The study of RNA interference and its therapeutic application are moving so fast I have almost given up trying to keep on top of it. But we should all pay more attention, because it appears that inhaled siRNA (short interfering RNA) may be an excellent rapid-response technology for disease outbreak situations.
(Update 7 Feb 06. Here are a couple of readable primers on siRNA from Answers.com and Wikipedia.)
Previous work has shown inhibition of flu viruses in cell culture and in mice using siRNA. The experiments in cell culture are nice for demonstrating basic biology, and the work in mice demonstrates physiological effectiveness, but both are a long way from showing anything safely works in humans. This is particularly true of the work in mice, because compounds are often delivered via a massive injection into a tail vein, which you just can't do in humans.
A paper in the January 2005 edition of Nature Medicine (PubMed) demonstrated inhibition of two negative stranded RNA viruses via nasal administration of siRNA in mice. Still in mice, but the authors are confident that the results show the technique is amenable to use with nasal inhalers in humans. Yes, you will probably sneeze immediately, but you will also probably be protected from the flu.
Bitko, et al., start off by noting that:
Viral infection of the respiratory tract is the most common cause of infantile hospitalization in the developed world with an estimated 91,000 annual admissions in the US at a cost of $300 million. [Respiratory syncytial virus (RSV)] and [parainfluenza virus (PIV)] are two major agents of respiratory illness; together, they infect the upper and lower respiratory tracts, leading to croup, pneumonia and bronchiolitis. RSV alone infects essentially all children within the first two years of life and is also a significant cause of morbidity and mortality in the elderly. Infants experiencing RSV bronchiolitis are more likely to develop wheezing and asthma later in life. Research towards effective treatment and a vaccine against RSV has been ongoing for nearly four decades with few successes. Currently, no vaccine is clinically approved for either RSV or PIV. Strains of both viruses also exist for nonhuman animals, causing loss to agriculture and the dairy and meat industries.
In short, respiratory viruses cause considerable disease and death, even in Western countries, and have an economic impact upwards of half a billion dollars per year in the US. Probably considerably more, if you start adding up productivity losses in the workplace.
After demonstrating previously that siRNA is efficient an antiviral in cell culture, Bitko, et al., set out to test the technology in mice. Here is where it gets really interesting: They used a free, web based algorithm from the Whitehead Institute to choose the sequences for their RNAs (follow the "about" link to see an explanation of design guidelines). The rules for designing siRNA are still being worked out, as evidenced by the fact that the best sequence experimentally was, "not suggested by several design engines, including those of Whitehead and Dharmacon, indicating the need for experimental verification of siRNAs." Not so surprising, given the novelty of the technology. But I'm fascinated by the fact that anyone could choose a sequence they want to inhibit and have access to design tools. And the tools will get much better, probably fairly quickly.
Following the design stage, RNA was chemically synthesized, presumably by Dharmacon, who produced RNA for the earlier paper. Rather than listing absolutely all the details of this work, here are the highlights:
- The RNA was administered nasally, with and without being complexed with transfection reagent TransIT-TKO.
- Mice were challenged with virus 4 hours later and then monitored.
- siRNA reduced viral titer by as much as 99.98%. The authors estimate that the transfection reagent free siRNA "was 70-80% as effective as siRNA complexed with TransIT-TKO."
- Mixed siRNA was effective against challenge with multiple viruses.
- siRNA was effective as a treatment after infection; "[Mice] receiving siRNA at subsequent days (day 2-4) showed gradually less and less protection, although we still observed substantial improvement of weight."
The authors sum up:
The principal finding of this paper is that appropriately designed siRNAs, applied intranasally, offer protection from respiratory infection, as well as providing considerable therapeutic value when administered after infection. We suggest that siRNAs, delivered by small particle aerosols in a simple hand-held inhaler, might prevent or cure pulmonary infections in humans.
I'll continue to post on this topic as I learn more.