De-Risking PET Radioligand Discovery for Neurodegenerative Diseases

This work sought to characterize novel and existing radioligands, assessing their utility and effectiveness for positron emission tomography (PET) imaging of neurodegeneration. Three proteins were targeted: 1) colony stimulating factor-1 receptor (CSF-1R); 2) glycogen synthase kinase-3 (GSK-3); and 3) transactivation response DNA-binding protein with molecular weight of 43 kDa (TDP-43). Utilizing autoradiography, a putative CSF-1R binding radioligand, [3H]CPPC, did not localize to microglia in the brain tissue of a preclinical model of neuroinflammation and percent displacement did not exceed 50% of signal, requiring use of spleen in screening assays. Selectivity assessment showed numerous binding sites which prevents an interpretation of signal contribution. Using similar methods, two radioligands targeting GSK-3, a kinase implicated in Alzheimer’s disease (AD) pathogenesis, were shown to quantify similar target densities (Bmax) with similar affinity (Kd) in human and rodent post-mortem homogenates. Investigation of human AD brain revealed possible differences in GSK-3 density compared to controls within females. In P301L mice, GSK-3 density in males was elevated compared to control, and a trend was recapitulated by in vivo PET, while females remained unchanged by both methods. GSK-3 PET research could inform the biological role of this critical kinase in pathogenesis of AD. Lastly, autoradiography identified two potential lead structures following evaluation of existing tau PET radioligands in post-mortem amyotrophic lateral sclerosis (ALS) tissues. Colocalization of specific binding with aggregate TDP-43 immunostaining was evaluated for isotopologues of 6 existing tau PET radioligands in varying structural classes. Three radioligands, [3H]MK-6240, [3H]GTP-1, and [3H]JNJ-067, showed no binding in ALS tissues. [3H]CBD-2115 specific binding was elevated but did not correlate to pathology. [3H]Flortaucipir binding was increased in the motor cortex compared to the cerebellum. Immunofluorescent staining with APN-1607 revealed no colocalization with a TDP-43 inclusion but significant binding to -amyloid. Despite revealing both positive and negative results an overarching objective of de-risking radioligands was achieved in each case by providing important information including selectivity, target density, specificity, or affinity. The results presented here are not unexpected due to challenges in developing imaging probes which has led to most putative radioligands failing prior to translation as successful clinical imaging ligands.

Department dc:contributor.department

Medical Science

Year dc:date.issued

2021

Author dc:creator

• Knight, Ashley Cherie

Advisor dc:contributor.advisor

• Vasdev, Neil

Handle dc:identifier.uri

http://hdl.handle.net/1807/108717

OAI identifier oai:identifier

oai:utoronto.scholaris.ca:1807/108717