Arrowhead and Alnylam Vying for Subcutaneous RNAi Delivery Success

The use of the intravenous route of administration for the currently leading systemic RNAi delivery technology, Tekmira’s SNALP technology, has been noted to be a drawback of the technology, especially for non-severe diseases and in therapeutic areas historically dominated by oral medicines (e.g. the cholesterol-lowering market).  As a result, the arrival of two delivery approaches that promise to allow for subcutaneous administration has been welcomed: Arrowhead’s Dynamic Polyconjugates (DPCs) and Alnylam GalNAc-siRNA conjugates which have shown data suggesting their clinical use for gene knockdown in the liver (at least initially; DPC with potential to go beyond the liver).  

A day ahead of Alnylam’s Roundtable on conjugate delivery, I thought it would be a good time to get into the mood and compare the two competing technologies.

Basic Chemistries

GalNAc-siRNAs consist of siRNAs to which a cluster of three N-acetylgalactosamine residues have been appended.  It is these GalNAcs that are recognized by the ASGPR receptor protein that is abundantly presented on hepatocytes.  The choice of three over just one or two GalNAcs is due to the synergistic binding of multiple GalNAcs to the receptor.

DPCs also comprise of siRNA conjugates, but involve an additional endosomolytic agent to facilitate siRNA release from the endosomes.  The two components can be mixed together so that the drug can be given as a single formulation.  This, however, also requires that both siRNA and endosomolytic agent end up in the same place.  For hepatocytes, this is achieved by conjugating the siRNA to a cholesterol moiety and the endosomolytic agent to GalNAc.  

The reason why two different targeting agents are employed are two-fold: reduced competition for the uptake receptor, and not requiring triantennal GalNAcs such as in Alnylam's case which seems to involve a quite costly chemistry. The reason why GalNAcs on the endosomolytic agent in DPCs are not so expensive is because as a polymer (a peptide in the latest versions) multiple mono-GalNAcs can be conjugated distributively and still achieve the same synergistic binding effect.

Potency and Safety

The Holy Grail in RNAi subcutaneous delivery appears to be to get formulations potent enough so that the desired level of knockdown can be achieved with volumes of 1ml or less: you can squeeze only that much liquid under your skin through a thin needle.

The first of Alnylam’s GalNAcs, ALN-TTRsc, achieves a 80% target gene knockdown (ED80) following repeat administration in preclinical animal studies.  This is below the (based on OTS 2012) 3mg/kg barrier that apparently would allow for 1ml or less volumes in humans.  What surprised me to see at the OTS meeting in late October was that the GalNAc potencies, both in rodents and non-human primates, varied quite a bit between the programs.  The TTR formulation actually had the poorest potency among the programs.  This surprised me even more so given that ALN-TTR01 and ALN-TTR02 (both SNALP programs) contained highly potent RNAi triggers.  In the case of PCSK9, ED50 of less than 0.1mg/kg were obtained.

It is possible that the differences are not just due to the natural sequence-specific differences in RNAi potency, but a result of advances in chemistry.  In particular, optimizing siRNA-conjugates for tissue/endosomal stability rather than serum stability as is often practiced in RNAi Therapeutics is critical.  Importantly, this consideration also applies to DPC technology.      

DPCs should be more potent than isolated GalNAc siRNAs.  This is because you are adding an endosomal release agent to the liver-targeted siRNA (e.g. GalNAc-siRNA) conjugate and unfacilitated release of nucleic acid out of endosomes is believed to be highly inefficient.

Arrowhead has reported various impressive potencies such as 99% knockdowns at sub-1mg/kg siRNA doses.  This to me is strong evidence of the superior potency of DPCs over GalNAc-siRNAs.  Moreover, it seems that DPCs may inherently require less frequent dosing compared to GalNAc-siRNAs for which Alnylam aims at weekly or twice monthly dosing.

What is unclear, however, is the amount and resulting safety and volume implications of the endosomal release agent.  In particular, the most impressive knockdown data seemed to involve saturating amounts of endosomal release agent (~6mg/kg).  The first-generation endosomal release agent, PBAVE, suffered from relatively high toxicity, partly as a result of premature unmasking in the blood.  It makes sense that the newer, ‘more natural’ peptide-based endosomolytic release agents are safer.  By contrast, assuming that the GalNAc sugar itself is harmless, I am not too concerned about the safety of Alnylam’s GalNAc conjugates.

In terms of potency, advantage Arrowhead, in terms of safety, advantage Alnylam.

Strategic Considerations

The challenge for Arrowhead will be to make the case of the benefit of increased complexity over GalNAc-siRNAs. Would the prospect of a 3- or 5-fold increase in potency e.g. be enough justification for the investment?  I say ‘prospect’ because Alnylam could obtain knockdown proof-of-concept data at least a year before Arrowhead, especially since Arrowhead is planning to conduct the first study with DPC (ARC-520 for HepB) in healthy volunteers and thus won’t be able to measure viral target knockdown.

In addition to potency, DPC has the important advantage that it may be a more widely applicable RNAi delivery platform.  This alone may tempt others to put some money down on the technology to see where it can go.

Although GalNAc-siRNAs and DPCs are currently clearly competing, there is also scope for them to synergize, especially in the area of oligonucleotide chemistry.  Curiously, Alnylam did seek access to DPCs earlier this year, supposedly for its evaluation in one of its 5x15^TM programs.  Learning about DPC siRNA chemistry may be of at least equal, if not considerably more value to Alnylam.

Which of the two delivery technologies do you prefer for target gene knockdown in the liver?  Take the survey on the top right-hand corner.