Friday, June 6, 2014

Game-Changing Potency Improvement for 2-Molecule DPC by Arrowhead

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?


Latebloomer said...


On p. 17 of the presentation, it looks like the use of the protease sensitive linker approx. *equals* the potency of the earlier/current version (PEG + pH sensitive linker + MLP), which makes me think that the key to the added potency is not the protease sensitive linker, but the change from PEG to NAG for the masking component (added to its existing targeting component).

I know that would then still beg the question of why also change to the protease sensitive linker if that's not part of the key to the new potency.

Maybe, like you mention, it's because it adds to safety since it's so specific to the endosome. (That certainly couldn't hurt.)

Dr. Z and I have been contemplating at least one other (additional?) possibility for turning to a protease sensitive linker. (I'll be looking forward to his posts on that.)


Another thought is that *if* changing to NAG for masking purposes is the key to the greater potency, then I don't see why that couldn't also add to the potency of the existing single molecule/subq version (which already uses the protease sensitive linker)...


Anonymous said...

Oh, Dirk, all the fools that you led to sell ARWR for TKMR.

You led them to sell the low risks/high rewards play, ARWR, to take on high risks/low rewards TKMR, all to prop up your exit from the latter.

In the coming months as market volatility increases we'll likely see TKMR a single digit stock as ARWR approaches triple digits.

Biotech investing is all about understanding and managing risks.

- investron

Latebloomer said...

Also, on the very last page of the presentation, p. 26, it shows that PEG has a limitation that begins to affect potency at a certain point:

"PEG-MLP loses potency with longer than ~40 PEG"

Maybe that's an important factor in the switch to NAG for masking; to get beyond that limit for PEG?

(Thanks to Mariano for pointing out this page. It was beyond the "Thank you!" page, so I wouldn't have known it was there if Mariano hadn't pointed it out.)


Anonymous said...

Hey Dirk,

wondering if you listened to Anzalone's comments at Jefferies from other day. Thought I heard him say that ARWR can go to 3mg/kg with ARC520 and that is what Phase 1 ext was about.

bikerieder said...

Thx for the blog. One thing you forgot talking about: you are not talking about how to get patients immune system to work AGAIN.
ARWR goal is func cure HBV. The virus harmstrings the immune system to come over the virus. If you come with a 3-4 log KD candidate 'atom bombe', you could do a lot of damage! So ARWR way to come with a lower KD - e.g. 0.8 log - approach is much better, safer and responsible worthy. There is a live after the first dose. You better activate the sleeping immune system smoothly and this is the way ARWR is working on. As I said on an other day, potency is not efficacy. I like it much more to have 3-6 doses to get to fund cure AND an activated immune system than 3-4 log KD with lots of ??? about a 'bad' response of the immune system.
One other thing to talk about
After meeting with ARWR Mgmt, RBC was talking about phase 2a data in july 14. One dose cohort takes 85 days. Cohort 1 will be at that point circa at the end of june 14. Is there no need for a cohort 2 because they have achieved their goal! Confusing, but that would be a real good 'atom bombe'!

Latebloomer said...

Some further thoughts I posted earlier this morning...

If ARWR prefers to start off at a lower dose, and to gradually increase the dosage, then perhaps (also accounting for the possible additive effect), maybe they would prefer NOT to start off with a particularly long duration dosing schedule that would presumably occur with the present single molecule subq technology.

My guess is that they're still working on tweaking that technology, perhaps in part to lessen the amt of DPC backbone needed.

But whatever the case, even if they got the single molecule subq technology just where they wanted it, and were ready to bring it into the clinic... I could well imagine that they might still like a shorter duration alternative (of approx. a month) in treating HBV, at least to start out with.

Now if that shorter duration alternative also happened to allow for subq application…


Latebloomer said...


I hesitated to add that last part, but I do wonder if part of the reason for changing the linker with the two molecule version to a protease sensitive one might have to do with potential subq considerations for their two molecule version.

An article from 2012 - about Arrowhead's then advances in their DPC technology - mentioned that a protease sensitive linker was key for subq (presumably because there are acidic environments that a subq application would encounter that an IV application wouldn't, at least to the same extent, on the way to the endosome - thanks to Dr. Zoidberg, who often tweets under $ARWR, and with whom I corresponded a bit about this recently... he mentioned that the skin environment is acidic... look for a tweet and post or two about these things from him as well.) Here's that article:

I hesitated to mention this because if that is an aim of ARWR - to progress to a subq version of the double molecule application - then I figured they would be dealing with a similar challenge that ALNY was dealing with in seeing the single siRNA molecules through the hostile route after the subq injection, something ARWR may still be attempting to work through. See Dirk's write up about that recently:

But it may be that ARWR is not as concerned as ALNY with further stabilizing the siRNA molecule (beyond the chol linker)… IF enough of the siRNA can still make it intact to the end of the line, and can therefore be boosted with endosome escape via the DPC.


Anyway, just something to ponder, now that ARWR has made it to the point of needing much less DPC than they did before for the double molecule version, at least in targeting the liver, and have, in addition to that, added the protease sensitive linker (so key to subq application)... particularly if they feel that they'd like to treat HBV patients, at least in the beginning, with a gradually increasing dose, and, therefore, would perhaps rather not use the 2 month + peak KD duration single molecule subq version to start off with, as it currently exists (or perhaps even if they were to get that version to just what they wanted, and it was ready for the clinic).


Anonymous said...

is it just me or do I get the feeling Blogger (Dirk), is being fickle again and re-invested in ARWR? what happened to being all back in TKMR?-- We have seen this before when he left for arwr.. and guess who got to 30 first... ?Thats right Tekmira..

Dirk Haussecker said...

3mg/kg. I re-listened to the Jefferies presentation, and yes, it seems that they are considering it for the phase IIa (not sure about what's with the extension, but I would think that might have happened already) if they don't get to 1log HBsAg knockdown with 2mg/kg.

Latebloomer said...

I've listened to it again too, and the only difference I can tell from the past several public discussions about the Phase 2A is that Anzalone answered within his prepared talk what had been asked consistently via every Q & A: Are they prepared to go on to 3 mg/kg if needed?

It seems to me that he simply anticipated that ongoing question and said what they have always said: They are prepared to go on to 3 mg/kg if needed.


Anonymous said...

Sounds like old Dirk is up to his worn trick again.

What Dr. Zoidberg is tweeting is what I said on Yahoo! from the start of this ruckus, then giving Dirk H. the benefit of the doubt.

Now of course we know better, Dirk is really that dumb. He was intentionally manipulative and pumping TKMR while putting down ARWR all because of a bad trade.

- investron

by investron • Feb 28, 2014 8:55 AM

has ARC-520's Ph 2a trial as starting in February.

Today is the last day of February.

The information is listed as having been verified by ArrowHead.

So the official news could come at anytime, today, tomorrow, Monday, etc.

The important thing to remember is that ARC-520 is efficacious, stunningly so.

To the point that the Hepatitis B Foundation broke the news of the potential functional cure.

Yesterday Dirk H. of the RNAi Therapeutics Blog who had been pushing TKMR heavily even before the Asia Tides Conference tweeted that TKMR's 3rd gen LNP might lead to an even more potent HBV candidate.

This seemed to have caused an absolute pandemonium in the peanut gallery.

Some even claimed that certain tweets suggested Dirk H. switched from ARWR to TKMR.

I don't think it makes sense and doubt that he would make such a mistake with ARC-520 going into trial right now and TKMR's HBV a complete unknown and won't go into trial until next year at the earliest.

But of course anything is possible.

The reason it would not make sense to an investor is because ARC-520 is by now a known quantity.

Compare it to TKMR's candidate there is literally nothing to compare.

ARC-520 is stunningly efficacious, the advantage of something that might be even better is a diminishing return.

Timing is on the first mover's side.

Would you invest in something that is known to be good and happening now or something unknown that might just be better next year?

Anonymous said...

Where does the MRK IDIX buy out leave a HCV PI with others on the way like Benitec at Dirk? Even without a lab they must carry some kind of value for a buy out.

Anonymous said...

The probability is high at this point that 1 mg/kg is proving efficacious in the present trial!!!

- investron

Anonymous said...

investron • 49 seconds ago

Think About This

The results of Ph 2a cohort 1 @ 1 mg/kg are pretty much known to management by now because of the way RNAi works.

It doesn't matter if the study is blinded, the data make it obvious.

Point no. 1 - The chimp was off the chart absolute worst case far beyond what is seen in typical patients. 2 mg/kg knocked it down to evidence of possible immune reactivation.

Point no. 2 - The 2 mg/kg was half mismatch for the chimp's viral genotype, effectively cutting it down to 1 mg/kg.

Point no. 3 - Humans are said to require lower dosages than primate models.

Point no. 4 - ARC-520 is being administered in combination with entecavir.

Here's an answer to why 3 mg/kg was not pursued in Ph 1, IT WAS NOT NEEDED!

When 2 mg/kg showed no ill effect it was already beyond what was required in management's mind.

Ever since people got dosed the management sounded extraordinarily confident and comfortable.

This became even more marked after Ph 2a began.

And now management is going to town to all the investor conferences talking about the huge market prize in front.

Frankly, if the results so far have been disappointing I would not do that.

At this point I'm willing to make the bet that 1 mg/kg is efficacious.

And I don't think I'm the only one who figured this out.

Somebody(ies) has been accumulating this stock like crazy there is hardly any pull back.

Sentiment: Strong Buy

- investron

By Dirk Haussecker. All rights reserved.

Disclaimer: This blog is not intended for distribution to or use by any person or entity who is a citizen or resident of, or located in any locality, state, country or other jurisdiction where such distribution, publication, availability or use would be contrary to law or regulation or which would subject the author or any of his collaborators and contributors to any registration or licensing requirement within such jurisdiction. This blog expresses only my opinions, they may be flawed and are for entertainment purposes only. Opinions expressed are a direct result of information which may or may not be accurate, and I do not assume any responsibility for material errors or to provide updates should circumstances change. Opinions expressed in this blog may have been disseminated before to others. This blog should not be taken as investment, legal or tax advice. The investments referred to herein may not be suitable for you. Investments particularly in the field of RNAi Therapeutics and biotechnology carry a high risk of total loss. You, the reader must make your own investment decisions in consultation with your professional advisors in light of your specific circumstances. I reserve the right to buy, sell, or short any security including those that may or may not be discussed on my blog.