Arrowhead Research Lifts Veil on Recent Progress in DPC Delivery

Four years after Mirus scientists published their seminal paper in polymer-mediated siRNA delivery, and 3 years after the acquisition of the Dynamic Polyconjugate (DPC) technology by Roche, Arrowhead Research, the new owner of the technology, finally lifted the veil on recent progress in bringing DPC technology to the clinic.

The White Paper reveals, for the first time, that DPCs have been successfully delivered to the livers of non-human primates, and moreover that they may also have use for applications outside the liver, one of the theorized attractions of DPCs based on their small size. Reading between the lines, however, it is also possible to identify some of the factors that have delayed clinical translation, and despite the progress and substantial investments by Roche, it remains an open question when the first DPC-based RNAi Therapeutic will enter clinical development.

Short circulation times were one of the deficiencies of 1^st generation DPCs. In order to reach tissues besides the liver, it is critical to achieve circulation times that are long enough so that the drug gets a chance to find and accumulate in its target tissues. Although DPCs on paper seemed to incorporate the features necessary for achieving such long circulation times, it came somewhat as a surprise when a DPC imaging paper last year showed DPC circulation times to be quite poor (Mudd et al., 2010). Although the particles were still able to accumulate in the liver, the data also raised questions whether the causes of the unexpected pharmacokinetics could have other consequences besides impacting biodistribution, for example in terms of safety.

It turns out that the short circulation times were the result of the premature exposure of the chemical groups that were supposed to shield the membranolytic functionalities of the DPCs outside their target cells. This would also explain why there seem to have been toxicity issues not just due to the poor biodegradability of the polymers, but also because such premature exposure renders DPCs as troublesome as many of the positively charged first-generation polymer approaches.

The White Paper indicates that the instability (and biodegradability) issue has been remedied to some degree such that the longer-circulating DPCs now show first promise for delivery outside the liver. But DPCs may also be competitive for delivery to the liver considering the following performance in non-human primates (excerpt from the White Paper):

‘Latest generation DPCs are remarkably efficacious in rats and non-human primates with ED80 values of ~0.1 mg/kg siRNA after a single dose. Increasing the dose two-fold in non-human primates results in >99% knockdown with a duration of effect of nearly 7 weeks. This is a 10-fold increase in efficacy compared to first generation DPCs containing PBAVE polymer. Latest generation DPCs are also better tolerated and have therapeutic indices of >10 in non-human primates as calculated from ED80 and NOAEL values.’

A 99% knockdown with 0.2mg/kg for 7 weeks- I’m impressed! It will be important to publish these data so that it is possible to see which model system was used, whether the 99% knockdown was seen for pretty much the 7 weeks, what the tox/tolerability profile was, the route of administration, and finally an explanation why a simple 2-fold increase in dosage had such a dramatic effect on knockdown efficacy.

The White Paper is also a reminder that for RNAi delivery technologies to be viable, there needs to be efficient scale-up. Apparently, manufacturing was, and possibly still is, a major issue with DPCs. At least the initial formulations had to be purified so extensively such that the yield became unacceptable for clinical translation and commercialization. Besides general liposomal expertise, quality manufacturing, of course, is what has always differentiated RNAi delivery company Tekmira from its competition.

My sense is that DPCs still have the potential to become an important delivery alternative. However, it is also clear that the path of DPCs was a tough one and, in the absence of guidance from Arrowhead Research, I expect additional delays (2-3 years?) before we will see the first DPC-based RNAi Therapeutic candidate in the clinic. This view is possibly shared by Alnylam as $10M in upfront and an increase of about $10M in annual operating costs would have been a small price to pay for Alnylam if DPCs were as advanced as Alnylam’s current systemic delivery workhorse, Tekmira’s SNALPs.

Until then, Arrowhead Resesarch needs to hit the ground running on the business development front given the increase in expenses that come with the 40+ research team in Wisconsin. The $15M facility with Lincoln Park Capital at least provides Arrowhead with increased financial flexibility. Ironically, it is positive clinical data from the RNAi Therapeutics candidates that are based on Tekmira’s SNALPs, DPCs most direct competitor that would greatly aid in that goal by re-igniting interest in the RNAi Therapeutics platform.

Arrowhead Research Acquires Roche’s RNAi Therapeutics Assets

The long wait is finally over. Almost a year* after infamously announcing its retreat from developing RNAi Therapeutics in-house as part of a corporate restructuring, Roche has sold its related assets to Arrowhead Research. That announcement sent shockwaves through the RNAi Therapeutics World as investors and potential licensing partners alike took this to mean that something was very wrong with RNAi Therapeutics technology. This led to a freeze in business development activities and precipitously falling valuations. Today’s news brings some closure to Roche’s sorry RNAi adventures and also promises a number of interesting revelations and developments in the months to come.

Arrowhead Research, of course, is already a well known player in RNAi Therapeutics through its Calando subsidiary. Arrowhead in my mind has suffered, however, from not realizing that in order to be a credible platform company, you actually need real scientists, and not just collect abstract intellectual property. It has always baffled me how Calando attempted to license its RONDEL delivery technology without even having a laboratory to overcome some of the apparent challenges of this technology.

With about 40 scientists that will join the company from Roche’s Madison, Wisconsin, site ('Mirus') this has obviously changed. This, however, also means that one should expect Calando’s RONDEL technology to be de-emphasized as these scientists will be more motivated to develop the Dynamic PolyConjugate (DPC) delivery technology they invented. As a reminder, DPCs have shown promise in mouse models for delivering siRNAs to selected cell types in the liver. It remains to be seen, however, whether this translates into primates, and whether DPCs have use beyond the liver. A recent imaging paper on DPCs suggests that the latter may be more challenging than one had first hoped (Mudd et al., 2010). Manufacturing and scale-up question will likely also need to be resolved first before entering clinical development.

A key question will be whether Alnylam will bless this spin-out or whether it will it try to undermine Arrowhead Research which has now gained access to, amongst other assets, Tekmira’s SNALP delivery technology and Alnylam’s own RNAi triggers. This could make the New Arrowhead a direct competitor of Alnylam, similar to Tekmira. While I am not sure yet whether Arrowhead management shares my view, but given the advancements of SNALP technology in the clinic, whilst DPCs have been stuck in pre-clinical for some time now, it would be obvious for Arrowhead to create near-term value by taking advantage of the well trodden path of advancing SNALP programs into the clinic. Of course, access to some of the newer lipids would be desirable.

For Arrowhead shareholders, this is likely a good deal given the depressed asset valuations in RNAi Therapeutics and Roche’s apparent desire to get out of RNAi Therapeutics. However, Roche will not be out entirely as it obtained a minority stake in Arrowhead and apparently retained licensing options and milestone/royalty streams to future products which are likely to be somewhat punitive considering that Arrowhead Research did not have much cash to offer.

Equally interesting for Arrowhead shareholders is a financing agreement with Lincoln Park Capital (LPC) under which LPC will provide up to $15M in funding over the next 3 years. Sounds familiar? If so, then you have probably seen LPC reach an almost identical deal with Marina Biotech just last week. The benefit for Arrowhead and Marina is that this source of capital brings some much-needed financial stability to the companies, just as Benitec used a similar arrangement with La Jolla Cove Investors to finance its recent transformation. One can expect, however, that LPC does not view their investments as charitable donations, and based on Benitec’s experience, it will both likely serve to support Arrowhead’s share price, but also somewhat cap its upside.

* An earlier version incorrectly referred to the waiting period as two years.

Following Late-Stage Pipeline Setbacks, Roche Cuts In-House RNAi Research as Part of Large-Scale Corporate Restructuring

The writing has certainly been on the wall. First, in an August 2010 interview, Roche’s Head of Pharma Research and Early Development, Jean-Jaques Garaud, was a bit lukewarm about the maturity of RNAi Therapeutics. While acknowledging that ‘siRNA' was a 'hot field’, this Roche executive who assumed his current role in 2009 following the retirement of Jonathan Knowles, stated that RNAi Therapeutics was a ‘bit like science fiction right now’, but that within 2-5 years Roche should make progress in that direction. Then in September, a tight-lipped Roche informed development partner Tekmira that it won’t meet its earlier guidance of filing for an LNP-enabled RNAi Therapeutics IND before the end of the year.

Of course, 2-5 years in today’s pharma world, a world challenged by patent expirations, poor pipeline productivity, ailing healthcare systems, and an ultra-conservative FDA, equates to ‘not relevant to our bottom line’ and are considered an investment with expected negative returns. Roche in particular has been hit hard recently by setbacks in their late-stage pipeline, as well as several blows to their all-important Avastin franchise which now makes the Genentech acquisition look a bit pricey. This is in stark contrast to what appeared to be the healthiest pipeline in the industry in 2007 when it took a license to Alnylam’s RNAi Therapeutics IP and when it acquired RNAi delivery company Mirus Bio in 2008.

So as Roche struggles to meet earnings expectations for the coming quarters, it is maybe not that surprising that as it cuts costs where it can, its in-house ‘siRNA’ research in Kulmbach, Germany, and Madison, Wisconsin, have to go as part of the large-scale re-structuring of the Company announced today.

I have today returned from the RNAiAsia meeting in Singapore, and it was during this trip that I have been particularly struck by just how conservative Big Pharma has become again when it comes to new therapeutic modalities. Only 3 years ago Big Pharma seemed committed not to repeat its mistake of not having participated early on in the development of recombinant proteins and monoclonal antibodies and then pay a hefty premium for acquiring biologics companies later on in the game, yet today the same companies don’t even want to give the impression that they want to lead in the unchartered waters of RNAi Therapeutics. Potential and promise are just that, and it seems like that there is no way around the conclusion that proof-of-concept has to be provided by the smaller pure-play companies before Big Pharma (re-)commits in a big way.

In addition, despite some attempts to improve Big Pharma’s capacity to innovate by setting up pseudo-independent ‘Centers of Excellence’, Big Pharma seems to be fundamentally organizationally challenged in maturing cutting-edge technologies like RNAi Therapeutics as a result of inflexible bureaucracies and insufficient/inefficient communication between the relevant groups within sprawling organizations.

I do not want to deny that some of Roche’s RNAi expectations were probably disappointed, and not all of it is Roche's fault. I wonder how mature Roche considered dynamic polyconjugate delivery to be when it purchased Mirus for $125M, or what exactly it got from Alnylam in return for over $300M in 2007. Not to say that these were outrageously overvalued deals if put in the proper context, but Roche management, watching their peers pull back from RNAi Therapeutics, must feel a bit exposed now.

The full impact of Roche’s own pull-back from internal RNAi research on its RNAi Therapeutics partners Alnylam and Tekmira remains to be seen. After having spent north of $500M on RNAi Therapeutics, it is hard for me to imagine that Roche will write this investment off just like that.

It is possible that Tekmira and Alnylam get a shot at Roche’s RNAi assets, including the one candidate that seems pretty close to the clinic (Tekmira reported Roche revenues in the latest quarter of $0.7M, mainly for the development of this candidate). With Severin Schwan, Roche's CEO, stating that Roche may ‘spin off’ or ‘find a partner’ for its RNAi assets, one may even speculate as to whether Roche has plans to float its RNAi Therapeutics division on the stock exchange. These are strange times in RNAi Therapeutics and the pharmaceutical industry and stranger things have happened.

RNAi Therapeutics delivery: not a zero-sum game

Shares of Tekmira briefly fell last week below 80c, a ~20% decline, following Roche’s purchase of fellow RNAi Therapeutics delivery company Mirus Bio. Initially, I did not draw a connection. If a reaction at all, then I would have expected the $125M price-tag to point out the value attached by the industry to promising RNAi delivery technologies in general, which “should have” resulted in a share price increase for Tekmira.

To my surprise, however, it then transpired that the Roche acquisition had been interpreted by some as a vote for DPC and against SNALP RNAi delivery, leading some to sell in a panic. This blog will briefly outline why, unlike is the case in the battle for core RNAi trigger IP, RNAi Therapeutics delivery is not a zero-sum game with room for more than just a handful of technology platforms.

As is becoming more evident by the day, RNAi Therapeutics offers the opportunity to address a very wide range of diseases. As all disease has a genetic element, regulating gene expression should always be able to modify or even correct a disease. To do this, however, one has to be able to knock down the desired genes in various different organs, tissues, and cell types. If there was a universal RNAi delivery technology, it would have to be able to physically contact every cell in the body, yet only knock down genes in the disease-related subset of cells.

Based on the limited publicly avaibable data, Mirus’ DPC technology actually promises to come close to that dream. It is very small in size (~20nm) and therefore satisfies at least one of the pre-requisites of achieving a broad biodistribution. Moreover, it has been shown to be capable of differentiating between the different cell types within an organ, in this case the liver, depending on which sugar had been added- all in the absence of apparent toxicity! It will clearly be exciting to learn more about the delivery profile of DPCs. Nevertheless, I doubt that DPC will necessarily be the delivery system of choice for every indication.

This is because the choice of a delivery system is not only determined by the ability to knock down a gene in a given cell, but also by factors such as overall maturity of the technology, safety as determined by the dose required to achieve such gene knockdown, the biodistribution and cell type-specific gene knockdown on a systemic level, as well as by cost, route of administration, and stability of the formulation.

Consider for example an RNAi Therapeutics against the kinesin spindle protein (KSP) that is being developed by Alnylam as a treatment for liver cancer. Although directly interfering with cell division by down-regulating KSP is seen as a very promising anti-cancer strategy, anti-KSP small molecule programs have indicated considerable dose-limiting toxicities that appear to be target specific. This, however, should not come as that much of a surprise since interfering with spindle function would be predicted to affect normally proliferating cells as well, particularly those of the hematopoietic system. Well, this dilemma is not new at all to the cancer field, and often the benefits outweigh the side-effects, but this example illustrates the value of a delivery technology such as SNALP-RNAi that can be tuned to deliver around 95% of the injected dose to the liver, including hepatic tumor tissue.

Certainly, Roche may elect DPC delivery over SNALP delivery (note: Roche has access to both) for some liver indications, but that type of competition has actually only decreased with Mirus Bio out of play. This is because all the other two dozen or so pharmaceutical companies equally interested in RNAi Therapeutics, will now have access to one less viable delivery technology. The incentive to gain access to the still fairly accessible RNAi delivery platforms with clinical potential has therefore never been greater. Roche, of course, enjoys the luxury of being able to choose between two technologies that, maybe in an effort to avoid the appearance of favoritism, it has valued essentially identically based on their $5M equity investment in Tekmira at $2.4 per share(Tekmira now has 51.6M shares outstanding).

Bottom line, due to the ever growing attraction of RNAi Therapeutics, any company, and that goes beyond Tekmira-SNALP, with clinically relevant RNAi delivery technology and a good IP package to protect it can rest assured that they own a very hot commodity.

RNAi Therapeutics portfolio update: With Roche’s validation of the value attributed to clinically relevant RNAi delivery technologies and with increased clinical visibility, I have slightly increased my position in Tekmira while paring back on the overweight ALNY position. In another attempt to capitalize on the idiosyncracies of the stock market, I have converted the RXi Pharmaceutical holdings back into CytRx shares (CytRx owns about 50% of RXi, but none of that is reflected in CytRx’s market cap) and added some more at the cost of largely cashing out of RNAi trigger competitor Silence Therapeutics (a token $1 investment is maintained to monitor stock performance).

Disclosure: Long Tekmira, Alnylam, Targeted Genetics, Oxford Biomedica.

Roche Nabs Mirus Bio’s RNAi Therapeutics Delivery Technology

Roche again...before this week will have come to an end, Roche has drastically changed the RNAi Therapeutics playing field leaving many heads in the RNAi space spinning.

You may think it is because I live in an RNAi Therapeutics bubble, but the Roche-Genentech press release and conference call to me clearly shows that a major motivation for the proposed taking private of Genentech was to broaden their RNAi Therapeutics efforts by bringing in a company with deep immunology and personalized cancer know-how, as well as being able to leverage Genentech’s monoclonal antibody capabilities for targeted RNAi Therapeutics delivery.

To this they now add for $125M, a sum that makes related companies look very cheap in comparison, the privately held Madison, Wisconsin, nucleic acid delivery company Mirus Bio. The jewel of Mirus Bio is their Dynamic PolyConjugates (DPCs), small, flexible designer particles for the targeted systemic delivery of siRNAs. Although the technology is relatively young and data scarce, from the PNAS publication last year (reviewed here in the RNAi Therapeutics blog) and conference presentations, DPCs are very competitive with liposomal technologies for delivery to the liver. Also very attractive from a safety and efficacy point-of-view is their apparent ability to selectively target silencing either to hepatocytes or Kupffer cells in the liver, depending on whether glucose or galactose-derivatives were attached. Although I haven’t seen data beyond the liver, the small size and modularity suggests that with the appropriate pharmacology it could well have applications for a number of other tissue types and organs and nicely complement larger nanoparticle delivery technologies.

It’s unlikely to be a coincidence that Roche is making all these moves in such short order. What has started with an IP license from Alnylam for basic access to RNAi mechanism of action, within 2 days they have now added to that one of the most coveted delivery technologies and scientific depth. The rapid moves by Roche means that fellow Big Pharmas like Pfizer, which had a non-exclusive license to DPCs, and Merck which had probably also been very interested in DPCs, now risk falling behind on delivery while their core RNAi IP has either not been secured yet (Pfizer) or is at best uncertain (Sirna Therapeutics/Merck). From a strategic perspective, it will be interesting whether due to their close relationships there will be any sharing/coordination of DPC technology with Alnylam and Tekmira, and for which indications Roche will employ the two leading delivery technologies (DPCs and SNALPs) both of which it has now immediate access to.

Today's acquisition is yet another piece of evidence that Roche is building their future on RNAi Therapeutics in a big and bold way. It's also encouraging that this comes a year after the Alnylam platform licensing agreement and suggests that they must have been pleased with what they have seen since.

2008: The Year of the Liver

It is true that next year could bring the first human proof-of-concept for an RNAi therapeutic. But since results from the experimental infection studies for ALN-RSV01 were originally expected in late 2007 and the results would be an overhang of work done in ’07, I find it more appropriate to pick a fresh candidate. It is also true that the lung may rival the liver in the number of programs moving into the clinic in ‘08 given recent data that suggest delivery of siRNAs to the lung by nebulizer to be fairly innocuous, and the fact that AstraZeneca and GSK should be busy working in that area following their agreements with Silence and Sirna Therapeutics, respectively.

Nevertheless, I feel that most systematic progress has been made in delivering RNAi to the liver with more progress likely to follow. There have now been a fair number of publications that clearly show target-specific RNAi knockdown in the liver, unlike maybe some of the lung studies where there were occasional data interpretation issues. Of course, in declaring 2008 ‘The Year of the Liver’, I should disclose that I am somewhat biased due to the hepatotropic focus of the laboratory that I work in.

For siRNA-mediated RNAi in the liver, I currently see three front-runners, although it is possible that among the considerable work that is ongoing and not yet published, others have reached a similar stage of technological maturity. These are the in-famous SNALPs, Mirus Bio’s “Dynamic PolyConjugates”, and MIT’s lipidoids. SNALPs, being worked on by Protiva, Tekmira, Alnylam, and Sirna/Merck, are probably 12-18 months ahead of the game and best characterized, whereas the data on PolyConjugates and lipidoids are very promising, but still somewhat spotty.

SNALPs, stable nucleic acid lipid particles, are set to enter the clinic in 2008 with programs in hypercholesterolemia and liver cancer (both Alnylam) and possibly a clinical candidate chosen by Tekmira. The hypercholesterolemia program is also an opportunity to get an early measure of therapeutic efficacy, possibly in ‘08. Protiva’s and Sirna/Merck’s intentions are less clear and I believe that Big Pharma often chooses to keep phase I programs secret for competitive reasons, so that it is theoretically possible that Sirna has already entered the clinic or is about to do so with the long-anticipated SNALP RNAi for Hepatitis C.

It is obvious that there were some delays in bringing SNALP RNAi to the clinic, and I believe that this is largely due to dosing and safety issues. Both of these concerns may come down to the propensity of SNALP liposomes to be taken up by immune cells such as Kupffer cells in the liver and plasmacytoid dendritic cells which a) function like a sink for the liposomes when they enter the liver so that SNALPs become available for entering the desired hepatocytes only after the sink is saturated, something that complicates dosing; and b) increases the risk of triggering unwanted cytokine responses. I am optimistic, however, that by varying the composition of SNALPs, safe and efficacious formulations may be found, particularly if the liposome uptake mechanism by the professional immune surveillance system were to be different from that by the hepatocytes, which I think is quite reasonable to assume. I therefore hope that the fact that so many scientists are working on SNALP RNAi is a sign of its promise rather than desperation. As such, the number of R&D staff at Tekmira has more than doubled from 17 to 39 in the year ending September 2006 largely due to work on SNALPs.

As I have written before, I am much taken by Mirus’ PolyConjugate work, although this is based on only a single paper published in the middle of ‘07 (see 24 July 07 Blog: “Mirus Scientists Publish Elegant Paper on Targeted siRNA Delivery to Hepatocytes”). The neat aspect of that work was that it showed that it should be possible to avoid the Kupffer cells in the liver and specifically target hepatocytes for knockdown with the help of carbohydrate ligands (I am curious whether similar targeting ligands would also work in the context of other formulations). As we know, this work is partnered with Pfizer, and I wonder whether the upcoming loss of exclusive marketing rights for their wonder-drug Lipitor will spur them into action here.

I am still waiting for more data on lipidoid-mediated RNAi which hopefully will become available soon as was indicated in the footnotes of an October 2007 paper on the effect of siRNA delivery on microRNA function. From both a business and scientific perspective, it will be interesting to see whether Alnylam may choose lipidoids over SNALPs for its first liver RNAi programs and the overlap of SNALP and lipidoid both in IP and manufacturing terms.

The liver may get further attention from the targeting of microRNA-122 for the treatment of hypercholesterolemia and Hepatitis C, and other programs on targeting certain microRNAs for the treatment of hepatocellular carcinoma. There are various programs by Santaris, Regulus and others that have progressed into larger animals and we may even see a first IND being filed for one of these applications by the end of 2008. Development in the antisense field, particularly the partnering of ISIS’ mipomersen, should also generate heightened awareness for RNA-based therapeutics of liver disease.

Other predictions for 2008:

1) Unpredicted delivery technologies
2) Pfizer finally makes its move
3) News on RNAi for wet AMD- good and/or bad?
4) Protiva-Tekmira dispute resolved (wishful thinking)

For a nice presentation on the powers of SNALPs by Protiva chief scientist Ian MacLachlan, please visit: http://mms.technologynetworks.net/RNAi06Presentations/maclachlan/Player.html

SomaGenics Reports on the Use of shRNAs for the Treatment of HCV

At the end of a day packed full with fun RNA science from the San Francisco Bay Area, Brian Johnston, President and CEO of SomaGenics presented some of his company’s data on the use of shRNAs for the treatment of HCV, historically one of the early therapeutic targets for RNAi.

Most shRNA approaches are based on DNA-directed expression of RNA hairpins in the nucleus that are then fed into the RNAi pathway. Somagenics’ approach is slightly different in that they are introducing the RNA directly into cells. Concentrating more on the science than IP issues here I only want to briefly remark that this probably overlaps quite a bit with the Hannon patents (licensed to both CytRx and Alnylam) on the use of shRNAs as RNAi triggers, both transcribed and synthetic.

The scope of the experiments were limited either to in vitro culture studies or its glorified in vivo counterpart, the hydrodynamic co-transfection experiment. Here, a large volume of RNA-containing liquid is injected into the tail vein of a mouse in a brief period of time, physically forcing the RNA into hepatocytes. It is of interest, that unlike the findings of the back-to-back papers of the Hannon and Rossi labs more than two years ago, hairpins (directed to the conserved HCV IRES) with minimal stems, 19 base-pairs, were generally more effective than those with extended 25 base-pair hairpins. While it has to be said that compared to the Hannon and Rossi papers which argued that the long stems that were processed by Dicer would increase RNAi efficacy, the sample size here was quite limited, it nevertheless shows that the processing of hairpins are often not as predictable as one would wish based on microRNA biology.

Johnston further reported that the shRNAs were at least as efficient as the corresponding siRNAs with some of the IC50s in the low picomolar range. I am curious how this will translate into in vivo uses for example because shRNAs, unlike classical siRNAs and Rossi-type two-stranded Dicer-substrates, contain the double-stranded RNA within one molecule which should make this structure thermodynamically quite stable. A potential disadvantage is that it is so much more expensive to synthetically make the ~42-55 nucleotide hairpin RNAs since cost and purity of manufacturing RNAs increases more than linearly after you reach a certain size, say 25 nucleotides. While these studies mostly used RNAs generated through biochemical synthesis in the test tube using recombinant phage RNA polymerases, for the clinic they are working together with Agilent to generate the “same” RNAs to scale. I say the “same” here since the phage polymerase leaves a triphosphate 5’ end while chemical synthesis does not. This is not a trivial issue here since 5’ modifications are known to influence RNAi processing. We will therefore have to wait how the transition from in vitro transcribed to synthetic shRNAs will affect the reproducibility of their data and potential concerns from the FDA. Similarly, some mention was made that shRNAs may tolerate RNA modifications less well than siRNAs, probably because they are subject to additional processing steps.

The use of more challenging animal models is also warranted before entering the clinic. It is therefore important for them to find suitable delivery solutions which they apparently have only started to. It was good to hear though hearing him mention Protiva/Tekmira’s SNALPs and Mirus Bio’s Dynamic PolyConjugates as probably the most advanced RNAi delivery platforms to the liver, reflecting my views on what’s out there in the literature. Overall, I like the fact that multiple RNAi approaches are in the pipeline to tackle HCV, namely siRNAs: Sirna/Merck; DNA-directed shRNAs: Tacere/Benitec and Nucleonics; and now synthetic shRNAs: SomaGenics. And maybe Tekmira/Protiva should harness the power of SNALP and officially nominate HCV as a clinical development program. I hope that the next 2 years should finally see programs moving into the clinic after HCV RNAi had been delayed in the wake of company-specific issues (Benitec; Merck/Sirna vs Protiva).

Mirus Scientists Publish Elegant Paper on Targeted siRNA Delivery to Hepatocytes

The ability to systemically administer siRNAs and functionally modulate gene expression in tissues of interest is considered by many an important step to opening up the therapeutic potential of RNAi to a wide range of diseases. The liver is arguably the best example where intravenously administered siRNAs have already been shown to potently knockdown target genes from mice to non-human primates.

The most promising delivery technologies so far (next to gene therapy vectors such as AAV which I will not discuss here) involve liposomes such as the SNALP particles which were first pioneered by Protiva scientists and subsequently became the subject of intense legal battles involving Protiva, Tekmira, Sirna Therapeutics, and Merck. While facilitating highly efficacious gene knockdown, toxicities such as liver enzyme elevations and non-linear relationships between knockdown and siRNA dosage have been repeatedly reported with these chemistries. While the toxicities were observed at relatively high dose levels, non-linear dose responses complicate the choice of the right dose for entering clinical trials.

One step in the right direction was taken when Daniel Anderson from the MIT reported earlier this year at the Keystone meeting the identification of a slightly different class of compounds which they termed “lipidoids”. Importantly, this class of chemistries appear to efficiently promote RNAi gene knockdown with little if any apparent toxicities and linear dose responses.

The just released paper by Rozema and colleagues from Mirus Bio Corporation (Rozema et al. PNAS Early Edition 24 July 2007: Dynamic PolyConjugates for targeted in vivo delivery of siRNA to hepatocytes) sheds light on some of the issues above and holds out a new paradigm for achieving safe and efficacious therapeutic RNAi knockdown in select cell populations of the liver. Rather than regarding drug delivery to the liver as a passive process given that the bulk of intravenously injected drugs will pass through it with a relatively high chance of entering resident cells there, the present approach involves attaching a simple galactose-derived ligand thereby actively targeting asialoglycoprotein receptors (ASGPr) displayed on hepatocytes.

Knockdown of ApoB100 and microscopic analysis of fluorescently tagged small double-stranded nucleic acids confirmed that hepatocytes were indeed efficiently transfected. Equally important, however, was their observation that Kupffer cells, a major population of macrophages in the liver that are implicated in many cases of drug-related liver toxicities, did not take up the siRNA mimicks, whereas non-ASGPr targeted particles were able to transfect surrounding cells, including Kupffer cells.

Taken together with the SNALP and lipidoid data, this study supports the hypothesis that Kupffer cells may act as a sink for certain siRNA formulations such as SNALPs, thereby not only causing non-linear dose responses, but also an immunogenic response and related toxicities. It also shows that it should be possible to design simple and small siRNA nanoparticles for RNAi delivery from relatively cheap materials. Although lipidoids appear to be a clinically viable technology already, it is comforting to know that alternative routes exist and gratifying to see almost daily improvements being made in the RNAi delivery field.

PS: An interesting aspect of this publication was data the authors obtained on ApoB knockdown. In addition to serving as a target for early proof-of-concept studies for systemic RNAi delivery, ApoB has been a favourite for treating hypercholesterolemia using both antisense and RNAi. ISIS Pharmaceuticals in particular has an anti-ApoB antisense compound in late phase II clinical trials. However, only a month ago, Alnylam reported that in their hands ApoB knockdown led to unacceptably high levels of fat accumulation in the liver (fatty liver phenotype), a finding that was confirmed in the present study. Moreover, both reports indicate that this was a siRNA sequence specific effect and was achieved using two different delivery methods. This physiologic response makes a lot of sense, since ApoB’s main role is in the export of cholesterol and triglycerides from the liver. Failure to export them should accumulate them in the liver.

This is a good example where RNAi can serve both as a target validation tool AND a platform for developing innovative drugs. Consequently, I fully support Alnylam’s decision to target PCSK9 in their hypercholesterolemia program instead, which is a genetically well validated target for reducing LDL-cholesterol and heart disease (see also Blogs from 6 May, 2007: “Preventing Heart Disease with RNAi Therapeutics”, and 9 May, 2007: “ISIS Copies Alnylam’s Heart Disease Strategy“). It is curious then that ISIS maintains and has published the absence of such a phenotype. For the sake of patients and stakeholders in ISIS, I can only hope they are right. The explanation for the apparent discrepancy? I cannot really offer a good one except to speculate on a fortuitous ISIS 301012 antisense off-target effect.