349PPolo-like kinase 1 inhibitor onvansertib synergizes with paclitaxel in breast cancer carrying p53 mutation

Abstract Background Somatic mutation in TP53 gene (mutp53) is a strong prognostic marker in breast cancer. Triple negative breast cancer (TNBC) is characterized by up to 80% mutp53 and the greatest overall genomic instability among subtypes. Polo-like kinase 1 (PLK1) regulates progression of cells through the G2 phase of the cell cycle. We hypothesize that mutp53 in the context of breast cancer can predict synergy to paclitaxel [p] plus onvansertib [o], an orally available highly selective Plk1 inhibitor. Methods Drugs were tested in a set of mutp53 breast cancer cells including TNBC SUM149 and SUM159, luminal SUM52 and T47D, and wtp53 TNBC SUM1315, luminal MCF7, and normal breast MCF10A. We performed cell proliferation assays in order to determine the IC50 (Inhibition Concentration of 50%) for each drug. Synergism was determined using the Chou-Talalay method, by determining the combination index (CI). Apoptosis was determined by PARP cleavage proteins detection in cells synchronized with double thymidine block. For the in vivo experiments, mice were injected with 5x106 SUM159 cells and randomly assigned to experimental condition (control, o, p, o+p). The primary endpoint was tumor volume (TV) measured longitudinally. We performed one-sided two-samples Wilcoxon test comparing the means of the adjusted TV measured at 10 days for p and o+p. Results TNBC cells were more sensitive to single agent o compared to luminal cells. Breast cell lines with mutp53 (SUM149, SUM159, SUM52 and T47D) were found to have Combination Index (CI) Conclusions Onvansertib in combination with paclitaxel showed promising results in a subset of breast cancer cells and animal model characterized by the presence of mutp53. Our preclinical results confirm mutp53 as predictive biomarker of synergy and set the feasibility of performing a clinical trial with the combination o+p for mutp53 breast cancer patients. Legal entity responsible for the study The authors. Funding K12 Paul Calabresi funding, K12 CA157688. Disclosure M. Erlander: Leadership role, Chief Scientific Officer (CSO): Trovagene Oncology. All other authors have declared no conflicts of interest.