N. Owusu-Brackett1, K. W. Evans1, A. Akcakanat1, E. Yuca1, E. Ileana Dumbrava1, F. Janku1, F. Meric-Bernstam1 1University Of Texas MD Anderson Cancer Center,Houston, TX, USA
Introduction: Phosphatase and tensin homologue (PTEN) negatively regulates the phosphatidylinositol 3-kinase (PI3K)/AKT/mTOR pathway. Triple negative breast cancers (TNBC) are often PTEN-deficient, making mTOR a compelling target. We evaluated the efficacy of catalytic mTOR inhibitor TAK228 alone and in combination with eribulin in triple negative breast cancers (TNBC) with and without PTEN loss.
Methods: We tested TAK228 in combination with eribulin in a panel of TNBC cell lines with cell proliferation assays. Western blot analysis was performed to assess PTEN status and PI3K pathway inhibition. In vivo, antitumor efficacy of TAK228 as a single agent as well as in combination with eribulin was evaluated.
Results: Five of eight triple negative breast cell lines were sensitive to TAK228, independent of PIK3CA/PTEN status. Western blotting demonstrated inhibition of mTORC1/2 signaling as demonstrated by decreased phosho-AKT, phospho-S6 and phospho-4EBP1. In vitro, TAK228 was synergistic with eribulin in all eight TNBC cell lines. The combination of TAK228 and eribulin did not enhance apoptosis but increased G2/M growth arrest (p<0.001). In vivo, TAK228 led to modest growth inhibition in TNBC derived xenografts with no tumor regression observed. In two TNBC PDXs with PTEN loss, one with intrinsic eribulin sensitivity, another eribulin resistance, TAK228 in combination with eribulin did not enhance in vivo efficacy. In a third PTEN-negative TNBC model, eribulin alone achieved disease stabilization, but the combination of TAK228 and eribulin led to significantly smaller tumor volumes compared to eribulin alone (p<0.001).
Conclusion: TAK228 enhances the antitumor efficacy of eribulin in TNBC models in vitro, and enhanced in vivo activity in selected models. Further study is needed to determine the potential of this combination, and optimal patient selection strategies.