Viewpoint pubs.acs.org/chemneuro
Classics in Neuroimaging: Imaging the Dopaminergic Pathway with PET Meghna Kanthan,† Paul Cumming,‡,§ Jacob M. Hooker,∥,⊥ and Neil Vasdev*,†,⊥ †
Division of Nuclear Medicine and Molecular Imaging, Massachusetts General Hospital, Boston, Massachusetts 02114, United States School of Psychology and Counselling and IHBI, Queensland University of Technology, Brisbane, Queensland 4000, Australia § QIMR Berghofer Medical Research Institute, Herston, Queensland 4006, Australia ∥ Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, Massachusetts 02129, United States ⊥ Department of Radiology, Harvard Medical School, Boston, Massachusetts 02115, United States ‡
ABSTRACT: The success of positron emission tomography (PET) for observing molecular processes underlying brain function and disease is underpinned by radiotracer chemistry. From the earliest applications of PET to measure dopamine synthesis capacity and the abundance of neuroreceptors and transporters, to the more recent topic of dynamic neurochemical imaging, interrogation of brain dopamine in conditions such as neurodegenerative diseases, schizophrenia, mood disorders, and addictions has been a driving force that challenges the ingenuity of radiopharmaceutical scientists. In fact, the pursuit of new ligands and reaction methods to address longstanding challenges has often been pioneered in the context of dopamine imaging. From this viewpoint, we highlight the unique history of imaging the dopaminergic pathway with PET, and present our interpretation of how this worldwide effort shaped and continues to drive the field of molecular imaging. KEYWORDS: PET, molecular imaging, dopaminergic pathway
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positions within a low molecular weight molecule has led to innovative new approaches and routes to radiochemical synthesis. One of the early successes of PET radiotracer discovery related to functional imaging of the dopaminergic pathway in vivo. Consequently, we offer a Viewpoint on how this early work on dopamine shaped the broader fields of radiochemistry, molecular imaging and drug discovery. Given this historical background, our Viewpoint aims to serve as a point of departure for a series of Viewpoints and Minireviews: Classics in Neuroimaging. The first phase of brain dopamine imaging drew upon the then-existing repertoire of pharmaceuticals as the starting point for radiolabeling, with major efforts initially focusing on the radiosynthesis of the dopamine synthesis tracer 18F-labeled 3,4dihydroxyphenyl-L-alanine (FDOPA) and other analogues.1 In the early 1970s, researchers at McMaster University Medical Centre in Canada worked on labeling the poorly activated aromatic ring of D,L-DOPA via the Balz-Schiemann reaction to prepare [18F]5-FDOPA. In 1983, the same laboratory reported the first functional PET imaging study to visualize dopamine synthesis in striata of the living human brain using [18F]6fluoro-L-DOPA ([18F]6-FDOPA).2 This tracer crosses the blood-brain barrier by facilitated diffusion and is enzymatically converted (decarboxylated) to [18F]6-fluorodopamine, which is retained within vesicles of dopamine neurons. [18F]6-FDOPA is
ositron emission tomography (PET) emerged as an influential tool for molecular imaging in the 1970s with the advent of instrumentation and computational procedures enabling source reconstruction based on the synchronous detection of pairs of gamma photons arising from the mutual annihilation of an electron (β−)−antielectron (β+). The innovations in instrumentation gave impetus for the synthesis of radiopharmaceuticals containing positron-emitting radionuclides in their structure. Indeed, radiotracer discovery over the past 40 years has been driven by parallel developments in neuropharmacology. In conjunction with refinements of radiochemistry, this endeavor has enabled the laboratory production of an ever-increasing repertoire of molecules targeting neurotransmitter systems. As such, nuclear medicine engendered the field of molecular neuroimaging, which inspired international efforts to discover a plethora of PET radiopharmaceuticals. Since the earliest clinical PET neuroimaging research studies, the brain dopaminergic pathway has retained a pivotal position in investigations of psychiatric and neurodegenerative disorders. New potent and selective drug scaffolds are being unveiled in concert with a burgeoning understanding of specific molecular imaging targets such as receptors, transporters and signal transduction pathways integral to dopamine signaling. These innovations have challenged radiochemists to explore the limits of available radiochemical methodologies to synthesize previously unobtainable radiopharmaceuticals. The need to label specific molecules with short-lived radionuclides (typically fluorine-18 (18F); t1/2 = 109.7 min) or carbon-11 (11C); t1/2 = 20.4 min) at particular © XXXX American Chemical Society
Received: July 6, 2017 Accepted: July 6, 2017
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DOI: 10.1021/acschemneuro.7b00252 ACS Chem. Neurosci. XXXX, XXX, XXX−XXX
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ACS Chemical Neuroscience
radiopharmaceuticals for imaging dopamine receptors. In 1984, [11C]3-N-methyl-spiperone, a dopamine D2-like receptor antagonist radiotracer was reported in a collaboration between Uppsala University in Sweden and Johns Hopkins University in the United States. This seminal work represents the first example of neuroreceptor mapping in the living human brain, and also unveiled the process of 11C-methylation via [11C]methyl iodide, which remains the most widely used radiochemical transformation in carbon-11 chemistry (Scheme 1B).4 The high receptor binding in brain seen with [11C]3-N-methylspiperone cannot be unambiguously attributed to dopamine receptors per se, but rather an admixture of dopamine and serotonin receptors. This deficiency motivated the continued development of dopamine receptor imaging agents. In 1985, an academic-pharmaceutical collaboration between the Karolinska Institute and Astra in Sweden reported on the synthesis of [11C]raclopride, via [11C]CH3I, which is still considered as the “gold-standard” selective dopamine D2/3 receptor radiopharmaceutical. Benzamide radiotracers such as [11C]raclopride enabled the measurement of acute fluctuations of endogenous dopamine as captured by changes in ligand binding in vivo. The original competition model proposed that changes in the radiotracer binding potential (BP) reflect altered occupancy of the D2/3 receptors by endogenous dopamine; the reversibility of benzamide binding to D2/3 sites affords a steady-state indication of receptor availability. While [11C]raclopride gives good quantitation in the striatum, where dopamine D2/3 receptors are abundant, the higher affinity homologues [18F]fallypride or [11C]FLB457 allow BP measurements in the thalamus and cortex, where receptor abundances are 10-fold less. Binding of the benzamide ligands is vulnerable to amphetamine-evoked dopamine release, which can reduce BP by as much as 50% in vivo. This phenomenon has proven a useful paradigm for investigating the dynamics of dopamine release. However, additional factors such as agonist-induced receptor internalization contribute to the competition between endogenous dopamine and PET ligands. Similar to other G-protein-coupled receptors, dopamine D2like receptors occur in a state of high affinity (functional state) and a state of low affinity for the endogenous agonist, dopamine. In contrast to dopamine agonist ligands that bind preferentially to the high affinity state of the dopamine D2 receptor, antagonists bind with similar affinity to both states. Over the past decade, agonist radiotracers have become considered to have inherently greater sensitivity than D2/3 antagonist radiotracers to competition from endogenous dopamine. Indeed, D2/3 agonist ligands such as (R)-2[ 11 C]methoxy-N-n-propylnorapomorphine, (−)-N-[ 11 C]propylnorapomorphine ([11C]NPA), and [11C]-(+)-4-propyl3,4,4a,5,6,10b-hexahydro-2H-naphtho[1,2-b][1,4]oxazin-9-ol ([11C]PHNO) exhibited greater sensitivity to competition from endogenous dopamine than did [11C]raclopride. The naphthoxazine dopamine agonist [11C]PHNO has been recharacterized as “D3-preferring”, which may make it particularly useful for studies of addiction, given the distribution of D3 receptors within ventral divisions of the striatum. It is noteworthy that a challenging radiochemical procedure was unveiled to synthesize [11C]PHNO and [11C]NPA, via multistep 11C-radiochemistry, which opened up the door to 11C-propylation in clinical research studies. Scheme 1C shows the synthesis of [11C](+)-PHNO. which involves a Grignard reaction with ethylmagnesium bromide and [11C]CO2 to form the intermediate
among the most widely studied PET radiopharmaceuticals. However, the interpretation of [18F]6-FDOPA PET images is complicated by several factors, which are primarily attributed to its metabolic degradation products. Despite certain limitations, [18F]6-FDOPA is employed the world over for diagnostic imaging studies, i.e., detection of nigrostriatal degeneration in Parkinson’s disease and related disorders. The classical use of [18F]6-FDOPA in brain-PET studies has been eclipsed in recent years due to its fitness for visualizing neuroendocrine tumors. The first clinical research studies with [18F]6-FDOPA paved the way for functional imaging of the dopaminergic pathway in vivo, and also shaped PET radiochemistry, specifically in relation to radiofluorination. The synthesis of [18F]6-FDOPA was initially achieved by direct fluorination of L-DOPA with [18F]F2 in anhydrous HF (Scheme 1) followed by semiScheme 1. Diverse Radiosyntheses of Ligands for Probing in the Dopaminergic Pathway
preparative HPLC purification, to separate impurities and another major product, [18F]2-FDOPA. Efforts to avoid superacidic media and production of undesirable fluoroisomers in the radiosynthesis of [18F]6-FDOPA led to regiospecific fluorodemetalation using [18F]F2 (or the milder agent, [18F]AcOF) with functionally protected L-DOPA precursors bearing a stannous (trialkylltin) or mercuric leaving groups at the 6-position of the aromatic ring. The practical radiosynthesis of [18F]6-FDOPA with no-carrier added [18F]fluoride has been an ongoing challenge,3 and has continued to inspire efforts to develop new general strategies for reactions of [18F]fluoride with electron-rich aromatics.4 Such methods include transition metal-mediated reactions with isolable aryl palladium or nickel complexes, copper-mediated fluorinations with aryl borate esters, oxidative fluorination with phenolic substrates, fluorination of diaryliodonium salts, sulfonium salts or diarylsulfoxides, deoxyfluorination, and iodonium ylide-based radiofluorination.4 The clinical translation of [18F]6-FDOPA was almost contemporaneous with the race to develop the first PET B
DOI: 10.1021/acschemneuro.7b00252 ACS Chem. Neurosci. XXXX, XXX, XXX−XXX
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ACS Chemical Neuroscience [11C]propionyl chloride. This acid chloride has to first be distilled for a coupling reaction between the despropyl precursor yielding the amide intermediate, followed by reduction with LiAlH4 to furnish the radiotracer. In light of its clinical and research potential, we and others are exploring methods to simply the radiochemistry of [11C]PHNO by use of [11C]CH3I, aiming to reduce the number of reaction steps, and generally to facilitate automation on commercial synthesis modules. Efforts to further our understanding of dopamine receptor subtypes continue to be the subject of radiopharmaceutical research. For example, the inconsistent findings with D2-like PET ligands of differing structural classes with respect to competition from dopamine have been perplexing. This may reflect distribution of differing binding properties of receptors in the plasma membrane and in intracellular microsomal compartment, which has been invoked to account for the invulnerability of butyrophenone ligands to competition. The above-mentioned issues (as well as off-target binding) similarly explain the invulnerability of dopamine D1 receptor antagonist ligands based on benzazepine derivatives, such as [11C]SCH23390 to competition from endogenous dopamine. Overall, development of D1 receptor ligands has been relatively neglected, albeit challenged by the above-mentioned issues and obtaining adequate selectivity over other central nervous system (CNS) targets. Nonetheless, dopamine D1-selective receptor agonists (i.e., (S)-[11C]N-methyl-NNC 01-0259) are under investigation in preclinical studies. Other efforts aim to develop D3 antagonist ligands and new probes for the low abundance D4 dopamine receptor subtype. Dopamine transporters in the plasma membrane have been imaged with radiolabeled tropanes, based on [11C]cocaine and related tropanes, such as [11C]PE2I, and their fluorinated analogues. The SPECT ligands 2β-carbomethoxy-3β-(4-[123I]iodophenyl)tropane) ([123I]beta-CIT) and N-ω-fluoropropyl-2β-carbomethoxy-3β-(4-[123I]iodophenyl) nortropane (DaTSCAN) are still widely used for clinical diagnosis of nigrostraital degeneration. Biogenic amine transporters in the synaptic vesicles (vesicular monoamine transporter 2; VMAT2) can be visualized in PET studies with the reserpine analogue [11C]dihydrotetrabenazine. Binding of this ligand reflects the composite of dopamine, serotonin, and noradrenaline innervations, but the preponderant component in striatum is attributed to dopamine terminals. Radiolabeled inhibitors of monoamine oxidase (MAO)-A and MAO-B have also been explored extensively in clinical PET research studies, with early work focusing on irreversibly binding ligands, [11C]-D2-clorgyline and [11C]-D2-(R)-deprenyl for MAO-A and MAO-B, respectively. Newer generation tracers such as [11C]harmine for MAO-A and [11C]SL2511.88 for MAO-B, have advanced to human imaging studies in patients with mood disorders.5 It is noteworthy that [11C]SL2511.88 was previously 11C-carbonylated via an intramolecular cyclization reaction with the unconventional reagent [11C]phosgene, but its radiochemistry has been simplified and validated for human translation by direct reaction of [11C]CO2 in a “fixation” reaction (Scheme 1D). This innovation should make MAO-B PET with reversibly binding radiotracers more accessible for clinical research. Throughout this Viewpoint, we have attempted to highlight some key examples for how the field of PET radiochemistry has been shaped by our emerging interests in understanding the dopaminergic pathway in vivo. A series of subsequent “Classics in Neuroimaging” Viewpoints and Minireviews on pivotal PET
radiopharmaceuticals which target G-protein coupled receptors, enzymes, or signal transduction pathways in the CNS will follow.
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AUTHOR INFORMATION
Corresponding Author
*Mailing address: Division of Nuclear Medicine and Molecular Imaging, Massachusetts General Hospital, 55 Fruit St., White 427, Boston, Massachusetts 02114, USA. Tel: 617-643-4736. Email:
[email protected]. ORCID
Jacob M. Hooker: 0000-0002-9394-7708 Neil Vasdev: 0000-0002-2087-5125 Author Contributions
M.K., P.C., J.M.H., and N.V. all contributed to writing this Viewpoint. Funding
N.V. thanks National Institute on Aging of the NIH for funding (R01AG054473). Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Cumming, P. (2007) Imaging Dopamine, Cambridge University Press. (2) Garnett, E. S., Firnau, G., and Nahmias, C. (1983) Dopamine visualized in the basal ganglia of living man. Nature 305 (5930), 137. (3) Edwards, R., and Wirth, T. (2015) [18F]6-fluoro-3,4-dihydroxy-Lphenylalanine − recent modern syntheses for an elusive radiotracer. J. Labelled Compd. Radiopharm. 58, 183−187. (4) Liang, S. H., and Vasdev, N. (2015) Total Radiosynthesis: Thinking outside the box to achieve multi-step reactions with shortlived radionuclides. Aust. J. Chem. 68 (9), 1319−1328. (5) Laruelle, M. (2012) Measuring dopamine synaptic transmission with molecular imaging and pharmacological challenges: The state of the art. In Molecular Imaging in the Clinical Neurosciences (Gründer, G., Ed.), Neuromethods, pp 163−203, Humana Press, New York.
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DOI: 10.1021/acschemneuro.7b00252 ACS Chem. Neurosci. XXXX, XXX, XXX−XXX
