Inhibition of DNA Repair Pathways and Induction of ROS Are Potential Mechanisms of Action of the Small Molecule Inhibitor BOLD-100 in Breast Cancer

Simple Summary BOLD-100 is a novel ruthenium-based drug that has shown anti-tumor effects in several human cancers. In this study, we investigated the biochemical changes associated with BOLD-100 in human breast cancer cell models to better understand how this drug inhibits cancer cell growth. We found that treatment with BOLD-100 induced chemically unstable reactive oxygen species (ROS) and reduced proteins that help repair DNA in breast cancer cells. When combined with other drugs that target the DNA repair pathway, such as olaparib, BOLD-100 promoted more DNA damage and cell death compared with olaparib alone in estrogen receptor negative (ER-) breast cancer cells. ER- breast cancer is an aggressive form of breast cancer without any specific targeted therapy. Thus, our findings provide the rationale for a novel combination therapeutic strategy with BOLD-100 for ER- breast cancer subtype that could be investigated in clinical trials. Abstract BOLD-100, a ruthenium-based complex, sodium trans-[tetrachloridobis (1H-indazole) ruthenate (III)] (also known as IT-139, NKP1339 or KP1339), is a novel small molecule drug that demonstrated a manageable safety profile at the maximum tolerated dose and modest antitumor activity in a phase I clinical trial. BOLD-100 has been reported to inhibit the upregulation of the endoplasmic reticulum stress sensing protein GRP78. However, response to BOLD-100 varies in different cancer models and the precise mechanism of action in high-response versus low-response cancer cells remains unclear. In vitro studies have indicated that BOLD-100 induces cytostatic rather than cytotoxic effects as a monotherapy. To understand BOLD-100-mediated signaling mechanism in breast cancer cells, we used estrogen receptor positive (ER+) MCF7 breast cancer cells to obtain gene-metabolite integrated models. At 100 μM, BOLD-100 significantly reduced cell proliferation and expression of genes involved in the DNA repair pathway. BOLD-100 also induced reactive oxygen species (ROS) and phosphorylation of histone H2AX, gamma-H2AX (Ser139), suggesting disruption of proper DNA surveillance. In estrogen receptor negative (ER−) breast cancer cells, combination of BOLD-100 with a PARP inhibitor, olaparib, induced significant inhibition of cell growth and xenografts and increased gamma-H2AX. Thus, BOLD-100 is a novel DNA repair pathway targeting agent and can be used with other chemotherapies in ER− breast cancer.