Induction of Triple-Negative Breast Cancer Cell Death and Chemosensitivity Using mTORC2-Directed RNAi Nanomedicine.

ABSTRACT: Alterations in the PI3K/mTOR signaling pathway are often seen in triple-negative breast cancers (TNBC), a breast cancer subtype characterized by limited molecularly targeted treatment options and poorer patient outcomes. We report that gene amplification or overexpression of the mTORC2-required cofactor RICTOR correlated with increased mTORC2 signaling and worse patient outcomes in clinical breast cancer expression datasets, supporting studies examining selective mTORC2 inhibition in TNBC. The mTOR kinase inhibitor PP242 blocks both mTORC1 and mTORC2, which decreases growth and survival of RICTOR-amplified TNBC cells. However, mTORC1 inhibition by PP242 causes resurgent PI3K signaling, limiting its therapeutic impact. In contrast, knockdown using siRNA designed against Rictor (siRictor) did not increase PI3K or mTORC1 signaling but potently blocked mTORC2 signaling, resulting in robust inhibition of tumor cell growth and survival. We developed siRictor-loaded nanoparticles to enable therapeutic testing of RICTOR silencing in TNBCs in vivo. Intravenous delivery of this mTORC2-selective nanomedicine achieved siRNA accumulation and on-target gene silencing in TNBC tumors in vivo. RICTOR silencing blocked tumor mTORC2 signaling and growth in multiple TNBC mouse models while also improving TNBC tumor response to chemotherapy. These findings support the further development of technologies for therapeutic RICTOR silencing as an effective approach for mTORC2-selective inhibition and treatment in TNBC. SIGNIFICANCE: We identified an mTORC2/Rictor-directed RNAi nanomedicine that cooperates with chemotherapy to enhance in vivo tumor cell killing in PI3K-active TNBCs.