A company developing therapeutics using RNA interference (RNAi) today (February 29) announced positive results of a clinical trial in humans ? marking a first for the much-touted promise of RNAi-based therapies.

Alnylam, based in Cambridge, Mass., exposed 88 male volunteers to respiratory syncytial virus, which affects mostly young children and the elderly. Half of the subjects received the RNAi treatment, delivered as a nasal spray, which blocked a protein made by the virus, while the other half received a placebo. The upshot: 67% of the treatment group became infected with the virus, while 88% of the placebo group were infected, the Wall Street Journal reports; though small, the difference was statistically significant.

Researchers have placed high hopes on the potential for RNAi-based therapies, but have struggled with delivery strategies. "With the positive results from GEMINI, we believe that Alnylam has demonstrated the first ever human proof of concept with an RNAi therapeutic," said John Maraganore, the compan"?s CEO, in a press release. The results are not published, but further details of the study will be described this weekend at the International Symposium on Respiratory Viral Infections in Singapore.