The Challenge

Being hailed as the biggest breakthrough in biology of the last decade or two and being reproducible in the Petri dish is no guarantee that drugs based on RNAi will successfully make it as drugs. Three main hurdles need to be taken for this to come true: 1) efficient delivery of the RNAi agent to the target cells, 2) managing off-target effects, and 3) endowing RNAi molecules with drug-like properties.

The efficient delivery of the RNAi agent is frequently cited as the main hurdle to the wide application of RNAi. While local delivery such as needle injection into the eye for ocular diseases or inhalation for lung-related conditions has a relatively high likelihood of success, systemic delivery, e.g. needed to reach metastatic cancer cells hidden in the body, is a taller order. However, a number of delivery methods are being tested, some borrowed from older oligonucleotide-based technologies like liposomes, some more innovative. It is likely that no one-fits-all solution will emerge, but strategies that are tailored to the specific disease. Particularly interesting are approaches where a synthetic siRNA is coupled to agents such as monoclonal antibodies or RNA aptamers which can selectively target cells. Hence, the specificity of the siRNA is compounded by the specificity of its delivery. This should also help in reducing the likelihood of potentially harmful off-target effects.

Off-targeting, the suppression of non-targeted genes is mainly a consequence of the siRNA acting like a microRNA. MicroRNAs are related endogenous 20-24 nucleotide small RNAs that recognise their targets through less than perfect complementarity. Although this typically does not downregulate target genes as dramatically as an efficient siRNA might do, nobody can be sure that a 2-3 fold reduction in a “random” gene will not have adverse side-effects. I should stress that this specificity is probably still much better than many other drug-classes today, but we would like to do better, especially when human health and the substantial resources needed for the development of a drug are at stake. Helped by the knowledge of the human genome, bioninformatics already can winnow down the number of potential off-targets, thus reducing the likelihood of an adverse side-effect. More lately, however, and as demonstrated by the work of Dharmacon scientists, it has become possible to modify the siRNA such that it will lose much of its microRNA abilities while retaining potent RNAi-like cleavage potential. I view this as a particular exciting development.

Chemical modification of siRNAs can also help to enhance its drug-like properties such as half-life, metabolism etc. However, it was only quite recently shown quite by Alnylam Pharmaceuticals, viewed by many as the leading RNAi Therapeutics company, that siRNAs may knock down genes with a profile that may allow dosing every 2 to 4 weeks. This is a great relief and opens up RNAi for many more, particularly chronic applications such as hypercholesterolemia, than would have been the case if the knockdown was limited up to 4 days after administration. The latter is typically observed with cultured cancer cell lines and is due to dilution of the siRNAs following frequent cell divisions. For some genetic diseases, viral gene therapy vectors for the expression of hairpin RNAs that are processed to siRNAs by the endogenous RNAi machinery are a further important option. They should allow for the more long-term expression of RNAi effector molecules, although the risk-benefit equation is shifted for DNA-based therapies.

In summary, many strategies are being explored to develop RNAi as a safe and efficient therapy. While human trials for each siRNA will have to be performed to evaluate its specific therapeutic value, as the risk of RNAi Therapeutics gets more manageable each day, it promises to become one of the most specific and versatile drug class to date.

The Promise

The development of RNA interference (RNAi) into a technology that is poised to revolutionise medicine has been breathtaking. Only 9 years after its discovery in worms by Fire and Mello, the 2006 Nobel Laureates in Physiology and Medicine, in 1998 and the subsequent realisation by Tuschl et al. that so called small interfering RNAs (siRNAs) can induce the same process in human cells in 2001, there are already 5 phase I clinical trials ongoing with many more to enter in the coming years. Scientists and clinicians alike seek to harness the power of RNAi for conditions ranging from age-related macular degeneration, viral infections (RSV, HIV, Hepatitis B and C, pandemic flu etc.), Huntington's Disease to cancer. This is because RNAi allows the specific down-regulation of gene products called mRNAs, and since it is safe to assume that virtually any disease interfaces with a gene regulatory network we should be able to modify, if not cure it by tweaking this network. These networks and the role of individual genes therein are being worked out by thousands of laboratories around the world and has been greatly aided by the sequencing of the human and other genomes and technologies to measure and modify gene activities (among them RNAi itself). As there is consequently no lack of validated gene targets, the main hurdle towards its wide application is thought to be the safe and efficacious delivery of the RNAi inducing agent to the site of disease. Luckily, RNAi can draw from the experience of antisense oligo and gene therapy and the exciting prospect of RNAi has energised some of the best minds to come up with innovative strategies. I will address these and other challenges in the next post before introducing the main players in the RNAi Therapeutics space.

For those who want to understand more about the basic mechanism and discovery of RNAi, a popular documentary can be found at PBS Nova: http://www.pbs.org/wgbh/nova/sciencenow/3210/02.html