Positron emission tomography imaging of (2R,3R)-5-[18F]fluoroethoxybenzovesamicol in rat and monkey brain: a radioligand for the vesicular acetylcholine transporter☆
Introduction
The vesicular acetylcholine transporter (VAChT) is a protein uniquely located in presynaptic vesicles of cholinergic neurons and is responsible for the transport of the neurotransmitter acetylcholine into the storage vesicles [1]. As a specific biochemical function of the cholinergic neuron, it has been a potential target for development of in vivo radioligands [2], [3] for single-photon emission computed tomography (SPECT) or positron emission tomography (PET) imaging of the potential losses of cholinergic neurons in degenerative neurological diseases. Successful imaging of the VAChT in the human brain, including losses in disease including Alzheimer's, has been reported using a SPECT radioiodinated ligand 5-[123I]iodobenzovesamicol ((−)-5-IBVM) [4], [5], [6], [7]. For potential use in PET, radioligands labeled with the positron-emitting radionuclides carbon-11 and fluorine-18 were reported as early as 1990 [8], [9] and there has been continued interest in further evaluation of both SPECT and PET radioligands for imaging of the VAChT binding site in the human brain [10], [11].
Most recently, Giboureau et al. [10] reported the in vivo evaluation in rats and primate brain of two potential PET radioligands, (2R,3R)-5-[18F]fluoroethoxybenzovesamicol ((−)-[18F]FEOBV, first reported in 1993 by Mulholland et al. [12]) and (2R,3R)-5-[18F]fluoropropoxybenzovesamicol (((−)-[18F]FPOBV). Using ex vivo dissection techniques for both radioligands, and in vivo PET imaging of [18F]FPOBV in monkey brain, these investigators concluded that both (−)-[18F]FEOBV and (−)-[18F]FPOBV were unsuitable as in vivo radioligands for the imaging for the VAChT in the mammalian brain. Their report directly contradicts our earlier and extensive rodent studies of (−)-[18F]FEOBV [13], which had consistently supported that it would be a viable candidate for further development as a PET imaging agent for human studies of the VAChT. As we have continued to pursue the development of (−)-[18F]FEOBV with the intention of eventual human applications, we felt it was necessary to revisit our in vivo studies of (−)-[18F]FEOBV to verify our earlier conclusions. We report here in vivo studies of (−)-[18F]FEOBV in mice, rats and nonhuman primates, including dynamic microPET imaging studies, which clearly demonstrate that (−)-[18F]FEOBV is indeed a successful imaging agent for the VAChT and is worthy of further development and implementation.
Section snippets
Synthesis of (−)-[18F]FEOBV
Syntheses of (−)-[18F]FEOBV and unlabeled (−)-FEOBV were done by slight modifications of the literature procedure [12]. Reaction of the tosyloxy precursor, (2R,3R)-5-(2-tosyloxyethoxy)benzovesamicol, with no-carrier-added [18F]fluoride ion in dimethylsulfoxide followed by purification using preparative high-pressure liquid chromatography provided the desired (−)-[18F]FEOBV with high radiochemical purity (>95%) and specific activities of 3500–13,300 Ci/mmol at the end of synthesis. For the
Regional mouse brain biodistribution and blocking studies
At 3 h after bolus intravenous injection of (−)-[18F]FEOBV, there was a clear heterogeneous distribution of radioactivity in the mouse brain (striatum>cortex>hippocampus>thalamus>cerebellum) consistent with the distribution of cholinergic synapses and VAChT. Administration of doses of cold (−)-FEOBV reduced the retention of radioactivity in regions of high VAChT concentrations (striatum and cortex) at the 5 and 10 μg/kg doses (Table 1).
Rat microPET imaging
Following bolus intravenous injection, (−)-[18F]FEOBV
Discussion
The synthesis of (−)-[18 F]FEOBV, in vitro characterization as a high affinity radioligand for the VAChT, and in vivo and ex vivo rodent studies had been reported from our laboratories more than a decade ago [12], [13]. From those studies, and others completed but not reported, we had concluded that (−)-[18F]FEOBV was a suitable candidate for further development into an in vivo PET imaging agent for the VAChT in the human brain, with potential applications in the study of cholinergic terminal
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2017, Bioorganic and Medicinal ChemistryCitation Excerpt :The tracer is a molecule that has been radiolabeled with a positron-emitting radionuclide, such as 11C and 18F. Several attempts at synthesizing a PET tracer for VAChT have been made but only a limited number have progressed to in vivo preclinical evaluations.17–24 All VAChT ligands synthesized thus far are structurally related to (±)-trans-2-(4-phenylpiperidino)-cyclohexanol (vesamicol, Fig. 1).
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This work was supported by grants from the National Institutes of Health (MH66506) and the Office of Science, US Department of Energy (DE-FG02-87ER60561).