Targeted nanoparticle destruction of cancer genes using RNAi

A new clinical report by a team of researchers from the California Institute of Technology (Caltech) describes the successful human application of targeted nanoparticles directly injected into patients’ bloodstreams to deliver small interfering RNAs (siRNAs) that disrupt the communication of genetic information via messenger RNA (mRNA). This process inhibits the expression of cancer-related genes through a dose-dependent mechanism, which is the first of its kind for an RNAi-based treatment.

Andrew Fire, a professor of pathology and genetics at Stanford University and Craig Mello a professor of molecular medicine at the University of Massachusetts Medical School, won the 2006 Nobel Prize in Physiology or Medicine for the 1998 discovery of RNAi, a cellular process where gene expression is silenced by introducing siRNA or single stranded RNA into a cell to stop mRNA from conferring its genetic signals to the cell’s ribosomes, which assemble proteins. Since the discovery, many scientists have hypothesized that RNAi could be used as a gene therapy to treat disorders like cancer because it is easier to target mRNA than it is to target specific proteins. Because proteins fold and re-fold, it is complicated to target a specific site on a protein. However, mRNA does not exhibit this behavior.

“In principle [it] means every protein is ‘druggable’ because its inhibition is accomplished by destroying the mRNA,” Mark Davis, a professor of chemical engineering at Caltech, said in a press release. “We can go after mRNAs in a designed way given all the genomic data that is and will become available.” However, clinical applications have been held back because researchers have failed to develop a mechanism to control the delivery and distribution of fragile siRNA in cells.

Now, Davis has developed a possible solution to the delivery and distribution problem. Throughout his career, Davis has focused on finding ways to deliver nucleic acids into cells through a systemic administration system. By adapting his previous work to RNAi, Davis has engineered a new four-component system that self assembles into a targeted siRNA-containing nanoparticle. The four parts of the system are a linear cyclodextrin-based polymer, a human transferring protein targeting ligand, a hydrophilic polymer, and siRNA. The human transferring protein targeting ligand engages transferring receptors on the surface of cancer cells and the hydrophilic polymer promotes nanoparticle stability in biological fluids. This siRNA delivery system is under clinical development by Calando Pharmaceuticals (Pasadena, CA).

Armed with this system, Davis and his colleagues proved the feasibility of using targeted siRNA nanoparticles to silence oncogenes in mice. “These nanoparticles are able to take the siRNAs to the targeted site within the body,” said Davis. Once the nanoparticles reach their target, they enter the cells and release the siRNAs.

With the success of these trials, Davis’ method was utilized for a human clinical trial for patients suffering from metastatic melanoma, which began treating patients in May 2008. The trail was sponsored by Calando Pharmaceuticals and is being performed at South Texas Accelerated Research Therapeutics and UCLA’s Jonsson Comprehensive Cancer Center. The clinical results have not yet been released.

Writing in Nature, Davis’ team reported that the delivery system was administered intravenously. Transmission electron microscopy was used to show that the nanoparticles were successful in infiltrating the tumor cells, and the siRNA affectively degraded the mRNA that encodes the cell-growth protein ribonucleotide reductase. The mRNA fragments created by the siRNA were the exact length and sequence the researches expected, based on the site targeted in the siRNA strand.

Davis’ team was also able to demonstrate that doses with higher nanoparticle concentration generated a higher number of nanoparticles inside the tumor cells. According to Davis, this is the first example of a dose-dependent response using targeted nanoparticles for RNAi.

“At the very least, we’ve proven that the RNAi mechanism can be used in humans for therapy and that the targeted delivery of siRNA allows for systemic administration,” said Davis. “It is a very exciting time.”

Funding for the researcher was provided by the National Cancer Institute, and the Daljit S. and Elaine Sarkaria Biomarker Laboratories. The paper, “Evidence of RNAi in humans from systemically administered siRNA via targeted nanoparticles,” was published March 21 in Nature.