Syntheses and Evaluation of Carbon-11- and Fluorine-18-Radiolabeled pan-Tropomyosin Receptor Kinase (Trk) Inhibitors: Exploration of the 4-Aza-2-oxindole Scaffold as Trk PET Imaging Agents

Tropomyosin receptor kinases (TrkA/B/C) are critically involved in the development of the nervous system, in neurological disorders as well as in multiple neoplasms of both neural and non-neural origins. The development of Trk radiopharmaceuticals would offer unique opportunities toward a more complete understanding of this emerging therapeutic target. To that end, we first developed [11C]GW441756 ([11C]9), a high affinity photoisomerizable pan-Trk inhibitor, as a lead radiotracer for our positron emission tomography (PET) program. Efficient carbon-11 radiolabeling afforded [11C]9 in high radiochemical yields (isolated RCY, 25.9% ± 5.7%). In vitro autoradiographic studies in rat brain and TrkB-expressing human neuroblastoma cryosections confirmed that [11C]9 specifically binds to Trk receptors in vitro. MicroPET studies revealed that binding of [11C]9 in the rodent brain was mostly nonspecific despite initial high brain uptake (SUVmax = 2.0). Modeling studies of the 4-aza-2-oxindole scaffold led to the su...