High-Dose Vitamin C: Preclinical Evidence for Tailoring Treatment in Cancer Patients

Abstract

High-dose vitamin C has been proposed as a potential therapeutic approach for patients with advanced tumors who failed previous treatment with chemotherapy. Due to vitamin C complex pharmacokinetics, only intravenous administration allows reaching sufficiently high plasma concentrations required for most of the antitumor effects observed in preclinical studies (>0.250 mM). Moreover, vitamin C entry into cells is tightly regulated by SVCT and GLUT transporters, and is cell type-dependent. Importantly, besides its well-recognized pro-oxidant effects, vitamin C modulates TET enzymes promoting DNA demethylation and acts as cofactor of HIF hydroxylases, whose activity is required for HIF-1α proteasomal degradation. Furthermore, at pharmacological concentrations lower than those required for its pro-oxidant activity (<1 mM), vitamin C in specific genetic contexts may alter the DNA damage response by increasing 5-hydroxymethylcytosine levels. These more recently described vitamin C mechanisms offer new treatment opportunities for tumors with specific molecular defects (e.g., HIF-1α over-expression or TET2, IDH1/2, and WT1 alterations). Moreover, vitamin C action at DNA levels may provide the rationale basis for combination therapies with PARP inhibitors and hypomethylating agents. This review outlines the pharmacokinetic and pharmacodynamic properties of vitamin C to be taken into account in designing clinical studies that evaluate its potential use as anticancer agent.

Keywords: 5-hydroxymethylcytosine; DNA damage; IDH1/2; PARP inhibitors; ROS; TET2; WT1; ascorbate; hypomethylating agents; vitamin C.

Figure 1

Flow chart of the inclusion and exclusion criteria applied for the selection of the articles cited in this review. * This number includes also articles providing background information that were retrieved by using specific PubMed queries.

Figure 2

Vitamin C has different chemical structures. At physiological pH, ascorbic acid loses a proton to form ascorbate, which can donate two electrons sequentially. Loss of the first electron (oxidation) generates ascorbate radical and the loss of the second electron produces dehydroascorbate. See text for further details.

Figure 3

Pro-oxidant effects of intravenously administered high-dose vitamin C. Intravenous administration (I.V.) of high-dose vitamin C (plasma ascorbate levels >1 mM) induces cytotoxic effects against cancer cells through different mechanisms of action. Ascorbate reacts with Fe³⁺ via Fenton reaction leading to the formation of Fe²⁺ that, by reacting with H₂O₂, produces highly damaging hydroxyl radicals. In the blood, H₂O₂ is eliminated by ROS scavenger systems [e.g., the glutathione peroxidase (GPx) and catalase] present on the erythrocyte membrane. In the extracellular matrix of the tumor microenvironment, hydroxyl radical accumulation can induce direct damage to cell membrane by lipid peroxidation; H₂O₂ can also enter into the cell by diffusion. Vitamin C enters into the cell by using different transporters: (1) via SVCT2 as ascorbate; (2) via GLUTs as DHA produced by ascorbate oxidation. Inside the cells, DHA is converted back to ascorbate decreasing GSH activity and NADPH. Tumor cells contain higher levels of labile iron (Fe²⁺) due to altered iron metabolism and high intracellular concentrations of ascorbate may favor the release of Fe²⁺ from ferritin. Tumors may also secrete in the extracellular matrix high amounts of ferritin from which Fe²⁺ can be released either directly by ascorbate or indirectly through the production of O_2. ROS accumulation induces ATP depletion and causes cancer cell death as a consequence of DNA damage, GAPDH inhibition and NAD+ depletion, metabolism alteration due to glycolysis blockade, tricarboxylic acid cycle (TCA) disruption and shift toward the pentose phosphate pathway (PPP).

Figure 4

Activity of high-dose vitamin C in tumors with TETs, IDH1/2 or WT1 altered pathways. TETs are a family of αKGDD enzymes involved in active DNA demethylation that catalyze the oxidation of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC). TET mutations (mostly TET2) result in nonfunctional forms of the enzyme leading to hypermethylation gene promoters. IDH1/2 enzymes catalyze the oxidative decarboxylation of isocitrate to α-ketoglutarate (αKG) that is required for the activity of multiple dioxygenases, including TETs. Gain-of-function mutations of IDH1/2 result in the overproduction of the oncometabolite 2-hydroxyglutarate (2-HG) that inhibits TET activity. WT1 interacts with TET2 and recruits it to the promoter of WT1-target genes stimulating their demethylation and expression. WT1 mutations hamper the ability of TET2 to bind to and induce transcriptional activation of WT1-target genes. High-dose vitamin C mimics TET demethylating activity and restores the normal DNA methylation pattern inhibiting tumor progression. See the text for further details.