. Therapeutic silencing of an endogenous gene by systemic administration of modified siRNAs. Nature. 2004 Nov 11;432(7014):173-8. PubMed.

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  1. Many groups have adopted siRNA or shRNA technologies as the method of choice for gene knockdown since it was first demonstrated that these techniques work for mammalian cells. Although there are previous demonstrations that one can inject siRNA duplexes and evoke systemic effects, the paper by Soutschek et al. improves things further by increasing efficiency of delivery as well as duplex stability. For those of us who work on neurodegenerative diseases, the question now is, “Will it work in brain?” Perhaps not: most of the cholesterol in the brain is synthesized locally and relatively small amounts are taken up through the blood-brain barrier. However, it seems likely that the general concept may be useful. By using molecules that are permeable to the blood-brain barrier, one could imagine peripherally administered siRNA complexes being delivered to the brain quite efficiently. Given that the biodistribution of many natural and artificial molecules is known, one could even design compounds that act as postcodes for different organs. The challenge then would be to develop sufficiently efficient siRNA molecules that could be safely used at lower doses to target gene expression in vivo.

    View all comments by Mark Cookson
  2. Immediately following the description that RNA interference (RNAi) works in mammalian cells, it was recognized that such specific inhibitors of gene expression held tremendous potential as drugs (1). A wealth of papers describing the inhibition of disease-causing or disease-associated genes soon followed (reviewed in 2,3). As scientists sought to extend these successes to animal models, a predictable but nevertheless difficult obstacle arose: unmodified small interfering RNAs were not easily delivered to the relevant tissues in vivo. Even delivery to relatively accessible organs such as the liver required techniques not amenable to clinical application in humans. Now Soutschek et al. present evidence that small interfering RNA (siRNA), delivered via a clinically acceptable route (IV), can inhibit gene expression with predictable and physiologically relevant results in vivo (4). The large and frequent doses of siRNA required, coupled with the potentially confounding technique of administering a cholesterol-containing drug for a lipid disorder, make it unlikely that this gene-specific approach will replace conventional drugs for hypercholesterolemia in the near future. But the broader implication of this study is that siRNA can be delivered to cells in vivo with simple modifications of the RNA backbone and coupling of the siRNA to cholesterol. For diseases in tissues accessible to intravenous administration, siRNA therapeutics may soon be a reality.

    What does this study mean for brain diseases? Clearly the CNS is less accessible than the liver or gut. The blood-brain barrier would probably exclude siRNA delivered intravenously, though cholesterol (or other lipid) conjugation might improve uptake by neurons if injected into the cerebrospinal fluid or brain parenchyma. On the bright side, effective siRNAs against neurodegenerative disease genes such as APP, tau, BACE1 and SOD1, to name a few, have already been developed and tested in cell culture (5-7). Further, a notable in vivo success was recently achieved when a mouse model of spinocerebellar ataxia type 1 (in which neurodegeneration is caused by polyglutamine expansion) was treated by viral delivery of siRNA. In this model, viral delivery to a minority of Purkinje cells led to a therapeutic benefit (8). Current viral vectors must be directly injected into the brain and may prove to be inefficient for diseases with more widespread pathology such as Alzheimer disease, but this growing body of work shows that in vitro validated siRNAs can indeed work in vivo if delivered to the right neurons.

    Additional and complementary modes of delivery may be crucial to widespread application of RNAi-based therapy to brain diseases. More hope is offered this week by a report demonstrating that conjugation of the peptide Penetratin1 to siRNA results in efficient delivery to primary neurons in culture (9). If it proves generally true that siRNA delivery can be enhanced by the addition of carrier molecules, this may open the door to tissue-specific targeting of gene-specific drugs. What we currently do not know is whether siRNAs will prove useful for chronic, slowly progressive conditions such as neurodegenerative diseases in which treatment duration will need to be long-term. Nonetheless, the tools provided by these two new studies will allow rigorous testing of the efficacy and safety of siRNA in animal models. They constitute a significant step toward turning siRNAs into viable drugs.

    View all comments by Henry Paulson

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  1. A Step Toward Therapeutic RNA Interference