In vivo activation of PEGylated long circulating lipid nanoparticle to achieve efficient siRNA delivery and target gene knock down in solid tumors

We developed a lipid nanoparticle formulation (LNPK15) to deliver siRNA to a tumor for target gene knock down. LNPK15 is highly PEGylated with 3.3% 1,2-distearoyl-sn-glycero-3-phosphatidylethanolamine-N-(polyethylene glycol-2000) (PEG-DSPE) and shows a long duration: the half-lives of siRNA in LNPK15 were 15.2 and 27.0 h in mice and monkeys, respectively. Although LNPK15 encapsulating KRAS-targeting siRNA (LNPK15/KRAS) had very weak KRAS gene knock down activity in MIA PaCa-2 cells in vitro, LNPK15/KRAS showed a strong anti-tumor efficacy in MIA PaCa-2 tumor xenograft mice after intravenous administration at 5 mg/kg twice weekly. KRAS mRNA and protein knock down was observed in tumor tissue, suggesting on-target antitumor efficacy. In order to elucidate the in vitro-in vivo discrepancy, we performed ex vivo knock down assay using serum samples obtained after intravenous administration of LNPK15/KRAS to mice and monkeys. The collected samples were added to MIA PaCa-2 cells, and KRAS gene knock down was evaluated after a 24-h incubation period. The knock down efficacy was weak (approximate to 20%) with serum samples at initial sampling point (2 h), and it became much stronger (similar to 90%) with serum samples at later time points. Lipid composition of LNPK15 in the serum samples was also investigated. Among the five lipids incorporated in LNPK15, PEG-DSPE was degraded more rapidly than siRNA and the other lipids in both mice and monkeys. In vitro lipase treatment of LNPK15/KRAS also hydrolyzed PEG-DSPE and enhanced knock down activity. From these results, it was concluded that LNPK15 acquires increased knock down activity after undergoing PEG-DSPE hydrolysis in vivo, and that is the key mechanism to achieve both long circulation and potent knock down efficiency. We also proposed an in vitro assay system using lipase for quality control of LNP to ensure biological activity.