At the TIDES meeting last
month, Arrowhead Research presented advancements with its intravenous
2-molecule DPC delivery approach for gene silencing in the liver. By
adding a protease-sensitive hydrophilic extension to the
endosomolytic peptide (MLP), the company has improved both the safety of the
delivery approach, and even more importantly so, the potency of
2-molecule DPC.
The new formulation can be expected to mediate deep
knockdown at the 2mg/kg endosomolytic peptide dosage at which
Arrowhead Research stopped their dose-escalation for the HBV product
candidate ARC520 which is based on the original 2-molecule DPC chemistry. The reason why I have come to be on the fence regarding the future of ARC520, especially in a competitive environment, is that I consider 2mg/kg insufficient to mediate the
multi-log HbsAg knockdowns that might be desirable. If they had increased the dose to 3mg/kg, or better 4mg/kg, I would have a much more positive outlook on ARC520 (maybe this is still in the cards pending the outcome of the phase IIa result in Q3).
It is important to keep in
mind that with DPC, it is the amount of endosomolytic escape agent
that is rate-limiting for knockdown potency and dose-limiting for
safety. The amount of (otherwise very safe) RNAi trigger by contrast is in vast excess,
just to make sure that no potency is wasted. With first-generation
DPC 2.0 (2-molecule), it was from around 2mg/kg for both Factor VII
and HBV mRNAs that target knockdown started to take off. The fact that the same can be observed for various target genes supports the notion that as long as you have a fairly potent RNAi
trigger, it is the amount of endosomolytic escape agent that
determines that depth of the knockdown.
There was no reason for
Arrowhead Research not to explore higher doses if they had no
trepidations about the safety profile. Somewhat
contrary to this notion, the company had claimed seeing no preclinical
tox at up to 10mg/kg.
Nevertheless, the thought
process behind the improved 2-molecule DPC version leads me to
believe that safety has been a concern. This is because the reason
for adding the hydrophilic extension to the endosomolytic backbone
was to limit non-specific, potentially toxic interactions with
membranes outside the endosomes. This minimization of non-specific
membrane interactions was confirmed in a test tube membrane
interaction assay.
In a second step, Arrowhead
scientists sought to get rid of the hydrophilic extension in the
endosome so that it would not inhibit membrane disruption there. For
this, the hydrophilic extension was rendered susceptible to cleavage
by proteases that are found only in endosomes. With this trick, they regained the
potency of the original DPC 2.0 version, but with the improved safety feature.
Going one step further and
truly making lemonade out of lemons, they replaced PEG as the
hydrophilic extension with more of GalNAc targeting ligand: whereas
DPC 2.0 exhibited 40% and >90% gene silencing at 1 and 3mg/kg
endosomolytic agent, respectively, the improved version DPC 2.1
achieved a >90% silencing at 1mg/kg already.
Given the increase in
potency, even if the safety profile was not enhanced with DPC2.1, a conservative assumption, a
2mg/kg dose should now facilitate very potent gene silencing in the
liver. This bodes particularly well for the 2nd
development program that Arrowhead will disclose this month. As a
follow-up to ARC520 in HBV...? I believe the company would be better
advised to wait for a single molecule, subcutaneous version, the
ultimate future of DPC.
Question of the day: why does adding more GalNAc to an already GalNAc-targeted molecule enhance gene silencing? Could it be that they added a trivalent GalNAc (like Alnylam), and not more monovalent GalNAc as on the endosomolytic backbone?
