siRNA design principles

RNA interference is a natural process that cells use to switch genes off. A small interfering RNA, or siRNA, is a short double stranded RNA of around twenty one base pairs. One strand, the guide, is loaded into a protein complex called RISC. RISC then finds messenger RNA that matches the guide and cuts it, silencing the gene.

Because RISC recycles, a single loaded guide strand can destroy many copies of its target message. This catalytic action is part of why siRNA drugs can be dosed as rarely as twice a year.

Naked RNA is fragile. Therapeutic siRNAs are extensively modified with two prime O methyl and two prime fluoro sugars and phosphorothioate linkages at the ends, tuned so the duplex resists nucleases while still being accepted by RISC. The modification pattern is as important to potency as the sequence itself.

The breakthrough that made siRNA drugs practical was conjugation to GalNAc, a sugar that binds a receptor found almost exclusively on liver cells. A GalNAc tagged siRNA is taken up efficiently by the liver after a simple injection under the skin, concentrating the drug where it is needed. Extending this kind of targeted delivery beyond the liver is one of the field's central challenges.