Delivering Nanoparticles to the Cell Nucleus
While a great deal of the potential for nanotechnology to improve cancer therapy lies with the ability of nanoparticles to deliver drug payloads directly to tumors, an equally important consideration is whether nanoparticles can then get their drug payload to their intended target inside tumor cells. Now, a team of investigators from the Northwestern University Center for Cancer Nanotechnology Excellence (Northwestern CCNE) has developed star-shaped nanoparticle that can deliver a drug directly to a cancer cell's nucleus—an important feature for many potential anticancer therapies.
The Northwestern CCNE team, led by Teri Odom, also reported that it was able to directly image at nanoscale dimensions how nanoparticles interact with a cancer cell's nucleus. Dr. Odom and her collaborators published their results in the journal ACS Nano.
"Our drug–loaded gold nanostars are tiny hitchhikers," said Dr. Odom. "They are attracted to a protein on the cancer cell's surface that conveniently shuttles the nanostars to the cell's nucleus. Then, on the nucleus' doorstep, the nanostars release the drug, which continues into the nucleus to do its work."
The nanoparticle is made of gold and shaped much like a star, with five to 10 points. The particle's large surface area allows the researchers to load a high concentration of drug molecules onto the nanostar. The drug used in the study is a short piece of single–stranded DNA, known as an aptamer, which like an antibody binds tightly to a specific molecular target. Approximately 1,000 of these aptamers are attached to each nanostar's surface.
The DNA aptamer serves two functions. First, it binds to nucleolin, a protein overexpressed in cancer cells and found both on the cell surface and within the cell nucleus. Then, when released from the nanostar, the DNA aptamer also acts as the drug itself.
Bound to the nucleolin, the drug–loaded gold nanostars take advantage of the protein's role as a shuttle within the cell and hitchhike their way to the cell nucleus. The researchers then direct ultrafast pulses of light -- similar to that used in LASIK surgery -- at the cells. The pulsed light cleaves the bond attachments between the gold surface and the DNA aptamers, which then can enter the nucleus.
In addition to allowing a large amount of drug to be loaded, the nanostar's shape also helps concentrate the light at the points, facilitating drug release in those areas. Drug release from nanoparticles is a difficult problem, Odom said, but with the gold nanostars the release occurs easily. That the gold nanostar can deliver the drug without needing to pass through the nuclear membrane means the nanoparticle is not required to be a certain size, offering design flexibility.
Using an electron microscope, Odom and her team found their drug–loaded nanoparticles dramatically change the shape of the cancer cell nucleus. What begins as a nice, smooth ellipsoid becomes an uneven shape with deep folds. They also discovered that this change in shape after drug release was connected to cells dying and the cell population becoming less viable -- both positive outcomes when dealing with cancer cells.
Since this initial research on human ovarian and cervical cancer cells, the researchers have gone on to study effects of the drug–loaded gold nanostars on 12 other human cancer cell lines. The effect was much the same. "All cancer cells seem to respond similarly," Odom said. "This suggests that the shuttling capabilities of the nucleolin protein for functionalized nanoparticles could be a general strategy for nuclear–targeted drug delivery."
Odom envisions the drug–delivery method, once optimized, could be particularly useful in cases where tumors are fairly close to the skin's surface, such as skin and some breast cancers. (The light source would be external to the body.) Surgeons removing cancerous tumors also might find the gold nanostars useful for eradicating any stray cancer cells in surrounding tissue
This work, which is detailed in a paper titled, "Direct observation of nanoparticle–cancer cell nucleus interactions," was supported in part by the NCI Alliance for Nanotechnology in Cancer, a comprehensive initiative designed to accelerate the application of nanotechnology to the prevention, diagnosis, and treatment of cancer. An abstract of this paper is available at the journal's website.