One of the most promising molecular agents for therapy of cancer and other diseases is siRNA molecules, which can be designed to silence the expression of specific genes, such as the genes that are responsible for the cancerous properties of tumor cells. But before siRNAs can be used in therapy, it will be necessary to develop methods to protect these fragile molecules from degradation and to get them inside the cells where they need to be. We have reported several successes that researchers have had using different types of nanoparticles to protect and transport siRNA molecules. Another promising nanotech approach to harnessing the potential of siRNA molecules is to pack them on the surface of gold nanoparticles. From the National Cancer Institute’s Alliance for Nanotechnology in Cancer “Pack ‘Em In—Gold Nanoparticles Improve Gene Regulation“:
Investigators at Northwestern University have found that packing small interfering RNA (siRNA) molecules onto the surface of a gold nanoparticle can protect siRNAs from degradation and increase their ability to regulate genes involved in cancer. As a result of this discovery, cancer researchers have at their disposal a relatively straightforward method of delivering these potent gene-regulating agents into targeted cells.
Chad Mirkin, Ph.D., principal investigator of the Nanomaterials for Cancer Diagnostics and Therapeutics Northwestern University Center for Cancer Nanotechnology Excellence [and winner of the 2002 Foresight Institute Feynman Prize in Nanotechnology in the Experimental category], led the research team that developed the methods needed to create these densely packed siRNA-nanoparticle conjugates. The investigators published their results in the Journal of the American Chemical Society [abstract].
One of the difficulties in working with potentially therapeutic siRNA molecules is that they are highly unstable, particularly in the presence of even trace levels of enzymes called nucleases that break down nucleic acids. To ensure that the surface of the gold nanoparticles was devoid of any nucleases, the investigators developed a harsh stripping method. To the researchers’ surprise, this treatment had no effect on the optical or physical properties of the nanoparticles. The researchers also found that without this pretreatment, they were unable to add any RNA molecules to nanoparticles. With the pretreatment, the resulting 13-nanometer gold nanoparticles held an average of 34 siRNA molecules each.
Using confocal microscopy, the investigators were able to watch the nanoparticles enter cultured tumor cells. More importantly, the researchers also showed that once inside the cell, the siRNA was able to escape from the nanoparticle surface and inactivate its gene target. The amount of gene silencing achieved with the siRNA-nanoparticle construct was double that observed when cells were treated with siRNA alone. The investigators note that other experiments suggest that this boost in therapeutic efficacy arises because of improved siRNA stability when associated with gold nanoparticles.