August 28, 2006
Nanoparticles Successfully Deliver Protein-Suppressing RNA-Based Therapies
Small pieces of nucleic acid, known as siRNAs (short interfering RNAs), can turn off the production of specific proteins, a property that makes them one of the more promising new classes of anticancer drugs in development. Two reports in the journal Clinical Cancer Research now show that nanoparticles may be the vehicle of choice for delivering these agents into tumor cells.
"Short interfering RNA is a great technology we can use to silence genes, shutting down production of harmful proteins," said Anil Sood, M.D., of the University of Texas M. D. Anderson Cancer, who together with colleague Gabriel Lopez-Berestein, M.D., led one of the two studies. "It works well in the lab, but the question has been how to get it into tumors." Short pieces of RNA don't make it to a tumor without being injected directly, and injection methods used in the lab are not practical for clinical use.
As their therapeutic agent, the investigators used an siRNA that targets a protein known as focal adhesion kinase (FAK) that helps ovarian cancer cells survive and spread. They packaged this siRNA inside a nanoscale liposome. Getting the siRNA inside tumor cells is important, Sood said, because the targeted protein, FAK, is inside the cell, rather than on the cell surface where most proteins targeted by cancer drugs are found. "Targets like FAK, which are difficult to target with a drug, can be attacked with this liposomal siRNA approach, which penetrates deeply into the tumor," Sood said.
TMice implanted with three human ovarian cancer cell lines derived from women with advanced cancer were treated for 3 to 5 weeks. They received liposomes that contained either the FAK siRNA, a control siRNA, or were empty. Some mice received siRNA liposomes plus the chemotherapy docetaxel. Mice receiving the FAK-silencing liposome had reductions in mean tumor weight ranging from 44 to 72 percent compared with mice in the control groups. Combining the FAK-silencing liposome with docetaxel boosted tumor weight reduction to the 94 to 98 percent range. These results also held up in experiments with ovarian cancer cell lines resistant to docetaxel and the chemotherapy drug cisplatin.
In addition to its anti-tumor effect, the researchers found that the therapeutic liposome attacked the tumor's blood supply, especially when combined with chemotherapy. By inducing programmed cell death, or apoptosis, by angiogenic blood vessel cells, the treatment steeply reduced the number of small blood vessels feeding the tumor, cut the percentage of proliferating tumor cells and increased apoptosis among cancer cells. Based on these results, the M. D. Anderson research team plans to begin safety testing as the next step toward human clinical trials.
In the meantime, an international research team has shown that carbon nanotubes can also deliver siRNA agents into malignant cells and suppress tumor growth. Yongsheng Chen, Ph.D., from Nankai University, in Tianjin, China, and Richard Roden, Ph.D., from Johns Hopkins University School of Medicine, led the effort to deliver an siRNA that targets the protein telomerase reverse transcriptase using positively charged single-walled carbon nanotubes. The researchers chose this type of carbon nanotube because negatively charged siRNA molecules would bind tightly to the surface of the nanotubes and because previous research had shown that these types of nanotubes easily cross the cell membrane without associated toxicities.
Initial experiments with tumor cells growing in culture showed that siRNA-nanotube complexes did enter the malignant cells easily. More importantly, once inside the cells the siRNA was able to stop production of telomerase reverse transcriptase and suppress proliferation of the treated cells. In followup experiments, the investigators injected the siRNA-nanotube conjugates directly into tumors, which had the effect of markedly reducing the size and weight of the tumors. The researchers note that while the direct injection technique might be useful with some types of cancer, a more efficient approach would be to add a tumor-targeting agent to the surface of the carbon nanotubes
The work targeting ovarian tumors, which was supported in part by the National Cancer Institute, is detailed in a paper titled, “Focal adhesion kinase targeting using in vivo short interfering RNA delivery in neutral liposomes for ovarian carcinoma therapy.” Investigators from University of California at Los Angeles also participated in this study. An abstract of this paper is available through PubMed.
The work using carbon nanotubes as a delivery vehicle, which was supported in part by the National Cancer Institute, is detailed in a paper titled, “Delivery of telomerase reverse transcriptase small interfering RNA in complex with positively charged single-walled carbon nanotubes suppresses tumor growth.” An abstract of this paper is available through PubMed.