Design of Bioconjugates and Prochelators to Target Dysregulated Iron Homeostasis in Cancer

Abstract

Iron is an essential element for tumor progression and metastasis. High iron demand is observed in cancers to accommodate their rapid growth rate since iron plays a vital role in DNA biosynthesis. This aberrant iron homeostasis is a hallmark of cancer, motivating the development of iron chelation approaches in cancer chemotherapy. Although iron-binding molecules have demonstrated promising antiproliferative effects in clinical trials, side effects were observed during iron chelator treatments. The off-target toxicity is attributed to indiscriminate metal-binding and induction of oxidative stress. As such, this dissertation aims to provide novel strategies to address these issues. We utilized prochelation and bioconjugation approaches to inactivate the iron-binding molecules in the extracellular environment so that thebinding ability can only be restored intracellularly and preferentially in cancer cells. These prochelators and bioconjugated prochelators demonstrated an improved specificity in cultured cells when compared to traditional chelation-based compounds.Chapter 1 of this dissertation provides a brief overview of the iron uptake mechanism in humans. Common diseases associated with iron are discussed, including iron overload and neurodegeneration. Next, the roles of iron in cancer biology are discussed, such as malignant initiation, metathesis, and drug resistance. Moreover, we provide an overview of clinically investigated iron chelators and reported metal prochelators.Chapter 2 focuses on two newly synthesized thiosemicarbazone prochelators, in which a binding thiolate donor is masked by a disulfide bond, requiring intracellular reduction to release active chelators that readily bind intracellular iron. The prochelator series in this study features methyl-substituted thiosemicarbazone or imidazole-2-thione moieties to increase lipophilicity for better membrane permeability. Interestingly, these new prochelators covalently modify bovine serum albumin through native mass spectrometry experiments, and their antiproliferative activities were observed at submicromolar levels. In Chapter 3, a new class of glycoconjugated aroylhydrazone (AH1) prochelators is described. The ligand and monosaccharides are connected through a disulfide-based linker, which is essential for intracellular release of the AH1 ligand. The goal of developing glycoconjugation strategy is to enhance the selectivity of iron chelation to cancers by exploiting the overexpressed glucose transporter 1 in malignant cells. The molecules described in Chapter 2 and 3 are examples of thiolate chelators utilizing disulfide mask for prochelator design. In Chapter 4, a new prochelation approach is described, which is inspired by a commercially available tetrazolium dye that is used for cellular viability assays. Instead of masking one binding donor atom on the ligand structure, the approach wraps the metal-binding unit into a cationic tetrazolium core. The synthesized tetrazolium-based prochelators are chemically inert in serum but can be reduced in cells, liberating formazan chelators to bind intracellular iron. The biological assays demonstrate that tetrazolium prochelators are preferably activated/reduced in cancer cells, providing a favorable therapeutic selectivity. In addition, quinoline-based tetrazolium prochelators are reported. These new prochelators are shown to be more toxic than the pyridyl analogs because their reduction potentials are more biologically accessible. The final Chapter 5 describes the design of formazan chelators as contrast agents for biological iron detection through photoacoustic imaging techniques. This chapter presents preliminary data on the synthesis of water-soluble formazan ligands and their ability to bind iron in aqueous solutions.