Design and synthesis of [alpha]-aminoxy acid-based cation transporters and their applications as anti-cancer and antibacterial agents

Abstract

By selectively modulating charged ions across cellular membranes, natural ion channels control membrane potential, which underpins elementary cellular functions, such as proliferation, cell volume, secretion and excitability. Synthetic compounds that are capable of mimicking ion channel functions are believed to have potential therapeutic values in targeting channelopathies as well as cancers. However, previously reported synthetic cation transporters were designed mainly based on large scaffolds, such as crown ethers, oligophenyl rods, calix[4]arene, pillar[n]arene and cyclic peptides, and they exhibited limited ion selectivity (especially potassium to sodium selectivity) and applications in living systems. In this thesis, easily accessible α-aminoxy acids have been employed for the construction of efficient and selective ion transporters. By using heterochiral α- aminoxy acids, dipeptide 2.1 has been developed, which self-assembles into ion channel with good potassium selectivity (PK +/ PCl − ≈ 14 and PNa + / PCl − ≈ 2). With bis-CF3 substitution at the N-terminal group, potassium carriers 2.4 and 2.5 with up to 27-fold improved potassium transport activity have been obtained. In addition, fluorinated monomeric amino acids with lower molecular weights have been demonstrated to be a versatile platform for the construction of efficient ion transporters with variable selectivity. By using this platform, highly selective potassium transporter 3.2, sodium potassium co-transporter 3.8, potassium chloride co-transporter 3.9 and selective chloride transporter 3.13 have been discovered. In addition, the potential applications of those ion transporters as anti-cancer and antibacterial agents have been explored. Cancer stem cells (CSCs) are subpopulations within tumors that are responsible for tumor growth and tumorigenesis. They are resistant to conventional treatment, which may result in tumor metastasis and relapse. Natural potassium ionophores, such as salinomycin and nigericin have been reported to be anti-cancer agents due to their selective toxicity against several kinds of cancer stem cells. However, CSCs, like ovarian cancer stem cells, with high expression of ABC drug transporters are resistant to these natural ionophores. In this thesis, I have shown that synthetic cation transporters 3.2 and 3.8 can selectively induce the death of ovarian CSCs with up to 50-fold selectivity to ovarian cancer cells and normal cells. Detailed mechanism studies have revealed that by facilitating proton and potassium ion translocation, these cation transporters can directly induce mitochondrial membrane depolarization, production of reactive oxygen species, mitochondrial morphology change, mitochondrial respiration attenuation, and disrupt lysosome pH, which finally lead to cell autophagy arrest and apoptotic cell death. These results demonstrate that my synthetic cation transporters with a simplified and highly modular scaffold represent a promising approach for cancer therapy. I also present in this thesis that cation transporters 2.4, 2.5, 3.2, 3.8 and 3.9 are effective against Gram-positive bacteria including drug resistant strains MRSA and VRE. Some of them have shown antibacterial activity comparable to commercial antibiotic daptomycin. These molecules also exhibited excellent selectivity to bacteria over mammalian cells. All these studies strongly indicate the potentials of α-aminoxy acid-based small ion transporters for antibacterial applications.