272 - The Role of Ribonucleotide Reductase (RR) in Breast Cancer and the Therapeutic Potential of the RR Inhibitor Didox

Abstract

Therapeutic targeting of estrogen receptor function is a front line treatment in breast cancer. However, acquired endocrine resistance develops in the majority of hormone-responsive breast cancers and frequently involves activation of the PI3K/AKT pathway. We identified that AKT-overexpressing breast cancer cells upregulate RRM2 expression. RRM2 overexpressing MCF-7 cells exhibited altered ER status and increased EGFR, HER2, Bcl-2, and NF-kB activated proteins that were reversed with siRRM2 or using the RR inhibitor, Didox (3,4 – dihydroxybenzohydroxamic acid). We evaluated various breast cancer subtypes and found that all six Triple Negative Breast Cancer (TNBC) cell lines expressed high levels of RRM2. Although doxorubicin (DOXO) is an effective agent for the treatment of TNBC, its efficacy is limited by cardiotoxicity. The cardiotoxic effect of DOXO is due to formation of a free radical toxic metabolite. The RR inhibitor Didox has been shown to be a potent free radical scavenger and iron chelator that provides the capabilities to alleviate the cytotoxic profile of DOXO, especially the cardiotoxicity, as well as enhance DOXO efficacy. In this current study, MDA-MB-468 and MDA-MB-231 TNBC cells were treated with DOXO alone or DOXO plus Didox. We report that Didox worked synergistically with DOXO and reduced the DOXO IC50 values by more than half in each cell line. Utilizing MDA-MB-468 xenografts, we determined that treatment with DOXO alone provided about a 70% lower tumor volume and Didox alone yielded about 50% lower tumor volume as compared to vehicle treatment. Importantly, adding Didox in combination with DOXO significantly inhibited the tumor size and resulted in a 90% lower final tumor volume. One indicator of cardiotoxicity is increased heart weights. We found that the heart weight of DOXO treated alone was significantly increased and including Didox in the treatment prevented this increase. Collectively, these results identify that adding the RR inhibitor Didox in combination with DOXO may be used to treat TNBC by enhancing the tumor treatment as well as alleviating the cardiotoxic effects of DOXO.