In Vivo Visualization and Quantification of Brain Heat Shock Protein 90 with [¹¹C]HSP990 in Healthy Aging and Neurodegeneration

Heat shock protein 90 (Hsp90) is essential for maintaining cellular proteostasis and may play an important role in the development of neurodegenerative proteinopathies. Therefore, we aimed to develop an Hsp90-specific PET brain tracer to quantify Hsp90 expression in the brain in vivo in order to explore its potential as a biomarker for neurodegenerative disease characterization and to support Hsp90-targeted drug development. Methods: We developed the radiosynthesis of (R)-2-amino-7-(4-fluoro-2-(6-(methoxy-¹¹C)pyridin-2-yl)phenyl)-4-methyl-7,8-dihydropyrido[4,3-d]pyrimidin-5(6H)-one, [¹¹C]HSP990, and validated the tracer using in vitro autoradiography, in vitro brain homogenate saturation binding, ex vivo biodistribution, and in vivo PET imaging in rodent models of Alzheimer disease (AD) and Parkinson disease versus healthy age-matched and young controls. Human brain samples from AD patients and healthy subjects were included in our in vitro binding studies. A nonhuman primate PET brain study with arterial blood sampling was conducted under baseline and blocking conditions. Results: In vitro and in vivo [¹¹C]HSP990 studies in rodents and a nonhuman primate revealed saturable Hsp90 binding pools in natural killer lymphocytes, bone marrow, and notably the brain, where the highest binding was observed, particularly in gray matter. Blocking studies indicated that saturable Hsp90 in natural killer lymphocytes considerably influences the pharmacokinetics of Hsp90-targeting probes, which is critical for Hsp90 drug development. In vitro [³H]HSP990 brain homogenate saturation binding assays suggested that the tracer binds a distinct subfraction of the total Hsp90 pool, which is significantly diminished in both rodent and human AD brain tissue compared with age-matched controls. In vivo PET imaging confirmed reduced [¹¹C]HSP990 brain binding on aging and an even stronger decrease in AD mice, suggesting that Hsp90 depletion may impair protein quality control and accelerate proteinopathies. Conclusion: [¹¹C]HSP990 is a promising Hsp90-specific tracer and reveals strong Hsp90 binding in the brain. Uniformly reduced tracer binding was observed in AD brain tissue compared with age-matched controls. [¹¹C]HSP990 holds potential as a biomarker for neurodegenerative disease characterization and progression, and it may aid in patient stratification and therapy monitoring. Human [¹¹C]HSP990 PET neuroimaging studies are under way to investigate whether these findings translate to humans.