Abstract 2706: Therapeutic silencing of Rictor using siRNA nanoparticles to selectively block mTORC2 signaling in triple negative breast cancer
Authors: Patel Shrusti S. , Hoogenboezem Ella N. , Yu Fang , Sorets Alex G. , Cherry Fiona K. , Lo Justin H. , Francini Nora , d'Arcy Richard A. , Cook Rebecca S. , Duvall Craig L. 
Published: April 2023

Triple negative breast cancers (TNBCs) are aggressive malignancies with a high rate of recurrence even after neoadjuvant chemotherapy (NAC) and resection. Nearly 50% of NAC-treated TNBC patients harboring residual disease display mutations in the phosphatidyl inositol-3 kinase/mechanistic target of rapamycin (PI3K/mTOR) pathway, correlating PI3K/mTOR with chemoresistance. The protein kinase mTOR functions within two distinct protein complexes—mTORC1 and mTORC2. Inhibition of mTORC1 is ineffective in TNBCs. However, less is known regarding mTORC2 as a TNBC drug target, though mTORC2 is known to regulate tumor cell survival and motility/metastasis in other cancers. Here, we leverage siRNA technology to block expression of the mTORC2 obligate cofactor, Rictor, to test the hypothesis that selective mTORC2 inhibition will improve TNBC response to chemotherapy.

siRNA-carrying nanoparticle complexes (si-NPs) continue to face systemic siRNA delivery challenges such as limited stability, off-target toxicities, and suboptimal tumor accumulation. Most conventional si-NPs are designed as “binary” systems comprising the siRNA cargo and the polymeric carrier that encapsulates it. Ternary si-NPs containing siRNA, an NP core-forming polymer (50B), and an NP surface-forming polymer (20kPEG-50B) have the potential to improve tumor silencing activity because of the participation of both polymers in siRNA encapsulation and endosome disruptive activity. Through concomitant structure-function optimization of 50B core polymer ratio (50B:20kPEG-50B) and molecular weight, we identified multiple lead ternary si-NPs with potent gene silencing activity, enhanced siRNA loading stability, and minimal toxicity. Following intravenous (i.v) treatment in vivo, all lead ternary si-NPs displayed increased circulation half-life, tumor uptake, and tumor gene silencing relative to the more canonical binary si-NPs. Critically, a single 1 mg/kg i.v. injection of our optimized formulation, 50B8-DP100 si-NPs, resulted in 80% tumor Rictor knockdown in orthotopic MDA-MB-231 tumors in mice.

Therapeutic efficacy of 50B8-DP100 si-NPs, harboring siRNA against Rictor (siRictor-NPs) was then tested in models of TNBC. siRictor-NPs decreased cell growth and enhanced apoptosis in multiple TNBC cell lines. In an orthotopic HCC70 TNBC model, i.v. treatment with siRictor-NPs revealed substantial inhibition of mTORC2 activity in tumors by western analysis. siRictor-NP treatment also inhibited tumor growth to 52% relative to siControl-NP treatment. Preliminary in vitro studies further indicate that combining chemotherapy with siRictor-NPs improves TNBC cell killing over chemotherapy alone. Together, this work provides strong evidence for the therapeutic potential of Rictor knockdown and identifies a novel nanotechnology for the treatment of PI3K-active TNBC.

Citation Format: Shrusti S. Patel, Ella N. Hoogenboezem, Fang Yu, Alex G. Sorets, Fiona K. Cherry, Justin H. Lo, Nora Francini, Richard A. d'Arcy, Rebecca S. Cook, Craig L. Duvall. Therapeutic silencing of Rictor using siRNA nanoparticles to selectively block mTORC2 signaling in triple negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2706.