Saturday, May 17, 2008
RNAi Therapeutics versus Antisense- Where Delivery Makes a Difference
Certainly, progress in both areas in the last 3-5 years has mutually benefitted the investment climate for both technologies as it has heightened interest and increased confidence in RNA therapeutics in general. However, the two technologies also compete for investment dollars with many of the same investors, which are typically upbeat about the future of gene-based medicines but unsure where to place their bets, allocating their investments based on where they see most promise. One issue that often comes up in making this decision is the observation that while for systemic applications antisense, as practiced in the most advanced programs today, is typically administered without a particular delivery formulation, the development of specialized delivery technology is frequently cited as the key challenge for RNAi to realize its ultimate therapeutic potential.
Antisense for gene knockdown works largely by two mechanisms: interfering with translational initiation (e.g. AVI Biopharma's morpholinos) or through an RNase H-type mechanism (e.g. Santaris and ISIS Pharmaceuticals). For this, the key factor is to achieve efficient hybridization of a single-stranded oligonucleotide antisense with its target mRNA which either prevents productive ribosome association to the mRNA (inhibition of translation initiation) or may be recognized as a substrate for the RNase H enzyme which may degrade the RNA portion of the mRNA-DNA duplex, but normally functions in the degradation of the RNA primer during DNA replication.
Various oligonucleotide chemistries have been developed to optimize these processes for in vivo applications. Essentially all of these are single-stranded oligos of which the sugar phosphate backbone is heavily modified to a) increase their in vivo stability; b) improve their pharmacokinetics and avoid rapid renal excretion by promoting their association with components of the blood; c) similarly allows them to be retained in tissues; d) facilitate crossing of cell membranes; and finally e) increase their target mRNA binding. By contrast siRNAs, because of their charge and more rigid double-stranded nature and with apparently some exceptions that include mucosal epithelia, do not cross cell membranes efficiently on their own and therefore need to be specially formulated for most applications.
The use of unformulated antisense is consistent with their mechanism of action. Since antisense does not harness a naturally existing endogenous gene silencing pathway, it relies on achieving concentrations of oligonucleotides in the target tissue over a prolonged period of time that are high enough such that, as a result of the rules of thermodynamics, a sufficient fraction of target mRNA will be recognized. Similarly, unlike RNAi, the specificity of antisense is largely governed by biophysics and benefits only relatively little from biological proof-reading.
In practice, to achieve the necessary tissue concentrations, patients are typically dosed frequently at the initiation of therapy so that the tissue concentrations reach steady-state therapeutic levels. Targeted delivery of antisense into cells of interest may allow one to achieve a knockdown earlier, but any benefit would only be short-lived as antisense is not retained in specific gene silencing complexes but will soon redistribute according to their partition coefficient throughout the entire tissue so that ultimately similar amounts have to be administered and a formulation would only be a nuisance with little benefit.
By contrast, RNAi harnesses an endogenous and catalytic gene silencing mechanism, which means that once it has been delivered, either by conjugation or in nanoparticles into the cytosol, they are efficiently recognized and stably incorporated into the RiSC silencing complex to achieve prolonged gene silencing. In fact, measurable RNAi-mediated gene silencing can be observed at siRNA concentrations so low that it becomes difficult to detect them (e.g. fluorescently-tagged siRNAs by microscopy). This means that as the majority of siRNAs that do not reach the cytoplasm may disappear quite rapidly, the total exposure of the body to the nucleic acid can be much lower compared to antisense which should be beneficial both in terms of safety and pharmacodynamics (activity profile of drug over time).
This is not to say that chemical modification is not practiced in RNAi. However, unlike in antisense, the purpose of modification in RNAi is mainly to avoid triggering innate immune responses, making the siRNA sufficiently stable so that they survive their journey into their target cells, and also to stabilize them as part of RiSC (Merck has been talking about that concept on several occasions); and as we learn more about the biochemistry of endogenous RNA silencing pathways, modification is also increasingly used to increase the inherent biological specificity of RNAi. Unfortunately, it is surprising to me that compared to RNAi only very little, if at all, is reported about the specificity of antisense and I would be grateful if somebody here could point out pertinent studies that I should be aware of.
Targeted delivery may also avoid unnecessary drug exposure of non-target tissues. For unformulated antisense, no matter what the indication and target tissue, the biodistribution is essentially the same, and toxicities of the liver and kidney due to extended exposure to large amounts of the heavily modified antisense compounds is well known.
Certainly, improving the therapeutic index is an important issue for RNAi Therapeutics, too, but as the many transgenic mouse models which express ample and highly efficient RNAi throughout their life without causing overt toxicity attest, ultimately the improvement in the therapeutic index of RNAi is not limited by its very mechanism of action.
While it is a certainty that antisense companies will come out with 4th and 5th generation antisense technology, advances after decades of antisense research aiming to improve target mRNA recognition will only be marginal and based on trying out yet more nucleic acid modifications, although it appears to be a challenge to improve upon the efficacy of probably the most potent antisense modification that have now been known for a while, namely LNAs and their derivatives.
While RNAi efficacy in animals has already surpassed that of antisense for applications of the liver and lung as well as other tissues, I am confident that future advancements in RNAi will be more than marginal. For example, even as recent liposomal formulations achieve 90% gene knockdown in the liver at 1mg/kg, this still means that only about 1 in 10,000 siRNAs that have reached the liver makes it into the cytoplasm (assuming it takes about 1000 cytosolic siRNAs to achieve that level of knockdown according to a recent presentation by Phil Sharp). Alone a better understanding of the endosomal uptake of these nanoparticles, which is only in its infancy and starting to be explored, should allow for more than incremental improvements in the therapeutic index of RNAi Therapeutics.
And if you are still undecided on where the future is heading, numerous transfection studies in vitro where it can be assumed that equal amounts of antisense and siRNAs are present in cells, have shown that RNAi is quite a bit more potent on a mole-by-mole basis comnpared to antisense.
I am aware that some in the antisense community, including investors, may take offense with this blog, but since I am often asked about this issue, I think a more straightforward approach is better than to keep beating about the bush. And, of course, there is always the comment section.
Disclaimer: This blog is not intended for distribution to or use by any person or entity who is a citizen or resident of, or located in any locality, state, country or other jurisdiction where such distribution, publication, availability or use would be contrary to law or regulation or which would subject the author or any of his collaborators and contributors to any registration or licensing requirement within such jurisdiction. This blog expresses only my opinions, they may be flawed and are for entertainment purposes only. Opinions expressed are a direct result of information which may or may not be accurate, and I do not assume any responsibility for material errors or to provide updates should circumstances change. Opinions expressed in this blog may have been disseminated before to others. This blog should not be taken as investment, legal or tax advice. The investments referred to herein may not be suitable for you. Investments particularly in the field of RNAi Therapeutics and biotechnology carry a high risk of total loss. You, the reader must make your own investment decisions in consultation with your professional advisors in light of your specific circumstances. I reserve the right to buy, sell, or short any security including those that may or may not be discussed on my blog.