New delivery technology paves way for disease therapies - RNA

New delivery technology paves way for disease therapies
May 26, 2005
Harvard College
A new way to administer therapeutic RNA molecules that efficiently guides them to cells throughout the body is being reported by researchers at the Harvard-affiliated CBR Institute for Biomedical Research and Harvard Medical School (HMS). The technique couples the homing ability of antibodies and an avid RNA-binding protein from sperm to deploy the RNAs in the bloodstream, where they can find and slip into their target cells.
The technique will speed the clinical application of RNA interference (RNAi), a recently discovered process of selective gene silencing that is expected to yield exquisitely focused therapies for many human diseases, including cancer and AIDS. Development of RNA therapeutics has been hung up on the practical matter of how to deliver these drugs to inaccessible organs and tissues, and in what way that cells will be able to absorb them.
"This method allows us to inject an RNA drug intravenously and get it throughout the body, and then to deliver it to certain cells very specifically and with very high efficiency," said Judy Lieberman, senior investigator at the CBR Institute for Biomedical Research and professor of pediatrics at HMS, and principal author of the study.
Lieberman and her colleagues used their new system to show for the first time that infusing small interfering RNAs (siRNAs) coupled to cancer-seeking antibodies slows the growth of tumors in mice while leaving the surrounding normal tissue untouched. The researchers also demonstrate that siRNAs targeted to immune system T cells infected with HIV, but not healthy T cells, block viral growth. Their work will appear May 22 online in Nature Biotechnology.
"We are developing reagents that are useful for targeting immune cells, erythroid cells, liver cells, and brain cells. The ability to target antibodies is pretty unlimited, and that's why it's so attractive, since we can think of whole classes of disease where the RNA interference approach might be useful," Lieberman said.
RNAi therapies are based on a natural cell process that controls the fundamental flow of information in cells: The genetic code contained in DNA is translated into messenger RNA, which then makes proteins. siRNAs silence genes by binding to messenger RNA and marking it for destruction. Because RNA, like DNA, is composed of repeating bases that define a unique pattern for each gene, siRNAs can be synthesized in the laboratory to match any gene sequence. This gives researchers the ability to come up with a therapeutic RNA that turns off just one disease-causing gene in an organism while leaving normal genes untouched.
To disburse these potentially powerful medicines, the researchers engineered human antibodies to carry their siRNA cargo by adding protamine, a protein long recognized for its prodigious DNA-carrying ability. Protamine abounds in sperm, where it functions to compress long DNA molecules into the tiny package that will eventually be delivered to the egg. The antibody-protamine fusions were first developed 10 years ago by Wayne Marasco, an HMS associate professor of medicine at the Dana-Farber Cancer Institute and a co-author on the paper. "We were using the antibodies to deliver DNA to cells for gene therapy, but we recognized early on that this protein could carry RNA as well as DNA. Sure enough, when we performed the siRNA experiments, it worked like a charm," Marasco said.
Compared with other delivery systems, the antibody-based method is safer, offers more flexibility, and delivers more active siRNA to cells, according to Lieberman, who was the first to successfully use RNAi in an animal, in 2003. In that study, delivery of the RNAi to mice required pumping the solution into the blood under high pressure, a procedure that is too dangerous to use in humans. The new technique does not carry such risk, and in fact, the researchers saw no signs of toxicity after infusion of their antibody-siRNA complex.
Because the siRNA binds protamine reversibly, antibodies and siRNAs can easily be mixed and matched to any disease situation. "It's possible to change the specificity of the antibody, or change the siRNA, or use a cocktail of siRNAs. For example, if a patient had a tumor and it mutated, you could quickly tailor the delivery to the new situation," Lieberman explained.