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.
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.
No comments:
Post a Comment