Cytotoxicity of ascorbate to cancer cells: mathematical modelling, mechanisms and clinical implications

Abstract

Studies over the last few decades provided supportive evidence for further research on the use of intravenous administration of high-dose ascorbate (0.5 -1.5 grams per kilogram of body weight) in cancer treatment. Although intravenous ascorbate showed limited anticancer activity in the recent clinical trials, its potential beneficial effects were noted, such as improvement in quality of life, reduction of chemotherapy-associated toxicity, reduction of inflammation and stabilization of cancer progression. These observations were in agreement with the outcomes of three cancer patients who declined chemotherapy and received intravenous ascorbate as an alternative in a preliminary clinical case study (Chapter 4). It was commonly accepted that the cytotoxicity of ascorbate was mediated by hydrogen peroxide (H2O2). However, our analysis and interpretation of the published results were not consistent with this being the sole mechanism of cytotoxicity, and an additional dehydroascorbate-mediated mechanism was proposed in this thesis (Chapter 1).In order to have a holistic understanding of the differential effects of ascorbate treatment in in vitro, animal and human studies, the treatment-induced changes in the intracellular H2O2 concentrations of the targeted cells should be compared. However, precise measurement of intracellular H2O2concentrations in vivo is difficult. In Chapter 2, a mathematical model was established for estimating the extracellular and intracellular H2O2 concentrations in a biological system that was challenged with H2O2 and/or an agent that could induce H2O2 production (such as ascorbate). Simulation of the mathematical model was carried out with the values of variables estimated from the published scientific literature and the experimental data obtained from this study. Several predictions were made and validated with the experimental results. One simulation predicted a limited increase of intracellular H2O2 concentration in a typical intravenous ascorbate treatment, which should be insufficient to induce cell death in tumours. The mathematical model also provided methods for estimating intrinsic intracellular H2O2 production rate and intracellular H2O2 concentration based on extracellular H2O2 concentration and other cell-dependent parameters, which were easier to be obtained with the current experimental methods.In Chapter 3, it was demonstrated in vitro that the cytotoxicity of ascorbate was mediated by both H2O2 and dehydroascorbate (DHA). The associations between the responses of cells to ascorbate (0 -30 mM) and some extracellular, cellular and molecular variables, such as cell culture medium, serum, glucose, pyruvate, cell cycle, cell volume and the expression and activities of H2O2 ii decomposing enzymes catalase and glutathione peroxidase, were investigated. Pyruvate but not glucose showed protective effect on cells in DHA treatment. Increased oxygen reactive species (ROS) levels in the DHA-treated cells was observed. These results supported our hypothesis that DHA might be able t...