US scientists have successfully completed a study where they showed targeted nanoparticles injected directly into a patient’s bloodstream navigated into tumors, delivered double-stranded small interfering RNAs and turned off a gene that drives cancer growth.

You can read about the study, by researchers from the California Institute of Technology in Pasadena (Caltech), the University of California, Los Angeles (UCLA), and others, in the 21 March advance online issue of Nature.

The results reported in this study are from a Phase 1 clinical trial that began treating patients with nanoparticles in May 2008. As well as intending to establish scientific proof of concept in humans, like all Phase 1 trials, the goal is to test safety and determine toxicity levels of the therapy. The trial is being sponsored by Calando Pharmaceuticals, a Caltech startup company.

A UCLA statement describes the study as the first to prove that a targeted nanoparticle can be used as an experimental therapeutic in human cancer tumors: it demonstrates the “feasibility of using both nanoparticles and RNA interference-based therapeutics in patients”.

Another first by the team is that they showed the therapeutic can be used in a dose-dependent fashion: the more nanoparticles they injected, the more they found in the cancer cells.

In 2006, American scientists Andrew Fire and Craig Mello won the Nobel Prize for medicine for their discovery of RNA interference (RNAi), the mechanism by which double strands of RNA silence genes by targeting the messenger RNAs (MRNAs) that code proteins.

Fire and Mello first reported their discovery in a 1998 Nature study, and since then there have been high hopes that this way of silencing genes could be developed to treat diseases like cancer.

The reason RNAi could be so powerful is that it does not target a protein directly but the mechanism that codes the protein. Targeting proteins with therapeutics is tricky as often the target areas can be inacessible, perhaps tucked away inside three-dimensional folded structures. But RNAi offers the opportunity to target the mRNA that encodes the information for making the protein: destroy the mRNA and you effectively switch off the corresponding gene and the production of its particular protein.

Lead author Dr Mark E Davis, the Warren and Katharine Schlinger Professor of Chemical Engineering at Caltech, told the press that in principle:

“Every protein now is druggable because its inhibition is accomplished by destroying the mRNA.”

“And we can go after mRNAs in a very designed way, given all the genomic data that are and will become available,” he added.

However, as is often the case, what looks straightforward in theory is fraught with obstacles when you try and apply it in practice. One such difficulty, when trying to apply RNAi technology to humans is, how do you deliver such tiny, fragile molecules, the small interfering RNAs (siRNAs), to the tumors?

Senior author Dr Antoni Ribas, an associate professor of medicine and surgery and a researcher at UCLA’s Jonsson Comprehensive Cancer Center, said:

“There are many cancer targets that can be efficiently blocked in the laboratory using siRNA, but blocking them in the clinic has been elusive.”

Davis and colleagues had a solution: they had already been working on ways to deliver nucleic acids into cells before RNAi was discovered. They eventually came up with a method featuring four components, one of which is a unique polymer that can assemble itself into a targeted nanoparticle that carries siRNA.

Davis explained that their nanoparticles can take the siRNAs into the targeted site within the body, and when they reach their target, the cancer cells inside the tumor, the nanoparticles enter the cells and release the siRNAs.

The researchers used a new method developed at Caltech to find and image the nanoparticles inside cells biopsied from the tumors of several patients taking part in the trial.

They also found that the more nanoparticles a patient was given, the more were present in the tumor cells: thus establishing there was a dose-dependent response.

But what was even better, said Davis, was they found evidence the siRNAs had done their job: in the cells they analyzed, which had been targeted to prevent production of the cell-growth protein ribonucleotide reductase, they found the corresponding mRNA had been degraded. Thus effectively the siRNAs had silenced the gene that was fuelling cancer growth.

Davis explained that this was the first time that anyone has found an RNA fragment from patient cells showing that the RNAi mechanism had severed the mRNA at exactly the correct base:

“It proves that the RNA interference mechanism can happen using siRNA in a human,” said Davis.

Ribas said:

“This research provides the first evidence that what works in the lab could help patients in the future by the specific delivery of siRNA using targeted nanoparticles.”

“We can start thinking about targeting the untargetable,” he added.

However, the researchers stressed that while these results are promising, it is still early days and there is a lot of work still to do. However, they are hoping these findings will open the door for future “game-changing” therapeutics that attack cancer and other diseases at the genetic level.

“Evidence of RNAi in humans from systemically administered siRNA via targeted nanoparticles.”
Mark E. Davis, Jonathan E. Zuckerman, Chung Hang J. Choi, David Seligson, Anthony Tolcher, Christopher A. Alabi, Yun Yen, Jeremy D. Heidel, Antoni Ribas.
Nature, published online 21 March 2010.

Sources: UCLA, Nobel Foundation.

Written by: Catharine Paddock, PhD