¹⁸F-based PET Imaging

In the last decades, the field of nanomedicine has received much attention because of the close relevance of nanoparticles and polymers to biological and pathophysiological problems. Initially, nanoparticles (NPs) or polymers were designed for MIR, CT and optical imaging, or simple drug delivery. On the other hand, NPs and polymers provide a nearly ideal platform for drug delivery or functional imaging techniques (PET or SPET). Not only are they crucial for patient screening/selection and monitoring in personalized therapeutic approaches, but they are also excellent tools to study pharmacological profiles and targeted drug delivery systems. In addition, polymer-drug couples used for chemotherapy are radiolabeled and their biodistribution can be studied by PET imaging.

¹⁸F for PET Diagnosis

Because fluorine-18 (¹⁸F) has a high positron intensity (97%) and optimal positron emission energy (0.635 MeV), it has a short linear range in tissue, enabling high-resolution PET imaging. In addition, an appropriate half-life is advantageous for imaging quality and radiation safety, despite the availability of various positron-emitting radionuclides such as carbon-11 (¹¹C), fluorine-18 (¹⁸F), gallium-68 (⁶⁸Ga), etc., due to the proper half-life (109.8 min) of ¹⁸F. Therefore, ¹⁸F is considered to be one of the most desirable and widely used nuclides for PET imaging. A number of ¹⁸F radiotracers have been developed for diagnostic purposes in cancer therapy and novel tracers for neurological diseases. Among them, ¹⁸F -fluorodeoxyglucose (¹⁸F) FDG is the most used radiotracer and has emerged as a powerful PET tool for the detection of various neurological and tumor-related diseases.

¹⁸F: A radionuclide with diverse range of radiochemistry and synthesis strategies for target based PET diagnosis. (Goud NS, et al., 2019)

¹⁸F-labeled Dendrimers as a PET Nucleophile

Nanomaterials not only serve as carriers for drug delivery, but also carry and deliver radionuclides, becoming an emerging field for malignancy imaging and treatment. Due to its desirable imaging properties and good availability, ¹⁸F is a very attractive radionuclide for radiolabeling of nanoparticles (NPs) and polymers. Different types of NPs and polymers require different coupling strategies to ensure fast, stable, and high yield of radioconjugation. There are many possibilities for the attachment of radionuclides to these systems, either by direct or synthetic labeling groups. ¹⁸F covalently linked to NP or polymers is the most widely used PET nuclide. In the case of ¹⁸F, direct radiolabeling provides only low radiochemical yields (RCYs). Therefore, novel coupling reactions are often performed using (novel) ¹⁸F-labeled prosthetic groups. As a nanomaterial, dendrimers can also be radiolabeled with ¹⁸F for PET imaging. A dendrimer possessing a disulfide linker is generated into a dendrimer with two more thiol groups for conjugation to biotin. The ¹⁸F-fluorination is performed after attaching the trifluoro-boroaryl moieties to the NH2 terminal group of the dendrimer. Finally, the boroaryl functionalized dendrimer is incubated with ¹⁸F-fluoride in glacial acetic acid, and its radiochemical yield can be as high as 55%, which can be used for targeted imaging of HER-2 cells.

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Reference

1. Goud NS.; et al. Fluorine-18: A radionuclide with diverse range of radiochemistry and synthesis strategies for target based PET diagnosis. Eur J Med Chem. 2020, 187:111979.