**3.2 Radiofluorination**

For routine PET imaging, fluorine-18 represents the near ideal radionuclide with its half-life of 109.8 min and low β<sup>+</sup> -energy (0.64 MeV). Due to this low positron energy, it has a short positron linear range in tissue, leading to particularly high spatial resolution in PET imaging. Furthermore, compared to other short lived radionuclides, such as 11C, its half-life is long enough to allow syntheses and imaging procedures to be extended over hours, enabling kinetic studies and high-quality metabolite and plasma analysis.

Efficient 18F-labeling of peptides and proteins often comprises a multistep process involving labeling and purification of a prosthetic group (synthon) and subsequent conjugation of the 18F-labeled synthon to the peptide/protein with or without activation. If necessary, the 18F-conjugate is purified by a final purification step. Over the years, a variety of prosthetic groups have been developed ranging from amine-reactive groups such as *N*-succinimidyl-4-[18F]fluorobenzoate ([18F]SFB) [35] to chemoselective groups like 4-[18F]fluorobenzaldehyde and [18F]FDG ([18F]

#### **Figure 7.**

*The auxiliary groups Bolton-hunter reagent, SIB, SIPC, and SGMIB for the radioiodination of peptides.*

fluorodeoxyglucose) [36, 37] that react with an aminooxy- or hydrazine-modified peptides. A special prosthetic group approach that has been explored in 18F-labeling chemistry is the click chemistry methodology. Click chemistry appeared to be an effective method to radiolabel peptides and proteins, because the click reaction is fast, bioorthogonal, chemo- and regioselective, results in relatively high yields and can be performed in aqueous media. This copper(I)-catalyzed azide-alkyne cycloaddition reaction has been exploited in radiopharmaceutical chemistry by several research groups [38–44]. A variant of the copper(I)-catalyzed click reaction is the copper-free click reaction that does not require the use of the cytotoxic metal. Reactions of electron-deficient tetrazines with ring-strained trans-cyclooctenes or norbenes have been investigated [45–49]. Click reactions, Cu(I)-catalyzed and copper-free, appeared to be powerful and versatile reactions for the synthesis of 18F-labeled peptides and proteins.

In spite of the variety of possibilities for introducing 18F, a major drawback of the 18F-labeling methods described above is that they are laborious, (require azeotropic drying of the fluoride and multiple purification steps) and are thus time consuming. In search of a kit-based 18F-labeling method, new 18F-labeling strategies based on fluorine-silicon [50–56], fluorine-boron [57–59], and fluorine-phosphorus [60] have been developed.

A facile chelator-based approach was developed wherein 18F is first attached to aluminum as Al18F, which is then complexed in a chelating agent attached to the peptide, forming a stable Al18F–chelate peptide complex in an efficient 1-pot process [61].
