Nanoparticles Provide Optimal Gene Silencing, Potential for Liver Disease Treatment

Chemical engineers, inspired by tiny particles that carry cholesterol through the body, have designed nanoparticles that can deliver bits of genetic material that turn off disease-causing genes. This new application, known as RNA interference (RNAi), has the potential for treating cancer and other disorders. However, delivering enough RNA to treat the diseased tissue, while avoiding side effects in the rest of the body, has been complicated.

The new particles, developed at the Massachusetts Institute of Technology (MIT; Cambridge, MA, USA), which encase short strands of RNA within a sphere of proteins and fatty molecules, silence target genes in the liver more efficiently than any earlier delivery system, the researchers found in a study of mice. “What we’re excited about is how it only takes a very small amount of RNA to cause gene knockdown in the whole liver. The effect is specific to the liver—we get no effect in other tissues where you don’t want it,” stated Dr. Daniel Anderson, an associate professor of chemical engineering and a member of MIT’s Koch Institute for Integrative Cancer Research.

Dr. Anderson is senior author of an article describing the particles in the Proceedings of the National Academy of Sciences of the United States of America (PNAS) the week of February 10, 2014. Dr. Robert Langer, a Koch Institute professor at MIT, is also an author. The team of scientists, which included those from Alnylam Pharmaceuticals (Cambridge, MA, USA) additionally discovered that the nanoparticles could effectively silence genes in nonhuman primates. The technology has been licensed to a company for commercial development.

RNA interference is a naturally occurring process that scientists have been trying to manipulate since its discovery in 1998. Tiny pieces of RNA, called short interfering RNA (siRNA), switch off specific genes inside living cells by destroying the messenger RNA molecules that carry DNA’s instructions to the remainder of the cell.

Scientists hope this approach could offer new treatments for diseases caused by single mutations, such as cancer or Huntington’s disease, by blocking mutated genes that encourage cancerous behavior. However, developing RNAi therapies has proven challenging because it is difficult to deliver large quantities of siRNA to the right location without causing side effects in other tissues or organs.

Drs. Anderson and Langer, in earlier research, demonstrated they could block multiple genes with small doses of siRNA by wrapping the RNA in fatlike molecules called lipidoids. In their latest research, the researchers tried to improve upon these particles, making them more efficient, more selective, and safer, according to Yizhou Dong, a postdoc at the Koch Institute and lead author of the study. “We really wanted to develop materials for clinical use in the future,” he says. “That’s our ultimate goal for the material to achieve.”

The design stimulus for the new particles came from the natural world—in particular, small particles known as lipoproteins, which transport cholesterol and other fatty molecules throughout the body. Similar to lipoprotein nanoparticles, the MIT investigators’ new lipopeptide particles are spheres whose outer membranes are composed of long chains with a fatty lipid tail that faces into the particle. In the new particles, the head of the chain, which faces outward, is an amino acid. Strands of siRNA are carried inside the sphere, surrounded by more lipopeptide molecules. Molecules of cholesterol embedded in the membrane and an outer coating of the polymer PEG (polyethylene glycol) help to stabilize the structure.

The researchers adjusted the particles’ chemical properties, which determine their behavior, by varying the amino acids included in the particles. There are 21 amino acids found in multicellular organisms; the researchers created about 60 lipopeptide particles, each containing a different amino acid linked with one of three chemical groups: an aldehyde, an acrylate, or an epoxide. These groups also contribute to the particles’ behavior.

The researchers then assessed the particles’ ability to block the gene for a blood clotting protein known as factor VII, which is generated in the liver by cells called hepatocytes. Gauging factor VII levels in the bloodstream reveals how effective the siRNA silencing is. In that first screen, the most efficient particle contained the amino acid lysine linked to an epoxide, so the researchers created an additional 43 nanoparticles similar to that one, for further testing. The best of these compounds, known as cKK-E12, achieved gene silencing five times more efficiently than that achieved with any previous siRNA delivery vehicle.

In a separate experiment, the researchers delivered siRNA to block a tumor suppressor gene that is expressed in all body tissues. They found that siRNA delivery was very specific to the liver, which should minimize the risk of off-target side effects. “That’s important because we don’t want the material to silence all the targets in the human body,” Dr. Dong remarked. “If we want to treat patients with liver disease, we only want to silence targets in the liver, not other cell types.”

In nonhuman primate testing, the researchers revealed that the particles could effectively silence a gene called TTR (transthyretin), which has been implicated in diseases including senile systemic amyloidosis, familial amyloid polyneuropathy, and familial amyloid cardiomyopathy.

The MIT team is now trying to determine about how the particles behave and what occurs to them once they are injected, to further enhance the particles’ performance. They are also working on nanoparticles that target organs other than the liver, which is more problematic because the liver is a natural destination for foreign substances filtered out of the blood.

Related Links:
Massachusetts Institute of Technology
Alnylam Pharmaceuticals