Targeting mutant p53 with COTI-2: A new approach for the treatment of patients with triple-negative breast cancer?

e13121 Background: The identification of a targeted therapy for patients with triple-negative breast cancer (TNBC) is one of the most urgent needs in breast cancer therapeutics. Since the p53 gene is mutated in approximately 80% of TNBC, it is an attractive therapeutic target for this subset of breast cancer. COTI-2 is a new, orally available, small molecule that acts by reactivating mutant. The compound is currently undergoing phase I trials in patients with gynecological and H&N cancers. The aim of this study was to investigate COTI-2 as a new treatment for TNBC. Methods: Cell viability was determined by MTT assay. p53 protein levels were quantified by ELISA. P53 binding was measured by surface plasmon resonance (SPR). Combination index (CI) values were calculated using Calcusyn software. Results: Using a panel of 18 breast cell lines, TNBC cells were significantly more responsive to COTI-2 than non-TNBC cells (p =0.04). In contrast, response was independent of cell line ER or HER2 status. Significantly lower IC50 values were found in p53 mutant compared to p53 wild-type cells (p =0.001). Additionally, a significant inverse correlation was found between IC50 values and p53 protein levels (p= 0.035, r= -0.51), ie, the higher the endogenous p53 protein level, the more sensitive the cell line was to COTI-2. By staining with antibodies specific for correctly folded WT p53 (PAb1620) or unfolded mutant p53 (PAb240), we showed that COTI-2 bound to and induced refolding of mutant p53. Further evidence of COTI-2 binding to mutant p53 was obtained using SPR. To enhance response, COTI-2 was combined with several cytotoxic agents. Highly synergistic growth inhibition, i.e. CI < 1, was found when COTI-2 was combined with doxorubicin in 6 different cell lines. In addition, docetaxel plus COTI-2 was synergistic in 4/6 cell lines, carboplatin plus COTI-2 was synergistic in 3/6, while cisplatin plus COTI-2 was synergistic in 2/6 cell lines. Conclusions: We conclude that targeting mutant p53 with COTI-2 is a potential approach for treating p53-mutated TNBC and should now be investigated in a clinical trial. In selecting such patients for treatment, mutant p53 or high levels of p53 protein should be used as predictive biomarkers.