Mutant p53 as a therapeutic target for the treatment of triple-negative breast cancer: Prelinical investigation with the anti-p53 drug, APR-246.

1082 Background: Despite intensive efforts, a validated targeted therapy for triple-negative breast cancer (TNBC) remains elusive. One of the most frequent genetic alterations identified in TNBC is mutation in the p53 gene, which is found in > 80% of samples. The aim of this study was therefore to investigate the potential value of the mutant p53 reactivating compound, APR-246 for the treatment of TNBC. Methods: Cell viability was determined using the MTT assay. p53 protein levels were determined using Western blot and ELISA while both p63 and p73 levels were measured by Western blotting. Results: Based on a panel of 23 breast cancer cell lines, significantly lower IC50 values were found in p53 mutant compared to p53 wild-type cells (p= 0.014). Furthermore, a significant inverse correlation was found between IC50 values and p53 protein levels (p= 0.0001, r= -0.76). We also related response with levels of the p53 family members, p63 and p73. A significant inverse correlation was found between p63 protein levels and IC50 values (p= 0.01, r= -0.55). In contrast, no correlation was seen between p73 protein level and IC50 values. In an attempt to enhance response, APR-246 was combined with different cytotoxic agents. Both docetaxel and carboplatin showed an additive effect in combination with APR-246. However, this additive effect was cell line-dependent, i.e., docetaxel plus APR-246 was additive in MDA-MB-453 cells (CI = 0.92) cells but not in MDA-MB-468 cells (CI = 1.7), while carboplatin plus APR-246 was additive in MDA-MB-468 cells (CI = 0.94) but not in MDA-MB-453 cells (CI = 1.2). Conclusions: Based on our finding, either the mutational status of the p53 gene or p53 protein levels may be used to predict response to APR-246. Clinical trials investigating this agent should incorporate either or both of these measurements as potential predictive biomarkers.