Abstract 3833: Molecular mechanisms by which BAY 80-6946 induces apoptosis in breast tumor cells as single agent or in combination

Abstract

Abstract PI3K/PTEN/AKT pathway has been found frequently activated and mutated in human breast cancer, which contributes to the development and progression of breast cancer as well as drug resistance. As genetic alteration of PIK3CA and PTEN as well as PI3K pathway activation are observed in almost all breast cancer categories, it is important to define the strategy for the development of PI3K inhibitors in breast cancer. BAY 80-6946 is a highly potent and selective pan-class I PI3K inhibitor in PhI clinical development. Molecular profiling of BAY 80-6946 in 24 breast tumor cell lines indicated that tumor cells with either PIK3CA mutation and/or HER2 expression were extremely sensitive to BAY 80-6946 with an average IC50 value of 17 (n=7) and 19 nM (n=8), respectively. In contrast wild type PIK3CA and HER2- breast tumor cells (n=12) were relatively insensitive or resistant to BAY 80-6946 with an average IC50 value of 773 nM. With regard to apoptosis induction, BAY 80-6946 demonstrated superior activity against other PI3K pathway inhibitors tested in PIK3CA mutant breast tumor cells. We found that the expression of Bcl-2, but not Mcl-1 nor survivin determined the sensitivity to apoptosis. Tumor cells that lack Bcl-2 expression immediately underwent apoptosis after exposure to BAY 80-6946. On the other hand, cells expressing high level of Bcl-2 such as T47D (PIK3CAmut) were resistant to the apoptosis induced by BAY 80-6946, despite a potent anti-proliferative activity of BAY 80-6946 (single-digit nM IC50). Combining BAY 80-6946 with ABT737 sensitized T47D cells to apoptosis, further supporting our hypothesis. On the other hand, PTEN-null, EGFRhigh and Bcl2 negative MDA-MB-468 tumor cells, are resistant to apoptosis induction by BAY 80-6946. Using shRNA and inhibitor against mTOR (TORC1 or TORC2), we identified an mTOR-dependent, PI3K-independent phosphorylation of AKT at S473, which might cause tumor cells resistance to the PI3K inhibition by BAY 80-6946. Rictor knockdown or combination with rapamycin can sensitize MDA-MB-468 to BAY 80-6946 with the respect to inhibition of proliferation and induction of apoptosis. Interestingly, the dual PI3K/mTOR inhibitor BEZ-235 could not induce apoptosis in MDA-MB-468, suggesting that the ratio of PI3K vs mTOR inhibition might be important in apoptosis induction. In conclusion, BAY 80-6946 was extremely effective to induce apoptosis in breast cancer cells expressing Her2 and/or PIK3CA mutation in the absence of Bcl-2. BAY 80-6946 in combination with anti-Bcl agents or mTOR inhibitors might be the promising approach to achieve tumor responses in Bcl-2 positive tumors or EGFR expressing/PTEN-null breast tumors, respectively. These findings provide a rationale to develop personalized therapies for the treatment of molecular subtypes of breast cancer.