Molecular Imaging of Sirtuin1 Expression-Activity in Rat Brain Using Positron-Emission Tomography-Magnetic-Resonance Imaging with [18F]-2-Fluorobenzoylaminohexanoicanilide

Development and Characterization of a Novel Carbon-11 Labeled Positron Emission Tomography Radiotracer for Neuroimaging of Sirtuin 1 with Benzoxazine-Based Compounds

Introduction

Sirtuins (SIRTs) comprise a group of histone deacetylase enzymes crucial for regulating metabolic pathways and contributing significantly to various disease mechanisms. Sirtuin 1 (SIRT1), among the seven known mammalian homologs, is extensively investigated and understood, playing a key role in neurodegenerative disorders and cancer. This study focuses on potential as a therapeutic target for conditions such as Parkinson's disease (PD), Huntington's disease (HD), and Alzheimer's disease (AD).

Methods

Utilizing positron emission tomography (PET) as a noninvasive molecular imaging modality, we aimed to expedite the validation of a promising sirtuin 1 inhibitor for clinical trials. However, the absence of a validated sirtuin 1 PET radiotracer impedes clinical translation. We present the development of [11C]1, and 11C-labeled benzoxazine-based derivative, as a lead imaging probe. The radiosynthesis of [11C]1 resulted in a radiochemical yield of 31 ± 4%.

Results

Baseline studies demonstrated that [11C]1 exhibited excellent blood-brain barrier (BBB) penetration capability, with uniform accumulation throughout various brain regions. Self-blocking studies revealed that introducing an unlabeled compound 1, effectively blocking sirtuin 1, led to a substantial reduction in whole-brain uptake, emphasizing the in vivo specificity of [11C]1 for sirtuin 1.

Discussion

The development of [11C]1 provides a valuable tool for noninvasive imaging investigations in rodent models with aberrant sirtuin 1 expression. This novel radiotracer holds promise for advancing our understanding of sirtuin 1's role in disease mechanisms and may facilitate the validation of sirtuin 1 inhibitors in clinical trials.

Bioluminescence Assay of Lysine Deacylase Sirtuin Activity

Lysine acylation can direct protein function, localization, and interactions. Sirtuins deacylate lysine towards maintaining cellular homeostasis, and their aberrant expression contributes to the pathogenesis of multiple pathological conditions, including cancer. Measuring sirtuins' activity is essential to exploring their potential as therapeutic targets, but accurate quantification is challenging. We developed 'SIRTify', a high-sensitivity assay for measuring sirtuin activity in vitro and in vivo. SIRTify is based on a split-version of the NanoLuc® luciferase consisting of a truncated, catalytically inactive N-terminal moiety (LgBiT) that complements with a high-affinity C-terminal peptide (p86) to form active luciferase. Acylation of two lysines within p86 disrupts binding to LgBiT and abates luminescence. Deacylation by sirtuins reestablishes p86 and restores binding, generating a luminescence signal proportional to sirtuin activity. Measurements accurately reflect reported sirtuin specificity for lysine acylations and confirm the effects of sirtuin modulators. SIRTify effectively quantifies lysine deacylation dynamics and may be adaptable to monitoring additional post-translational modifications.

Sirtuins 1 and 2 in the Acute Period After Photothrombotic Stroke: Expression, Localization and Involvement in Apoptosis

Sirtuins (SIRTs) are NAD+- dependent histone deacetylases. They are involved in a variety of biological pathways and are thought to be a promising target for treating several human disorders. Although evidence is piling up to support the neuroprotective role of SIRTs in ischemic stroke, the role of different sirtuin isoforms needs further investigation. We studied the effects of photothrombotic stroke (PTS) on the expression and localization of sirtuins SIRT1 and SIRT2 in neurons and astrocytes of the penumbra and tested the activity of their selective and non-selective inhibitors. SIRT1 levels significantly decreased in the penumbra cells nuclei and increased in their cytoplasm. This indicated a redistribution of SIRT1 from the nucleus to the cytoplasm after PTS. The expression and intracellular distribution of SIRT1 were also observed in astrocytes. Photothrombotic stroke caused a sharp increase in SIRT2 levels in the cytoplasmic fraction of the penumbra neurons. SIRT2 was not expressed in the penumbra astrocytes. SIRT1 and SIRT2 did not co-localize with TUNEL-positive apoptotic cells. Mice were injected with EX-527, a selective SIRT1 inhibitor; SirReal2, selective SIRT2 inhibitor or salermide, a nonspecific inhibitor of SIRT1 and SIRT2. These inhibitors did not demonstrate any change in the infarction volume or the apoptotic index, compared to the control samples. The studies presented indicate the involvement of these sirtuins in the response of brain cells to ischemia in the first 24 h, but the alterations in their expression and change in the localization of SIRT1 are not related to the regulation of penumbra cell apoptosis in the acute period after PTS.

Solid-Phase Synthesis of Selectively Mono-Fluorobenz(o)ylated Polyamines as a Basis for the Development of 18F-Labeled Radiotracers

Polyamines are highly attractive vectors for tumor targeting, particularly with regards to the development of radiolabeled probes for imaging by positron emission (PET) and single-photon emission computed tomography (SPECT). However, the synthesis of selectively functionalized derivatives remains challenging due to the presence of multiple amino groups of similar reactivity. In this work, we established a synthetic methodology for the selective mono-fluorobenz(o)ylation of various biogenic diamines and polyamines as lead compounds for the perspective development of substrate-based radiotracers for targeting polyamine-specific membrane transporters and enzymes such as transglutaminases. For this purpose, the polyamine scaffold was constructed by solid-phase synthesis of the corresponding oxopolyamines and subsequent reduction with BH₃/THF. Primary and secondary amino groups were selectively protected using Dde and Boc as protecting groups, respectively, in orientation to previously reported procedures, which enabled the selective introduction of the reporter groups. For example, N¹-FBz-spermidine, N⁴-FBz-spermidine, N⁸-FBz-spermidine, and N¹-FBz-spermine and N⁴-FBz-spermine (FBz = 4-fluorobenzoyl) were obtained in good yields by this approach. The advantages and disadvantages of this synthetic approach are discussed in detail and its suitability for radiolabeling was demonstrated for the solid-phase synthesis of N¹-[¹⁸F]FBz-cadaverine.

Molecular imaging of NAD+ -dependent deacetylase SIRT1 in the brain

INTRODUCTION

Aging is an inevitable physiological process and the biggest risk factor of Alzheimer's disease (AD). Developing an imaging tracer to visualize aging-related changes in the brain may provide a useful biomarker in elucidating neuroanatomical mechanisms of AD.

METHODS

We developed and characterized a new tracer that can be used to visualize SIRT1 in brains related to aging and AD by positron emission tomography imaging.

RESULTS

The SIRT1 tracer displayed desirable brain uptake and selectivity, as well as stable metabolism and proper kinetics and distribution in rodent and nonhuman primate brains. This new tracer was further validated by visualizing SIRT1 in brains of AD transgenic mice, compared to nontransgenic animals.

DISCUSSION

Our SIRT1 tracer not only enables, for the first time, the demonstration of SIRT1 in animal brains, but also allows visualization and recapitulation of AD-related SIRT1 neuropathological changes in animal brains.

Hypothalamic GHR-SIRT1 Axis in Fasting

Many aspects of physiological functions are controlled by the hypothalamus, a brain region that connects the neuroendocrine system to whole-body metabolism. Growth hormone (GH) and the GH receptor (GHR) are expressed in hypothalamic regions known to participate in the regulation of feeding and whole-body energy homeostasis. Sirtuin 1 (SIRT1) is the most conserved mamma-lian nicotinamide adenine dinucleotide (NAD+)-dependent protein deacetylase that plays a key role in controlling life span and sensing nutrient availability in the hypothalamus in response to caloric restriction. However, the interaction between GHR signaling and SIRT1 in the hypothal-amus is not established. In the arcuate nucleus (ARC) of the hypothalamus, the anorexigenic proopiomelanocortin (POMC)-expressing neurons and the orexigenic agouti-related protein (AgRP)-expressing neurons are the major regulators of feeding and energy expenditure. We show that in the ARC, the majority of GHR-expressing neurons also express SIRT1 and respond to fasting by upregulating SIRT1 expression. Accordingly, hypothalamic upregulation of SIRT1 in response to fasting is blunted in animals with GHR deletion in the AgRP neurons (AgRPEYFPΔGHR). Our data thus reveal a novel interaction between GH and SIRT1 in responses to fasting.

Noninvasive quantification of SIRT1 expression-activity and pharmacologic inhibition in a rat model of intracerebral glioma using 2-[18F]BzAHA PET/CT/MRI

Background

Several studies demonstrated that glioblastoma multiforme progression and recurrence is linked to epigenetic regulatory mechanisms. Sirtuin 1 (SIRT1) plays an important role in glioma progression, invasion, and treatment response and is a potential therapeutic target. The aim of this study is to test the feasibility of 2-[¹⁸F]BzAHA for quantitative imaging of SIRT1 expression–activity and monitoring pharmacologic inhibition in a rat model of intracerebral glioma.

Methods

Sprague Dawley rats bearing 9L (N = 12) intracerebral gliomas were injected with 2-[¹⁸F]BzAHA (300–500 µCi/animal i.v.) and dynamic positron-emission tomography (PET) imaging was performed for 60 min. Then, SIRT1 expression in 9L tumors (N = 6) was studied by immunofluorescence microscopy (IF). Two days later, rats with 9L gliomas were treated either with SIRT1 specific inhibitor EX-527 (5 mg/kg, i.p.; N = 3) or with histone deacetylases class IIa specific inhibitor MC1568 (30 mg/kg, i.p.; N = 3) and 30 min later were injected i.v. with 2-[¹⁸F]BzAHA. PET-computerized tomography-magnetic resonance (PET/CT/MR) images acquired after EX-527 and MC1568 treatments were co-registered with baseline images.

Results

Standard uptake values (SUVs) of 2-[¹⁸F]BzAHA in 9L tumors measured at 20 min post-radiotracer administration were 1.11 ± 0.058 and had a tumor-to-brainstem SUV ratio of 2.73 ± 0.141. IF of 9L gliomas revealed heterogeneous upregulation of SIRT1, especially in hypoxic and peri-necrotic regions. Significant reduction in 2-[¹⁸F]BzAHA SUV and distribution volume in 9L tumors was observed after administration of EX-527, but not MC1568.

Conclusions

PET/CT/MRI with 2-[¹⁸F]BzAHA can facilitate studies to elucidate the roles of SIRT1 in gliomagenesis and progression, as well as to optimize therapeutic doses of novel SIRT1 inhibitors.