One Step Radiosynthesis of 6-[18F]Fluoronicotinic Acid 2,3,5,6-Tetrafluorophenyl Ester ([18F]F-Py-TFP): A New Prosthetic Group for Efficient Labeling of Biomolecules with Fluorine-18

The labeling of biomolecules for positron emission tomography (PET) with no-carrier-added fluorine-18 is almost exclusively accomplished using prosthetic groups in a two step procedure. The inherent complexity of the process renders full automation a challenge and leads to protracted synthesis times. Here we describe a new F-18-labeled prosthetic group based on nicotinic acid tetrafluorophenyl ester. Reaction of [F-18]fluoride at 40 degrees C with the trimethylammonium precursor afforded 6-[F-18]fluoronicotinic acid tetrafluorophenyl ester ([F-18]F-Py-TFP) directly in 60-70% yield. [F-18]F-Py-TFP was conveniently purified by Sep-Pak cartridge prior to incubation with a peptide containing the RGD sequence. The desired conjugate was formed rapidly and in good yields. An in vitro receptor-binding assay for the integrin alpha(v)beta(3) was established to explore competition with peptide and peptidomimetic prepared from F-Py-TFP with I-125-echistatin. The nonradioactive conjugates were found to possess high binding affinities with calculated K-i values in the low nanomolar range.