Abstract
Over 20 years have passed since siRNA was brought to the public's attention. Silencing genes with siRNA has been used for various purposes, from creating pest-resistant plants to treating human diseases. In the last six years, several siRNA therapies have been approved by the FDA, which solely target disease-inducing proteins in the liver. The extrahepatic utility of systemically delivered siRNA has been primarily limited to preclinical studies. While siRNA targeting the liver comprises relatively simple ligand-siRNA conjugates, siRNA treating extrahepatic diseases such as cancer often requires complex carriers. The complexity of these extrahepatic carriers of siRNA reduces the likelihood of their widespread clinical use. In the current report, we initially demonstrated that a linear histidine-lysine (HK) carrier of siRNA, injected intravenously, effectively silenced luciferase expressed by MDA-MB-435 tumors in a mouse model. This non-pegylated linear peptide carrier was easily synthesized compared to the complex cRGD-conjugated pegylated branched peptides our group used previously. Notably, the tumor-targeting component, KHHK, was embedded within the peptide, eliminating the need to conjugate the ligand to the carrier. Moreover, brief bath sonication significantly improved the in vitro and in vivo silencing of these HK siRNA polyplexes. Several other linear peptides containing the -KHHK- sequence were then screened with some carriers of siRNA, silencing 80% of the tumor luciferase marker. Additionally, silencing by these HK siRNA polyplexes was confirmed in a second tumor model. Not only was luciferase activity reduced, but these siRNA polyplexes also reduced the Raf-1 oncogene in the MDA-MB-231 xenografts. These simple-to-synthesize, effective, linear HK peptides are promising siRNA carriers for clinical use.