Tandem Delivery Method Increases Intracellular Availability of Cholesterol-Encapsulated siRNA

A recent paper described a new approach that improved the delivery of therapeutic small interfering RNA (siRNA) by over 500-fold and allowed greater than 90% reduction in target gene expression in mice and, for the first time, high levels of gene knockdown in nonhuman primates.

Effective delivery of small interfering (siRNA) to the cell nucleus has been a major obstacle in the development of drugs based on RNA interference. One of the first attempts to overcome this obstacle utilized intravenous injection of cholesterol-conjugated siRNA (chol-siRNA). Although studies in mice revealed target gene knockdown in the liver, delivery was relatively inefficient, requiring three daily injections of chol-siRNA particles to obtain measurable reduction in gene expression.

Investigators at Arrowhead Research Corporation (Madison, WI, USA) had previously described the development of a targetable, polymer-based siRNA delivery platform named dynamic polyconjugate (DPC) that enabled efficient siRNA delivery to liver hepatocytes after intravenous injection. In the current paper, which was published in the December 2012 issue of the journal Nucleic Acid Therapeutics, they reported combining this polymer with chol-siRNA.

Chol-siRNA was taken up preferentially by the liver but became encapsulated in membrane-bound globules called endosomes and was unable to reach the cells' DNA to exert its gene silencing effect. The investigators solved this problem by co-injecting the DPC polymer together with chol-siRNA particles. Once taken up by liver cells DPC broke open the endosomes and released the encapsulated siRNA. Knockdown of gene expression was shown to be absolutely dependent on the presence of hepatocyte-targeting ligand on the polymer, the cognate hepatocyte receptor, and the cholesterol moiety of the siRNA. The increase in efficacy was not dependent on interaction between the chol-siRNA and the polymer before injection or in the bloodstream prior to contact with the target cell.

Use of this tandem delivery platform resulted in an increase of more than 500-fold in the levels of available siRNA and allowed over 90% reduction in target gene expression in mice and, for the first time, high levels of gene knockdown in non-human primates.

These results have important implications for the design and manufacture of siRNA delivery systems for clinical use, while the simplicity of the formulation and efficacy of this mode of siRNA delivery should prove beneficial in the use of siRNA as a therapeutic agent.

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Arrowhead Research Corporation