Radiolabeled (4-Fluoro-3-Iodobenzyl)Guanidine Improves Imaging and Targeted Radionuclide Therapy of Norepinephrine Transporter–Expressing Tumors

Synthesis and preliminary evaluation of benzylaminoimidazoline derivatives as novel norepinephrine transporter ligands

A series of benzylaminoimidazoline derivatives was synthesized and evaluated for norepinephrine transporter (NET) targeting. Among them, N-(3-iodobenzyl)-4,5-dihydro-1H-imidazol-2-amine (Compound 9) displayed the highest affinity for NET (IC50  = 5.65 ± 0.97 μM). The corresponding radiotracer [125 I]9 was further prepared by copper-mediated radioiodination and evaluated both in vitro and in vivo. The cellular uptake results suggested that [125 I]9 was specifically taken up by the NET-expressing SK-N-SH cell line. Biodistribution studies showed that [125 I]9 accumulated in the heart (5.54 ± 1.24 %ID/g at 5 min p.i. and 0.79 ± 0.08 %ID/g at 2 h p.i.) and adrenal gland (14.83 ± 3.47 %ID/g at 5 min p.i. and 3.87 ± 0.24 %ID/g at 2 h p.i.). The uptake in the heart and adrenal gland could be significantly inhibited by preinjection of desipramine (DMI). These results indicated that the benzylaminoimidazoline derivatives retained affinity for NET, which could provide structure-activity relationship data for further studies.

Imaging pheochromocytoma in small animals: preclinical models to improve diagnosis and treatment

Pheochromocytomas (PCCs) and paragangliomas (PGLs), together referred to as PPGLs, are rare chromaffin cell-derived tumors. They require timely diagnosis as this is the only way to achieve a cure through surgery and because of the potentially serious cardiovascular complications and sometimes life-threatening comorbidities that can occur if left untreated. The biochemical diagnosis of PPGLs has improved over the last decades, and the knowledge of the underlying genetics has dramatically increased. In addition to conventional anatomical imaging by CT and MRI for PPGL detection, new functional imaging modalities have emerged as very useful for patient surveillance and stratification for therapy. The availability of validated and predictive animal models of cancer is essential for translating molecular, imaging and therapy response findings from the bench to the bedside. This is especially true for rare tumors, such as PPGLs, for which access to large cohorts of patients is limited. There are few animal models of PPGLs that have been instrumental in refining imaging modalities for early tumor detection, as well as in identifying and evaluating novel imaging tracers holding promise for the detection and/or treatment of human PPGLs. The in vivo PPGL models mainly include xenografts/allografts generated by engrafting rat or mouse cell lines, as no representative human cell line is available. In addition, there is a model of endogenous PCCs (i.e., MENX rats) that was characterized in our laboratory. In this review, we will summarize the contribution that various representative models of PPGL have given to the visualization of these tumors in vivo and we present an example of a tracer first evaluated in MENX rats, and then translated to the detection of these tumors in human patients. In addition, we will illustrate briefly the potential of ex vivo biological imaging of intact adrenal glands in MENX rats.

Novel [18F]-Labeled Meta-Bromobenzylguanidine Derivatives: Potential Positron Emission Tomography Imaging Probes for the Norepinephrine Transporter

To develop novel norepinephrine transporter (NET)-targeting positron emission tomography (PET) probes with optimal pharmacokinetic properties, a series of meta-bromobenzylguanidine derivatives was synthesized. 4-Fluorodiethoxyethane-3-bromobenzylguanidine (compound 12) showed relatively good affinity for the NET (IC50 = 1.00 ± 0.04 μM). The corresponding radiotracer 18F-12 was prepared in high radiochemical purity (>98%) via a three-step method. The in vitro cellular uptake results demonstrated that 18F-12 was specifically taken up by NET-expressing SK-N-SH cells by the uptake-1 mechanism. Biodistribution studies in mice showed that 18F-12 exhibited high cardiac uptake (10.45 ± 0.66 %ID/g at 5 min p.i. and 6.44 ± 0.40 %ID/g at 120 min p.i.), faster liver clearance, and a lower dose of absorbed radiation than [123I]-labeled meta-iodobenzylguanidine ([123I]MIBG). Small animal PET imaging confirmed the high heart-to-background ratio of 18F-12 and the uptake-1 mechanism specific for the NET in rats, indicating its potential as a promising PET radiotracer for cardiac sympathetic nerve imaging.

Initial Evaluation of AF78: a Rationally Designed Fluorine-18-Labelled PET Radiotracer Targeting Norepinephrine Transporter

Purpose

Taking full advantage of positron emission tomography (PET) technology, fluorine-18-labelled radiotracers targeting norepinephrine transporter (NET) have potential applications in the diagnosis and assessment of cardiac sympathetic nerve conditions as well as the delineation of neuroendocrine tumours. However, to date, none have been used clinically. Drawbacks of currently reported radiotracers include suboptimal kinetics and challenging radiolabelling procedures.

Procedures

We developed a novel fluorine-18-labelled radiotracer targeting NET, AF78, with efficient one-step radiolabelling based on the phenethylguanidine structure. Radiosynthesis of AF78 was undertaken, followed by validation in cell uptake studies, autoradiography, and in vivo imaging in rats.

Results

[¹⁸F]AF78 was successfully synthesized with 27.9 ± 3.1 % radiochemical yield, > 97 % radiochemical purity and > 53.8 GBq/mmol molar activity. Cell uptake studies demonstrated essentially identical affinity for NET as norepinephrine and meta-iodobenzylgaunidine. Both ex vivo autoradiography and in vivo imaging in rats showed homogeneous and specific cardiac uptake.

Conclusions

The new PET radiotracer [¹⁸F]AF78 demonstrated high affinity for NET and favourable biodistribution in rats. A structure-activity relationship between radiotracer structures and affinity for NET was revealed, which may serve as the basis for the further design of NET targeting radiotracers with favourable features.

Electronic supplementary material

The online version of this article (10.1007/s11307-019-01407-5) contains supplementary material, which is available to authorized users.

Sequential Ir/Cu-Mediated Method for the Meta-Selective C-H Radiofluorination of (Hetero)Arenes

This article describes a sequential Ir/Cu-mediated process for the meta-selective C-H radiofluorination of (hetero)arene substrates. In the first step, Ir-catalyzed C(sp²)-H borylation affords (hetero)aryl pinacolboronate (BPin) esters. The intermediate organoboronates are then directly subjected to copper-mediated radiofluorination with [¹⁸F]tetrabutylammonium fluoride to afford fluorine-18 labeled (hetero)arenes in high radiochemical yield and radiochemical purity. This entire process is performed on a benchtop without Schlenk or glovebox techniques and circumvents the need to isolate (hetero)aryl boronate esters. The reaction was automated on a TracerLab FX_FN module with 1,3-dimethoxybenzene and a meta-tyrosine derivative. The products, [¹⁸F]1-fluoro-3,5-dimethoxybenzene and an ¹⁸F-labeled meta-tyrosine derivative, were obtained in 37 ± 5% isolated radiochemical yield and >99% radiochemical purity and 25% isolated radiochemical yield and 99% radiochemical purity, and 0.52 Ci/μmol (19.24 GBq/μmol) molar activity (A_m), respectively.

Fluorinations of unsymmetrical diaryliodonium salts containing ortho-sidearms; influence of sidearm on selectivity

Activated aromatics were reacted with two different fluoroidoane reagents 1 and 2 in the presence of triflic acid to prepare only the para-substituted diaryliodonium salts. With fluoroiodane 1 the unsymmetrical diaryliodonium salts contained an ortho-propan-2-ol sidearm, whereas the alcohol sidearm was eliminated to form an ortho-styrene sidearm in the reaction with fluoroiodane 2. Only the diaryliodonium salts containing a styrene sidearm were fluorinated successfully to deliver para-fluorinated aromatics in good yields.

Selective synthesis of L-2-[18 F]fluoro-alpha-methylphenylalanine via copper-mediated 18 F-fluorination of (mesityl)(aryl)iodonium salt

L-2-[18 F]fluoro-alpha-methylphenylalanine (2-[18 F]FAMP) is a promising amino acid tracer for positron emission tomography (PET) imaging, yet the low production yield of direct electrophilic radiofluorination with [18 F]F2 necessitates further optimization of the radiolabeling process. This paper describes a two-step preparation method for L-2-[18 F]fluoro-alpha-methylphenylalanine (2-[18 F]FAMP) starting from [18 F]fluoride. The (Mesityl)(L-alpha-methylphenylalanine)-2-iodonium tetrafluoroborate precursors with various protecting groups were prepared. The copper-mediated 18 F-fluorination of the iodonium salt precursors successfully produced 2-[18 F]FAMP. The highest radio chemical conversion of 57.6% was noted with N-Piv-protected (mesityl)(aryl)iodonium salt in the presence of 5 equivalent of Cu (OTf)2 . Subsequent deprotection with 57% hydrogen iodide produced 2-[18 F]FAMP within 120 min in 21.4 ± 11.7% overall radiochemical yield with >95% radiochemical purity and an enantiomeric excess >99%. The obtained 2-[18 F]FAMP showed comparable biodistribution profiles in normal mice with that of the carrier-added 2-[18 F]FAMP. These results indicate that usefulness of copper mediated 18 F-fluorination for the production of 2-[18 F]FAMP, which would facilitate clinical translation of the promising tumor specific amino acid tracer. Individual facilities could adopt either production method based on radioactivity demand and equipment availability.

Copper-Mediated Late-stage Radiofluorination: Five Years of Impact on Pre-clinical and Clinical PET Imaging

PURPOSE

Copper-mediated radiofluorination (CMRF) is emerging as the method of choice for the formation of aromatic C-18F bonds. This minireview examines proof-of-concept, pre-clinical, and in-human imaging studies of new and established imaging agents containing aromatic C-18F bonds synthesized with CMRF. An exhaustive discussion of CMRF methods is not provided, although key developments that have enabled or improved upon the syntheses of fluorine-18 imaging agents are discussed.

METHODS

A comprehensive literature search from April 2014 onwards of the Web of Science and PubMed library databases was performed to find reports that utilize CMRF for the synthesis of fluorine-18 radiopharmaceuticals, and these represent the primary body of research discussed in this minireview. Select conference proceedings, previous reports describing alternative methods for the synthesis of imaging agents, and preceding fluorine-19 methodologies have also been included for discussion.

CONCLUSIONS

CMRF has significantly expanded the chemical space that is accessible to fluorine-18 radiolabeling with production methods that can meet the regulatory requirements for use in Nuclear Medicine. Furthermore, it has enabled novel and improved syntheses of radiopharmaceuticals and facilitated subsequent PET imaging studies. The rapid adoption of CMRF will undoubtedly continue to simplify the production of imaging agents and inspire the development of new radiofluorination methodologies.

Molecular imaging of norepinephrine transporter-expressing tumors: current status and future prospects

The human norepinephrine transporter (hNET) is a transmembrane protein responsible for reuptake of norepinephrine in presynaptic sympathetic nerve terminals and adrenal chromaffin cells. Neural crest tumors, such as neuroblastoma, paraganglioma and pheochromocytoma often show high hNET expression. Molecular imaging of these tumors can be done using radiolabeled norepinephrine analogs that target hNET. Currently, the most commonly used radiopharmaceutical for hNET imaging is meta-[123I]iodobenzylguanidine ([123I]MIBG) and this has been the case since its development several decades ago. The γ-emitter, iodine-123 only allows for planar scintigraphy and single photon emission computed tomography imaging. These modalities typically have a poorer spatial resolution and lower sensitivity than positron emission tomography (PET). Additional practical disadvantages include the fact that a two-day imaging protocol is required and the need for thyroid blockade. Therefore, several PET alternatives for hNET imaging are actively being explored. This review gives an in-depth overview of the current status and recent developments in clinical trials leading to the next generation of clinical PET ligands for imaging of hNET-expressing tumors.

Recent advances in radiotracers targeting norepinephrine transporter: structural development and radiolabeling improvements

The norepinephrine transporter (NET) is a major target for the evaluation of the cardiac sympathetic nerve system in patients with heart failure and Parkinson's disease. It is also used in the therapeutic applications against certain types of neuroendocrine tumors, as exemplified by the clinically used ^123/131I-MIBG as theranostic single-photon emission computed tomography (SPECT) agent. With the development of more advanced positron emission tomography (PET) technology, more radiotracers targeting NET have been reported, with superior temporal and spatial resolutions, along with the possibility of functional and kinetic analysis. More recently, fluorine-18-labelled NET tracers have drawn increasing attentions from researchers, due to their longer radiological half-life relative to carbon-11 (110 min vs. 20 min), reduced dependence on on-site cyclotrons, and flexibility in the design of novel tracer structures. In the heart, certain NET tracers provide integral diagnostic information on sympathetic innervation and the nerve status. In the central nervous system, such radiotracers can reveal NET distribution and density in pathological conditions. Most radiotracers targeting cardiac NET-function for the cardiac application consistent of derivatives of either norepinephrine or MIBG with its benzylguanidine core structure, e.g. ¹¹C-HED and ¹⁸F-LMI1195. In contrast, all NET tracers used in central nervous system applications are derived from clinically used antidepressants. Lastly, possible applications of NET as selective tracers over organic cation transporters (OCTs) in the kidneys and other organs controlled by sympathetic nervous system will also be discussed.