Parenteral ascorbate as a cancer therapeutic: a reassessment based on pharmacokinetics

Abstract

Significance: Ewan Cameron reported that ascorbate, given orally and intravenously at doses of up to 10 g/day, was effective in the treatment of cancer. Double-blind placebo-controlled clinical trials showed no survival advantage when the same doses of ascorbate were given orally, leading the medical and scientific communities to dismiss the use of ascorbate as a potential cancer treatment. However, the route of administration results in major differences in ascorbate bioavailability. Tissue and plasma concentrations are tightly controlled in response to oral administration, but this can be bypassed by intravenous administration. These data provide a plausible scientific rationale for the absence of a response to orally administered ascorbate in the Mayo clinic trials and indicate the need to reassess ascorbate as a cancer therapeutic.

Recent advances: High dose ascorbate is selectively cytotoxic to cancer cell lines through the generation of extracellular hydrogen peroxide (H2O2). Murine xenograft models confirm a growth inhibitory effect of pharmacological concentrations. The safety of intravenous ascorbate has been verified in encouraging pilot clinical studies.

Critical issues: Neither the selective toxicity of pharmacologic ascorbate against cancer cells nor the mechanism of H2O2-mediated cytotoxicity is fully understood. Despite promising preclinical data, the question of clinical efficacy remains.

Future directions: A full delineation of mechanism is of interest because it may indicate susceptible cancer types. Effects of pharmacologic ascorbate used in combination with standard treatments need to be defined. Most importantly, the clinical efficacy of ascorbate needs to be reassessed using proper dosing, route of administration, and controls.

FIG. 1.

Bioavailability of orally administered ascorbate is tightly controlled compared to intravenous administration. (A) Plasma ascorbate concentrations as a function of dose in men and women. Dotted line indicates average daily dose from intake of five servings of fruit and vegetables. [Reprinted by permission from Levine et al. (44, 45)]. (B) Plasma concentrations of ascorbate after oral and intravenous dosing in a single patient. Baseline plasma ascorbate values are represented by a dashed line. (Top) Plasma ascorbate concentrations (μM) after receiving 200 mg intravenous (•) or oral (○) ascorbate as a function of time. (Bottom) Plasma ascorbate concentrations (μM) after receiving 1250 mg intravenous (•) or oral (○) ascorbate as a function of time. [Reprinted by permission from Levine et al. (44)].

FIG. 2.

Millimolar plasma ascorbate concentrations are predicted following intravenous administration. Predicted plasma ascorbate concentrations as a function of time following infusion of variable doses of intravenous ascorbate. Pink line represents plasma concentration following oral administration of maximum dose of ascorbate (18 g/day). [Reprinted by permission from Padayatty et al. (60)]

FIG. 3.

Pharmacologic ascorbate concentrations are selectively cytotoxic to cancer cell lines in vitro. Effect of pharmacologic concentrations of ascorbate on oncogenic and normal cell lines. Cells were exposed to increasing concentrations of ascorbate (0–20 mM, pH 7) for 2 h, then washed and cultured for an additional 24–48 h in growth media. EC50 represents the effective concentration of ascorbate required to kill 50% of cells. EC50 was determined by either MTT or Alamar Blue viability assays. [Reprinted by permission from Chen et al. (19)].

FIG. 4.

Pharmacologic ascorbate generates H₂O₂ formation in a rat glioblastoma model. H₂O₂ formation in extracellular fluids in 9L tumor-bearing nude mice. Mice were treated with 4 g/kg of parenteral ascorbate by intraperitoneal injection and fluids were gathered from probes in the extracellular tumor tissue (square, ▪) or subcutaneous space (circle, •) at 30 min intervals. H₂O₂ concentrations were determined using fluorescence spectroscopy. [Reprinted by permission from Chen et al. (19)]. H₂O₂, hydrogen peroxide.

FIG. 5.

Pharmacologic ascorbate generates H₂O₂ in extracellular fluid leading to production of ROS. Proposed mechanism of ascorbate-mediated H₂O₂ formation in blood and extracellular fluid. After ingestion or injection, ascorbate is equally distributed between blood (left) and the extracellular fluid (right). In the extracellular fluid, the oxidation of ascorbate concomitantly forms the ascorbate radical and reduces a protein-centered metal (In this example, Fe³⁺ is reduced to Fe²⁺). Fe²⁺ then interacts with oxygen to form superoxide, which undergoes dismutation to H₂O₂. H₂O₂ then interacts with another transition metal to generate ROS, including the highly reactive hydroxyl radical, via Fenton chemistry. The generation of H₂O₂ is largely inhibited in blood by catalase and GSH peroxidase, both of which are primarily found in erythrocytes (dashed lines). [Reprinted by permission from Chen et al. (18)]. GSH, glutathione; ROS, reactive oxygen species.

FIG. 6.

Pharmacologic ascorbate decreaseses tumor growth in mice. Growth and weight of murine colon carcinoma HT 29 tumors after exposure to pharmacologic ascorbate. (a) Tumor growth of murine colon carcinoma in mice treated by daily intraperitoneal injection of 1000 mg/kg (dashed lines) or placebo (solid lines). (b) After 1 month of daily treatment with either placebo, 1000 mg/kg, 100 mg/kg or 15 mg/kg of intraperitoneal ascorbate, mice were sacrificed and the tumors were weighed. *Indicates statistical significance of p<0.05. [Reprinted by permission from Belin et al. (3)].

FIG. 7.

Pharmacologic ascorbate enhances the effect of gemcitabine in pancreatic cancer xenograft models. Comparison of gemcitabine and ascorbate treatments on PANC-1 tumor volumes in mice. Mice were intraperitoneally injected with either saline, 4 g/kg of pharmacologic ascorbate, 30 g/kg (A) or 60 g/kg (B) of gemcitabine, or both ascorbate and gemcitabine at the indicated concentrations every 4 days. **indicates a p value<0.002 using a paired two-tailed t-test. Reprinted by permission from Espey et al. (24). To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 8.

Pharmacologic ascorbate concentrations are attainable and safe in patients. (A) Plasma concentrations in patients given intravenous ascorbate over a period of 90 min. Infusion doses are (from top to bottom) 1.5, 0.9, 0.6, 0.4, 0.2, and 0.1 g/kg. The data points represent the mean value of plasma concentration from either five or six patients as a function of time. Peak plasma values>20 mM are transiently attainable in humans. [Reprinted by permission from Hoffer et al. (35)]. (B) Pre- and post-treatment tumor size in millimeters for metastatic pancreatic cancer patients treated with pharmacologic ascorbate (50, 75, or 100 g per infusion three times weekly) in combination with intravenous gemcitabine (1 g/m² once per week) and erlotinib (100 mg per day orally). Gemcitabine was administered for seven consecutive weeks; ascorbate and erlotinib were administered for 8 weeks [Reprinted by permission from Monti et al. (56)].