Project Summary/Abstract Metastatic breast cancer (MBC) is a deadly disease and novel therapeutic approaches are urgently needed. The proposal aims to test and characterize a novel therapeutic strategy for selectively damaging breast cancer cells with a genetic abnormality in the tumor suppressor p53 gene that commonly occurs in MBC. Genetic alteration in p53 drives cancer development and speeds metastatic progression. We found that p53-mutant cancer cells accumulate DNA damage in response to treatment with nucleotide analogues in part due to dysregulation in the DNA repair process. We further discovered that poly(ADP-ribose) polymerase (PARP) inhibitors selectively amplify DNA damage and increase toxicity induced by nucleotide analogues in p53- mutant cancer cells. We developed a novel methodology evaluating this response by flow cytometry. The proposed novel combination strategy was further validated in animal MBC models. Thus, unlike all prior approaches that depend upon mutant p53 as a target, our novel strategy exploits vulnerability of the identified dysregulation of DNA repair in p53-mutant cancer cells. We hypothesize that a combination therapy of nucleotide analogues with PARP inhibitors will selectively eliminate p53-mutant MBC. The proposal will examine in clinically relevant models of MBC a combination of two drugs that have never been tested together. We will assess the contribution of various hot-spot mutants of p53 in the observed response. Successful completion of the project will support a Phase I clinical trial testing the novel drug combination. This research has a strong potential to transform breast cancer treatment and significantly reduce the mortality associated with metastatic disease.