The second day of the Meeting was filled with cutting-edge science related to RNAi drug development with emphasis on RNAi mechanism, delivery, and cytokine responses triggered by various nucleic acid classes.
Tuschl kicked off the day by presenting data on the systemic identification of Argonaute interaction partners (mostly published data) and a new approach towards identifying microRNA targets. In contrast to microRNA target identification algorithms which heavily rely on sequence conservation, Tuschl and colleagues identified RNAs pulled down in Argonaute immuno-precipitations and then sequencing. Validating the approach, these RNAs were enriched for sequences with seed targets of the most abundantly expressed microRNAs in the tested cell line (1.8x enrichment over random). He finally reported on small molecule screens aimed at identifying inhibitors of microRNA maturation, however, with less success. It appears to me that he should stick to his guns and work at inhibiting microRNAs and their precursors instead by nucleic acids, such as antagomirs, which have proven to be much more potent and specific inhibitors of microRNA activity.
Ingo Roehl (Roche Gmbh, former Alnylam Europe) gave a nice presentation on progress in the development of analytical chemistries to support DMPK/PD (Drug Metabolism and Pharmacokinetics/dynamics) studies of RNAi Therapeutics. It was encouraging to see significant improvements that now allow siRNA and endogenous microRNA quantitations in the picomole-femtomole range in biological samples at high-throughput. In contrast to methods used by other groups, their coupled HPLC mass-spec setup allows for the discrimination between intact siRNAs and their degradation products. However, further improvements in sensitivity are desired as the potency of siRNAs means that biological activity can be observed long after siRNA levels drop below the level of quantitation.
Peter Linsley from Rosetta Inpharmatics (a subsidiary of Merck), which gained early “notoriety” for being the first group to point out the problem of off-targeting by siRNAs some years ago, highlighted the dilemma posed by the fact that the off-target signature of a given siRNA is extremely different in mouse and man. This means that mice will not function as a safety model for off-target toxicity studies. Furthermore, by limiting oneself to cross-species specific siRNAs for the sake confirming the treatment in the animal model, many siRNAs with better potencies and off-targeting signatures in humans may ultimately be missed. Also of interest is the fact that by leveraging its genomic expression profiling capabilities, Rosetta Inpharmatics has now established microRNA overexpression signatures for over 150 microRNAs, raising the question when Merck will officially announce its entry into the microRNA therapeutics arena.
Following on from his identification of SID-1 in being required for systemic RNAi in worms by acting as an siRNA channel, Craig Hunter (Harvard University) presented data to support a model in which siRNAs enter and exit cells via SID-1 by diffusion, whereas other proteins such as SID-2 are involved in binding RNAs so as to increase their concentration close to the siRNA channel. That this is relevant for the development of RNAi therapeutics is highlighted by the recent Nature Biotech paper by Dr. Stoffel from the ETH Zurich and Alnylam where data suggested a role for SID-1 in taking up siRNAs following docking of siRNA-studded lipoprotein particles to their cellular receptors. In addition to walking his audience the published data-heavy paper, Stoffel stated that he saw no reason why it should not be possible to create artificial lipoprotein particles containing siRNAs and targeting agents.
Instead of Dinah Sah or David Bumcrot presenting Alnylam’s progress on cancer RNAi therapeutics, Rachel Myers stood in to give a general overview of the approach Alnylam takes to RNAi-based drug development. Since little new primary data was presented, I thought it is noteworthy that both Ingo Roehl and Myers gave scientific credit for the development of the hotly contested SNALP development to Protiva Biotherapeutics and that the delivery technology used in last week's Nature paper on microRNA competition by siRNAs was called "Alnylam proprietary”. Also, it was a great relief for me to hear that Alnylam had reproduced all of Sailen Barik’s data on the RSV-RNAi treatment paradigm, including efficacy of siRNAs after RSV infection. Indeed, siRNAs reduce viral titers up to 3 days after infection in a mouse model where maximal viral burden is seen by day 4 of the infection. Moreover, illustrating that the upcoming proof-of-concept studies for RSV in the experimental infection model is not merely an academic RNAi therapeutics de-risking exercise, the patients that Alnylam expects to treat with ALN-RSV01 will have infection in the upper respiratory tract with little or no exposure in the lung. Consequently, ALN-RSV01 will be a combination of treating upper respiratory tract infection and prophylaxis for the lung. Everybody, of course, is quite restless now to learn about the outcome of the phase II experimental infection results which appear to be on track and from which will be presented in detail early next year without precluding a PR on the results in December.
Before the session on RNAi/oligonucleotide delivery, a number of speakers spoke about recent findings on the immunogenic properties of various forms of RNAs, including siRNAs. The importance of considering the expression pattern of the main oligonucleotide receptors (the endosomal TLR-3,7,8,9 and the cytosolic RIG-I and MDA-5), their localization, and their exact ligands became evident and will inform modification strategies aimed at avoiding the induction of unwanted cytokine responses following siRNA administration which is something that Rachel Myers noted Alnylam is still trying to get a better handle on (see the delay in the pandemic flu program). Other groups, however, such as Gunther Hartmann’s group from Bonn, embrace the immunogenic properties of some, particularly unmodified siRNAs and would like to combine it with their silencing activity for treating cancer and viral infections. Some encouraging data in that regard were presented.
Song Li (Pittsburgh) presented the use of neutral lipids for delivering a variety of oligonucleotides to the pulmonary circulation. Like others at the meeting, this approach illustrates a trend away from using cationic liposomes which have been associated with interacting with components of the blood and may trigger certain toxicities. Delivery to endothelial cells of larger vessels and capillaries was particularly efficient, and his group is currently collaborating with ISIS on targeting endothelin-1 for treating hypertension. Encouragingly, he was able to report on good gene knockdowns and quite impressive in vivo efficacy data, such as the reduction of hypoxia-induced right ventricular hypertrophy.
Similarly impressive in vivo efficacy data were then reported by Klaus Giese from Silence Therapeutics, mostly located in the host city of Berlin. Without showing all the controls, tumor burden, metastasis, cell proliferation were all strongly reduced following systemic delivery of siRNAs in a number of mouse tumor models. Although I cannot agree with his repeated claims on the uniqueness of their Atu-siRNAi design and related IP claims, the Atuplex delivery technology certainly deserves more credit. Unlike other lipid-based delivery methods, siRNAs here associate with the liposomes externally thus allowing liposomal charge to be modulated from cationic, via neutral, to anionic. Of practical importance, these particles can be lyophylised for storage and shipping and then resuspended without loss of silencing activity. Progress has also been made on lung delivery and a number of pre-clinical programs are being pushed forward into the clinic.
The founder and CEO of the nucleic acid delivery company Novosom, Steffen Panzner, continued the string of impressive in vivo efficacy data in a mouse model of RA (inflammation of the paws) following delivery of their proprietary siRNA-loaded Smarticles, a charge-reversible liposomal delivery technology. One problem of using the more desirable, in terms of safety, neutral and anionic lipids, is their reduced siRNA binding affinity. Novosome’s amphoteric liposomes circumvent this problem by binding siRNAs at low pHs, at which point they are positively charged, and rapidly shifting them to higher pHs for closing the liposomes. During this process, most of externally bound siRNAs are shaved off, with the additional benefit of consequently minimising endosomal exposure of the siRNAs which may trigger TLR7-mediated cytokine responses.
The scientific day was closed by a talk from Alan Sachs of Merck who set out to emphasise that despite a dearth of recent information on Sirna’s/Merck’s preclinical and clinical RNAi therapeutics pipeline, that they were more than ever committed at developing RNAi therapeutics. Unfortunately, this was not followed by a presentation of primary scientific data and specific examples, but rather how Merck thinks about RNAi drug development in general and repeated appeals of please partnering with Merck, particularly in the area of targeted siRNA delivery. It appears to me that Merck would like to rival Alnylam in accessing the best minds in the RNAi drug delivery field and is offering to send out their siRNAs for free so that others can formulate them and report back on their findings without any strings attached as to the use of these results. Sachs feels that Merck, through their expertise on the genomics and genetics of gene expression through Rosetta Inpharmatics, that it knows best which targets are most suitable to go after for drug development. Since siRNAs allow essentially any gene to be targeted, he like other feels that this is the natural drug development platform to be harnessed and thinks about 21 month development timelines from gene identification to preclinical proof-of-concept given the availability of suitable biomarkers. While searching for more targeted delivery solutions and collaborators, Merck is meanwhile establishing a platform for siRNA delivery to the liver and, encouraged by ISIS' 301012 results, is clearly motivated in targeting ApoB100 with siRNAs for the treatment of hypercholesterolemia. I would not be surprised at all, to see them join the PCSK9 frenzy as well. Being second in a $50-60B should not be that bad.
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