Noting that RNA interference and its potential to shut offany gene in the body has been a focus of scientists since 1998, the team setout to find a safe and effective way to deliver short strands of RNA that canbind with and destroy messenger RNA, which carry instructions from the nucleus.The study resulting from these efforts appears in the Oct. 9 issue of Nature Biotechnology.
The study describes the merging of in-vivo RNA interference with recent insight into monocyte biology,opening a new translational avenue to approach the many diseases driven byrecruitment of these cells," says Daniel Anderson, associate professor in theHarvard-MIT Division of Health Sciences and Technology.
Anderson was joined by MIT Prof. Robert Langer, as well asscientists from Massachusetts General Hospital, Harvard Medical School, Brighamand Women's Hospital, Alnylam Pharmaceuticals, the Harrison School of Pharmacyand Seoul National University in South Korea.
In the study, the researchers described how they deliveredshort strands of RNA packaged in a layer of lipidoids, or fat-like molecules. Inflammatorymonocytes depend on the chemokine receptor CCR2 for distribution to injuredtissue and stimulate disease progression, the researchers note in the study.Monocytes are recruited by a molecule called MCP1 that's released at injurysites. MCP1 binds to a protein on the surface of monocytes called the CCR2receptor, stimulating the cells to travel to the injury site and launchinflammation.
Precise therapeutic targeting of this inflammatory monocytesubset could spare innate immunity's essential functions for maintenance ofhomeostasis—thus limiting unwanted effects, according to the MIT team.
"Most clinically approved drugs may have molecularspecificity, but broadly target cells in tissues they distribute to, includingcells that should be spared to avoid unwanted side effects. For example,steroids have been tested in patients with myocardial infarction withdisastrous outcomes, and their side effects (fluid retention, diabetes) makethem an unlikely candidate for treating cardiovascular disease," they pointout. "Other potent immunosuppressive drugs such as methotrexate, used fortreatment of rheumatoid arthritis, also lack cellular specificity. Therefore,while these drugs suppress inflammation, they also diminish protectivefunctions of the immune system which are involved in the resolution ofinflammation, wound healing and defense against infection.
In general, delivery of drugs by nanoparticles can increasetheir concentration at the site of action, which may partially explain theadvantage in efficiency when we compared siCCR2 treatment with small-moleculeCCR2 inhibitors."
For this study, the researchers designed an RNA sequencethat blocks the gene for the CCR2 receptor. Without that receptor, themonocytes do not respond to MCP1, so in theory, the treatment should block muchof the inflammatory response.
What they discovered when testing the RNA nanoparticles inmice with atherosclerosis and cancer is that inflammation was greatlyreduced—in fact, even tumors grew more slowly in the treated mice. In addition,they also found reduced inflammation when they treated mice that had recentlyhad a heart attack.
Specifically, the treatment attenuated their number inatherosclerotic plaques, reduced infarct size following coronary arteryocclusion, prolonged normoglycemia in diabetic mice after pancreatic islettransplantation and resulted in reduced tumor volumes and lower numbers oftumor-associated macrophages. Taken together, the siRNA nanoparticlemediatedCCR2 gene silencing in leukocytes selectively modulates functions of innateimmune cell subtypes and may allow for the development of specificanti-inflammatory therapy, the researchers wrote in their paper.
The researchers note that their approach is distinct fromprevious work on gene silencing in leukocytes.
"To our knowledge, this is the first demonstration of siRNAdelivery to the inflammatory Ly-6C high monocyte subset," they wrote in thepaper.
Anderson says that with these findings, the team has "a lotof steps now." The team is now developing manufacturing techniques that couldconsistently yield large numbers of identical particles, which would benecessary for potential clinical trials.
"One of the things we are interested in is translating thisstuff to people. We're interested in delivering RNA to tumors and to the liver,and reengineering the chemistry of these particles to make them more effectiveand safer to hit the cells," he says.
Anderson notes that the researchers have a long workingrelationship with Alnylam Pharmaceuticals as a commercial partner.
