A Solid-Phase Route to18F-Labeled Tracers, Exemplified by the Synthesis of [18F]2-Fluoro-2-deoxy-D-glucose

18 F-Fluorination of a supported 2-(aryl-di-tert-butylsilyl)-N-methyl-imidazole for indirect 18 F-labeling of a VH H single-variable domain

Anchoring an imidazole-di-tert-butyl-arylsilane possessing an azido group to a polystyrene resin provided a heterogeneous precursor that was radiolabeled easily using aqueous [18 F]fluoride. After optimizing the conditions (i.e., using DMSO as solvent and heating at 160°C for 15 min), the desired [18 F]fluorosilane was obtained in 24% radiochemical yield (RCY) and 78% radiochemical purity (RCP) using solid-phase extraction as sole purification. Then, this compound was conjugated by strain-promoted alkyne-azide cycloaddition to a model single-variable domain possessing a cyclooctyne tag, yielding to the desired 18 F-labeled bioconjugate in 2% RCY and >95% RCP after purification by a size exclusion chromatography.

Radiometallation and photo-triggered release of ready-to-inject radiopharmaceuticals from the solid phase

The efficient, large-scale synthesis of radiometallated radiopharmaceuticals represents an emerging clinical need which, to date, is inherently limited by time consuming, sequential procedures to conduct isotope separation, radiochemical labeling and purification prior to formulation for injection into the patient. In this work, we demonstrate that a solid-phase based, concerted separation and radiosynthesis strategy followed by photochemical release of radiotracer in biocompatible solvents can be employed to prepare ready-to-inject, clinical grade radiopharmaceuticals. Optimization of resin base, resin loading, and radiochemical labeling capacity are demonstrated with ⁶⁷Ga and ⁶⁴Cu radioisotopes using a short model peptide sequence and further validated using two peptide-based radiopharmaceuticals with clinical relevance, targeting the gastrin-releasing peptide and the prostate specific membrane antigen. We also demonstrate that the solid-phase approach enables separation of non-radioactive carrier ions Zn²⁺ and Ni²⁺ present at 10⁵-fold excess over ⁶⁷Ga and ⁶⁴Cu by taking advantage of the superior Ga³⁺ and Cu²⁺ binding affinity of the solid-phase appended, chelator-functionalized peptide. Finally, a proof of concept radiolabeling and subsequent preclinical PET-CT study with the clinically employed positron emitter ⁶⁸Ga successfully exemplifies that Solid Phase Radiometallation Photorelease (SPRP) allows the streamlined preparation of radiometallated radiopharmaceuticals by concerted, selective radiometal ion capture, radiolabeling and photorelease.

2-Deoxy-D-Glucose: A Novel Pharmacological Agent for Killing Hypoxic Tumor Cells, Oxygen Dependence-Lowering in Covid-19, and Other Pharmacological Activities

The nonmetabolizable glucose analog 2-deoxy-D-glucose (2-DG) has shown promising pharmacological activities, including inhibition of cancerous cell growth and N-glycosylation. It has been used as a glycolysis inhibitor and as a potential energy restriction mimetic agent, inhibiting pathogen-associated molecular patterns. Radioisotope derivatives of 2-DG have applications as tracers. Recently, 2-DG has been used as an anti-COVID-19 drug to lower the need for supplemental oxygen. In the present review, various pharmaceutical properties of 2-DG are discussed.

Last-step 18F-fluorination of supported 2-(aryl-di-tert-butylsilyl)-N-methyl-imidazole conjugates for applications in positron emission tomography

Aiming for potential applications in positron emission tomography, fully automated productions of ¹⁸F-labelled bioconjugates were achieved using heterogenous precursors obtained by anchoring imidazole-di-tert-butyl-arylsilanes to a polystyrene resin. The reactions were performed using either "batch" or "flow" procedures, avoiding both the time-consuming azeotropic drying and HPLC purifications usually required.

The Longer Route can be Better: Electrosynthesis in Extended Path Flow Cells

This personal account provides an overview of work conducted in my research group, and through collaborations with other chemists and engineers, to develop flow electrolysis cells and apply these cells in organic electrosynthesis. First, a brief summary of my training and background in organic synthesis is provided, leading in to the start of flow electrosynthesis in my lab in collaboration with Derek Pletcher. Our work on the development of extended path electrolysis flow reactors is described from a synthetic organic chemist's perspective, including laboratory scale-up to give several moles of an anodic methoxylation product in one day. The importance of cell design is emphasised with regards to achieving good performance in laboratory electrosynthesis with productivities from hundreds of mg h^-1 to many g h^-1 , at high conversion in a selective fashion. A simple design of recycle flow cell that can be readily constructed in a small University workshop is also discussed, including simple modifications to improve cell performance. Some examples of flow electrosyntheses are provided, including Shono-type oxidation, anodic cleavage of protecting groups, Hofer-Moest reaction of cubane carboxylic acids, oxidative esterification and amidation of aldehydes, and reduction of aryl halides.

Original synthesis of radiolabeling precursors for batch and on resin one-step/late-stage radiofluorination of peptides

Radiolabeling of peptides with fluorine-18 is hurdled by their chemical sensitivity and complicated processes. Original triflyl-pyridine intermediates afforded ammonium precursors that were radiolabeled at low temperature. From that study, a generic tag has been designed to allow a simple one-step/late-stage radiolabelling of peptides. The strategy has been transposed to an automated "on-resin" radiolabelling.

Synthesis of [123I]-iodometomidate from a polymer-supported precursor with a large excluded volume

We report on the synthesis of [¹²³I]-iodometomidate from a soluble polymer-supported precursor with a large extent volume. The precursor enabled the synthesis of the [¹²³I]-iodometomidate with high quality without use of HPLC purification.

Solid-supported reagents composed of a copolymer possessing 2-O-sulfonyl mannosides and phase-transfer catalysts for the synthesis of 2-fluoroglucose

We described the synthesis of a solid-supported co-polymer possessing mannosides and phase-transfer catalysts and synthesis of 2-fluoroglucoside from it. We first prepared a soluble copolymer from two allene monomers possessing a precursor for the synthesis of 2-fluoroglycose and a crown ether. The copolymerization of the monomers via the π-ally nickel-catalyst smoothly proceeded at room temperature to provide a desired copolymer without decomposition of the sulfonate esters. The copolymer exhibited high reactivity towards fluorination in comparison with a conventional precursor. We next synthesized the solid-supported copolymer by using the solid-supported initiator attached with TentaGel® resins. TentaGel® enabled polymerization under stirring with stirring bar without decomposition. The solid-supported copolymer exhibited comparable reactivity towards fluorination in comparison with the soluble copolymer. In addition, it can be easily separated from the reaction vessel by filtration.

In vitro and in vivo characterization of 2-deoxy-2-18F-fluoro-D-mannose as a tumor-imaging agent for PET

2-Deoxy-2-(18)F-fluoro-d-mannose ((18)F-FDM) is an (18)F-labeled mannose derivative and a stereoisomer of (18)F-FDG. Our preliminary study demonstrated that (18)F-FDM accumulated in tumors to the same extent as (18)F-FDG, with less uptake in the brain and faster clearance from the blood. However, detailed studies on the uptake of (18)F-FDM in tumors have not been conducted. We undertook this study to establish a practical method of (18)F-FDM synthesis based on an (18)F-nucleophilic substitution (SN2) reaction and to advance the biologic characterization of (18)F-FDM for potential application as a tumor-imaging agent.

METHODS

We synthesized 4,6-O-benzylidene-3-O-ethoxymethyl-1-O-methyl-2-O-trifluoromethanesulfonyl-β-D-glucopyranoside as a precursor for the nucleophilic synthesis of (18)F-FDM. The precursor was radiofluorinated with (18)F-KF/Kryptofix222, followed by removal of the protecting groups with an acid. (18)F-FDM was purified by preparative high-performance liquid chromatography and then subjected to in vitro evaluation regarding phosphorylation by hexokinase as well as uptake and metabolism in AH109A tumor cells. The in vivo properties of (18)F-FDM were examined in Donryu rats bearing AH109A tumor cells by biodistribution studies and imaging with a small-animal PET system.

RESULTS

We radiosynthesized (18)F-FDM in sufficient radiochemical yields (50%-68%) with excellent purities (97.6%-98.7%). (18)F-FDM was phosphorylated rapidly by hexokinase, resulting in 98% conversion into (18)F-FDG-6-phosphate within 30 min. Tumor cells showed significant uptake of (18)F-FDM with time in vitro, and uptake was dose-dependently inhibited by D-glucose. (18)F-FDM injected into tumor-bearing rats showed greater uptake in tumors (2.17 ± 0.32 percentage injected dose per gram [%ID/g]) than in the brain (1.42 ± 0.10 %ID/g) at 60 min after injection. PET studies also revealed the tumor uptake of (18)F-FDM (quasi-standardized uptake value, 2.83 ± 0.22) to be the same as that of (18)F-FDG (2.40 ± 0.30), but the brain uptake of (18)F-FDM (1.89 ± 0.13) was ≈ 30% lower than that of (18)F-FDG (2.63 ± 0.26).

CONCLUSION

We prepared (18)F-FDM with good radiochemical yield and purity by an SN2 reaction. We demonstrated that (18)F-FDM had adequate tumor cell uptake by a metabolic trapping mechanism and can afford high-contrast tumor images with less uptake in the brain, indicating that (18)F-FDM has almost the same potential as (18)F-FDG for PET tumor imaging, with better advantages with regard to the imaging of brain tumors.

Standardization of fluorine-18 manufacturing processes: new scientific challenges for PET

In [(18)F]fluoride chemistry, the minute amounts of radioactivity taking part in a radiolabeling reaction are easily outnumbered by other reactants. Surface areas become comparably larger and more influential than in standard fluorine chemistry, while leachables, extractables, and other components that normally are considered small impurities can have a considerable influence on the efficiency of the reaction. A number of techniques exist to give sufficient (18)F-tracer for a study in a pre-clinical or clinical system, but the chemical and pharmaceutical understanding has significant gaps when it comes to scaling up or making the reaction more efficient. Automation and standardization of [(18)F]fluoride PET tracers is a prerequisite for reproducible manufacturing across multiple PET centers. So far, large-scale, multi-site manufacture has been established only for [(18)F]FDG, but several new tracers are emerging. In general terms, this transition from small- to large-scale production has disclosed several scientific challenges that need to be addressed. There are still areas of limited knowledge in the fundamental [(18)F]fluoride chemistry. The role of pharmaceutical factors that could influence the (18)F-radiosynthesis and the gaps in precise chemistry knowledge are discussed in this review based on a normal synthesis pattern.