Researchers believe that their findings could lead to improved treatments for gastrointestinal disorders such as inflammatory bowel disease.
The researchers - including members from the Massachusetts Institute of Technology (MIT) in Cambridge, and Brigham and Women's Hospital in Boston, MA - report their findings in the journal Gastroenterology.
The ability to block or enhance the production of target proteins inside cells holds promise as a treatment for many diseases.
The main advantage is speed. For example, people with GI disorders such as inflammatory bowel disease (IBD) must often endure treatments that involve holding an enema in the colon for hours while medication is absorbed naturally.
Jets from bursting bubbles propel RNA into cells
The new study focuses on using ultrasound to deliver nucleic acids such as RNA. Scientists are interested in using RNA to treat disease because small strands of RNA, called short interfering RNA (siRNA), can be used to silence specific genes.
- Estimates suggest that IBD affects between 1 million and 1.3 million people in the United States
- Lack of standard criteria means that it can sometimes be misdiagnosed
- IBD is found to be more common in developed countries.
RNA strands must enter cells to do their work. This presents a problem because they are big - much larger than the small-molecule drugs the team had tested before.
RNA also degrades rapidly in the GI tract, which contains an abundance of enzymes in the form of nucleases that quickly chop up any stray RNA.
Therefore, delivery via enema and relying on natural diffusion results in minimal RNA passing into cells.
Some groups have tried to overcome the degradation problem by packaging the RNA into particles, or making enzyme-resistant forms.
The researchers wanted to see whether their ultrasound method could work for RNA, and thus overcome the need to modify or encapsulate it.
As established in their previous work, the team showed that at low frequencies, ultrasound produces an effect called "transient cavitation," in which small bubbles form in the liquid inside the GI tract.
When the bubbles touch tissue they burst, emitting tiny jets that propel the agent into the cells.
Ultrasound reduced TNF alpha by up to 10 times
The researchers tested the method in mice with colitis, a form of IBD in which the colon is inflamed. They used siRNA, which blocks production of tumor necrosis factor alpha (TNF alpha), a cell-signaling protein that promotes inflammation.
The researchers delivered the unmodified RNA to the colon in a solution of water via enema. At the same time, they delivered a half-second burst of ultrasound. The mice were treated in this way for 6 days, with a range of ultrasound frequencies.
Compared with mice that only received RNA, the mice that also received ultrasound showed a seven- to 10-fold reduction in TNF alpha, and their inflammation was almost eliminated.
As expected, unmodified RNA showed no side effects, and there were no adverse effects from the ultrasound.
In another set of experiments, the team showed the method also works for messenger RNA (mRNA). These molecules are even bigger than siRNA and help transfer genetic code from DNA to make proteins.
In these tests, the researchers delivered a firefly gene for bioluminescence with the associated mRNA, and they found that colons of mice that also received ultrasound had "levels of bioluminescence 11-fold greater than colons of mice given the mRNA alone."
The team suggests that the method could be developed further to stimulate cells to produce enzymes or other proteins that are missing in diseases caused by enzyme deficiency.
"Having the ability to deliver large amounts of this material, which is not modified in any way, really opens up the possibility to have a positive impact on patients down the line."
Co-senior author Dr. Giovanni Traverso, Brigham and Women's Hospital