Synthesis and Characterization of Technetium-99m-Labeled Tropanes as Dopamine Transporter-Imaging Agents

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Abstract
In the development of novel Tc-99m-labeled tropane derivatives as dopamine transporter (reuptake site)-imaging agents, a series of neutral and lipophilic complexes containing bis(aminoethanethiol) as a neutral complexing moiety for a [99mTc]TcO3+ center core was successfully prepared. Biological evaluation of the Tc-99m-labeled complexes 13−16 as central nervous system (CNS) dopamine transporter-imaging agents was reported. Synthesis of the tropane derivatives was achieved by stepwise reactions adding two aminoethanethiol units. The final free thiol ligands were obtained by deblocking the 4-methoxybenzyl protecting group with Hg(OAc)2 to obtain trifluoroacetate salts. All of the Tc-99m complexes, with the exception of 16, displayed good initial brain uptake and selective uptake in the striatal area, where dopamine transporters are concentrated. One of the compounds, [2-[[2-[[[3-(4-chlorophenyl)-8-methyl-8-azabicyclo[3.2.1]oct-2-yl]methyl](2-mercaptoethyl)amino]ethyl]amino]ethanethiolato(3−)-N2,N2‘,S2,S2‘]oxo-[1R-(exo-exo)]- [99mTc]technetium, [99mTc]TRODAT-1 (13), displayed the highest initial uptake in rat brain (0.4% at 2 min post iv injection); the striatal/cerebellar (ST/CB) ratio reached 2.8 at 60 min after an iv injection. The specific uptake in rat brain can be blocked by pretreating rats with a competing dopamine transporter binding agent, β-CIT (RTI-55, 2β-carbomethoxy-3β-(4-iodophenyl)tropane; iv, 1 mg/kg), which reduced the regional brain uptake ratio (ST/CB) to 1.2. In contrast, the specific striatal uptake was not affected by pretreating rats with a noncompeting ligand, haldol (iv, 1 mg/kg). After an iv injection of 9 mCi of [99mTc]TRODAT-1 (13), in vivo images of baboon brain using single-photon emission-computed tomography exhibited excellent localization in striatum (basal ganglia), where dopamine neurons are known to be concentrated. This series of compounds may provide a convenient source of short-lived imaging agents for routine diagnosis of CNS diseases (i.e., Parkinson's disease) in which changes in the dopamine transporter concentration are implicated.

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