Strategies to ‘Hydrophobize’ Systemic siRNA Vectors and Selectively Inhibit mTORC2 in Breast Tumors Through RNA Interference
Werfel, Thomas Anthony
In theory, siRNAs can inhibit every known cancer-causing gene through sequence-specific RNA interference. However, almost twenty years after the discovery of RNA interference, the use of siRNAs as targeted molecular medicines remains challenging due to comprehensively poor pharmacokinetic properties of siRNA. Naked siRNA molecules are rapidly excreted through the urine and cannot inherently enter cells or access the cytosol through endosomal escape, resulting in limited bioavailability within tumor cells after systemic administration. Strategies to complex negatively-charged siRNA into cationic polyion complexes (polyplexes) have been effective for the treatment of diseases in the liver where polyplexes naturally biodistribute. But the same polyplexes have shown limited success in oncology due to rapid disassembly within the kidneys, off-target accumulation within the liver, and limited on-target accumulation within tumor tissue. Thus, a broader set of polyplex physicochemical parameters remain to be optimized in order to improve siRNA delivery to tumors after systemic administration. Here, we show that fine-tuning hydrophobic stabilizing forces of siRNA polyplexes, through altering either the polymer carrier or siRNA molecule, can simply and effectively improve siRNA bioavailability and accumulation within solid breast tumors. In both cases, increased polyplex stability through the optimization of core hydrophobicity decreased rapid renal clearance and led to appreciable increases in blood circulation, tumor accumulation, and intratumoral siRNA bioactivity. Our updated siRNA polyplex technology enabled the first selective, therapeutic silencing of mTORC2 in HER2-amplified breast tumors. Due to the prominent role of mTORC2 within the oncogenic PI3K-Akt-mTOR pathway, selective mTORC2 inhibition slowed tumor growth through the induction of cell death and cooperated with the HER2 receptor tyrosine kinase inhibitor, lapatinib, to kill HER2-amplified tumor cells and halt tumor growth. In sum, this work systematically elucidates the impact of core hydrophobicity on siRNA polyplex performance in vivo, illustrates the broad potential for therapeutically inhibiting currently ‘undruggable’ cancer-causing oncogenes, and highlights the specific therapeutic potential of selectively inhibiting mTORC2 as a tumor cell killing strategy in HER2-amplified breast cancers.