[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

Chopra A.

Publication Details

Table Icon

Table

In vitro Rodents

Background

[PubMed]

Dopamine is a neurotransmitter that modulates a variety of human functions such as motion, cognition, emotions, and the peristaltic reflexes in the gastrointestinal tract. The transport of this molecule at the neuron pre- and postsynaptic junctions is mediated by an axonal membrane dopamine transporter (DAT) that regulates dopamine levels within the synaptic cleft (1). The process of dopamine release and reuptake within the synaptic cleft facilitates nerve transmission. The striatum region of the brain has a high density of DAT and is rich in dopamine. Psychotic drugs such as cocaine, amphetamines, and methylphenidate (MPH) target DAT, which suggests that the dopamine system modulates the mood of an individual (1-3). Also, changes in DAT are of particular interest in individuals with Parkinson’s disease (PD), which is caused by the loss of neurons bearing these transporters in the basal ganglia and substantia nigra regions of the striatum (4). Development of various imaging ligands that specifically bind to DAT has been of interest to understand the functioning of these transporters and also to diagnose and monitor the treatment of PD. Dopamine receptors and transporters have also been suggested to play a role in the development of the heritable and disruptive attention deficit hyperactivity disorder (ADHD) that is sometimes observed in children (5). Although several ligands are available that can be used for DAT imaging, they have a limited availability and require cyclotron-produced radionuclides that have short half-lives (6). In addition, these ligands require from a few hours to >18 hours to reach a suitable concentration at the imaging target area (4).

In an effort to alleviate the limitations observed with the cyclotron-produced radionuclides, DAT ligands labeled with 99mTc were developed for research and possibly for clinical use. These ligands are neutral lipophilic complexes that contain N-(alkylthiolate)tropane, aminobis(ethylthiolate), and a center core of 99mTc (4). The cocaine derivative [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) is a ligand that belongs to this group. Hu et al. suggested the possible benefit of 99mTc-TRODAT-1 for an early and differential diagnosis of PD (7).

The application of 99mTc-TRODAT-1 for the imaging of DAT is reviewed in this chapter.

Synthesis

[PubMed]

The synthesis of 99mTc-TRODAT-1 has been described by Meegalla et al. (4). Due to the complex intermediate compounds synthesized to obtain 99mTc-TRODAT-1, only general terms have been used in this chapter. The reader should refer to the original publication for complete names (4).

Briefly, the synthesis of a 3β-(p-chloro-)tropane ester and the corresponding carboxylic acid was performed as detailed elsewhere (8, 9). A corresponding acyl chloride was obtained by the action of oxalyl chloride on the carboxylic acid at room temperature. This compound was treated with an amine to obtain the corresponding amide. A secondary amine was prepared by diborane reduction of the amide and converted to a tertiary amide by alkylation with alkyl chloride. The amide functions of compound 9 were reduced with BH3•THF to obtain the amine compound 11a. TRODAT-1 was obtained by deprotection of compound 11a with Hg(OAc)2.

To radiolabel with 99mTc, TRODAT-1 was dissolved in ethanol and HCl, and a Sn-glucoheptonate solution containing EDTA was successively added to it. Subsequently, a saline solution containing 99mTc-pertechnetate was added to the mixture, which was then heated to 100oC for 30 min. The mixture was then cooled to room temperature and neutralized with a saturated solution of NaHCO3. The 99mTc-TRODAT-1 complex was extracted from the aqueous solution with ethyl acetate, passed through a column of Na2SO4, and the ethyl acetate was removed under a flow of nitrogen. The resultant residue was dissolved in ethanol and purified by high-performance liquid chromatography. The radiochemical yield of the reaction was 88% with a radiochemical purity of 98%. The specific activity of the radiolabeled compound was not provided in the publication.

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

The affinity constant (Ki) of 99mTc-TRODAT-1 in rat brain sections was determined using the surrogate molecule rhenium-TRODAT-1 (Re-TRODAT-1) to be 14.1 nM (10). In another study with rat tissue homogenates, the Ki of 99mTc-TRODAT-1 for DAT and the serotonin transporter was determined using Re-TRODAT-1 to be 14.1 ± 2.1 and 360 ± 44 nM, respectively (11).

Animal Studies

Rodents

[PubMed]

Hwang et al. investigated the effect of DAT competitors 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), levadopa (L-DOPA), N-methyl-2β-carbomethoxy-3β-(4fluorophenyl)tropane (CFT, WIN 35,428), and MPH on the biodistribution of 99mTc-TRODAT-1 in mice (12). Compared to controls, mice treated with MPTP and L-DOPA did not show significant differences in radiolabel uptake in bilateral striata of the brain. Almost no 99mTc-TRODAT-1 binding was observed in mice treated with CFT or MPH. The investigators concluded that 99mTc-TRODAT-1 could probably be used for clinical investigation of neurological disorders such as PD.

Acton et al. used 99mTc-TRODAT-1 to demonstrate that DAT binding could be quantified and studied accurately in mice with ultrahigh-resolution single-photon emission computed tomography (SPECT) (13). Similarly, Chen et al. demonstrated that 99mTc-TRODAT-1 could be used to study DAT in a rodent model that used rats treated with 6-hydroxydopamine (14). Fang et al. showed that 99mTc-TRODAT-1 was a safe and useful imaging agent to investigate localization of presynaptic DAT in rodent brain (15). Dresel et al. showed that 99mTc-TRODAT-1 binding in rats was affected only if the animals were pretreated with drugs that significantly increased dopamine levels or competed with DAT binding (16). The investigators suggested that prior knowledge of drugs being administered to patients could help interpret DAT status in individuals being evaluated by SPECT with 99mTc-TRODAT-1 in a clinical setting.

Other Non-Primate Mammals

[PubMed]

No publications are currently available.

Non-Human Primates

[PubMed]

In an effort to study the pre- and postsynaptic dopamine systems in a primate model, the technique of dual SPECT with 99mTc-TRODAT-1 and 123I-labeled iodobenzamide was used with baboons (17) and Formosan rock monkeys (18). From these studies it was concluded that the use of a dual-isotope technique could be an effective method to gain a better insight into the dopaminergic system. In another study, the uptake ratio of 99mTc-TRODAT-1 in the striatum versus the cerebellum of normal and hemi-Parkinsonian monkeys was determined (7). The investigators demonstrated that the striatum/cerebellum ratio of 99mTc-TRODAT-1 uptake in the hemi-Parkinsonian animals was significantly lower compared to that of controls.

Xiao et al investigated the effect of an acute morphine injection on the functional changes of DAT in rhesus monkeys (19). A rapid downregulation of DAT in the morphine-treated monkeys was observed, and this was only partially reversible with cessation of morphine. The investigators suggested that the striatum is an effective target of morphine and that DAT function could be used as an indicator of morphine addiction.

Human Studies

[PubMed]

99mTc-TRODAT-1 scintigraphy was shown to have a high sensitivity and specificity to measure the gradual loss of DAT in PD patients (20). The investigators suggested the use of this radiolabeled compound for the possible diagnosis of PD. In another study, the possible use of SPECT with 99mTc-TRODAT-1 to distinguish between PD and essential tremor was suggested (21).

The distribution volume ratios (DVRs) of 99mTc-TRODAT-1 binding affinity was determined from SPECT data for patients with low and high depressive symptoms (22). Data obtained from this study suggested that the 99mTc-TRODAT-1 binding affinity to DAT may be associated with the depressive effect. The study also supported an earlier study (23) that showed an elevated 99mTc-TRODAT-1 DVR in patients in a high depressive state. In a pilot study designed to investigate the effect of estrogen replacement therapy (ERT) on DAT availability in the brains of healthy postmenopausal women, an increase in DAT in the putamen region of the brain was observed (24). This indicated a possible association of ERT with improvement in PD and late-onset schizophrenia.

The possible role of the degree of 99mTc-TRODAT-1 binding to striatal DAT and the response to MPH in patients with attention deficit hyperactivity disorder (ADHD) was investigated (25). Data from the study showed that individuals with ADHD who have an elevated striatal DAT responded better to MPH therapy than those with low DAT levels. On the basis of these results, the possible use of DAT measurements to predict a response to MPH therapy was suggested. In another study that used 99mTc-TRODAT-1, it was shown that the decrease in the number of DAT binding sites in ADHD patients receiving MPH therapy correlated with an improvement in clinical symptoms (26).

A new formulation of 99mTc-TRODAT-1 was evaluated in a phase-I human clinical trial to characterize striatal DAT binding and to determine if the physiological age and sex of trial participants had an effect on binding (6). The investigators observed an age- and sex-dependency (especially in postmenopausal women) for the binding of this radionuclide. On the basis of this data, the investigators suggested that the age and sex of individuals being studied with 99mTc-TRODAT-1 should be taken into consideration while data after scintigraphy is evaluated.

Supplemental Information

[Disclaimers]

Toxicology

Pharmacology

Dosimetry

NIH Support

Parts of the studies presented in this chapter were funded by NIH grant #s AG-17524, DA-09469, NS24538 and NS18509.

References

1.
Amara S.G. , Kuhar M.J. Neurotransmitter transporters: recent progress. Annu Rev Neurosci. 1993; 16 :73–93. [PubMed: 8096377]
2.
Goeders N.E. , Smith J.E. Cortical dopaminergic involvement in cocaine reinforcement. Science. 1983; 221 (4612):773–5. [PubMed: 6879176]
3.
Volkow N.D. , Wang G.J. , Fowler J.S. , Gatley S.J. , Ding Y.S. , Logan J. , Dewey S.L. , Hitzemann R. , Lieberman J. Relationship between psychostimulant-induced "high" and dopamine transporter occupancy. Proc Natl Acad Sci U S A. 1996; 93 (19):10388–92. [PMC free article: PMC38394] [PubMed: 8816810]
4.
Meegalla S.K. , Plossl K. , Kung M.P. , Chumpradit S. , Stevenson D.A. , Kushner S.A. , McElgin W.T. , Mozley P.D. , Kung H.F. Synthesis and characterization of technetium-99m-labeled tropanes as dopamine transporter-imaging agents. J Med Chem. 1997; 40 (1):9–17. [PubMed: 9016323]
  • 5. Thapar, A., M. O'Donovan, and M.J. Owen, The genetics of attention deficit hyperactivity disorder. Hum Mol Genet, 2005. 14 Spec No. 2: p. R275-82. [PubMed: 16244326]
    • 6.
    Koch W. , Pogarell O. , Popperl G. , Hornung J. , Hamann C. , Gildehaus F.J. , Seelos K. , Lewis D. , Favit A. , Tatsch K. Extended studies of the striatal uptake of 99mTc-NC100697 in healthy volunteers. J Nucl Med. 2007; 48 (1):27–34. [PubMed: 17204696]
    7.
    Hu P. , Chen L. , Zhang H.Q. , Li J.R. , Liang H. Single photon emission computer tomography of dopamine transporters in monkeys and humans with 99mTc-TRODAT-1. Chin Med J (Engl). 2004; 117 (7):1056–9. [PubMed: 15265382]
    8.
    Carroll F.I. , Abraham P. , Lewin A.H. , Parham K.A. , Boja J.W. , Kuhar M. Isopropyl and phenyl esters of 3 beta-(4-substituted phenyl)tropan-2 beta-carboxylic acids. Potent and selective compounds for the dopamine transporter. J Med Chem. 1992; 35 (13):2497–500. [PubMed: 1619622]
    9.
    Meltzer P.C. , Liang A.Y. , Brownell A.L. , Elmaleh D.R. , Madras B.K. Substituted 3-phenyltropane analogs of cocaine: synthesis, inhibition of binding at cocaine recognition sites, and positron emission tomography imaging. J Med Chem. 1993; 36 (7):855–62. [PubMed: 8464040]
    10.
    Kung M.P. , Stevenson D.A. , Plossl K. , Meegalla S.K. , Beckwith A. , Essman W.D. , Mu M. , Lucki I. , Kung H.F. [99mTc]TRODAT-1: a novel technetium-99m complex as a dopamine transporter imaging agent. Eur J Nucl Med. 1997; 24 (4):372–80. [PubMed: 9096087]
    11.
    Dresel S.H. , Kung M.P. , Huang X. , Plossl K. , Hou C. , Shiue C.Y. , Karp J. , Kung H.F. In vivo imaging of serotonin transporters with [99mTc]TRODAT-1 in nonhuman primates. Eur J Nucl Med. 1999; 26 (4):342–7. [PubMed: 10199939]
    12.
    Hwang J.J. , Liao M.H. , Yen T.C. , Wey S.P. , Lin K.J. , Pan W.H. , Chen J.C. , Ting G. Biodistribution study of [99mTc] TRODAT-1 alone or combined with other dopaminergic drugs in mice with macroautoradiography. Appl Radiat Isot. 2002; 57 (1):35–42. [PubMed: 12137024]
    13.
    Acton P.D. , Choi S.R. , Plossl K. , Kung H.F. Quantification of dopamine transporters in the mouse brain using ultra-high resolution single-photon emission tomography. Eur J Nucl Med Mol Imaging. 2002; 29 (5):691–8. [PubMed: 11976810]
    14.
    Chen Y.K. , Liu R.S. , Huang W.S. , Wey S.P. , Ting G. , Liu J.C. , Shen Y.Y. , Wan F.J. The role of dopamine transporter imaging agent [99mTc]TRODAT-1 in hemi-parkinsonism rat brain. Nucl Med Biol. 2001; 28 (8):923–8. [PubMed: 11711311]
    15.
    Fang P. , Wu C.Y. , Liu Z.G. , Wan W.X. , Wang T.S. , Chen S.D. , Chen Z.P. , Zhou X. The preclinical pharmacologic study of dopamine transporter imaging agent [99mTc]TRODAT-1. Nucl Med Biol. 2000; 27 (1):69–75. [PubMed: 10755648]
    16.
    Dresel S.H. , Kung M.P. , Plossl K. , Meegalla S.K. , Kung H.F. Pharmacological effects of dopaminergic drugs on in vivo binding of [99mTc]TRODAT-1 to the central dopamine transporters in rats. Eur J Nucl Med. 1998; 25 (1):31–9. [PubMed: 9396872]
    17.
    Dresel S.H. , Kung M.P. , Huang X.F. , Plossl K. , Hou C. , Meegalla S.K. , Patselas G. , Mu M. , Saffer J.R. , Kung H.F. Simultaneous SPECT studies of pre- and postsynaptic dopamine binding sites in baboons. J Nucl Med. 1999; 40 (4):660–6. [PubMed: 10210226]
    18.
    Ma K.H. , Huang W.S. , Chen C.H. , Lin S.Z. , Wey S.P. , Ting G. , Wang S.D. , Liu H.W. , Liu J.C. Dual SPECT of dopamine system using [99mTc]TRODAT-1 and [123I]IBZM in normal and 6-OHDA-lesioned formosan rock monkeys. Nucl Med Biol. 2002; 29 (5):561–7. [PubMed: 12088726]
    19.
    Xiao Z.W. , Cao C.Y. , Wang Z.X. , Li J.X. , Liao H.Y. , Zhang X.X. Changes of dopamine transporter function in striatum during acute morphine addiction and its abstinence in rhesus monkey. Chin Med J (Engl). 2006; 119 (21):1802–7. [PubMed: 17097035]
    20.
    Weng Y.H. , Yen T.C. , Chen M.C. , Kao P.F. , Tzen K.Y. , Chen R.S. , Wey S.P. , Ting G. , Lu C.S. Sensitivity and specificity of 99mTc-TRODAT-1 SPECT imaging in differentiating patients with idiopathic Parkinson's disease from healthy subjects. J Nucl Med. 2004; 45 (3):393–401. [PubMed: 15001678]
    21.
    Wang J. , Jiang Y.P. , Liu X.D. , Chen Z.P. , Yang L.Q. , Liu C.J. , Xiang J.D. , Su H.L. 99mTc-TRODAT-1 SPECT study in early Parkinson's disease and essential tremor. Acta Neurol Scand. 2005; 112 (6):380–5. [PubMed: 16281920]
    22.
    Newberg A. , Amsterdam J. , Shults J. Dopamine transporter density may be associated with the depressed affect in healthy subjects. Nucl Med Commun. 2007; 28 (1):3–6. [PubMed: 17159542]
    23.
    Brunswick D.J. , Amsterdam J.D. , Mozley P.D. , Newberg A. Greater availability of brain dopamine transporters in major depression shown by [99m Tc]TRODAT-1 SPECT imaging. Am J Psychiatry. 2003; 160 (10):1836–41. [PubMed: 14514499]
    24.
    Gardiner S.A. , Morrison M.F. , Mozley P.D. , Mozley L.H. , Brensinger C. , Bilker W. , Newberg A. , Battistini M. Pilot study on the effect of estrogen replacement therapy on brain dopamine transporter availability in healthy, postmenopausal women. Am J Geriatr Psychiatry. 2004; 12 (6):621–30. [PubMed: 15545330]
    25.
    la Fougere C. , Krause J. , Krause K.H. , Josef Gildehaus F. , Hacker M. , Koch W. , Hahn K. , Tatsch K. , Dresel S. Value of 99mTc-TRODAT-1 SPECT to predict clinical response to methylphenidate treatment in adults with attention deficit hyperactivity disorder. Nucl Med Commun. 2006; 27 (9):733–7. [PubMed: 16894328]
    26.
    Dresel S. , Krause J. , Krause K.H. , LaFougere C. , Brinkbaumer K. , Kung H.F. , Hahn K. , Tatsch K. Attention deficit hyperactivity disorder: binding of [99mTc]TRODAT-1 to the dopamine transporter before and after methylphenidate treatment. Eur J Nucl Med. 2000; 27 (10):1518–24. [PubMed: 11083541]