocampus compared to the cerebellum peaked at 2.7 and 1.6, respectively, for [11C]6, and 3.7 and 2.7, respectively, for [11C]9, which were reduced by administration of a dose of SB-242084. Our results support the candidacy of [11C]6 and [11C]9 for further study as radioligands for in vivo quantitation of 5-HT2C sites by PET.

A series of imidazo[1,2-b]pyridazine derivatives were synthesized and evaluated for binding to amyloid plaques in vitro using synthetic aggregates of Aβ1−40. Binding affinities of these compounds were found to range from 11.0 to >1000 nM, depending on the various substitution patterns in the 6-position and 2-position. 2-(4′-Dimethylaminophenyl)-6-(methylthio)imidazo[1,2-b]pyridazine (4) showed high binding affinity (Ki = 11.0 nM) and might be useful for the development of novel positron emission tomography radiotracers for imaging Aβ plaques.

Reboxetine analogs with methyl and fluoroalkyl substituents at position 2 of the phenoxy ring 1–4 were synthesized. In vitro competition binding demonstrated that 1–4 have a high affinity for the norepinephrine transporter (NET) with Ki’s = 1.02, 3.14, 3.68, and 0.30 nM (vs [3H]nisoxetine), respectively. MicroPET imaging in rhesus monkeys showed that the relative regional distribution of [11C]1 and [11C]4 is consistent with distribution of the NET in the brain, while [18F]2 and [18F]3 showed only slight regional differentiation in brain uptake. Especially, the highest ratios of uptake of [11C]1 in NET-rich regions to that in caudate were obtained at 1.30–1.45 at 45 min, and remained relatively constant over 85 min. Pretreatment of the monkey with the selective NET inhibitor, desipramine, decreased the specific binding for both [11C]1 and [11C]4. PET imaging in awake monkeys suggested that anesthesia influenced the binding potential of [11C]1 and [11C]4 at the NET.