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Nano RNA Delivery

Novel delivery agents could mean a more targeted way to turn off disease genes.
April 29, 2008

An experimental and potentially powerful way to fight disease, called RNA interference (RNAi), could now be closer to reality, as researchers at MIT and Alnylam, a biotech company based in Cambridge, MA, have addressed a key obstacle to effectively delivering the treatment to targeted cells. The researchers report a method for quickly synthesizing more than a thousand different lipid-like molecules and screening them for their ability to deliver short RNA molecules to cells. They’ve shown that some of these delivery agents are 10 times as effective at delivering RNA than previous methods were.

Playing the numbers: A new synthesis method made it possible to rapidly make more than a thousand new lipid-like molecules; a few of the vials containing the molecules are shown here. Some of the molecules proved to be effective at delivering new RNA-based treatments.

RNAi, which was first discovered in 1998, has attracted considerable attention as a potential treatment for a wide range of ailments, including cancer, viral infections, genetic diseases, and even heart attacks. Short RNA strands introduced into the cytoplasm of cells block the action of specific genes, while leaving other cellular mechanisms unaffected. This gives scientists a precise tool to stop the expression of specific proteins associated with disease. “You want to shut down just the bad gene–nothing else,” says Robert Langer, a professor of chemical engineering at MIT who led the work developing the new delivery agents. “Most drugs have side effects, in part because of a lack of this type of specificity.” Langer is a member of Alnylam’s scientific advisory board. The work was published this week in Nature Biotechnology.

But since 2001, when RNAi was first demonstrated in mammals, only six RNAi-based therapies have reached clinical trials, and none have yet been approved for use. One big thing holding back RNAi therapy, Langer says, is the lack of an effective delivery mechanism. If RNA is introduced into the bloodstream, the body quickly attacks the RNA and prevents it from reaching the cytoplasm of diseased cells. Progress on finding new delivery agents has been a slow and painstaking process.

The MIT researchers, however, developed a way to make more than a thousand different delivery agents in parallel using a simple, one-step chemical process. And that allowed the team to quickly discover effective delivery molecules, including several that surprised the researchers. “We wouldn’t have necessarily sat down and said, this is a structure that’s going to work,” says Daniel Anderson, a research associate at the David H. Koch Institute for Integrative Cancer Research at MIT. “It was only by making and testing over a thousand that we were able to get to that place.”

The researchers began with the observation that certain lipid molecules had properties that made them attractive for delivering RNA. Since these were difficult to synthesize, the team turned to amine-based molecules that were similar in many ways, but easier to make. By combining amino molecules with alkyl-akrylates and alkyl-acrylamides, the researchers were able to make more than a thousand different lipid-like molecules. These molecules have properties that cause them to assemble into nanoscale capsules called liposomes, which can encapsulate RNA molecules. The researchers then tested the ability of the liposomes to deliver RNA to the cytoplasm of cells by using a screening method they developed that involves blocking the bioluminescence in genetically engineered cells.

In tests on mice, the best of these delivery agents were 10 times as effective at delivering RNA to treat a respiratory ailment, compared with existing methods that deliver the RNA directly to the lungs without it being encapsulated. The researchers also demonstrated that the agents worked for delivering RNA through the bloodstream to the liver and various tissues. What’s more, initial tests in primates showed promising results. Perhaps most important, Anderson says, is the fact that the team can make several variants of a delivery agent, which could make it more likely that one of them will survive all the stages of clinical trials.

The researchers are working with Alnylam, which will be moving the delivery agents toward clinical trials. They’re also continuing to sort through new variants of the molecules to find ones that work better, and they’re testing the effectiveness of the delivery agents for targeting different diseases and for delivering different therapeutics in addition to RNA.

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