RNA interference (RNAi) is making great strides as an indispensable strategy for target-specific knockdown of gene expression. Emerging second-generation technologies represent advances in RNAi design, efficiency, and efficacy. Yet, daunting obstacles remain, such as how to deliver RNAi compounds to the right targets in the right amounts.
Two recent meetings, Select Biosciences’ “RNAi World Congress” and CHI’s “Beyond Genome,” highlighted new and innovative strategies for overcoming delivery hurdles. Advances include utilizing tiny delivery vehicles called nanoparticles, replacing traditional ideas about RNAi compound size, and even drawing on the new field of epigenetics for changing DNA expression without altering the gene sequence.
Successful delivery of RNAi faces a number of challenges, reported Tod Woolf, Ph.D., president and CEO RXi Pharmaceuticals (www.rxipharma.com). “Delivery of RNA oligos is a complicated situation but also fascinating. The first issue is one of exposure. That is, if the RNAi compound is to be injected, for instance, will the tissue targeted be exposed to the RNAi? If the RNA is injected and delivered intraperitoneally or intravenously it goes to the blood, where it is taken up by the kidneys and removed. for the most part, standard injections don’t have a prayer of turning off genes with standard RNAi compounds.”
Exposure issues also depend on the type of tissue targeted. “You can consider local delivery, such as injecting RNAi into the eye for treating age-related macular degeneration. It also can work with injections directly into the central nervous system or with routes such as inhalation or even topical.”
A second hurdle is cytoplasmic uptake. “Overcoming exposure problems doesn’t mean the RNA can get into the cytoplasm of cells,” Dr. Woolf explained. “This is a separate issue, as is the stability of the RNAi compound itself. RNA needs to be carefully designed and chemically stabilized.”
Nanotransporters used for RNA delivery can potentially solve these problems, he reported. “Our nanotransporters are chemicals of defined size that are mixed with an RNAi compound to form minute particles for delivery into target tissues. The nanotransporter has a core to which layers are added by chemical synthesis. The final layer has a positive charge allowing it to attract and bind negatively charged RNAi compounds.”
RXi has successfully utilized its nanotransporters for delivery into mouse liver as a treatment for amyotrophic lateral sclerosis (ALS). “We have been able to knockdown the gene for superoxide dismutase1 (SOD1) in a mouse model of ALS. This molecule normally neutralizes oxygen molecules that damage cells. Although ALS can be associated with more than 100 different mutations in SOD1, we found that RNAi-mediated knockdown of SOD1 had a benefit in an animal model of ALS.”