Pharmacological Ascorbate as a Means of Sensitizing Cancer Cells to Radio-Chemotherapy While Protecting Normal Tissue

Abstract

Chemoradiation has remained the standard of care treatment for many of the most aggressive cancers. However, despite effective toxicity to cancer cells, current chemoradiation regimens are limited in efficacy due to significant normal cell toxicity. Thus, efforts have been made to identify agents demonstrating selective toxicity, whereby treatments simultaneously sensitize cancer cells to protect normal cells from chemoradiation. Pharmacological ascorbate (intravenous infusions of vitamin C resulting in plasma ascorbate concentrations ≥20 mM; P-AscH^-) has demonstrated selective toxicity in a variety of preclinical tumor models and is currently being assessed as an adjuvant to standard-of-care therapies in several early phase clinical trials. This review summarizes the most current preclinical and clinical data available demonstrating the multidimensional role of P-AscH^- in cancer therapy including: selective toxicity to cancer cells via a hydrogen peroxide (H₂O₂)-mediated mechanism; action as a sensitizing agent of cancer cells to chemoradiation; a protectant of normal tissues exposed to chemoradiation; and its safety and tolerability in clinical trials.

Figure 1.. Ascorbate (vitamin C) acid-base and redox chemistry.

(A) xAscorbate exists primarily as the ascorbate anion (AscH⁻) at physiologic pH (99.4% as AscH⁻; 0.06% as AscH₂; and 0.004% as Asc²⁻). As the pH increases, however, the dianion form (Asc²⁻) increases logarithmically. (B) Autoxidation of the ascorbate monoanion and dianion produces the ascorbate radical (Asc^•−) that can undergo further oxidation to form DHA. The rate constant for the autoxidation of the monoanion is approximately one million times smaller than that of the dianion; k = 3 × 10² M⁻¹ s⁻¹ vs. 3 × 10⁻⁴ M⁻¹ s⁻¹ for Asc²⁻ and AscH⁻, respectfully. This will result in a very low value for k_3-obs ≈ 1 × 10⁻² M⁻¹ s⁻¹ (pH 7.4) for Rxn 3. Thus, metal catalyzed oxidation will be the dominant mechanism for the oxidation of ascorbate at near-neutral pH, reactions 4 – 7.

Figure 2.. Differences in H₂O₂ metabolism and redox-active iron metabolism underlie the selective toxicity of pharmacological ascorbate.

The proposed mechanisms underlying the selective toxicity of pharmacological ascorbate has as its center the production of H₂O₂ upon its oxidation. This is generally non-toxic to normal cells due to a high capacity to metabolize H₂O₂ in conjunction with well-regulated iron-metabolism. These properties limit the levels of redox-active, labile iron and the associated production of oxidizing free radicals. In normal cells the absence of ascorbate-mediated oxidative distress allows the reducing capabilities of ascorbate as an antioxidant to surface. Thus, inhibition of chemoradiation-mediated oxidative distress can manifest itself, thereby protecting normal tissue; i.e. a state of oxidative eustress. In contrast, decreased capacity of cancer cells to remove H₂O₂ as well as cancer-cell specific disruptions in iron metabolism result in increased levels of labile iron leading to significant oxidative distress and the selective sensitization of cancer cells to chemoradiation.

Figure 3.. Pharmacological ascorbate protects C57Bl/6NHsd mice from radiation-mediated skin toxicity.

(A) Utilizing a target irradiator developed in-house (See methods.), an average of 13.25 ± 0.005 Gy was delivered to the right lung field. Irradiation was unrestricted in the dorsal-ventral axis, allowing dorsal skin overlying the radiation field to be used to monitor skin toxicity. Ascorbate, or equivalent dose of NaCl, was administered IP for two days prior and for two weeks following radiation treatment. Mice were monitored on a binary scale (present/not present) for (B) achromotrichia and/or (C) alopecia. Each datum is representative of the percent of mice within that group that displayed the phenotype at that time-point (n = 6 mice per group).