Arrowhead Research Patent Application Shows Ample Experience with Triantennary GalNAc-siRNAs

[Warning: this blog entry is not about to discuss a very recent development, but rather is intended to compare the liver gene knockdown technologies by Arrowhead Research and Alnylam based on a review of the patent literature; for the non-technical folks, a mention of Novartis towards the end might be of interest]. 

It has become clear that the DPC technology by Arrowhead Research, especially their 2-molecule-version used in ARC520 for chronic HepB, and Alnylam’s GalNAc-siRNAs share a number of features.  Based on patent application by Arrowhead Research that published last summer, there is evidence that the company has ample first-hand experience with the platform used by Alnylam.  Importantly, the data show that the addition of an endosomal release agent greatly increases the potency of GalNAc-siRNAs.

Large increase in potency with endosomal release polymers

Previously, I had speculated that simple GalNAc-siRNA conjugates as advertised by Alnylam have insufficient potency.  Accordingly, patent application US2012/0136042A1 by Alnylam showed that whereas simple GalNAc-siRNA conjugates had no or very little knockdown activity, the addition of a lipidic pharmacokinetic modulator with some endosomal release activity such as cholesterol allowed for more robust activity (see first image below).

Demonstrating the superiority of DPC delivery technology for gene knockdown in the liver, at least in terms of potency, the patent application by Arrowhead Research shows that even so, the activity of a GalNac-lipid-siRNA pales in comparison to its use along an endosomal release polymer: no knockdown with GalNAc-palmitoyl-siRNA alone, but an 80% knockdown when given together with the polymer (see table).

Note that a range of lipids, including cholesterol were evaluated in that patent application.  Also note that the preferred GalNAc-conjugation was the same triantennary GalNAc structure as used in Alnylam’s programs.  

Intellectual property consideration

While the Alnylam patent application claims priority to sometime in 2007, the Arrowhead patent application claims priority to sometime in 2010.  Given that the data suggest that Arrowhead should have chosen the triantennary GalNAc-cholesterol-siRNA backbone for ARC520 for maximum potency with the 2-molecule DPC approach, instead of the cholesterol-siRNA that they eventually chose, it is possible that IP concerns played a role in that decision. 

Another, non-exclusive explanation might have been manufacturing cost concerns which should favor simple cholesterol conjugates over triantennary GalNAcs.  This would also be justified in that it is the toxicity from the endosomal release peptide and not the RNAi trigger that is expected to be rate-limiting in terms of toxicity.  In other words, to compensate for the inferior potency of cholesterol-siRNA along the release polymer, you just give more of it.

In any case, given the overlapping research activities of the two companies as evidenced by the patent applications, possibly partly the result of the former Alnylam-Roche partnership, it will be interesting to follow the patent prosecutions to find out to which extent the patents by Arrowhead Research could impair the freedom-to-operate and novelty of Alnylam’s platform, both with regard to GalNAc3-siRNAs and GalNAc3-lipid-siRNAs.

An interesting player in this convoluted situation is Novartis.  Assuming Novartis has access to GalNAc-siRNAs from Alnylam, they might be able to combine them with the endosomal release polymers from Arrowhead Research for optimal DPC2.0 knockdown activity (of course, that assumes they take some sort of license from Arrowhead Research).  It’s time for Novartis to show their RNAi delivery hand anyway lest they suffer the same fate as their peers' with their RNAi investments losing all of their value.

Single-molecule subQ DPC

The potentially convoluted IP situation is another, albeit secondary reason, why I greatly look forward forward to Arrowhead Research adopting for their upcoming development candidates the new old single-molecule DPC technology for which they had shown very impressive non-human primate data at last year’s OTS meeting. 

Because the GalNAc residues in the single-molecule DPCs are distributed along the peptide, there is no need for a triantennary GalNAc cluster for similar hepatocyte targeting potency.  Moreover, PK modulation can be achieved by modifying the polymer without the need for direct modification of the RNAi trigger.

And with regard to ARC520- don’t get me wrong. The intravenously administered ARC520 is still an exciting candidate with good activity, it’s just not as potent as it could have been.  I therefore look forward to seeing a second-generation candidate enter clinical development once clinical proof-of-concept for the immune reactivation hypothesis has been formally obtained.  Such a candidate would have much increased potency (at least 10x) and could be administered subcutaneously.  A high-quality problem to have.