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]<b>9</b>), a high affinity photoisomerizable pan-Trk inhibitor, as a lead radiotracer for our positron emission tomography (PET) program. Efficient carbon-11 radiolabeling afforded [¹¹C]<b>9</b> 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]<b>9</b> specifically binds to Trk receptors <i>in vitro</i>. MicroPET studies revealed that binding of [¹¹C]<b>9</b> in the rodent brain was mostly nonspecific despite initial high brain uptake (SUV_max = 2.0). Modeling studies of the 4-aza-2-oxindole scaffold led to the successful identification of a small series of high affinity fluorinated and methoxy derivatized pan-Trk inhibitors based on our lead compound <b>9</b>. Out of this series, the fluorinated compound <b>10</b> was selected for initial evaluation and radiolabeled with fluorine-18 (isolated RCY, 2.5% ± 0.6%). Compound [¹⁸F]<b>10</b> demonstrated excellent Trk selectivity in a panel of cancer relevant kinase targets and a promising <i>in vitro</i> profile in tumors and brain sections but high oxidative metabolic susceptibility leading to nonspecific brain distribution <i>in vivo</i>. The information gained in this study will guide further exploration of the 4-aza-2-oxindole scaffold as a lead for Trk PET ligand development

Identification of [¹⁸F]TRACK, a Fluorine-18-Labeled Tropomyosin Receptor Kinase (Trk) Inhibitor for PET Imaging

Changes in expression and dysfunctional signaling of TrkA/B/C receptors and oncogenic Trk fusion proteins are found in neurological diseases and cancers. Here, we describe the development of a first ¹⁸F-labeled optimized lead suitable for in vivo imaging of Trk, [¹⁸F]­TRACK, which is radiosynthesized with ease from a nonactivated aryl precursor concurrently combining largely reduced P-gp liability and improved brain kinetics compared to previous leads while displaying high on-target affinity and human kinome selectivity

A Kinome-Wide Selective Radiolabeled TrkB/C Inhibitor for in Vitro and in Vivo Neuroimaging: Synthesis, Preclinical Evaluation, and First-in-Human

The proto-oncogenes <i>NTRK1/2/3</i> encode the tropomyosin receptor kinases TrkA/B/C which play pivotal roles in neurobiology and cancer. We describe herein the discovery of [¹¹C]-(<i>R</i>)-<b>3</b> ([¹¹C]-(<i>R</i>)-IPMICF16), a first-in-class positron emission tomography (PET) TrkB/C-targeting radiolabeled kinase inhibitor lead. Relying on extensive human kinome vetting, we show that (<i>R</i>)-<b>3</b> is the most potent and most selective TrkB/C inhibitor characterized to date. It is demonstrated that [¹¹C]-(<i>R</i>)-<b>3</b> readily crosses the blood–brain barrier (BBB) in rodents and selectively binds to TrkB/C receptors in vivo, as evidenced by entrectinib blocking studies. Substantial TrkB/C-specific binding in human brain tissue is observed in vitro, with specific reduction in the hippocampus of Alzheimer’s disease (AD) versus healthy brains. We additionally provide preliminary translational data regarding the brain disposition of [¹¹C]-(<i>R</i>)-<b>3</b> in primates including first-in-human assessment. These results illustrate for the first time the use of a kinome-wide selective radioactive chemical probe for endogenous kinase PET neuroimaging in human