Data from Manganoporphyrins Increase Ascorbate-Induced Cytotoxicity by Enhancing H<sub>2</sub>O<sub>2</sub> Generation

Abstract

<div>Abstract<p>Renewed interest in using pharmacological ascorbate (AscH<sup>−</sup>) to treat cancer has prompted interest in leveraging its cytotoxic mechanism of action. A central feature of AscH<sup>−</sup> action in cancer cells is its ability to act as an electron donor to O<sub>2</sub> for generating H<sub>2</sub>O<sub>2</sub>. We hypothesized that catalytic manganoporphyrins (MnP) would increase AscH<sup>−</sup> oxidation rates, thereby increasing H<sub>2</sub>O<sub>2</sub> 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<sup>−</sup> oxidation as determined by the concentration of ascorbate radical [Asc<sup>•−</sup>] and the rate of oxygen consumption. At concentrations that had minimal effects alone, combining MnPs and AscH<sup>−</sup> 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 H<sub>2</sub>O<sub>2</sub>. MnPs increased steady-state concentrations of Asc<sup>•−</sup> upon <i>ex vivo</i> addition to whole blood obtained either from mice infused with AscH<sup>−</sup> or patients treated with pharmacologic AscH<sup>−</sup>. Finally, tumor growth <i>in vivo</i> was inhibited more effectively by combining MnT4MPyP with AscH<sup>−</sup>. We concluded that MnPs increase the rate of oxidation of AscH<sup>−</sup> to leverage H<sub>2</sub>O<sub>2</sub> flux and ascorbate-induced cytotoxicity. <i>Cancer Res; 73(16); 5232–41. ©2013 AACR</i>.</p></div>