Manganoporphyrins Increase Ascorbate-Induced Cytotoxicity by Enhancing H2O2 Generation

Renewed interest in using pharmacological ascorbate (AscH(-)) to treat cancer has prompted interest in leveraging its cytotoxic mechanism of action. A central feature of AscH(-) action in cancer cells is its ability to act as an electron donor to O-2 for generating H2O2. We hypothesized that catalytic manganoporphyrins (MnP) would increase AscH(-) oxidation rates, thereby increasing H2O2 fluxes and cytotoxicity. Three different MnPs were tested (MnTBAP, MnT2EPyP, and MnT4MPyP), exhibiting a range of physicochemical and thermodynamic properties. Of the MnPs tested, MnT4MPyP exerted the greatest effect on increasing the rate of AscH(-) oxidation as determined by the concentration of ascorbate radical [Asc(center dot-)] and the rate of oxygen consumption. At concentrations that had minimal effects alone, combining MnPs and AscH(-) synergized to decrease clonogenic survival in human pancreatic cancer cells. This cytotoxic effect was reversed by catalase, but not superoxide dismutase, consistent with a mechanism mediated by H2O2. MnPs increased steady-state concentrations of Asc(center dot-) upon ex vivo addition to whole blood obtained either from mice infused with AscH(-) or patients treated with pharmacologic AscH(-). Finally, tumor growth in vivo was inhibited more effectively by combining MnT4MPyP with AscH(-). We concluded that MnPs increase the rate of oxidation of AscH(-) to leverage H2O2 flux and ascorbate-induced cytotoxicity. Cancer Res; 73(16); 5232-41. (C) 2013 AACR.