The promise, pitfalls and progress of RNA-interference-based antiviral therapy for respiratory viruses.

Advances in the understanding of RNA biological processing and control are leading to new concepts in human therapeutics with practical implications for many human diseases, including antiviral therapy of respiratory viruses. So-called 'non-coding RNA' exerts specific and profound functional control on regulation of protein production and indeed controls the expression of all genes through processes collectively known as RNA interference (RNAi). RNAi is a naturally occurring intracellular process that regulates gene expression through the silencing of specific messenger RNAs (mRNAs). Methods are being developed that allow the catalytic degradation of targeted mRNAs using specifically designed complementary small interfering RNAs (siRNA). siRNAs are now being chemically modified and packaged into advanced delivery systems so as to acquire drug-like properties and the ability to deliver their effects systemically. Recent in vivo studies have provided proofs of the concept that RNAi may be useful therapeutically. Much of the design of these siRNAs can be accomplished bioinformatically, thus potentially expediting drug discovery and opening new avenues of therapy for many uncommon, orphan, or emerging diseases. Theoretically, any disease that can be ameliorated through knockdown of any endogenous or exogenous protein is a potential therapeutic target for RNAi-based therapeutics. Lung diseases in general are attractive targets for RNAi therapeutics, since the location of affected cells increases their accessibility to topical administration of siRNA, and respiratory viral infections are particularly attractive targets for RNAi-based drug discovery and development. RNAi therapeutics have been shown to exert potent antiviral effects against respiratory syncytial virus (RSV), parainfluenza, influenza, coronaviruses, measles and human metapneumoviruses in vitro and in vivo. Recently, a double-blind placebo-controlled clinical trial of an RNAi-based therapeutic against RSV demonstrated that this technology has therapeutic activity, representing the first proof-of-concept test of efficacy for RNAi's therapeutic effect in humans. This review discusses the science behind RNAi and the potential practical issues in applying this technology to various respiratory viral diseases.