We will develop the first potent, selective and brain-penetrant positron emission tomography (PET) ligand for cholesterol 24-hydroxylase (CYP46A1) to facilitate CNS drug discovery. The CYP46A1 enzymatic conversion of brain cholesterol into 24S-hydroxycholesterol is the major elimination mechanism to maintain brain cholesterol homeostasis. Disturbances in CYP46A1 is implicated in the physiopathology of neurological and neurodegenerative diseases such as schizophrenia, Huntington?s disease and Alzheimer?s disease. Pharmacological modulation of CYP46A1 represents an attractive therapeutic approach. PET is capable of quantifying biochemical processes in vivo, and a suitable CYP46A1 ligand would substantially improve our understanding of CYP46A1- mediated cholesterol homeostasis under physiopathological conditions otherwise inaccessible by ex vivo (destructive) analysis. Quantification of CYP46A1 in living brain by PET would provide the assessment of distribution, target engagement and dose occupancy of new CYP46A1-targeted neurotherapeutics. To date, no successful examples have been demonstrated to image CYP46A1 for clinical use, representing a significant deficiency of our ability to study this target in vivo. Therefore, we propose to develop a novel PET ligand that can fill this void, as the first translational imaging tool. Our ligand [11C]CYP-507 demonstrated the first prototype for imaging towards CYP46A1, but was discontinued due to marginal binding specificity and low brain penetration. In our next generation, we successfully identified a lead molecule, CYP-812, which showed high binding affinity and selectivity towards CYP46A1 over all other CYP isoforms and major CNS targets. An 11C-isotopologue of CYP-812 was synthesized and preliminary PET studies confirmed that we have overcome the major obstacles for CYP46A1 ligand development by achieving: 1) reasonable and regional-specific brain uptake; 2) moderate target specificity. Though CYP-812 is a promising lead molecule for the development of new CYP46A1-targeted PET ligands, further optimizations aimed at higher brain permeability, improved potency and binding specificity with proper brain kinetics are sought for translational cross-species (rodents and nonhuman primates) imaging studies to achieve optimal CYP46A1 quantification for drug discovery and clinical translation. As specific goals, we will design and prepare a focused library of CYP46A1 modulators amenable for labeling with 11C or 18F, and evaluate their ability to quantify CYP46A1 activity and changes during drug challenge in rodents and nonhuman primates, as well as autoradiography and biological validation in postmortem human brain tissues. The impact of this work is not only to develop the first potent and selective CYP46A1 PET ligand for the study of disease-related biological processes, but also ultimately, via PET imaging validation in higher species, to advance this ligand for potential clinical translation and monitor target response of novel neurotherapeutics. Relevance: This proposal has the potential to improve public health and help patients suffering from CNS diseases through the discovery of neurotherapeutics using CYP46A1-targeted PET ligands.

Public Health Relevance

As the burden of neurodegenerative diseases and neurological disorders is high, there is a critical need to develop novel PET ligands to image illness-related biological processes in the brain. This work will develop key imaging tools to advance our understanding of central cholesterol homeostasis and facilitate drug discovery.

National Institute of Health (NIH)
National Institute on Aging (NIA)
Research Project (R01)
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Drug Discovery for the Nervous System Study Section (DDNS)
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Hsiao, John
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Massachusetts General Hospital
United States
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