PET examination of [(11)C]5-methyl-6-nitroquipazine, a radioligand for visualization of the serotonin transporter

Radiochemistry for positron emission tomography

Positron emission tomography (PET) constitutes a functional imaging technique that is harnessed to probe biological processes in vivo. PET imaging has been used to diagnose and monitor the progression of diseases, as well as to facilitate drug development efforts at both preclinical and clinical stages. The wide applications and rapid development of PET have ultimately led to an increasing demand for new methods in radiochemistry, with the aim to expand the scope of synthons amenable for radiolabeling. In this work, we provide an overview of commonly used chemical transformations for the syntheses of PET tracers in all aspects of radiochemistry, thereby highlighting recent breakthrough discoveries and contemporary challenges in the field. We discuss the use of biologicals for PET imaging and highlight general examples of successful probe discoveries for molecular imaging with PET - with a particular focus on translational and scalable radiochemistry concepts that have been entered to clinical use.

New methodologies for the preparation of carbon-11 labeled radiopharmaceuticals

PURPOSE

This short review aims to cover the more recent and promising developments of carbon-11 (11C) labeling radiochemistry and its utility in the production of novel radiopharmaceuticals, with special emphasis on methods that have the greatest potential to be translated for clinical positron emission tomography (PET) imaging.

METHODS

A survey of the literature was undertaken to identify articles focusing on methodological development in 11C chemistry and their use within novel radiopharmaceutical preparation. However, since 11C-labeling chemistry is such a narrow field of research, no systematic literature search was therefore feasible. The survey was further restricted to a specific timeframe (2000-2016) and articles in English.

RESULTS

From the literature, it is clear that the majority of 11C-labeled radiopharmaceuticals prepared for clinical PET studies have been radiolabeled using the standard heteroatom methylation reaction. However, a number of methodologies have been developed in recent years, both from a technical and chemical point of view. Amongst these, two protocols may have the greatest potential to be widely adapted for the preparation of 11C-radiopharmaceuticals in a clinical setting. First, a novel method for the direct formation of 11C-labeled carbonyl groups, where organic bases are utilized as [11C]carbon dioxide-fixation agents. The second method of clinical importance is a low-pressure 11C-carbonylation technique that utilizes solvable xenon gas to effectively transfer and react [11C]carbon monoxide in a sealed reaction vessel. Both methods appear to be general and provide simple paths to 11C-labeled products.

CONCLUSION

Radiochemistry is the foundation of PET imaging which relies on the administration of a radiopharmaceutical. The demand for new radiopharmaceuticals for clinical PET imaging is increasing, and 11C-radiopharmaceuticals are especially important within clinical research and drug development. This review gives a comprehensive overview of the most noteworthy 11C-labeling methods with clinical relevance to the field of PET radiochemistry.

Cross-coupling reactions as valuable tool for the preparation of PET radiotracers

The increasing application of positron emission tomography (PET) in nuclear medicine has stimulated the extensive development of a multitude of new radiotracers and novel radiolabeling procedures with the most prominent short-lived positron emitters carbon-11 and fluorine-18. Radiolabeling with these radionuclides represents a remarkable challenge. Special attention has to be paid to synthesis time and specific labeling techniques due to the short physical half life of the respective radionuclides ¹¹C (t_1/2 = 20.4 min) and ¹⁸F (t_1/2 = 109.8 min). In the past, numerous transition metal-catalyzed reactions were employed in organic chemistry, even though only a handful of these coupling reactions were adopted in radiochemical practice. Thus, the implementation of modern synthesis methods like cross-coupling reactions offers the possibility to develop a wide variety of novel radiotracers. The introduction of catalysts based on transition metal complexes bears a high potential for rapid, efficient, highly selective and functional group-tolerating incorporation of carbon-11 and fluorine-18 into target molecules. This review deals with design, application and improvement of transition metal-mediated carbon-carbon as well as carbon-heteroatom cross-coupling reactions as a labeling feature with the focus on the preparation of radiolabeled compounds for molecular imaging.

Synthesis, radiosynthesis, and biological evaluation of carbon-11 and fluorine-18 labeled reboxetine analogues: potential positron emission tomography radioligands for in vivo imaging of the norepinephrine transporter

Reboxetine analogues with methyl and fluoroalkyl substituents at position 2 of the phenoxy ring 1-4 were synthesized. In vitro competition binding with [(3)H]nisoxetine demonstrated that 1-4 have a high affinity for the norepinephrine transporter (NET) with K(i)'s = 1.02, 3.14, 3.68, and 0.30 nM, respectively. MicroPET imaging in rhesus monkeys showed that the relative regional distribution of [(11)C]1 and [(11)C]4 is consistent with distribution of the NET in the brain, while [(18)F]2 and [(18)F]3 showed only slight regional differentiation in brain uptake. Especially, the highest ratios of uptake of [(11)C]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 [(11)C]1 and [(11)C]4. PET imaging in awake monkeys suggested that anesthesia influenced the binding potential of [(11)C]1 and [(11)C]4 at the NET.

Synthesis and biological evaluation of one novel technetium-99m-labeled nitroquipazine derivative as an imaging agent for serotonin transporter

Imaging of serotonin transporter (SERT) by positron emission tomography (PET) or single-photon emission-computed tomography (SPECT) in humans would provide useful information in diagnosis and therapy of several neurodegenerative and neuropsychiatric disorders. 6-Nitroquipazine is a highly potent and selective inhibitor of the SERT. For the development of new (99m)Tc-labeled 6-nitroquipazine derivatives as SERT imaging agents, novel [N-[2-((3-(4-(6-nitroquinolin-2-yl)piperazin-1-yl)propyl)(2-mercaptoethyl)amino]-acetyl-2-aminoethanethiolato] [(99m)Tc]technetium (V) oxide ((99m)Tc-MAMA-3-PQ) and its rhenium analog were synthesized and characterized. (99m)Tc-MAMA-3-PQ displayed high initial brain uptake (0.52% ID/organ at 2 min post-injection (pi)) and relatively fast washout in mice (0.09% ID/organ at 60 min pi). The regional brain distribution studies in rats showed high-specific binding ratios at 60 min pi. Maximum regional contrast ratio observed for thalamus/cerebellum was 2.94, followed by 2.62 for hypothalamus/cerebellum. These encouraging results lead us to further explore its derivatives as new imaging agents for the SERT in the brain.

Synthesis and in vivo evaluation of halogenated N,N-dimethyl-2-(2'-amino-4'-hydroxymethylphenylthio)benzylamine derivatives as PET serotonin transporter ligands

N, N-dimethyl-2-(2'-amino-4'-hydroxymethylphenylthio)benzylamine (38), substituted on ring A, was reported to display high binding affinity and selectivity to the human brain serotonin transporter (SERT). In an attempt to explore the potential of compounds substituted on ring B of the phenylthiophenyl core structure, three derivatives of 38 were synthesized: N, N-dimethyl-2-(2'-amino-4'-hydroxymethyl-phenylthio)-5-fluorobenzylamine (35), N, N-dimethyl-2-(2'-amino-4'-hydroxymethyl-phenylthio)-5-bromobenzylamine (36), and N, N-dimethyl-2-(2'-amino-4'-hydroxymethyl-phenylthio)-5-iodobenzylamine (37). The in vitro binding studies in cells transfected with human SERT, norepinephrine transporter (NET), and dopamine transporter (DAT) showed that 35, 36, and 37 exhibited high SERT affinity with K is (SERT) = 1.26, 0.29, and 0.31 nM (vs [(3)H]citalopram), respectively. [(11)C]-(35), [(11)C]-(36), and [(11)C]-( 37) were prepared by methylation of their monomethyl precursors 16, 17, and 18, with [(11)C]iodomethane in 28, 11, and 14% radiochemical yields, respectively. The microPET images of [(11)C]-(35), [(11)C]-(36), and [(11)C]-(37) showed high uptake in the monkey brain regions rich in SERT with peak midbrain to cerebellum ratios of 3.41, 3.24, and 3.00 at 85 min post-injection, respectively. In vivo bindings of [(11)C]-(35), [(11)C]-(36), and [(11)C]-(37) were shown to be specific to the SERT as displacement with citalopram (a potent SERT ligand) reduced radioactivity in SERT-rich regions to the cerebellum level. These results suggest that [(11)C]-(35), [(11)C]-(36), and [(11)C]-(37) could be potential agents for mapping human SERT by PET and radiolabeling 37 with iodine-123, which could afford the first SPECT SERT imaging agent exhibiting fast kinetics.

Carbon-11 labeling chemistry based upon [11C]methyl iodide

Radiochemistry with the short-lived positron emitter 11C (half-life 20.38 min) represents special challenges in terms of synthesis time and labeling techniques. The recent developments in 11C radiochemistry have steadily expanded the number of 11C labeled compounds. This chapter addresses selected chemical and technical aspects of 11C chemistry based on the readily available labeling precursors [11 C]methyl iodide and, to a lesser extent, [11C]methyl triflate. Special emphasis is placed on heteroatom methylation reactions and 11C-C bond formations.

[(11)C]MADAM, a new serotonin transporter radioligand characterized in the monkey brain by PET

The aim of this study was to explore the potential of a new selective serotonin transporter (5-HTT) inhibitor, N,N-dimethyl-2-(2-amino-4-methylphenylthio)benzylamine (MADAM, K(i)=1.65 nM), as a PET radioligand for examination of 5-HTT in the nonhuman primate brain. MADAM was radiolabeled by an N-methylation reaction using [(11)C]methyl triflate and the binding was characterized by PET in four cynomolgus monkeys. Metabolite levels in plasma were measured by gradient high-performance liquid chromatography (HPLC). The radiochemical incorporation yield of [(11)C]MADAM was 75-80% and the specific radioactivity at the time of administration was 34-652 GBq/micromol (n=8). The highest uptake of radioactivity was observed in striatum, thalamus, mesencephalon, and the lower brainstem. Lower binding was detected in neocortex and the lowest radioactive uptake was found in the cerebellum. This distribution is in accordance with the known expression of 5-HTT in vitro. The fraction of the total radioactivity in monkey plasma representing unchanged [(11)C]MADAM was 20% at 45 min after injection, as measured by gradient HPLC. Pretreatment measurements, using unlabeled citalopram, GBR 12909, and maprotiline, as well as a displacement measurement, using unlabeled MADAM, confirmed that [(11)C]MADAM binds selectively and reversibly to 5-HTT, and support the use of the cerebellum as reference region. The present characterization of binding in the monkey brain suggests that [(11)C]MADAM is a potential PET radioligand for quantitative studies of 5-HTT binding in the human brain.

Methoxyphenylethynyl, methoxypyridylethynyl and phenylethynyl derivatives of pyridine: synthesis, radiolabeling and evaluation of new PET ligands for metabotropic glutamate subtype 5 receptors

We have synthesized three different PET ligands to investigate the physiological function of metabotropic glutamate subtype 5 receptors (mGluR5) in vivo: 2-[(11)C]methyl-6-(2-phenylethynyl)pyridine ([(11)C]MPEP), 2-(2-(3-[(11)C]methoxyphenyl)ethynyl)pyridine ([(11)C]M-MPEP) and 2-(2-(5-[(11)C]methoxypyridin-3-yl)ethynyl)pyridine ([(11)C]M-PEPy). [(11)C]Methyl iodide was used to label the compounds under basic conditions, and a Pd(0) catalyst was applied to label [(11)C]MPEP in a Stille coupling reaction. In vivo microPET imaging studies of the functional accumulation of radiolabeled ligands were conducted in 35 rats (Sprague-Dawley, 8 weeks old male, weight of 300 g). Specific binding was tested using pre-administration of unlabeled mGluR5 antagonist 2-methyl-6-(2-phenylethynyl)pyridine (MPEP) (10 mg/kg iv 5 min before radioactivity injection). In the radiolabeling of [(11)C]MPEP, [(11)C]M-MPEP and [(11)C]M-PEPy, a specific radioactivity of 700-1200 mCi/micromol and over 97% radiochemical purity were obtained. The microPET studies showed these three radiolabeled mGluR5 antagonists having the highest binding in the olfactory bulb followed by striatum, hippocampus and cortex. Pre-administration of the mGluR5 antagonist MPEP induced a 45.1% decrease in [(11)C]MPEP binding, a 59.7% decrease in [(11)C]M-MPEP binding and an 84.6% decrease in [(11)C]M-PEPy binding in the olfactory bulb at 5 min. The feasibility of synthesizing high-affinity and high-selectivity ligands for mGluR5 receptors and their suitability as PET imaging ligands for mGluR5 receptors in vivo are demonstrated.

Frontal and limbic metabolic differences in subjects selected according to genetic variation of the SLC6A4 gene polymorphism

Allelic variants in the promoter region of the serotonin transporter (5-HTT) gene have been implicated in several psychiatric disorders and personality traits. In particular, two common alleles in a variable repeat sequence of the promoter region (SLC6A4) have been differentially associated with a display of abnormal levels of anxiety and affective illness in individuals carrying the "s" allele. The aim of this study was to compare the basal cerebral metabolic activity of non-psychiatric subjects in fronto-limbic structures to determine whether differences exist in basal metabolic activity within this functional polymorphism. PET scans with fluorine-18 fluorodeoxyglucose as radiotracer were performed in 71 non-psychiatric subjects previously screened for psychopathology and subsequently genotyped for SLC6A4; PET images were compared with SPM2 according to s/s (n = 27), s/l (n = 25), and l/l (n = 19) groups considering a significance threshold in a priori selected areas of P < 0.001 and an extent threshold > or =5 voxels. The analysis showed an effect of interest among the three genotype groups in right anterior cingulate gyrus (ACC), left middle frontal gyrus, and left posterior cingulate gyrus (PCC). Comparison between l/l vs. s/s showed increased metabolism for l/l in left middle frontal gyrus and an increase for s/s in right ACC and left PCC. Comparison between s/s vs. s/l showed an increase for s/s in left PCC and right ACC. Increased basal metabolism in fronto-limbic structures for the s/s group may be conceived as an "overactive metabolic state" of these structures, possibly related to an increased susceptibility for developing an anxiety-depression spectrum disorder.

Functional neuroimaging of genetic variation in serotonergic neurotransmission

Serotonin (5-hydroxytryptamine; 5-HT) is a potent modulator of the physiology and behavior involved in generating appropriate responses to environmental cues such as danger or threat. Furthermore, genetic variation in 5-HT subsystem genes can impact upon several dimensions of emotional behavior including neuroticism and psychopathology, but especially anxiety traits. Recently, functional neuroimaging has provided a dramatic illustration of how a promoter polymorphism in the human 5-HT transporter (5-HTT) gene, which has been weakly related to these behaviors, is strongly related to the engagement of neural systems, namely the amygdala, subserving emotional processes. In this commentary, we discuss how functional neuroimaging can be used to characterize the effects of polymorphisms in 5-HT subsystem genes on the response of neural circuits underlying the generation and regulation of mood and temperament as well as susceptibility to affective illness. We argue that in time, such knowledge will allow us to not only transcend phenomenological diagnosis and represent mechanisms of disease, but also identify at-risk individuals and biological pathways for the development of new treatments.