Role of labile iron in the toxicity of pharmacological ascorbate.

Pharmacological ascorbate has been shown to induce toxicity in a wide range of cancer cell lines. Pharmacological ascorbate in animal models has shown promise for use in cancer treatment. At pharmacological concentrations the oxidation of ascorbate produces a high flux of H 2 O 2 via the formation of ascorbate radical (Asc •- ). The rate of oxidation of ascorbate is principally a function of the level of catalytically active metals. Iron in cell culture media contributes significantly to the rate of H 2 O 2 generation. We hypothesized that increasing intracellular iron would enhance ascorbate-induced cytotoxicity and that iron chelators could modulate the catalytic efficiency with respect to ascorbate oxidation. Treatment of cells with the iron-chelators deferoxamine (DFO) or dipyridyl (DPD) in the presence of 2 mM ascorbate decreased the flux of H 2 O 2 generated by pharmacological ascorbate and reversed ascorbate-induced toxicity. Conversely, increasing the level of intracellular iron by preincubating cells with Fe-hydroxyquinoline (HQ) increased ascorbate toxicity and decreased clonogenic survival. These findings indicate that redox metal metals, e.g., Fe 3+ /Fe 2+ , have an important role in ascorbate-induced cytotoxicity. Approaches that increase catalytic iron could potentially enhance the cytotoxicity of pharmacological ascorbate in vivo . [ABSTRACT FROM AUTHOR]