RNA interference (RNAi) - also known as gene silencing - involves the use of double stranded RNA (dsRNA). Once inside the cell, this material is processed into short 21-26 nucleotide RNAs termed siRNAs that are used in a sequence-specific manner to recognise and destroy complementary RNA, effectively switching off the activity of the gene and halting production of the protein for which it codes.
The major use of RNAi reagents is in research but it partially overlaps that of drug discovery and therapeutic development.
Ireland's Research and Markets, which compiled the report, noted that the markets for RNAi are difficult to define as no RNAi-based product is in clinical development yet. It is estimated to be $300 million (€251m) currently and will increase to $400 million in 2005 and $850 million by the year 2010.
Meanwhile, the value of the drug discovery market based on RNAi can be assessed at $500 million currently with increase to $650 million in 2005 and further doubling to $1 billion in the year 2010. And even if only a handful of products get into the market by the year 2010, this market will expand to $3.5 billion, based on revenues from sales of RNAi-based drugs
The report compares RNAi with other antisense approaches using oligonucleotides, aptamers, ribozymes, peptide nucleic acid and locked nucleic acid.
Various RNAi technologies are described, along with design and methods of manufacture of siRNA reagents. These include chemical synthesis by in vitro transcription and use of plasmid or viral vectors. Other approaches to RNAi include DNA-directed RNAi (ddRNAi) that is used to produce dsRNA inside the cell, which is cleaved into siRNA by the action of Dicer, a specific type of RNAse III. MicroRNAs are derived by processing of short hairpins that can inhibit the mRNAs. Expressed interfering RNA (eiRNA) is used to express dsRNA intracellularly from DNA plasmids.
Delivery of therapeutics to the target tissues is a crucial consideration, and is one of the primary reasons why it has taken so long for antisense-based therapeutics to emerge as a realistic new product class. The report notes that siRNAs can be delivered to cells in culture by electroporation or by transfection using plasmid or viral vectors. Meanwhile, in vivo delivery of siRNAs can be carried out by injection into tissues or blood vessels or use of synthetic and viral vectors, according to R&M.
Because of its ability to silence any gene once the sequence is known, RNAi has been adopted as the research tool to discriminate gene function. After the genome of an organism is sequenced, RNAi can be designed to target every gene in the genome and target for specific phenotypes. Several methods of gene expression analysis are available and there is still need for sensitive methods of detection of gene expression as a baseline and measurement after gene silencing.
To this end. RNAi microarrays have been devised that can be tailored to meet the needs for high throughput screens for identifying appropriate RNAi probes, used to analyse gene function and identify new drug targets. And with the advent of vector-mediated siRNA delivery methods it is now possible to make transgenic animals that can silence gene expression stably, providing powerful models for exploring disease processes.
RNAi can be rationally designed to block the expression of any target gene, and this includes genes for which traditional small molecule inhibitors cannot be found. Areas of therapeutic applications include virus infections, cancer, genetic disorders and neurological diseases.
However, despite the exquisite selectivity of the technology, side effects can result from unintended interaction between an siRNA compound and an unrelated host gene. So if RNAi compounds are designed poorly, there is an increased chance for non-specific interaction with host genes that may cause adverse effects in the host.