Susceptibility of cancer cells to β-lapachone is enhanced by ionizing radiation

To reveal the interaction between β-lapachone (β-lap) and ionizing radiation (IR) in causing clonogenic death in cancer cells and to elucidate the potential usefulness of β-lap treatment in combination with radiotherapy of cancer. FSaII tumor cells of C3H mice were used. The cytotoxicity of β-lap alone or in combination with IR in vitro was determined using clonogenic survival assay method. The IR-induced changes in the expression and the enzymatic activity of NAD(P)H:quinone oxidoreductase (NQO1), a mediator of β-lap cytotoxicity, were elucidated and the relationship between the NQO1 level and the sensitivity of cells to β-lap was investigated. The combined effect of IR and β-lap to suppress tumor growth was studied using FSaII tumors grown subcutaneously in the thigh of C3H mice. β-Lap caused clonogenic death of FSaII tumor cells in vitro in a dose- and time-dependent manner. When cells were treated first with β-lap and then exposed to IR in vitro, the resultant cell death was only additive. On the contrary, exposing cells to IR at 2.5 Gy first and then treating the cells with β-lap killed the cells in a synergistic manner. Importantly, the 2.5 Gy cells were sensitive to β-lap as long as 10 h after irradiation, which was long after the sublethal radiation damage was repaired. Irradiation of FSaII cells in vitro with 2.5 Gy significantly increased the expression and enzymatic activity of NQO1. The growth delay of FSaII tumors caused by an intraperitoneal injection of β-lap in combination with 20 Gy irradiation of tumor was significantly greater than that caused by β-lap or 20 Gy irradiation alone. The sensitivity of cells to β-lap is dependent on NQO1 activity. IR caused a long-lasting increase in NQO1 activity in cancer cells, thereby sensitizing cells to β-lap and treatment of experimental mouse tumors with IR and β-lap suppressed tumor growth in a synergistic manner. The combination of β-lap and radiotherapy is a potentially effective regimen for the treatment of human cancer.