The discovery of RNA interference (RNAi) in mammalian cells (Fire, et al. Nature 391:806-811 (1998)) has allowed for the development of short interfering RNA (siRNA) therapeutics (Elbashir, et al. Nature 411:494-8 (2001)), which have the potential to treat a wide variety of human diseases, including viral infections and cancer, through genetic modulation. Theoretically, siRNA can be used to alter the expression of nearly any gene in the body through the silencing of complementary messenger RNA. Such precise genetic control offers a broad therapeutic potential that is typically not attainable using conventional small molecule drugs. siRNA delivery vehicles must negotiate a number of obstacles in vivo prior to delivering their payload to target cells. In addition to escorting therapeutic cargo through the bloodstream and extracellular matrix, delivery vehicles must mediate siRNA transport across the cellular membrane of the target cell as well as to facilitate endosomal escape prior to lysosomal digestion (Akinc, et al. J. Gene. Med. 7:657-63 (2005)). It is only once these barriers have been breached that siRNA can interact with the RNAi machinery within the cytoplasm and trigger the gene silencing process (Whitehead, et al. Nature Rev. Drug Discov. 8:129-38 (2009)).
A select number of delivery systems have previously been reported to deliver siRNA for the treatment of a variety of disease targets in vivo, including hypercholesterolemia (Frank-Kamenetsky, et al. Proc. Natl. Acad. Sci. USA 107:1864-9 (2010); Love, et al. Proc. Natl. Acad. Sci. USA 26:431-42 (2008)), liver cirrhosis (Sato, et al. Nature Biotechnol. 26:431-42 (2008)), Ebola virus (Geisbert, et al. Lancet 375:1896-1905 (2010)), and cancer (Huang, et al. Proc. Natl. Acad. Sci. USA 106:3426-30 (2009)). Unfortunately, RNAi success in vivo has not consistently translated to success in the clinic. Because siRNA must be dosed repeatedly to achieve therapeutic effect, ideal delivery vehicles will offer a substantial therapeutic window in order to ensure the broadest clinical application. Although some materials have been identified that allow for potent gene silencing at siRNA doses as low as 0.01 mg/kg (Love, et al. Proc. Natl. Acad. Sci. USA 107:1864-9 (2010)), their clinical potential has been limited due to a lack of delivery vehicle degradability. There exists a continuing need for non-toxic, biodegradable, biocompatible lipids that can be used to transfect nucleic acids and other therapeutic agents. Such lipids would have several uses, including the delivery of siRNA.