Arrowhead Publishes SubQ Delivery Technology to Go Beyond the Liver

In late 2012, Arrowhead Research shocked the Oligonucleotide Therapeutics world when it presented spectacularly potent and prolonged gene knockdown data in non-human primates using a subcutaneously administered single-molecule Dynamic Polymer Conjugate (DPC) formulation.  This arguably represented the most elegant delivery technology at the time.  Moreover, also due to its small, but not too small size (10-20nm) and slight negative charge, it provided us with a glimpse into the future of systemic RNAi delivery for regulating genes beyond the liver.

It certainly got my full attention and made me invest almost 100% of my stock portfolio back then.

Unfortunately, despite the validation in non-human primates which suggested clinical readiness would not be far off, the subQ DPC technology has seemingly struggled to reach clinical/commercial maturity. Not only Arrowhead’s lead development candidate, ARC520 for HBV, but also its second development candidate, ARC-AAT for AAT-related liver disease, was still based on the intravenously administered two molecule DPC version.  Although the reasons for the delays were never really disclosed, a few comments here and there made it seem very likely that chemistry and manufacturing issues were behind the delay.

Back to the Future

Last week, Arrowhead Research finally published a paper showing that single molecule DPC is still alive and kicking (Rozema et al.2015) and is progressing towards clinical application.  In essence, the new single-molecule subQ DPC prototype comprises of a membrane-active polymer which has been masked from premature cytotoxic interactions by pegylation and cell-targeting ligands that are added via protease-sensitive bonds; as before, the highly modified/stabilized RNAi triggers are appended by disulfide chemistry. 

The DPC is made in a 4-step process followed by a purification step to remove unwanted side-products and reactants.  The latter step is apparently important when going into primates.

The new old DPCs are thus distinguished from the intravenous version not only in that it combines the RNAi trigger and endolysosomal release polymer in a single molecule, but most importantly by the nature of its triggered release mechanism.  Whereas in the former DPC generations triggered release was dependent on changes in pH such as they occur when a DPC is endosomally taken up, they are now responsive to the presence of certain proteases in lysosomes. 

pH-dependent formulations apparently suffered from instabilities both in the body and during storage.  This was adequate for targeting genes in the liver because of the ready access of macromolecules in the circulation to this organ following intravenous administration, but not when the DPC first has to reach the circulation from the subcutaneous space and when less well accessible target organs are the ultimate destination.

Accordingly, non-liver single-molecule DPCs of the latest publication had impressive circulation half-times of the intact, protected molecule of 11 hours.  Similarly, such DPCs are stable for at least a year both in solution and when lyophilized. 

The extra-hepatic potential thus facilitated by increased stability now needs to be demonstrated by finding suitable targeting ligands and I’m sure Arrowhead has been busy working on that.  It should be noted that for target tissues where high concentrations comparable to the liver are unlikely to be achieved following systemic delivery, the extra kick that comes from an explicit release chemistry could provide a critical advantage over competing approaches.  These include simple conjugates of the GalNAc-type and probably also self-delivering RNAi trigger chemistries which incorporate ‘milder’ release chemistries (like lipid tails).

Knockdown lasting for weeks and months

The most impressive demonstration of the single molecule DPC performance in the Rozema paper came from the primate studies.  Here, a single administration of 0.5mg/kg 2’-O-methyl/F-modified RNAi trigger led to a highly potent knockdown (peak knockdown >95%) of liver expressed Factor VII with >80% knockdown of 2 and 4 months following subcutaneous and intravenous administration, respectively.

Following the 2012 delays and some uncertainties around what was really new and old in the recent publication, I am somewhat hesitant to declare that subQ DPC is now fully de-risked and ready-to-go.  In that regard, it would be helpful to learn more about the tox profile of the new molecules and related to that which polymers will be eventually used (e.g. 2-molecule with melittin-like peptide, a polyacrylate in the publication).

Nevertheless, since Arrowhead has said that the new 2015 development candidate may be from the subQ line of DPCs (or if not going after a extra-hepatic target) one would think that the most important challenges have now been overcome.

The Tide May Have Turned for ARWR

Arrowhead Research emerged as a major RNAi Therapeutics player due to its- at least publicly- single-minded focus on HBV.  During this time (early 2013-early 2014) it saw a meteoric rise in its stock by more than 10-fold.

Management got so caught up by their own campaign of pushing Arrowhead Research as an HBV stock that they set themselves up for failure by setting overly ambitious goals for that program. 

As a result, the stock plummeted almost as rapidly as it had risen first by the Fed-induced biotech sell-off in spring 2014, and especially after first clinical results (see here and here) of ARC520 in HBV-infected patients did not live up to the hyped-up expectations.  90% HBsAg knockdowns had been the stated goal for a single-dose 2mg/kg.  This was despite preclinical studies which suggested that more than 2mg/kg of the endosomolytic DPC component was needed to achieve such robust knockdowns.

While my jaws certainly dropped in disbelief when I heard this, in my mind this has to be chalked up to a lack of full understanding of their company's own technology rather than gross misconduct.

…but for me it has always been subQ, subQ, subQ, extrahepatic

While I very much liked the fact that Arrowhead Research was at the very cutting edge of the ‘HBV-The-Next-HCV'  wave, what originally got me all fired up about Arrowhead Research was an OTS presentation in late 2012 where they presented impressive (robust and long-lasting) knockdown in non-human primates with a subcutaneous, most likely single-molecule version of their DPC delivery technology.  Knockdown that was more potent than anything out there (Alnylam GalNAc-STC at the time) combined with the convenience of subcutaneous instead of intravenous administration.  The latter is practiced with their more advanced two-molecule DPC version underlying ARC520 and ARC-AAT in the clinic already.

Single-molecule DPCs should also be the foundation for reaching tissues beyond the liver, making the transition back to single-molecule DPC all the more valuable.  Given that the liver has been solved for oligonucleotide therapeutics with Alnylam’s and ISIS’ GalNAcs, opening up new tissues to RNAi is obviously all the more attractive.

It is unclear what held the company back from taking the non-human primate achievements almost 3 years ago into the clinic.  Scale-up manufacturing issues rank highest on my list of possibilities.

Company guides for 2015 IND for either subQ liver or extrahepatic i.v. candidate  

During this week’s Q4 earnings conference call, the company indicated that they have finally achieved long-awaited technological breakthroughs so that we can now expect them to file an IND for either a liver target using for the first time a subcutaneous DPC formulation or an IND for an extra-hepatic target. 

Correction/clarification (2 Feb 2015): The company contacted me to clarify that what they said was that they will file an IND in 2015, and in addition to that, nominate a new development candidate that will either be extrahepatic or a subQ liver candidate.

Interestingly, if the extrahepatic program should make it to the finish line first, it would still be administered intravenously, which leads me to believe that it is a target in the kidney which I consider the only other obvious target tissue amenable to 2-molecule DPC.  If the target cell is not the proximal tubule cell, it would suggest that Arrowhead has identified a GalNAc-ASGPR-type ligand-receptor pair for the kidney.

ARWR 2015 playbook

Be it as it may, the prospect of both a highly competitive delivery technology for the liver and the availability of a new target tissue makes this a highly attractive re-entry point into ARWR.  At $6+ down from the mid $20s not even a year ago and with almost half of its valuation in cash, I do not see much downside from the 3-4mg/kg results of ARC520 to be reported in Q2 2015. 

Personally, I expect an 80-90% knockdown at 4mg/kg, but since I have no idea how the market would react to an 80% knockdown, the results are a coin toss to me, but with a somewhat larger upside (up to $14) than downside (down to $5) from here.

If the stock trades down, but somewhat dependent on the safety data, it may be an opportunity to snap up ARWR for the ARC-AAT phase I results coming up by the end of the year.  I consider ARC-AAT a very robust program with increased knockdown potency compared to ARC520 and much less ambiguity around what an X% knockdown means.

 

Right now, Arrowhead Research is an ARC520-only story and that should change once ARC-AAT becomes recognized as a medically and commercially very attractive product candidate (e.g. an orphan indication with an estimated 100.000 patient population in the US alone).  And I am convinced that I'm not the only investor to recognize subQ and extra-hepatic as the ultimate value drivers for ARWR all of which could propel the stock back to its 2014 highs over the next year.