RNAi therapeutics start acting when a short piece of double-stranded RNA (somewhere between 20 and 25 or so nucleotides) enters a cell. In the cytoplasm, the RNA bumps into an enzyme called dicer, which acts like a samurai sword-wielding ninja, chopping the dsRNA into smaller pieces known as small interfering RNAs. These siRNA unzip into two strands. One of the strands gets picked up by a group of different proteins known as the RNA-induced silencing complex. The entire package hunts down strands of messenger RNA inside a cell that complement the contained siRNA. Once that complementary strand is found, a group of enzymes chop up the matching mRNA. Without that mRNA, the corresponding protein can’t get made. Since most diseases are the result of problematic proteins – either faulty construction of a necessary protein or too much of a good thing – ridding cells of certain proteins might lessen their consequences or, in the case of some infectious diseases, cure certain conditions altogether. Figure from Robinson, R. (2004) PLoS Biology 2, e28.
After Fire and Mello’s influential paper, basic researchers flocked to RNAi. It was enough to win the two researchers the 2006 Nobel Prize in Physiology or Medicine, an unheard of turnaround in a novel field.
But years before Fire and Mello accepted their prize in Stockholm, RNAi also had caught the notice of pharmaceutical companies. The ability to silence specific genes, thereby ridding the body of pesky disease-causing proteins, also could provide unprecedented gains for therapies. Though about two-thirds of pharmaceutical targets currently are considered undruggable – with no small molecule currently identified or no way to specifically hit a target without causing other unspecific and undesirable effects – RNAi could provide a way to home in on a desired target through its gene sequence, making more targets druggable.
The problem, Krieg says, is that delivering double-stranded RNA has proven incredibly tricky. RNAs that are too long provoke an interferon response that muddies any effect of the RNAs themselves – a holdover from the earlier days of evolution when double-stranded RNA automatically equaled a viral attack. RNAs that are too short might not be enough to prompt sufficient interference. Naked RNAs are vulnerable to degrading RNAses circulating in blood and tissues. Finding a way to coat RNAs of the right size and sequence now has become a field in itself.
“People thought, ‘Here we have this platform in which we’re going to identify genes for breast cancer or chronic obstructive pulmonary disease or Alzheimer’s, and then we’ll have these RNAi compounds that we can give to patients, and they’ll go where we want them to go and the patients will get better,’” Krieg says. “With more experience working with this, they realized that it’s not going to work that way.”
Delivery seems to have proven tricky for other big pharma companies as well, even those that are sticking with the technology. Over e-mail, Alan Sachs, the global head of exploratory and translational science at Merck, noted that the company had explored more than 300 different delivery technologies for a range of disease targets. But although Merck acquired RNAi biotech Sirna Therapeutics in 2006 for the astronomical sum of $1.15 billion, the company has yet to have any RNAi therapeutic candidates in clinical trials.
“Merck recognized from the outset that developing RNAi therapeutics would be a long-term investment and not a quick path to blockbuster drugs,” he says, adding that the company “is taking a careful and steady approach to RNAi.”