Direct fluorine-18 labeling of heat-sensitive biomolecules for positron emission tomography imaging using the Al18F-RESCA method

Positron emission tomography (PET) is a quickly expanding, non-invasive molecular imaging technology, and there is high demand for new specific imaging probes. Herein, we present a generic protocol for direct radiolabeling of heat-sensitive biomolecules with the positron-emitting radioisotope fluorine-18 (¹⁸F) using the aluminum fluoride restrained complexing agent (Al¹⁸F-RESCA) method. The Al¹⁸F-RESCA method combines the chemical advantages of a chelator-based radiolabeling method with the unique physical properties of the radionuclide of choice, fluorine-18. Proteins of interest can be conjugated to RESCA via amine coupling using (±)-H₃RESCA-TFP, followed by purification using size-exclusion chromatography (SEC). Next, RESCA-derivatized biomolecules can be labeled in one step, at room temperature (~20 °C) in an aqueous medium with aluminum fluoride (Al¹⁸F). Al¹⁸F-labeled proteins can be obtained with moderate (12-17 GBq/µmol) to good (80-85 GBq/µmol) apparent molar activity, depending on the starting activity of ¹⁸F^-. In addition, satisfactory radiochemical yields (35-55%, non-decay corrected) and high radiochemical purity (>98%, using gel filtration or solid-phase purification) are obtained. The mild radiolabeling procedure takes 0.5 h to complete and can be used for direct labeling of vector molecules such as Peptides, protein scaffolds, and engineered antibody fragments.