Selected Alkylating Agents Can Overcome Drug Tolerance of G-like Tumor Cells and Eradicate BRCA1-Deficient Mammary Tumors in Mice.

We aimed to characterize and target drug-tolerant BRCA1-deficient tumor cells that cause residual disease and subsequent tumor relapse. We studied responses to various mono- and bifunctional alkylating agents in a genetically engineered mouse model for -mutant breast cancer. Because of the large intragenic deletion of the gene, no restoration of BRCA1 function is possible, and therefore, no BRCA1-dependent acquired resistance occurs. To characterize the cell-cycle stage from which mammary tumors arise after cisplatin treatment, we introduced the fluorescent ubiquitination-based cell-cycle indicator (FUCCI) construct into the tumor cells. Despite repeated sensitivity to the MTD of platinum drugs, the -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-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 finding that the nimustine MTD, among several alkylating agents, is the most effective in eradicating -mutated mouse mammary tumors. Our data show that targeting G-like cells is crucial for the eradication of BRCA1/p53-deficient tumor cells. This can be achieved with selected alkylating agents such as nimustine. .