F-18 Labeling Protocol of Peptides Based on Chemically Orthogonal Strain-Promoted Cycloaddition under Physiologically Friendly Reaction Conditions

We introduce the high-throughput synthesis of various F-18-labeled peptide tracers by a straightforward F-18-labeling protocol based on a chemo-orthogonal strain promoted alkyne azide cycloaddition (SPAAC) using aza-dibenzocyclootyne-substituted peptides as precursors with F-18-azide synthon to develop peptide based positron emission tomography (PET) molecular imaging probes. The SPAAC reaction and subsequent chemo-orthogonal purification reaction with azide resin proceeded quickly and selectively under physiologically friendly reaction conditions (i.e., toxic chemical reagents free, aqueous medium, room. temperature, and pH approximate to 7), and provided four F-18 labeled tumor targetable bioactive peptides such as cyclic Arg-Gly-Asp (cRGD) peptide, bombesin (BBN), c-Met binding peptide (cMBP), and apoptosis targeting peptide (ApoPep) in high radiochemical yields as direct injectable solutions without any HPLC purification and/or formulation processes. In vitro binding assay and in vivo PET molecular imaging study using the F-18-labeled cRGD peptide also demonstrated a successful application of our F-18-labeling protocol.