Monday, October 1, 2012

Roche Reveals Antibody-Targeted DPC and SNALP Data

Arrowhead Research has stated that it is about to lift the veil on the RNAi Therapeutics research at Roche through a number of publications.  As you remember, Arrowhead Research made a daring splash a year ago when it acquired much of the Roche RNAi assets for about a cent on the dollar invested in RNAi Therapeutics by Roche.  I will be covering the revelations on this blog as I had once considered Dynamic PolyConjugates one of the more promising systemic RNAi delivery technologies- albeit at least 3-4 years behind Tekmira's SNALP technology in terms of clinical translation and validation.

It was actually Roche (and not Arrowhead/Madison) that fired the opening shot with a study on the use of bispecific antibodies for targeted siRNA delivery (Schneider et al. Molecular Therapy- Nucleic Acids: Targeted siRNA delivery and mRNA knockdown mediated by bispecific digoxigenin-binding antibodies). 

Antibody-targeted siRNA delivery has been a concept that has been around for a while.  The overall industry sentiment on this topic is that while it is still worthwhile pursuit, there have been problems with replicating some of the early data concerning simple antibody-siRNA conjugates and electrostatic complexes.  Stability and cytoplasmic penetration have been the two main issues.

The latter obstacle was also encountered when Roche simply added siRNAs conjugated to digoxigenin (DIG) to a bispecific antibody recognizing both the small molecule DIG (thereby binding the siRNA) and a cell surface protein: the siRNA was specifically delivered to the cell expressing the cell surface protein, but there was no gene silencing, presumably due to lack of endosomal escape into the cytoplasm.

This, of course, is not all that surprising, although, in all fairness, it is not just the old antibody-siRNA literature, but also work from the aptamer-siRNA field and the GalNAc data by Alnylam that suggest that certain cell surface receptor uptake pathways allow for such simple delivery.

To further facilitate gene silencing, the bispecific antibody technology was then applied to DPCs and SNALPs by  linking DIG to the polymer backbone (DPC) and PEG-lipid (SNALP), respectively, with the siRNA either covalently bound to the backbone or enclosed in the aqueous SNALP interior.  The masked cationic polymer in the DPC and the cationic liposomes were thus tasked with overcoming the endosomal release challenge.

Targeted SNALPs for Gene Knockdown in Vascular Endothelia

Indeed, gene silencing could now be observed in tissue culture with both approaches.  Next, the authors tested out the concept in the much more challenging in vivo animal setting.  Data for the targeted SNALPs were reported.   

Mouse seeded with human tumor cells were administered SNALPs (employing Tekmira’s ’57.1’ formulation ratio, KC2 ionizable lipid, and in-line mixing) targeted towards the VEGFR2 receptor that is abundantly expressed on the endothelial cells of the vasculature.  Impressively, such constructs not only maintained the expected endothelial silencing efficiency of SNALPs, but greatly enhanced it: a 40% versus almost 80% gene silencing of the CD31 endothelial marker gene.

Endothelial cells are a good initial target cell population for this approach as these complex, and rather large (>100nm) nanoparticles have easy access.  The approach also does not rely on positive charge for cellular uptake which could be a safety advantage over competing approaches.

The reason why the performance of DPCs in this model was not described is unclear.  It is possible that DPCs simply did not work.  It is also possible that Roche did not want to steal Arrowhead’s thunder and held off on publishing the data.  Pointing in that direction was the fact that the siRNA sequence and, more importantly, modification was not described in the paper.  Not only would this have been crucial for interpreting the innate immune stimulation data (it seemed to be absent), but also an important factor when it comes to uptake that involves the direct exposure of the siRNA to endosomal endonucleases.

Overall, it was a fun paper to read and quite a bit more innovative than many of the RNAi research that has been conducted by Big Pharma bar Merck. Antibody-targeted RNAi delivery is certainly not dead. The cationic lipoplexes by Silence Therapeutics could get competition, although Atuplex enjoys a considerable, multi-year head-start.

Comment on Last Week’s ‘Patent Victory’ by Alnylam

Last week, a US judge ruled that, based on the contracts between Alnylam and Tekmira, Alnylam has standing in enforcing certain exclusively licensed patents.  Tekmira tried to duck this infringement lawsuit by claiming that as the licensor of these patents, it was immune from such enforcement.  The judge disagreed saying that such immunity should have been explicitly stated in their agreement.  In my opinion, this technical decision is not entirely surprising.  All it means is that the case will now be tested in more detail, representing another cash drain on Tekmira.  That financial pressure is Alnylam's main aim in filing the lawsuit is also shown by the company expanding the case into Canada, where Tekmira actually conducts its business.

The ruling does not, however, predict whether Alnylam will prevail on the merits.  This should boil down to the question of whether Alnylam’s right to the patents extend to target validation and not just RNAi therapeutics.  As the Alnylam field is defined as “the treatment, prophylaxis and diagnosis of diseases in
humans using an RNAi Product or miRNA Product", it obviously does not.  

The decision also does not reduce the damage that Tekmira could claim as part of the much more important Trade Secret litigation where Tekmira has charged financial damage arising from Alnylam trying to cut out Tekmira from the financial benefits of SNALP, e.g. when dealing with Big Pharma companies such as Takeda and Novartis.  I do not recall that Tekmira accused Alnylam of making it impossible for Tekmira to close RNAi Therapeutics deals directly with Big Pharma, without Alnylam.  An important distinction.

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By Dirk Haussecker. All rights reserved.

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