Background: mTOR is an atypical Ser/Thr kinase involved in regulating major cellular functions, such as nutrients sensing, growth, proliferation and angiogenesis. Current FDA approved clinical mTOR inhibitors only inhibit mTORC1 and have several limitations. First, inhibiting mTORC1 releases the negative feedback of PI3K-AKT signaling and therefore activates upstream AKT signaling. Secondly, allosteric inhibitors of rapalogs block the phosphorylation of some but not all mTORC1 downstream substrates. In particular phosphorylation of 4EBP1 is largely insensitive to rapalogs. In order to address these issues, “second generation” ATP competitive catalytic inhibitors including TAK228 against mTORC1/C2 have also been developed and are now in active in various stages of clinical trials.

Purpose: Resistance of breast cancers (BC) to targeted hormone receptor inhibitors often occurs through deregulation of the PI3K-AKT-mTOR pathway. Mutations or loss of function of upstream regulators such as TSC1/2, LKB1, or components of the PI3K pathway such as PIK3CA, AKT, or PTEN have been reported in most types of human tumors including BC. Here, we investigated the effects of TAK228 alone or in conjunction with letrozole in ER+ BC cell lines.

Methodology: Anti-proliferative, apoptotic, cell cycle, and intracellular signaling effects of TAK228 alone and in combination with letrozole were evaluated in ER +/PIK3CA mutated and ER+/PTEN mutated BC cell lines.

Results: 1) TAK228 caused a strong differential growth inhibition in ER+/PIK3CA or ER+/PTEN mutated or ER+ (WT of PI3K pathway molecules) BC cell lines by 3D-ON-TOP clonogenic assay and real-time monitoring in an IncuCyte Zoom. Inhibition is greater when TAK228 was combined with letrozole. 2) Administration of TAK228 induced cell cycle G0/G1 arrest and resulted in increased apoptosis in a dose-dependent manner. 3) qRT-PCR data showed that pro-apoptotic transcript, PUMA and cell survival transcripts SURVIVIN & STATHMIN mRNA expressions were significantly increased and decreased respectively following the treatment of TAK228. 4) CYCLIN D1 mRNA expression was significantly decreased in both 24 & 48 hrs following the treatment of TAK228. 5) TAK228 is mechanistically distinct from everolimus by a more potent and comprehensive inhibition of the PI3K-AKT-mTOR signaling network: a) avoided S6K inhibition-mediated feedback activation of PI3K-AKT and b) abrogated 4EBP1 phosphorylation.

Conclusion: Data showed that TAK228 effectively inhibited proliferation and enhanced apoptosis via inhibition of the PI3K-AKT-mTORC1/C2 signaling pathway and dual TORC1/2 inhibition also mitigated the feedback activation of AKT caused by selective inhibition of mTORC1, and because of this, TAK228 may yield greater therapeutic benefit in BC patients.

Citation Format: Pradip De, Jennifer H. Carlson, Tyler Jepperson, Casey Williams, Nandini Dey, Brian Leyland Jones. TAK228, a kinase inhibitor of mTORC1 and mTORC2, is highly effective in ER+ breast cancer model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3452.