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

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.

[18F]fluoride Activation and 18F-Labelling in Hydrous Conditions-Towards a Microfluidic Synthesis of PET Radiopharmaceuticals

18F-labelled radiopharmaceuticals are indispensable in positron emission tomography. The critical step in the preparation of 18F-labelled tracers is the anhydrous F-18 nucleophilic substitution reaction, which involves [18F]F- anions generated in aqueous media by the cyclotron. For this, azeotropic drying by distillation is widely used in standard synthesisers, but microfluidic systems are often not compatible with such a process. To avoid this step, several methods compatible with aqueous media have been developed. We summarised the existing approaches and two of them have been studied in detail. [18F]fluoride elution efficiencies have been investigated under different conditions showing high 18F-recovery. Finally, a large scope of precursors has been assessed for radiochemical conversion, and these hydrous labelling techniques have shown their potential for tracer production using a microfluidic approach, more particularly compatible with iMiDEV™ cassette volumes.

Fluorine-18 and Radiometal Labeling of Biomolecules via Disulfide Rebridging

Biomolecules labeled with positron-emitting radionuclides like fluorine-18 or radiometals like copper-64 and zirconium-89 are increasingly employed in nuclear medicine for diagnosis purposes. Given the fragility and complexity of these compounds, their labeling requires mild conditions. Besides, it is essential to develop methods inducing minimal modification of the tertiary structure, as it is fundamental for the biological activity of such complex entities. Given these requirements, disulfide rebridging represents a promising possibility since it allows protein modification as well as conservation of the tertiary structure. In this context, we have developed an original radiofluorinated dibromopyridazine dione prosthetic group for labeling of disulfide-containing biomolecules via rebridging. We employed it to radiolabel octreotide, a somatostatin analogue, and to radiolabel fragment antigen binding (Fab) targeting programmed death-ligand 1 (PD-L1), whose properties were then evaluated in vitro and in vivo by positron emission tomography (PET) imaging. We next extended our strategy to the radiolabeling of cetuximab, a monoclonal antibody, with various radiometals commonly used in PET imaging (zirconium-89, copper-64) by developing various rebridging molecules bearing the appropriate chelators. The stabilities of the radiolabeled antibody conjugates were assessed in biological conditions.

Impact of Radiolabeling Strategies on the Pharmacokinetics and Distribution of an Anti-PD-L1 PET Ligand

Molecular imaging with PET offers an alternative method to quantify programmed-death-ligand 1 (PD-L1) to accurately select patients for immunotherapies. More and more clinical and preclinical trials involve radiolabeling of antibody fragments for their desirably fast clearance and high tumor penetration. As the radiolabeling strategy can significantly impact pharmacokinetics and biodistribution, we explored in this work a site-specific radiofluorination strategy on an anti-PD-L1 fragment antigen-binding (Fab) and compared the pharmacokinetic and biodistribution properties with the same Fab labeled using stochastic radiolabeling chemistry. We applied an enzymatic bioconjugation mediated by a variant of the lipoic acid ligase (LplA) that promotes the formation of an amide bond between a short peptide cloned onto the C terminus of the Fab. A synthetic analogue of the enzyme natural substrate, lipoic acid, was radiolabeled with fluorine-18 for site-specific conjugation by LplA. We compared the biodistribution of the site-specifically labeled Fab with a stochastically labeled Fab on lysine side chains in tumor-bearing mice. The two methods of fluorination demonstrate a comparable whole-body biodistribution. The ⁸⁹Zr-labeled Fab had different biodistribution compared to either ¹⁸F-labeled Fab. We attribute the difference to [⁸⁹Zr] metabolism. Fab-LAP-[¹⁸F]FPyOctA therefore reflects better the true pharmacokinetic profile of the Fab.

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.

Recent Advances in Synthetic Methodologies to Form C-18F Bonds

Positron emission tomography (PET) is an important technique for the early diagnosis of disease. Due to the specific physical and chemical properties of Fluorine-18, this important isotope is widely used in PET for labelling and molecular imaging, and its introduction into medicine molecules could produce PET tracers. Developing with the development of organic synthetic methodologies, the introduction of Fluorine-18 into drug molecules efficiently and rapidly under mild conditions, and the formation of C-¹⁸F chemical bonds, has become one of the leading topics in both organic synthetic chemistry and radiochemistry. In this mini-review, we review a series of recent advances in the organic synthesis of C-¹⁸F bonds (2015–2021), including non-catalytic radiofluorinations via good leaving functional groups, transition metal-catalyzed radiofluorinations, and photo- or electro-catalytic synthetic radiofluorinations. As a result of the remarkable advancements in this field, organic synthetic methods for forming C-¹⁸F bonds are expected to continue growing.

Rapid 18F-labeling via Pd-catalyzed S-arylation in aqueous medium

We report radiolabeling of thiol-containing substrates via Pd-catalyzed S-arylation with 2-[¹⁸F]fluoro-5-iodopyridine, which is readily accessible using the "minimalist" radiofluorination method. The practicality of the procedure was confirmed by preparation of a novel PSMA-specific PET-tracer as well as labeling of glutathione, Aβ oligomer-binding RD2 peptide, bovine serum albumin and PSMA I&S.

Highlight selection of radiochemistry and radiopharmacy developments by editorial board (January-June 2020)

BACKGROUND

The Editorial Board of EJNMMI Radiopharmacy and Chemistry releases a biyearly highlight commentary to describe trends in the field.

RESULTS

This commentary of highlights has resulted in 19 different topics selected by each member of the Editorial Board addressing a variety of aspects ranging from novel radiochemistry to first in man application of novel radiopharmaceuticals.

CONCLUSION

Trends in radiochemistry and radiopharmacy are highlighted demonstrating the progress in the research field being the scope of EJNMMI Radiopharmacy and Chemistry.

Automated Synthesis of Fluorine-18 Labeled CXCR4 Ligand via the Conjugation with Nicotinic Acid N-Hydroxysuccinimide Ester (6-[18F]SFPy)

The C-X-C motif chemokine receptor 4 (CXCR4) is a seven-transmembrane G protein-coupled receptor that is overexpressed in numerous diseases, particularly in various cancers and is a powerful chemokine, attracting cells to the bone marrow niche. Therefore, CXCR4 is an attractive target for imaging and therapeutic purposes. The goal of this study is to develop an efficient, reproducible, and straightforward method to prepare a fluorine-18 labeled CXCR4 ligand. 6-[18F]Fluoronicotinic acid-2,3,5,6-tetrafluorophenyl ester (6-[18F]FPy-TFP) and nicotinic acid N-hydroxysuccinimide ester (6-[18F]SFPy) have been prepared using 'fluorination on the Sep-Pak' method. Conjugation of 6-[18F]SFPy or 6-[18F]FPy-TFP with the alpha-amino group at the N terminus of the protected T140 precursor followed by deprotection, yielded the final product 6-[18F]FPy-T140. The overall radiochemical yields were 6-17% (n = 15, decay-corrected) in a 90-min radiolabeling time with a radiochemical purity >99%. 6-[18F]FPy-T140 exhibited high specific binding and nanomolar affinity for CXCR4 in vitro, indicating that the biological activity of the peptide was preserved. For the first time, [18F]SFPy has been prepared using 'fluorination on the Sep-Pak' method that allows rapid automated synthesis of 6-[18F]FPy-T140. In addition to increased synthetic efficiency, this construct binds with CXCR4 in high affinity and may have potential as an in vivo positron emission tomography (PET) imaging agent. This radiosynthesis method should encourage wider use of this PET agent to quantify CXCR4 in both research and clinical settings.