RNA interference is one of the most fascinating biological discoveries of the last few years, and there’s excitement that it could lead to a new class of powerful drugs which would be an absolutely specific treatment both for viral diseases and cancers. But these drugs, based on short lengths of RNA, need to be introduced into the target cell. A recent paper in Nature Biotechnology – Potent and persistent in vivo anti-HBV activity of chemically modified siRNAs by Morissey et al (subscription required) – suggests that encapsulating the RNA in a liposome can do the job.
In the normal process of gene expression, the genetic code for is transferred from the cell’s DNA, where the information is stored, to the ribosome where the corresponding protein is made in the form of a molecule of RNA – messenger RNA. It turns out that there’s a naturally occurring cellular process that destroys messenger RNA when it’s been marked with a short piece of RNA which binds to it. This RNA interference process was named Science Magazine’s breakthrough of the year in 2002 (needs free registration). These short interfering RNA molecules can thus be used to inactivate one individual gene. To quote from a January 2004 article by Richard Robinson in Public Library of Science: Biology – RNAi Therapeutics: How Likely, How Soon? – “The clinical applications appear endless: any gene whose expression contributes to disease is a potential target, from viral genes to oncogenes to genes responsible for heart disease, Alzheimer’s disease, diabetes, and more.”
But bits of free RNA floating around the body are soon identified and destroyed – after all, they are most likely to originate in viruses. And the highly charged RNA molecule can’t penetrate the lipid bilayer that separates a cell from its surroundings. To quote from the Robinson article again: “stability and delivery are also the major obstacles to successful RNAi therapy, obstacles that are intrinsic to the biochemical nature of RNA itself, as well as the body’s defenses against infection with foreign nucleotides.” The Nature Biotechnology article describes the work of scientists from a pharmaceutical company trying to bring this technology to the clinic – Sirna therapeutics. They have shown that by using a lipid-based nanoparticle delivery system they can get good results treating hepatitis B virus in an animals. The delivery system is essentially a liposome, a self-assembled hollow shell formed by a phospholipid sheet which has folded round on itself to form an enclosed surface, but I suspect there’s quite a lot of art to selecting the mixture of lipids to use. This includes charged lipids which probably bind to the RNA, lipids to promote uptake of the delivery device by the cell, and lipids bound to protective polyethylene glycol hairs to disguise the liposomes from the body’s defenses.