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Small RNAs: Delivering the future
Nathan Blow1
Abstract
Drugs to treat diseases from cancer to AIDS could soon rely on short strands of RNA for their effects. But scientists must first work out how to navigate these fragments around the body. Nathan Blow reports.
The remarkable ability of short sequences of synthetic RNA to interfere with messenger RNA and thereby silence the activity of specific genes has proved incredibly helpful to geneticists wrestling with genetic function. And the push to harness this RNA interference (RNAi) for therapeutic use is now beginning to make headway. In the six years since the first paper reporting RNAi gene silencing in mammals was published1, at least six therapeutic programmes based on the concept have moved into clinical trials.
"Progress in the field of RNAi therapeutics has occurred remarkably fast," says John Maraganore, president and chief executive of Alnylam Pharmaceuticals in Cambridge, Massachusetts. But delivering the sequences remains a problem. Initial clinical trials relied on 'local delivery', directly introducing short interfering RNAs (siRNAs) into the specific tissue they were to treat. But for true therapeutic value, the siRNAs need to be introduced systemically.
"Systemic delivery is the major issue right now," says Alan Sachs, vice-president for RNA therapeutics based at Sirna Therapeutics, a wholly owned subsidiary of Merck in San Francisco.
Getting a small RNA to interfere with the right messenger RNA in the correct tissue and cell type at a safe, therapeutic level by systemic administration requires an exquisite degree of control — creating the need for different delivery vehicles and potentially even specialized targeting strategies. Animal studies2 have shown that it is possible for siRNAs delivered systemically to silence target genes. "What we have learned over the past couple of years is that systemic delivery of RNAi can be achieved, and there are a variety of methods that can be used to achieve it," says Maraganore. But he is also quick to note that there is no simple solution.
"If you inject naked siRNA into the blood, under normal pressure, it doesn't work," says Daniel Anderson of the Center for Cancer Research at the Massachusetts Institute of Technology (MIT) in Cambridge. Yet naked siRNA delivered directly into the lungs to treat respiratory syncytial virus (RSV) can profoundly reduce viral replication. This contrast highlights the chasm between local and systemic delivery of synthetic siRNAs.
