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Alnylam focused on local delivery when it began to develop RNA-based therapeutics. Its treatment for RSV in the lungs uses an inhaled synthetic RNA that triggers the destruction of a protein essential for the virus's replication. Inhalation allows high local concentrations of the RNA to be achieved, says Maraganore, while also taking advantage of naturally occurring mechanisms, such as pinocytosis, for uptake into the target cells.
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Liposomes offer one way of achieving systemic delivery of siRNAs.
Work by researchers at the company has also helped shed light on systemic delivery. In a 2004 study2, they affirmed for the first time the potential 'drug-like' properties of siRNAs when delivered systemically. The team used a synthetic RNA conjugated to cholesterol and stabilized with a partial phosphorothioate backbone and 2'-O-methyl sugar modifications on both the sense and antisense strands of the RNA.
Since this study, both lipid- and polymer-based vehicles for systemic delivery of siRNAs have been developed and tested. At Polyplus-transfection in Illkirch, France, researchers have taken advantage of the difference between cationic polymers and cationic lipids for systemic delivery to different organs. "We are interested in delivery to the lung and have used systemic administration of the cationic polymer polyethylenimine for delivery," says Patrick Erbacher, the company's chief scientific officer. "But for tumour injections, we use either a cationic polymer or a cationic lipid formulation."
With siRNAs conjugated to lipids or encapsulated in liposomes or lipid nanoparticles, several companies have achieved stable and efficient systemic delivery to organs including the liver, pancreas, kidneys and even to some types of tumour. And polymers that can complex with siRNA can deliver the short sequences to organs such as the lungs, spleen and kidneys.
