Stable nucleic acid lipid particles (SNALPs) are microscopic particles approximately 120 nanometers in diameter, smaller than the wavelengths of visible light. They have been used to deliver siRNAs therapeutically to mammals in vivo. In SNALPs the siRNA is surrounded by a lipid bilayer containing a mixture of cationic and fusogenic lipids, coated with diffusible polyethylene glycol.[1]
For purposes of research or treatment, the involvement of pattern recognition receptors of the innate immune system (specifically Toll-like receptors) is an inconvenience that greatly reduces the stability of the siRNAs and the required dose of SNALPs. SiRNAs delivered by SNALPs are subject to endosomal trafficking, potentially exposing them to TLR3 and TLR7, and can lead to activation of interferons and proinflammatory cytokines. Methods of preventing this include:
We were able to confer complete protection with either a pool of siRNAs encapsulated in SNALPs or individual SNALP siRNAs, depending on their relative potency ... [the most potent siRNA] ... conferred absolute protection, that is 100 percent survival, and also contributed to complete aviremia in the infected guinea pigs. So there was no detectable Ebola virus even though the animals had been inoculated with essentially 30,000 times the lethal infectious dose for the virus.
In May 2010, an application of SNALPs to the Ebola Zaire virus made headlines, as the preparation was able to cure rhesus macaques when administered shortly after their exposure to a lethal dose of the virus, which can be up to 90% lethal to humans in sporadic outbreaks in Africa. The treatment used for rhesus macaques consisted of three siRNAs (staggered duplexes of RNA), targeting three viral genes:
L polymerase:
m m m
5’-GUACGAAGCUGUAUAUAAAUU-3’
3’-AACAUGCUUCGACAUAUAUUU-5’
m m
VP24:
m m
5’-UCCUCGACACGAAUGCAAAGU-3’
3’-CUAGGAGCUGUGCUUACGUUU
m m m
VP35:
m m m
5’-GCAACUCAUUGGACAUCAUUC-3’
3’-AUCGUUGAGUAACCUGUAGUA-5’
m m m
where "m" marks positions modified by 2'-O-methylation. The SNALPs (around 81 nm in size here) were formulated by spontaneous vesiculation from a mixture of cholesterol, dipalmitoyl phosphatidylcholine, 3-N-[(ω-methoxy poly(ethylene glycol)2000)carbamoyl]-1,2-dimyrestyloxypropylamine, and cationic 1,2-dilinoleyloxy-3-N,N-dimethylaminopropane.[5]