Manganoporphyrins Increase Ascorbate-Induced Cytotoxicity by Enhancing H2O2 Generation

Abstract

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 O₂ 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^•−] 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^•− 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. ©2013 AACR.