We've all heard of the promise of RNA interference (RNAi) as a potential approach to treating disease, but how close are we to delivering on this promise? Writing in Nature, Vornlocher and colleagues now take us a step closer to the goal of selectively modifying gene activity for a clinical benefit.

Vornlocher and colleagues have demonstrated the in vivo silencing of an endogenous gene that encodes a protein of therapeutic relevance, apolipoprotein B (APOB), which is essential for the formation of low-density lipoprotein (LDL) and therefore implicated in a range of cardiovascular disorders. Although RNAi has been demonstrated in vivo before, previous models have either not targeted an endogenous gene or have been administered in a way that is not applicable to the human therapeutic setting. In this study, small interfering RNA (siRNA) molecules, which induce RNAi of genes with complementary sequences, were administered to mice by tail-vein injection, a route that can be readily translated to human patients.

Just as gene therapy and antisense approaches have been held back by problems of delivery, so too has RNAi. In the past, viral vectors have been used, among other delivery vehicles, to get siRNA to the target tissue. This study used chemical modifications to render the siRNA molecules more drug-like and stable within the body, and more likely to be taken up by cells. The conjugation of cholesterol to the 3′ end of siRNA molecules significantly improved their in vivo pharmacological properties, such as cellular delivery and half-life. Such improvements lend credence to the idea that further chemical modifications could improve the prospects of siRNA-based therapeutics.

To explore the in vivo effect of these ApoB-targeting, cholesterol-modified siRNAs, they were injected into mice that were fed a normal diet. Levels of the target mRNA were then measured in the liver and jejunum, key sites of APOB expression. The modified siRNA caused a significant reduction of ApoB mRNA in both tissues, which was reflected as a diminution of APOB protein levels. Desirable effects were seen at the physiological level too: siRNA treatment resulted in a 25% reduction in high-density lipoprotein levels, and a 40% reduction in LDL levels.

A key concern with studies of RNAi is that the observed results might be caused by nonspecific 'off-target' effects or the interferon response. Vornlocher and colleagues were able to eliminate this possibility, because two ApoB-specific siRNAs that target different regions of the ApoB mRNA resulted in similar effects. Furthermore, control siRNA, although present in the liver and jejunum at levels comparable to those of therapeutic siRNA, had no effect. The authors were also able to establish, through an analysis of APOB degradation products, that an RNAi mechanism of action was responsible for the experimental findings.

With the first demonstration of the in vivo silencing by siRNA of an endogenous gene through an RNAi mechanism — and using an administration route that could readily be applied to humans — in the bag, RNAi looks set to gain even further prominence as a research tool and potential therapeutic.