Abstract 2709: Leukosome-based siRNA strategy for targeting ribosomal protein L39 (RPL39) in metaplastic breast cancer
Authors: Chervo Maria Florencia , Mancino Chiara , Giordano Federica , Reddy Tejaswini P. , Qian Wei , Zhou Jianying , Guzman-Rojas Liliana , Rosato Roberto R. , Taraballi Francesca , Chang Jenny C. 
Published: April 2023

Abstract
Metaplastic breast cancer (MpBC) is a rare and highly aggressive subset accounting for &lt;5% of all BCs. Clinically, MpBCs exhibit the most dismal prognosis of all BC subtypes, with a median survival rate of 8 months in patients with metastatic disease. The main therapeutic option for MpBC remains standard chemotherapy, despite known resistance to most cytotoxic drugs. We recently reported that ~40% of MpBC patient samples display a gain-of-function oncogenic mutation (A14V) in ribosomal protein RPL39, which is responsible for treatment resistance, cancer stem cell self-renewal, and lung metastases. We also showed that high RPL39 levels were associated with worse survival in patients with MpBC. Collectively, these findings support the rationale of targeting this ubiquitous genetic marker in MpBC. This study was aimed to design an innovative targeted therapy against RPL39 by combining a small interfering RNA (siRNA) strategy (siRPL39) with biomimetic lipid nanoparticle (LNP) technology for in vivo delivery to MpBC cells. These LNPs referred to as “leukosomes” incorporate membrane proteins from circulating leukocytes to facilitate evasion of immune clearance, enhance tropism towards inflamed endothelium such as the tumor-associated one, and improve siRNA internalization into the tumor mass. For leukosomes synthesis, we adapted a microfluidic device to incorporate membrane proteins from leukocytes within lipid bilayers. This technique retained all the physical and biological features of leukosomes, combined with high siRNA loading efficiency. We evaluated in vitro uptake and efficacy of siRPL39-loaded leukosomes in MpBC cell lines. We found that fluorescently labeled siRPL39 was rapidly internalized by leukosomes into MpBC cells demonstrating efficient and uniform uptake without inducing cytotoxicity. Treatment of MpBC cells with siRPL39-loaded leukosomes exhibited significantly knockdown of RPL39 mRNA, further supporting that the observed siRNA internalization results in potent and specific gene silencing. Notably, siRPL39-loaded leukosomes inhibited in vitro cell proliferation as compared with control siRNA leukosomes, indicating a key role of RPL39 in MpBC growth. Next, we sought to evaluate functional gene silencing of siRPL39-loaded leukosomes in vivo. We established MpBC patient-derived xenografts (PDXs) in the mammary fat pad of humanized NSG mice. We revealed that leukosomes formulation enables siRPL39 accumulation and functional gene silencing in MpBC tumors after local administration, without causing overt toxicity. Overall, our findings highlight RPL39 as a novel therapeutic target in MpBC and provide proof of principle for the development of leukosome-based siRNA drugs to improve delivery to cancer cells. Ongoing studies are directed to evaluate biodistribution, in vivo silencing and therapeutic effects of siRPL39-leukosomes after systemic administration.
Citation Format: Maria Florencia Chervo, Chiara Mancino, Federica Giordano, Tejaswini P. Reddy, Wei Qian, Jianying Zhou, Liliana Guzman-Rojas, Roberto R. Rosato, Francesca Taraballi, Jenny C. Chang. Leukosome-based siRNA strategy for targeting ribosomal protein L39 (RPL39) in metaplastic 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 2709.