Disruption of the structural integrity of the SIRT1 network and interruption of SIRT1 function contribute to various age-related pathologies, such as Alzheimer’s disease (AD). SIRT1, therefore, provides a promising avenue for therapeutic intervention by constituting as a unique molecular link between aging and neurodegenerative disorders. However, most SIRT1 modulators have not shown benefits in the context of neurodegenerative disorders despite efficacy in cancer-related malignancies, largely due to lack of reliable tool to detect SIRT1-related molecular events underlying disease. Positron emission tomography (PET) imaging of SIRT1 would potentially allow us to investigate SIRT1-related pathological changes between normal and disease states and in vivo interactions of novel SIRT1 modulators with the target. As a result, there is a critical demand for the development of SIRT1 PET radioligands with appropriate performance characteristics for SIRT1 imaging in the brain.
Selisistat is a highly potent and selective small molecule inhibitor against SIRT1. It exhibits much lower inhibitory activity against SIRT2 and SIRT3 and shows no inhibitory effect on class I/II histone deacetylases (HDACs) and NAD glycohydrolase (NADase) at 100 μM. Based on these appropriate profile, we selected selisistat as a candidate ligand for further radiolabeling and in vivo imaging studies. However, because of the difficult on radiolabeling selisistat, the indole derivatives were chosen as the lead substrates for further development as imaging agents. Among these indole derivatives, SIRT1Cpd-3, which was confirmed by docking studies, exhibits favorable profile on SIRT1. Radiolabeling of the methyl group resulting in [11C]SIRT1Cpd-3, therefore, proves to be the most suitable strategy for the SIRT1 imaging studies in the brain. In the present study, we radiolabeled the indole derivative SIRT1Cpd-3, and demonstrated its specific binding as well as other appropriate biochemical properties in rodent and nonhuman primate (NHP) brains using PET imaging.
Radiosynthesis of [11C]SIRT1Cpd-3 was successfully achieved by palladium-catalyzed methylation with [11C]CH3I based on Suzuki-Miyaura coupling. [11C]SIRT1Cpd-3 was produced with a good radiochemical yield of 30 ± 5% (n = 10, decay corrected) in a total synthesis time of 40 ± 5 min from end of cyclotron bombardment. Analytical HPLC demonstrated that the radiochemical purities of [11C]SIRT1Cpd-3 were consistently greater than 99%. [11C]SIRT1Cpd-3 displayed desirable brain-blood barrier (BBB) penetration, brain uptake and selectivity, as well as stable metabolism and proper kinetics and distribution. Then, our probe was validated by visualizing SIRT1 in brains of AD transgenic mice, compared to nontransgenic animals, which demonstrated that [11C]SIRT1Cpd-3 is a promising tool to detect SIRT1 in brains of Alzheimer’s model animals. We also showed that SIRT1 is differentially expressed across various brain areas in our rodent and NHP animal models, and that midbrain and thalamus display high SIRT1 signals in both rodents and NHP, suggesting a conserved function of SIRT1.
[11C]SIRT1Cpd-3 not only enables the demonstration of SIRT1 in preclinical animal models but also allows visualization and recapitulation of AD-related SIRT1 changes in animal brains, supporting its promising clinical potential. Further studies, including target occupancy, binding kinetics and radiation dosimetry in NHP, are needed to promote its clinical translation for human imaging. Furthermore, because our data suggest [11C]SIRT1Cpd-3 as a useful tool to visualize AD-related SIRT1 changes in the brain, it is urgent and warrants future studies to detect SIRT1 changes in other aging-related pathways, e.g. calorie restriction and NAD supplementation. Collectively, our present results strongly support that our new SIRT1 PET tracer can be used as a potential biomarker to evaluate AD progression and enhance diagnosis for AD brains.
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Image/Figure Caption:
PET imaging studies in Papio Anubis baboon. Representative PET/MR images in the baboon brain (summed 60-90 min). Time-activity curves in representative baboon brain regions. Arterial plasma analysis. The radioactivity accumulation is presented as the standardized uptake value (SUV).
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Massachusetts General Hospital and Harvard Medical School