Background: Soft tissue sarcomas are a heterogeneous group of malignant tumors including more than 70 histological subtypes. We have recently demonstrated that dual targeting of PI3K/mTOR pathway is associated with significant anti-tumor activity in in vitro and in vivo models of leiomyosarcoma (LMS), one of the most frequent sarcoma subtype. Secondary resistance limits the efficacy of all targeted therapies. We have developed and characterized resistance models to a dual PI3K/mTOR inhibitor in order to describe underlying resistance mechanisms and develop alternative strategies.

Methods: To develop in vitro BEZ-235 (dual PI3K/mTOR inhibitor) resistance models, 3 LMS and 1 myxofibrosarcoma cell lines were exposed to increasing doses of BEZ-235 several months. To characterize the resistance models, sensitivity to BEZ-235 was assessed by MTT assay and Annexin V-PI staining. To test in vivo the resistance models, effects of BEZ-235 treatment were observed in mice with subcutaneous cell line-derived xenograft tumors for 3 weeks. The transcriptome profile of sensitive and resistant tumors to BEZ-235 was analyzed by using RNA-seq.

Results: After prolonged exposure of cell lines to BEZ-235, we obtained secondary resistant cell lines characterized by an IC50 value 5- to 21-fold higher than parental sensitive cells. Resistant cells were also significantly less sensitive to BEZ-235-induced apoptosis than parental cells. We confirmed the resistance patterns in vivo with a significantly higher anti-tumor effect of BEZ235 in sensitive cell-line derived xenograft group compared with resistant cell-line derived xenograft group. Deep transcriptome profiling indicated overexpression of stem cells related genes in resistant cells as well as a modification of the cellular metabolism. By using 1) RNAi-mediated suppression of pyruvate carboxylase (PC), a potential key player in our resistance models and 2) hallmarks of CSC like CD133 and enhanced aldehyde dehydrogenase (ALDH) activity, we confirmed that resistant tumors were characterized by a significantly higher proportion of cancer stem cells and by a metabolic shift with a strong deregulation of glucose metabolism.

Conclusions: Selection of CSC subpopulation and metabolism shift play a crucial role in secondary resistance to dual PI3K/mTOR inhibition in sarcoma. Strategies assessing anti-CSC agents to suppress acquired resistance may have major implications to improve efficacy of dual PI3K/mTOR targeting in human malignancies.

Citation Format: Benjamin Fourneaux, Aurélien Bourdon, Vanessa Chaire, Carlo Lucchesi, Marie Karanian, Raphael Pineau, Audrey Laroche, Antoine Italiano. Mechanisms of secondary resistance to dual PI3K/mTOR inhibition in sarcomas with complex genetics [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4106. doi:10.1158/1538-7445.AM2017-4106