New approaches to the labelling of biological targeting vectors with 18F

Positron emission tomography (PET) is currently the premiere molecular imaging technique for the detection and staging of cancer in vivo. Owing to the desirable nuclear properties of the short- lived positron emitter 18F, and the remarkable targeting potential of many peptides, proteins and nucleic acid- based compounds, the efficient radiolabelling of these sensitive biomolecules with 18F remains a fundamental objective in nuclear diagnostic medicine. Antisense imaging remains a promising tool for the identification and treatment of genetic diseases, particularly cancer. Herein, we expand the utility of the 2-[18F]fluoropyridine- bearing labelling agent [18F]FPy5yne to include nucleic acid- based probes for PET imaging. We describe the conjugation of [18F]FPy5yne by way of a copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction to a 5'-azide-modified antisense oligodeoxyribonucleotide, which was first prepared from a 5'-aminohexyl-modified DNA 20mer. This work also describes protocols suitable for the coupling of [18F]FPy5yne and mini-PEGylated analog PEG-[18F]FPyKYNE to azide- modified peptide receptor ligands derived from bombesin and neuropeptide Y. These 18F- labelled targeting vectors were assayed by collaborators for the detection of prostate, breast and brain cancer in mouse models by way of PET imaging and ex vivo autoradiography. Finally, a new approach for the 18F- labelling of biological molecules is introduced and investigated. Sulfonyl [18F]fluorides can be prepared under aqueous conditions and at room temperature, yet they have not yet been assayed as a potential means to 18F- label biomarkers. A general route was developed for the synthesis of bifunctional arylsulfonyl [18F]fluorides from their sulfonyl chloride precursors in 1:1 mixtures of MeCN, THF, or t-BuOH and Cs[18F]F/Cs2CO3 over 15 minutes at room temperature. In most cases, pyridine could be used to selectively degrade the precursor without significantly affecting observed yields. As proof-of-principle, 3-formylmesitylenesulfonyl [18F]fluoride was synthesized in excellent preparative yields and used to radiolabel an oxyamino-modified bombesin(6-14) analog. The 18F bioconjugate showed signs of both defluorination and modification in mouse serum.