Demystifying ddRNAi Trigger Design

A somewhat vexing problem with DNA-directed RNAi (ddRNAi) has been the heterogeneous mixture of small RNAs generated from a typical expression vector.  Not only will this compromise knockdown efficacy through competition for RISC loading, it also poses a safety risk by increasing the number of off-target genes.  For a long time, heterogeneous ddRNAi trigger processing had been accepted as a way of life- as long as the efficacy was right you would not waste too many thoughts on those multiple bands on your Northern blot.  

Choosing the double-strand length of a ddRNAi trigger has also been more art than science.  Different sizes, usually between 19 and 29bp facilitate potent gene knockdown, but there have been few studies looking at the consequence of size on the uniformity of hairpin processing.   

Next-generation sequencing technology is changing these attitudes.  In a paper that appeared last week in CELL, Gu et al. from the Kay lab in Stanford showed that next-gen sequencing is a powerful tool to detect which small RNA species are generated from a given ddRNAi template plus their relative quantities.  Moreover, through an iterative process of structural change and sequencing, a key structural feature causing the Dicer enzyme make just one predictable cut could be identified.  Without going into much detail, a simple 21 base-pair hairpin yields the purest results.

Commercial ddRNAi trigger landscape getting rusty  

While the clinical validation of SNALP delivery technology has allowed synthetic RNAi trigger-based Tekmira and Alnylam to turn around their fortunes and should also increase general interest in the technology, there seem to be no commercial players left in the ddRNAi Therapeutics arena able to champion and refine the platform. 

Benitec is pre-occupied with trying to monetize IP that is rapidly losing in value due to its age and as a result failing to become a real biotechnology company with a lab.  The two viral delivery companies that once had some ddRNAi ambitions, AMT (AAV) and Oxford Biomedica (lentivirus), either went out of business and/or lack the requisite RNAi molecular biology expertise.  And the other groups developing ddRNAi Therapeutics, including Calimmune and Genables are de facto one-product, disease-focused companies.  This is unfortunate as there should be room for at least one or two ddRNAi Therapeutics platform companies.

Such a company would have expanded on the basic dsRNA concept and would thus have kept its IP estate fresh, eventually forcing others to take a license as simply running out the patent clock would not have been an option.  Such IP would include discoveries like the one made by Gu and colleagues.  On a more positive note, it is possible that the current revival of gene therapy after a decade of neglect and scorn will eventually carry ddRNAi Therapeutics along with it.

Run-Down of Companies in the RNAi Therapeutics Portfolio

Alnylam: As the bellwether of RNAi Therapeutics due to its IP position, maturing pipeline, strong balance sheet and a generally broad outlook on RNAi Therapeutics, a must for those (institutional) investors with significant funds to invest in the RNA Therapeutics space. Data from Alnylam’s Huntington’s Disease and RSV programs suggest that they have potential on their own, independent of how they contribute to the learning of RNAi for CNS and lung disorders in general. Surely, the hiring pattern of Big Pharma argues that the perception of RNAi as a therapeutic modality has not gone out of fashion there, immediately adding potential licensees to Alnylam’s leading RNAi trigger portfolio.

However, as it is delivery that potential licensees and investors are increasingly paying attention to and even cash-rich Big Pharma/Biotech will question whether it should spend $300M for an RNAi trigger license now that there have been a few decisions that did not go in Alnylam’s favor, I am not sure whether we will see a simple pre-packaged RNAi trigger platform deal. Rather, such IP access may be packaged with access to Alnylam’s know-how on the delivery, chemistry, and safety of RNAi Therapeutics, somewhat reminiscent of the Roche deal and the Kulmbach component. To set up such deals may take longer, but ultimately provide more value not only for the licensee, but also for Alnylam. Certainly, positive SNALP clinical data should prove as a catalyst for these negotiations and the stock.

Tekmira: If you did not know already, my favorite RNAi Therapeutics investment right now. Pioneered the, in my opinion, most advanced systemic RNAi delivery technology, SNALP, which renders the Canadian $50M market cap company fully exposed to the major value drivers in the space near- to mid-term. The well validated ability to deliver oligonucleotides to the liver with SNALP will make Tekmira not only an attractive collaborator and acquisition target in RNAi Therapeutics, but should offer it new business opportunities outside the traditional siRNA structure. This includes various forms of microRNA mimics and inhibitors, immunostimulatory oligonucleotides, and oligos for targets based on emerging non-coding RNA pathways or other knockdown mechanisms. Mir-122 inhibition with SNALP may be an interesting pharmacologic alternative to the naked LNA-anti-miR122 by Santaris now in late phase I studies. Demonstrating the utility of SNALP outside the liver, such as for solid cancers and cells of the immune system (maybe by using targeted delivery) could further increase the perceived value of this conservatively managed company. With about two years’ worth of burn in a relatively good financial position.

I should temper my enthusiasm, however, as there are no sure things in biotech and the first use of SNALP in Man may well yield some unpleasant surprises and could dramatically change the outlook for the company. Similarly, it needs to think ahead about how to access a broader investor audience outside of the Canadian market as its own pipeline is growing in size and capital demand. A good bet nevertheless.

Benitec: Faces an uphill battle with regards to their core DNA-directed RNAi patent, essentially pitting it against mighty Fire-Mello. However, as time progresses and the ’099 Graham patent not getting any younger, I’m starting to have doubts as to how important this patent will prove to be. Other patents assigned to Benitec, the HIV programs, and potentially the Biomic collaboration may prove to be of more immediate practical value to the company. What is needed, of course, is a re-capitalization of the company.

Targeted Genetics: This cat has 8 lives. I had been quite confused that after almost everything scientific and clinical was going in Targeted Genetics’ favor, the company was rapidly approaching bankruptcy. Society and the investment world are not always fair, which is a warning to those investing just according to scientific principles. Now, Genzyme has come to the rescue, but it remains to be seen how committed the company is to its RNAi pipeline. It would make sense for RNAi to be part of a company focusing on diseases of the eye (and CNS) for which AAV and lentiviral gene therapies currently have most promise (the eye as the liver of DNA-directed RNAi).

RXi Pharmaceuticals: Experienced management and scientific team, access to Tuschl I and preferential treatment by the state of Massachusetts, yet for some reason very little drive towards the clinic and financially challenged. Instead of a clinical pipeline, a pipeline of ‘interesting’ RNAi trigger and delivery approaches. I’m still not sure about what their rights to Tuschl I are that have recently been characterized as ‘limited’ in scope. This, however, and the Massachusetts/Mello connection are probably the biggest draws for the stock. Still, without being able to offer complementary practical know-how I would think Big Pharma is not too anxious to access RXi as a partner.

Silence Therapeutics: Similar to RXi Pharmaceuticals, stands to greatly benefit depending on the messiness of the Tuschl patent outcomes where Silence’s ability to operate in the 21-23nucleotide space is at stake. Also reasonably successful in battling the patent that most likely imposes most constraints on the company, namely Kreutzer-Limmer. While nobody would doubt Alnylam’s freedom-to-operate (however questions have been raised as to the degree of being able to exclude), my fundamental question about Silence Therapeutics is whether what may be a patent work-around also makes for the best scientific approach. Their underlying patent application is based on quite limited data, so I have yet to be convinced of any real generally applicable scientific advantage of the Atu-RNAi design (nevermind, at least in terms of IP, my opinion does not matter much since the European and US patent offices appear to concur with Silence). Things have been looking up recently for the company and its Atu-027 program for advanced solid cancers has started phase I dosing. This program aims at silencing the PKN3 kinase in the endothelia of solid tumors which apparently inhibits metastatic spread through reduced lymphangiogenesis. An interesting approach towards RNAi cancer therapy and has been described in a detailed company publication late last year that provided reasonable support for bona fide in vivo gene knockdown using lipoplex delivery (Aleku et al., 2008). One interesting point I found in that publication was that in cynomolgous monkeys, the circulation time of the particles was greatly extended to what they found in rodents. This can only be a good thing for the prospect of lipid-based nanoparticles.

mdRNA: Together with Targeted Genetics, another unlikely survivor from the financial crisis coming from Seattle. Two deals with Big Pharma, one on delivery (Novartis), and one on siRNA structure (Roche), early this year contained enough upfront to give the company another couple of months to get itself on sounder footing. Similar to Silence Therapeutics, their main delivery approach consists of essentially the same chemistries as contained in SNALP and apparently lends itself to targeted delivery (which, however, is not a unique property of their technology). Also, I would be cautious about claims that putting UNA-modified nucleotides in the 3’ overhang of siRNAs would liberate them from claims in Tuschl II. UNA modifications appear to be a viable option for the siRNA modification toolbox, but I would be cautious in how far they are uniquely advantageous over other chemistries at last according to an excellent, comprehensive siRNA modification screen as published by Bramsen et al. this year. In any case, the fresh, and apparently well-connected management team can be congratulated for rescuing the company, and the scientific team for their skills in being able to rapidly adopt oligonucleotide modification and liposomal delivery skills at least to the degree that Big Pharma is curious enough to look under their hood. I would like to speculate that if RXi e.g. had built such practical skills in-house, we may have seen some deals that would not have been as dilutive as recent efforts to raise capital.

Rosetta Genomics: After having apparently staked their future on a blood-based test for colorectal cancer screening, it has reported that these plans have been slightly delayed due to technical issues. The poster on the colorectal cancer-related microRNAs in blood that had been presented previously certainly showed initial proof-of-concept for blood-based microRNA diagnostics, but more robust detection methods are needed in order to make such tests a commercial reality. It is debatable whether the one-shot strategy was a wise one, instead of churning out a series of tissue-based Dx albeit with a much smaller target market. If blockbuster products like a screening test for colorectal cancer were a primary business goal, then an alternative route may be to collaborate on Rx-Dx combinations which however are much onerous to develop than home-brew Dx and may require a partner like Roche. There may be a number of regulatory and health care reform issues that could affect the future prospects of being able to sell and get reimbursements for home-brew Dx. On a positive note, according to my literature, I have stumbled across enough references by Big Pharma on the potentially unique utilities of microRNA Dx that I believe the concept has well arrived in the minds of important constituencies for Rosetta. A pick-up in sales of their first products would also be welcome by investors. Due to a number of synergies, companies like Regulus may also be a good home for Rosetta Genomics.

ISIS Pharmaceuticals: There is certainly a flood of ISIS-related antisense in various stages of clinical development, some with interesting results indicating efficacy. Mipomersen meanwhile blazes the trail for ApoB as a target for hypercholesterolemia, and assuming it will confirm phase II results, I am curious about how much of the patient audience Genzyme is able to capture. This should also have implications for the financial potential of follow-on ApoB therapeutics. ISIS also was successful in monetizing their IP for ssRNAi with almost $21M (! a high number considering the stage of ssRNAi and other deals that Alnylam has done in the past) in upfront and near-term payments from Alnylam, while still being able to develop ssRNAi Therapeutics itself. OK, you know that I have some problems with how ISIS likes to interpret RNAi as an antisense technology, so please allow me this comment: if RNAi already was a single-stranded antisense technology, how is it then possible to claim ssRNAi as a separate technology without running afoul double-patenting rules? Anyway, I acknowledge that ISIS is on a good way of becoming a sustainable, profitable oligotherapeutics company and probably should be part of a diversified RNA Therapeutics portfolio.

Oxford Biomedica: Despite disappointing cancer vaccine results causing partner Sanofi-Aventis to give up on Trovax, Oxford Biomedica must have been able to impress Sanofi-Aventis with their core leniviral delivery technology (note: Trovax is not a lentiviral technology). Sanofi-Aventis thus seems to agree that lentiviral delivery has significant potential for applications of the CNS, including the eye. However, as I hear little about Oxford Biomedica using its IP and know-how in RNAi, I will consider replacing it with companies like Genesis R&D in my next portfolio update. There is certainly a lot of DNA-directed RNAi Therapeutics technology and IP looking for a well-funded home.

[Please note that the following is a run-down of my own impressions of companies in the portfolio and links are not necessarily provided for all major assertions. Please use the comments section if you believe that there are factual inaccuracies]

Portfolio Review: RNA Therapeutics Deals Help Sector amid Recession Fears

It seems the markets decided to wait for the New Year for the cracks from the credit crunch to finally start spreading throughout the economy. Not surprisingly, stocks have taken a plunge amid eye-popping write-offs by the major investment and commercial banks. Fortunately, however, the RNAi Therapeutics sector was largely unharmed, partly supported by a strong pharma sector considered a safe harbor in economically difficult times, but largely because of the two remarkable deals involving RNA-based drugs.

One was for ISIS’ mipomersen for the treatment of hypercholesterolemia, which although an antisense and not an RNAi compound further highlighted interest in RNA-based therapeutics in general; the other was for Tacere’s AAV-shRNA TT-033 gene therapy for HCV and represented the first licensing of an DNA-directed RNAi Therapeutics program by Big Pharma (Pfizer). This happened the same week that I attended a presentation by a VP responsible for RNA therapeutic drug development for a major pharmaceutical company, who admitted that the pharmaceutical industry was a failing business model which may only be saved by technologies such as RNAi (stressing especially the potential cost and time savings). Clearly, RNAi Therapeutics is and will continue to be a hot commodity in 2008 and beyond.

Interestingly, while earlier RNA therapeutics deals tended to lift the entire sector, two companies that particularly live on partnering hopes failed to participate in the rally: Silence Therapeutics (-11.4%) and Nastech Pharmaceuticals (-4.5%). It seems that the lift in Alnylam’s share price (+8.8%) indicates that investors are starting to focus on the proven players with solid balance sheets at a time when biotechs may find it difficult to raise funds in the capital markets.

Portfolio Transactions:

Sells:

Benitec: I will take profits from Benitec by selling 1/3 after it exploded on the Tacere deal (Benitec retains significant stakes in TT-033 after it spun out Tacere). This will also avoid Benitec becoming too large a position. Moreover, the exclusive license award to the fundamental Fire-Mello patents to Oxford Biomedica this week for lentivirally-delivered expressed RNAi further may be interpreted as a negative development for Benitec’s IP position on DNA-directed RNAi, including its phase I program for HIV which makes use lentiviral RNAi technology. Still, Pfizer has now repeatedly shown interest in Benitec’s RNAi technology, and one has to wonder whether more deals are in store.

Nastech: The company announced this week, without giving an explanation, that Philip Ranker resigned as CFO. My motivation for including Nastech in the portfolio was based on hopes for a successful spin-out of RNAi Therapeutics subsidiary MDRNA. A delay in the mdRNA spin-out, however, may force Nastech to seek the capital markets for a secondary, thereby risking massive dilution at current price levels. Seeing the CFO go at this juncture forces me to reduce my position from 8.8% to 5% of my portfolio.

Buys:

Oxford Biomedica: Four years ago, Oxford Biomedica first piqued my interest when they presented impressive data on targeting SOD1 for the treatment of a form of amyotrophic lateral sclerosis (ALS; subsequently published in Nature Medicine in 2005: Ralph et al. Nature Medicine 11:429). Finishing up my D.Phil. in Oxford I even enquired whether they were looking for scientists working on RNAi gene therapy, but was told that they were pursuing this more as a tool, not a therapy. Clearly, with the new license, the Alnylam of lentiviral gene therapy technology is now officially part of the RNAi Therapeutics community. A broad clinical pipeline, a solid financial position following a high-profile deal last year with Sanofi-Aventis for the phase II/III cancer immunotherapy Trovax, their track record of scientific accomplishments and an unparalleled IP estate in lentivirus technology are worth 6% of my model portfolio.

ISIS Pharmaceuticals: The remaining cash from the sales of Nastech and Benitec will be invested in ISIS since I believe that the markets have not fairly valued the Genzyme transaction. In the long-run, it will not only help validate ISIS’ antisense chemistry in investors’ eyes, but also finally gives ISIS the financial strength to realize the full potential of their technology.