RNAi: Send in the Nanobots!

The publication of the preliminary results of a small clinical trial of a new therapy called RNA interference (RNAi) online in the scientific journal Nature is causing quite a stir in the scientific community this week.  A team led by Professor Mark E. Davis at Caltech targeted the delivery of a nanoparticle only 70 nanometers in diameter containing small interfering RNA (siRNA) to cancer cells in three patients with metastatic melanoma, which reduced the levels of a protein called RRM2 that is required for the tumour growth.  This trial is the result of over a decade of research in organisms as diverse as nematode worms, mice and monkeys, but why is the result of this trial so noteworthy? And what is RNAi anyway?

If you have ever studied biology you will probably be familiar with the “central dogma of molecular biology”; it describes how our genes encode the proteins that are the building blocks, and indeed the builders, of all the cells in our bodies. The very short version is that our genes are made up of sequences of double stranded DNA consisting of the deoxyribonucleotides A,C, G and T, and these sequences are transcribed by a protein called RNA polymerase into matching sequences of the single stranded messenger RNA (mRNA) , made from the ribonucleotides A, C, G and U. Another protein complex known as the ribosome then translates the mRNA sequence into a corresponding sequence of amino acids that when completed make up a brand new protein.  Our new protein almost invariable undergoes further processing but we needn’t concern ourselves with that here.  RNAi is the process where an assembly of proteins named the  RNA-induced silencing complex (RISC) binds short double stranded segments of RNA that in turn target RISC to particular mRNA sequences to which they are complementary.  RISC breaks down the mRNA molecule, preventing production of its associated protein and effectively silencing the targeted gene. The beauty of RNAi is that it allows an organism to target specific mRNA molecules for destruction, and it is a mechanism for regulating the flow of genetic information whose importance we are still only beginning to appreciate.

RNAi was discovered only 12 years ago by Andrew Fire and Craig Mello through their basic research on the regulation of gene expression in the nematode worm Caenorhabditis elegans, a discovery which earned then the Nobel Prize in 2006. C.elegans is a popular model organism for scientists studying gene function and development, its small size and simple structure make it relatively easy to follow the fate of individual cells, while as an animal it shares many of its genes and biological processes with mammals.  This turned out to be the case with when in 2001 it was shown that RNAi helps mice to control hepatitis B infection, and scientists began to examine whether RNAi could be used therapeutically (1). To do this scientists made siRNA, an artificial version of the short double stranded segments of RNA that target RISC to complementary mRNA sequences, and early experiments in mice demonstrated that siRNA induced RNAi could reduce the levels of target proteins in mice.  The first human trials of RNAi began in 2004 for the treatment of wet age related macular degeneration and at first seemed promising, but suffered a setback when further research in mice revealed that the “naked” siRNA injected into the eye in these trials actually stimulated an immune response that was responsible for at least some of the benefits seen in earlier trials (2). This was a worry as an unwanted immune response might lead to an adverse reaction if the siRNA was injected into the bloodstream rather than a small part of the eye.

In recent years scientists have been developing technologies that allow injected siRNA to evade the immune system and target only those tissues where RNAi activity is desired,  reducing the quantity of siRNA that needs to be injected and also the risk of adverse  effects due to RNAi affecting off-target tissues. Mark E Davis, a professor of chemical engineering at Caltech and one of the scientists leading these efforts, uses polymers that assemble with siRNA to form a nanoparticle that resembles a tiny ball with siRNA at its centre.  The nanoparticle shell protects the siRNA from being broken down while it is circulating in the bloodstream, and then interacts with the cell membrane to help the siRNA enter a cell so that it can do its job.  Of course he didn’t want the nanoparticle to release its siRNA payload into any old cell so he attached a protein called transferrin as a targeting ligand to the nanoparticle. Tumour cells express far more of the transferrin receptor on their surfaces than normal cells, and the hope was that the nanoparticles would bind to tumour cells in preference to normal cells.  To test whether this would work Prof. Davis team injected the nanoparticles, containing a siRNA that targeted a cancer gene, into mice that had metastatic Ewing’s sarcoma(3). They observed that the transferrin labelled nanoparticle delivered the siRNA to the tumour cells, knocked down the activity of the target cancer gene and dramatically slowed tumour growth, and when the transferring ligand was removed his effect not seen.  They also observed that the nanoparticle did not stimulate the immune system or affect any of the major organs of the mouse, indicating that their method had solved safety problems seen in earlier RNAi trials.

Prof. Davis and his colleagues next needed to identify an appropriate target for human trials of their nanoparticle siRNA delivery system, and decided to target the M2 subunit of Ribonuclease reductase (RRM2), a protein that is required for cell division and which has recently been the subject of a lot of research as a target for anti-cancer drugs.   They first used in vitro studies to identify a siRNA sequence that effectively targeted the RRM2 mRNA, which they named siR2B+5, and then demonstrated in mice that this siRNA could block the production of RRM2 and reduce the growth of tumours (4).  As a final safety evaluation prior to human trials they injected different doses of their nanoparticle  containing siR2B+5 and labelled with transferrin to cynomologus monkeys, whose RRM2 mRNA is targeted by siR2B+5 in exactly the same way as in humans,  and found that it was safe and did not produce any unwanted effect on the immune system (5).

The human clinical trial reported this week confirmed that transferrin-labelled nanoparticle injected into the bloodstream were safely delivered siR2B+5 to the tumours of metastatic melanoma patients, and that the siRNA knocked down the production of RRM2 protein by RNAi (6).  Of course this is only a preliminary result, at this stage we don’t know to what extent this experimental treatment will reduce tumour growth in these patients, let alone if it will cure their cancer. If it is a success it will probably need to be combined with other anti-cancer drugs to be fully effective, so it is good to know that thanks to animal research other nanotechnology based drugs such as Lipoplatin are in clinical trials that offer more potent anti-cancer activity with less toxicity than existing anti-cancer drugs. Nonetheless to focus on this uncertainty would be to miss why this small trial is causing such excitement; for the first time scientists have shown that it is possible to target RNAi therapy to a particular tissue type within the body, and that is a breakthrough that opens up a whole new area of medicine. The era of RNAi medicine has begun!

Paul Browne

1)      McCaffrey A.P., et al. “RNA interference in adult mice.” Nature Volume 418, pages 38–39 (2002) DOI: 10.1038/418038a

2)      Castnotto D. and Rossi J.R. “The promises and pitfalls of RNA-interference-based therapeutics” Nature Volume 457(7228), pages 426-433 (2009) DOI:10.1038/nature07758

3)      Hu-Lieskovan S. et al. “Sequence-specific knockdown of EWS-FLI1 by targeted, nonviral delivery of small interfering RNA inhibits tumor growth in a murine model of metastatic Ewing’s sarcoma.” Cancer Re. Volume 65 (19), Pages 8984-8992 (2005) DOI:10.1158/0008-5472.CAN-05-0565

4)      Heidel J. et al. “Potent siRNA inhibitors of ribonucleotide reductase subunit RRM2 reduce cell proliferation in vitro and in vivo” Clin. Cancer Res. Volume 13(7), Pages 2207-2215 (2007) DOI: 10.1158/1078-0432.CCR-06-2218

5)      Heidel J. et al. “Administration in non-human primates of escalating intravenous doses of targeted nanoparticles containing ribonucleotide reductase subunit M2 siRNA.” Proc.Natl Acad. Sci. USA Volume 104(14), Pages 5715-5721 (2007) DOI: 10.1073/pnas.0701458104

6)      Davis M.E. “Evidence of RNAi in humans from systemically administered siRNA via targeted nanoparticles”  Nature Advance Online Publication 21 March 2010 DOI:10.1038/nature08956