Radiosynthesis of [18F]ATPFU: a potential PET ligand for mTOR

Mammalian target of rapamycin (mTOR) plays a pivotal role in many aspects of cellular proliferation, and recent evidence suggests that an altered mTOR signaling pathway plays a central role in the pathogenesis of aging, tumor progression, neuropsychiatric, and major depressive disorder. Availability of a mTOR-specific PET tracer will facilitate monitoring early response to treatment with mTOR inhibitors that are under clinical development. Towards this we have developed the radiosynthesis of [(18)F]1-(4-(4-(8-oxa-3-azabicyclo[3.2.1]octan-3-yl)-1-(2,2,2-trifluoroethyl)-1H-pyrazolo[3,4-d]pyrimidin-6-yl)phenyl)-3-(2-fluoroethyl)urea [(18)F]ATPFU ([(18)F]1) as an mTOR PET ligand. Synthesis of reference 1 and the precursor for radiolabeling, 4-(4-8-oxa-3-azabicyclo[3.2.1]-octan-3yl)-1-(2,2,2-trifluoroethyl)-1H-pyrazolo[3,4-d]pyrimidin-6yl)aniline (10), were achieved from beta-chloroaldehyde 3 in 4 and 5 steps, respectively, with an overall yield of 25-28%. [(18)F]Fluoroethylamine was prepared by heating N-[2-(toluene-4-sulfonyloxy)ethyl]phthalimide with [(18)F]fluoride ion in acetonitrile. [(18)F]1 was obtained by slow distillation under argon of [(18) F]FCH2CH2NH2 into amine 10 that was pre-treated with triphosgene at 0-5 °C. The total time required for the two-step radiosynthesis including semi-preparative HPLC purification was 90 min, and the overall radiochemical yield of [(18)F]1 for the process was 15 ± 5% based on [(18)F]fluoride ion (decay corrected). At the end of synthesis (EOS), the specific activity was 37-74 GBq/µmol (N = 6).