Thursday, September 5, 2013

Alnylam’s GalNAcs As Cholesterol-siRNA 2.0

That Alnylam bets its future on GalNAc-siRNAs is remarkable and indicates that the company feels the technology is clinically viable.  Accordingly, the preclinical animal data leaves open the possibility that the required volume of an effective dose of ALN-TTRsc, the first GalNAc-based clinical candidate, can be administered in a volume of 1ml or less, the stated goal of the company (results to be presented at HFSA, Sep22-25).  

While I view ALN-TTRsc as a cliffhanger in terms of commercial viability, not just because of the volume issue but also in light of the likely more effective SNALP option in the form of ALN-TTR02, the potency of GalNAc-based ALN-AT3 for hemophilia looks quite adequate.

Alnylam’s claim that a simple ligand-siRNA conjugate was able to mediate gene silencing in vivo required a rethinking of RNAi delivery by many since similar efforts involving especially aptamer-siRNA conjugates have been ‘viewed with skepticism’ to put it nicely.

Did Alnylam mislead the competition?

If you are a student and your funding body just spent $3000 to send you to a conference to learn about your subject, you might be forgiven if you came away with the notion that simple siRNA-conjuates such as Alnylam’s GalNAc-siRNAs can be effective (see e.g. slide 16 of the presentation by John Maraganore at the OTS last year).  The trick apparently is that with a magic ligand-receptor pair such as GalNAc-ASGPR all you need is to conjugate an RNAi trigger chemically modified for stability.  

Of course, the imaginary student here is only collateral damage as the real target audience of a presentation by Alnylam is its investors, potential partners, and competition.

As such, it may not surprise that according to patent application US2012/0136042A1, a simple GalNAc-siRNA conjugate has no activity on its own.  

Instead, silencing activity critically depends on the molecule also having a cholesterol (=lipophilic group) attached to it.  Since Alnylam had worked on such cholesterol-siRNA conjugates before (Soutschek et al. 2004), what now appears to be the actual GalNAc-siRNA structure can be viewed as an evolution of the cholesterol-siRNA which showed knockdown activity only at doses of 50mg/kg and higher.

DPCs by Arrowhead Research Take another Step

The 10-20 fold gains in potency with the addition of the GalNAc ligand shows how comparatively inefficient ApoE/LDL-based uptake systems are for liver-directed RNAi delivery.  The reason why SNALP is still more effective than GalNAc-siRNA is because cholesterol-siRNAs do an even poorer job when it comes to the next rate-limiting step in delivery: endosomal release.

With Arrowhead’s Dynamic Polyconjugates (DPCs) for liver-directed knockdown, you combine the best of both worlds: effective attachment to hepatocytes via multivalent GalNAc-ASGPR interactions and effective endosomal release through endosomolytic polymers/peptides.  

In fact, because of the potent chemistries used for the latter step, the endosomolytic agent has to be masked which initially made me a bit wary about the safety of DPCs (note: clinical safety data for ARC520 are about to be released, probably in October, and the preclinical safety performance looks good so far). 

According to Arrowhead’s research, this combination allows for more than 500-fold potency gains compared to cholesterol-siRNAs (Wong et al. 2012).  

Because the ratio of siRNA to endosomal-release peptide has to be held constant in the clinical studies of ARC520 which is based on the 2-molecule DPC version, you will not see these improvements in potency translated into clinical practice.  This is not all that critical anyway since ARC520 is administered intravenously and cholesterol-siRNAs should not be toxic even at elevated dosages.  Still, while I’m excited about ARC520 for all the stated reasons (see the HBV Knockdown Blog), for the sake of Arrowhead’s platform, I really would like to see the elegant single-molecule DPCs enter development.  This should not take much longer given the advanced data presented at OTS 2012.

But hey, maybe they have already advanced into development, but they are just being referred to as 'GalNAc-siRNA'.


Latebloomer said...

I know I'm showing my ignorance here, but I could use some help. Is the single version of the molecule the version in which the siRNA payload is attached to the DPC, as is shown in the picture on this page of their website?

...whereas, because, as described in the March webcast, it's easier and more cost effective to manufacture, they are currently using the unattached, co-injected version, at least to target liver cells?

If so, I didn't realize there was still an advantage in some way to the attached version... but if there is, I guess that's good to know that there are two versions, each with its own set of advantages?

But maybe I do not have the right idea here about the single and double molecule versions of DPC that you're writing about?


Dirk Haussecker said...

Linda, you are correct.

Anonymous said...

I guess from researcher's point of view that the single molecule version will be for another tissue.


tettrazini said...

Arrowhead’s two molecule DPC solution is beautiful in its simplicity. It is plenty potent, and seemingly safe. It surely is easier, cheaper, and less risky to develop than potential single-molecule DPCs. There is no shortage of liver targets that can be addressed with the co-injection paradigm. So what is the advantage for Arrowhead to develop a single-molecule solution that adds a whole level of complexity? If it ain’t broke, don’t fix it. At least until you have plenty of money in the bank.

I am confused by your oblique suggestion that ALNY is leaving a false impression about its science. Do you believe some of the data they presented as reflecting their platform science sprung from Arrowhead's DPC technology? That alone would be quite misleading, but even more so because ALNY only has a license for a single target.

Anonymous said...

Where does all this leave the Benitec's of this world who have pooled all of their resources, their shareholder funds and their future on double stranded DNA directed RNAi?

Dirk Haussecker said...

Tet, I appreciate your 'if it ain't broke don't fix it' comment. I don't know, however, whether a co-injection strategy would also work via the subcutaneous route of administration. So yes, while a number of indications are well served with the 2-molecule and likely intravenous version (just as is the case for SNALP), the single-molecule version provides extra target space and, get this, Big Pharma/the industry loves single-molecule approaches. For Arrowhead to build a franchise around ARC520, subQ and oral, both feasible IMO, are the next steps.

Anonymous said...

Benitec will have a bright future working with adult stem cell companies in addition to their own ddRNAi drugs. They are already working with Medistem and Regen Biopharma. Google it. Read what these companies say about Benitec's technology

Anonymous said...

Hi Dirk, thanks for another great article.

Could you please elaborate on your comment "aptamer-siRNA conjugates have been ‘viewed with skepticism’ to put it nicely".

Many thanks, Sebastian.

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