Data from Selected Alkylating Agents Can Overcome Drug Tolerance of G<sub>0</sub>-like Tumor Cells and Eradicate BRCA1-Deficient Mammary Tumors in Mice

Abstract

<div>Abstract<p><b>Purpose:</b> We aimed to characterize and target drug-tolerant BRCA1-deficient tumor cells that cause residual disease and subsequent tumor relapse.</p><p><b>Experimental Design:</b> We studied responses to various mono- and bifunctional alkylating agents in a genetically engineered mouse model for <i>BRCA1/p53</i>-mutant breast cancer. Because of the large intragenic deletion of the <i>Brca1</i> gene, no restoration of BRCA1 function is possible, and therefore, no BRCA1-dependent acquired resistance occurs. To characterize the cell-cycle stage from which <i>Brca1</i><sup>−/−</sup><i>;p53</i><sup>−/−</sup> mammary tumors arise after cisplatin treatment, we introduced the fluorescent ubiquitination-based cell-cycle indicator (FUCCI) construct into the tumor cells.</p><p><b>Results:</b> Despite repeated sensitivity to the MTD of platinum drugs, the <i>Brca1</i>-mutated mammary tumors are not eradicated, not even by a frequent dosing schedule. We show that relapse comes from single-nucleated cells delaying entry into the S-phase. Such slowly cycling cells, which are present within the drug-naïve tumors, are enriched in tumor remnants. Using the FUCCI construct, we identified nonfluorescent G<sub>0</sub>-like cells as the population most tolerant to platinum drugs. Intriguingly, these cells are more sensitive to the DNA-crosslinking agent nimustine, resulting in an increased number of multinucleated cells that lack clonogenicity. This is consistent with our <i>in vivo</i> finding that the nimustine MTD, among several alkylating agents, is the most effective in eradicating <i>Brca1</i>-mutated mouse mammary tumors.</p><p><b>Conclusions:</b> Our data show that targeting G<sub>0</sub>-like cells is crucial for the eradication of BRCA1/p53–deficient tumor cells. This can be achieved with selected alkylating agents such as nimustine. <i>Clin Cancer Res; 23(22); 7020–33. ©2017 AACR</i>.</p></div>