Mapping the Fundamental Metabolic Drivers of Inflammation with Molecular Imaging

Abstract

Inflammation and oxidative stress are tightly linked processes underlying the pathogenesis of a wide variety of diseases. Using molecular imaging, we can enhance our understanding of these mechanisms through non-invasive visualization of important mediators of disease at the molecular and cellular level. Aldehydes are toxic biproducts of inflammation and oxidative stress, formed by many metabolic pathways including lipid peroxidation, autoxidation, oxidative burst from inflammatory monocytes, polyamine catabolism, and drug metabolism. These aldehydes are highly reactive electrophiles, rapidly forming adducts with phospholipids, proteins, and DNA, regulating cell survival, autophagy, senescence, apoptosis, and necrosis. These endogenous aldehydes and their adducts have been implicated in many pathological conditions including neurodegenerative, metabolic, and cardiovascular diseases, cancer, drug toxicity, and traumatic brain injury. However, due to their transient and highly reactive nature, the direct investigation of aldehydes and their role in disease has been challenging. The contents of this thesis include the development and application of molecular imaging probes for fluorescence, positron emission tomography (PET), and magnetic resonance imaging (MRI) targeted toward the cellular and in vivo visualization of aldehydes in models of oxidative stress, inflammation, mild traumatic brain injury, and acetaminophen overdose. Using these probes, the direct mapping of endogenous production has been elucidated, and with this data I propose aldehydes are mediators and propagators of inflammatory and oxidative pathologies. Another metabolic driver of disease is the cellular switch from glucose to fructose metabolism (fructolysis). Fructose can be generated endogenously from glucose via the polyol pathway or consumed from dietary sources. Fructolysis has been identified as the driver of end-stage diabetes complications including retinopathy, nephropathy, and neuropathy. Fructose is also effective at protein glycation, leading to the production of advanced glycation end products (AGEs), which have been suggested as promotors of metabolic syndrome and chronic inflammatory conditions. The metabolism of fructose disrupts the delicate balance between antioxidant and pro-oxidant systems, leading to oxidative stress. Some cancers have demonstrated preference for fructose over glucose metabolism, making them difficult to visualize using standard PET imaging with [18F]-2-fluoro-2-deoxy-D-glucose ([18F]-FDG), a radiolabeled glucose analog for mapping of glycolysis. Herein, I describe a novel metabolic PET radiotracer, [18F]4-fluoro-4-deoxyfructose ([18F]4-FDF), for in vivo mapping of fructose metabolism. The utility of this radiotracer is demonstrated in mouse models of cancer and systemic inflammation and represents a powerful clinical tool for the non-invasive diagnosis of many diseases. The final topic of this thesis involves molecular imaging of inactivated bacteria to determine its trafficking and biodistribution in vivo. Site Specific Immunomodulators (SSIs) are a novel class of immunotherapy derived from bacteria that usually infect specific organs (Escherichia coli for the gut, and Klebsiella pneumoniae for the lungs). While it is understood that in keeping the macromolecular components of these inactivated bacteria modulates the innate immune system, it was previously unknown whether their biodistribution matched that of the live bacteria, and how they were trafficked within the body. The biodistribution of the SSIs was mapped in healthy mice through radiolabeling, and the involvement of phagocytosing cells was elucidated by immunohistochemical evaluation of local innate immune cells. Thus, by using molecular imaging, the inflammatory response to immunomodulating therapies was mapped in vivo by PET. In conclusion, this thesis highlights the pivotal role of molecular imaging in advancing our comprehension of inflammation and oxidative stress, shedding light on the intricate interplay of aldehydes in various diseases. Furthermore, the in vivo mapping of fructolysis with [18F]4-FDF contributes valuable insights into the metabolic drivers of diseases, offering potential diagnostic avenues for neuro and cardio-inflammatory conditions. Finally, the application of molecular imaging to unravel the trafficking and biodistribution of inactivated bacteria underscores its significance in mapping the inflammatory response to immunomodulating therapies, providing a new perspective on the intricate dynamics of disease pathogenesis.