Enhancing the antiproliferative effect of topoisomerase II inhibitors using a polypeptide inhibitor of c-Myc

Topoisomerase II inhibitors are widely used in cancer chemotherapy. However, their use is limited by severe adverse effects to normal tissues, including cardiotoxicity. One approach to reduce the cytotoxicity in normal tissues may be to sensitize cancer cells to the toxicity of these agents, allowing them to be administered in a lower and safer dose. A hallmark of many types of cancer is overexpression of c-Myc, and a molecule which targets c-Myc will affect the cancer cells more significantly than the normal tissues. This report demonstrates that pretreatment of cells with a polypeptide, which inhibits c-Myc transcriptional function causes cells to be more susceptible to the topoisomerase II inhibitors doxorubicin and etoposide. Inhibition of c-Myc and Max dimerization by this polypeptide leads to as much as a 2-fold reduction in the doxorubicin and etoposide IC50 in three different cell lines tested. Furthermore, the c-Myc inhibitor affects the cell cycle distribution of MCF-7 breast cancer cells by enhancing the G0/G1 accumulation induced by doxorubicin and etoposide. We have shown that this effect is not due to enhanced drug accumulation or inhibited drug efflux. Rather, it is likely due to the transcriptional consequences of c-Myc inhibition, specifically reduction in the levels of the polyamine synthesizing enzyme ornithine decarboxylase. In summary, our results suggest that polypeptides, which inhibit c-Myc transcriptional function, may prove to be a useful tool in combination therapy with topoisomerase II inhibiting drugs.