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Targeted Tropomyosin Receptor Kinase Radiotracers for Positron Emission Tomography Imaging
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- Author / Creator
- Bernard-Gauthier, Vadim
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Tropomyosin receptor kinases TrkA/B/C family supports neuronal growth, survival and differentiation during development, adult life and ageing. Downregulation of TrkA/B/C is a prominent hallmark of numerous neurological disorders including Alzheimer disease (AD). Abnormally expressed or overexpressed full length or fusion TrkA/B/C proteins which bear oncogenic potential and were shown to drive tumorigenesis in a variety of neurogenic and non-neurogenic human cancers are currently the focus of intensive clinical research. The study, both in oncology and neurology, of the spatiotemporal alterations in TrkA/B/C expression and density or the determination of target engagement of emerging antineoplastic kinase inhibitor drugs with those receptors in normal and diseased tissue is crucially needed but has however remained largely unexplored due to the lack of suitable non-destructive analytic tools. Here, we aim to develop and provide multi-species validation of carbon-11- and fluorine-18-labeled positron emission tomography (PET) radiotracers based on purposely designed small molecule kinase catalytic domain-binding inhibitors of TrkA/B/C. We first demonstrate that pan-Trk selective inhibitor scaffolds which target both the active DFG-in and inactive DFG-out kinase conformations can be rationally modified to yield suitable compounds for translation into PET radiotracers. In particular, the carbon-11 isotopologue of the preclinical 4-aza-2oxindole lead GW441756 is characterized as the first brain penetrant Trk radiotracer based on rodent PET experiments in vivo. Using human neuroblastoma tissue, it is also shown that type-I pan-Trk-selective radiotracers, including [11C]GW441756, enable tumor visualization selectivity, based on TrkB status in vitro. Subsequently, we show that impediments associated with the development of orthosteric tracers for intracellular in vivo neuroimaging of protein kinases can be addressed via thorough structure-activity relationship (SAR) screening such as generating a lead suitable for human use. From the screening of an imidazo[1,2-b]pyridazine-based pan-Trk inhibitors library, followed by the in vivo assessment of multiple radiotracers from this series, we provide the detailed evaluation of [11C]-(R)-IPMICF16 as the first TrkB/C-targeted lead radiotracer with suitable properties for neuroimaging in human. The evaluation presented includes PET imaging studies in four species from mice to first-in-human as well as Trk kinase inhibitor target engagement confirmation using the phase II clinical inhibitor entrectinib in mice. Relying on extensive human kinome analyses, we also show that (R)-IPMICF16 constitutes both the most potent and most selective TrkB/C inhibitor known to date. We furthermore demonstrate that this lead efficiently enables the discrimination of AD versus healthy control brains based on hippocampal binding in human in vitro. We present additional efforts in our exploration of Trk radiotracers with the development of an 18F-labeled quinazoline-based pan-Trk lead which displays suitable properties for in vivo translation and favorable Trk activities, including towards clinically relevant Trk oncogenic drivers such as TrkA-TPM3. Finally, current work in the second generation optimization of our [11C]-(R)-IPMICF16 clinical lead is described. Of interest, it is shown that a state-of-art copper-mediated 18F-fluorination technique can be used to secure the inactivated 18F-arene moiety of our new lead tracer, [18F]-(R)-IPMICF17. The Trk-targeted probes delineated here represent a novel class of molecular imaging radiotracers for the non-invasive and in-depth interrogation study of signal transduction at the interface of oncology and neurology. Globally, the results presented entail the further exploration of Trk radiotracers as a novel tool for the study and diagnosis of human neurodegenerative diseases and patients identification in the context of personalized oncology.
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- Graduation date
- Fall 2017
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- Type of Item
- Thesis
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- Degree
- Doctor of Philosophy
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- License
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.