Eph receptors are known to be overexpressed in various types of cancer and are therefore promising targets for tumor cell imaging by positron emission tomography (PET). In this regard, imaging could facilitate the early detection of Eph-overexpressing tumors, monitoring responses to therapy directed toward Eph, and thus improvement in patient outcomes. This work report the synthesis and evaluation of several fluorine-18-labeled peptides containing the SNEW and SWLAY amino acid motif, with high affinity for the EphA2 and B2 receptor, for their potential as radiotracers in the non-invasive imaging of cancer using PET. For the purposes of radiofluorination, EphA2- and EphB2-antagonistic peptides were varied at the C-terminus by the introduction of L-cysteine, and further by alkyne- or azide-modified amino acids. In addition, two novel bifunctional and bioorthogonal labeling building blocks [18F]AFP and [18F]BFP were applied, and their capacity to introduce fluorine-18 was compared with that of the established building block [18F]FBAM. Copper-assisted Huisgen 1,3-dipolar cycloaddition, which belongs to the set of bioorthogonal click chemistry reactions, was used to introduce both novel building blocks into azide- or alkyne-modified peptides under mild conditions. Finally, the depletion of copper immediately after radiolabeling is a highly important step of this novel methodology.

Eph receptors are known to be overexpressed in various types of cancer and are therefore promising targets for tumor cell imaging by positron emission tomography (PET). In this regard, imaging could facilitate the early detection of Eph-overexpressing tumors, monitoring responses to therapy directed toward Eph, and thus improvement in patient outcomes. This work report the synthesis and evaluation of several fluorine-18-labeled peptides containing the SNEW and SWLAY amino acid motif, with high affinity for the EphA2 and B2 receptor, for their potential as radiotracers in the non-invasive imaging of cancer using PET. For the purposes of radiofluorination, EphA2- and EphB2-antagonistic peptides were varied at the C-terminus by the introduction of L-cysteine, and further by alkyne- or azide-modified amino acids. In addition, two novel bifunctional and bioorthogonal labeling building blocks [18F]AFP and [18F]BFP were applied, and their capacity to introduce fluorine-18 was compared with that of the established building block [18F]FBAM. Copper-assisted Huisgen 1,3-dipolar cycloaddition, which belongs to the set of bioorthogonal click chemistry reactions, was used to introduce both novel building blocks into azide- or alkyne-modified peptides under mild conditions. Finally, the depletion of copper immediately after radiolabeling is a highly important step of this novel methodology.