Monday, September 23, 2013

A First Clinical Step Towards Subcutaneously Administered RNAi Therapeutics

Today, Alnylam reported more detailed results from their phase I study of ALN-TTRsc for the treatment of the cardiomyopathy form of TTR amyloidosis (press release here, data presentation here).  As telegraphed by COO Barry Greene at the recent Morgan Stanley Healthcare Conference, 5mg/kg and more of the subcutaneously administered ALN-TTRsc were required to obtain a persistent knockdown of 80% (5mg/kg) and more (10mg/kg dose).  

As these doses fail the 2.5mg/kg challenge the company had set itself earlier, increased injection volumes and injection numbers are being evaluated.  A phase II trial with ALN-TTRsc is planned to start at the end of the year with a pivotal phase III study slated to start in 2014.

Small potency differences to monkey studies matter for ALN-TTRsc

As I had indicated earlier, the preclinical monkey studies with ALN-TTRsc (see slide 7) left open the possibility that with an 80% target knockdown, ALN-TTRsc can be sufficiently effective at 2.5mg/kg which corresponds to the magical injection volume of 1ml for safe and convenient subcutaneous administration.  However, the phase I human data are shifted by 1 dose cohort meaning that 5.0mg/kg were required for an approx. 80% sustained knockdown (see slide 11 where 2.5mg/kg clearly missed).

If you are confused about the press release prefers which talks about an 87.5% 'mean' (n=3) knockdown ‘at nadir’ for the 5.0mg/kg dose, it is more appropriate for this indication to refer to the average knockdown level over time which was more like 80-82% (slide 11).  

Nevertheless, oozing with optimism, Alnylam set itself an even higher bar for ALN-TTRsc for future studies with a sustained target knockdown of 90%.  Such a level of knockdown was only achieved in this study at the very high dose of 10mg/kg.  Indicative that 10mg/kg is not a desirable dose, the company amended its study to evaluate 7.5mg/kg given as two 1.5ml injections in addition to 5.0mg/kg and 7.5mg/kg every 2 weeks to determine whether less frequent dosing is feasible.

Personally, I believe 5.0mg/kg twice a week would be a more promising dosing schedule as with higher doses there should be diminishing returns due to receptor saturation.


Safety OK, but questions raised

The reported safety profile did not yield apparent nasty surprises.  This is obviously very good in a phase I study of a new chemistry.  However, it has also become obvious that injection site reactions could become an issue.  Unlike ISIS Pharmaceuticals which has declared such reactions to be a cosmetic problem in the past, injection site reactions have to be closely watched as they could indicate systemic immune issues.

Despite the small numbers, the injection site reactions appear to be dose-related with 3 out of 3 subjects at the 10mg/kg exhibiting them during the 5 weeks of the study.
 
These observations are also relevant to the competitive DPC delivery technology by Arrowhead Research because both involve galactose conjugations.  In the case of the intravenous version of DPC, the galactose has been added to the endosomolytic peptide, while a cholesterol is added to the RNAi trigger administered separately. 

Initially, I had thought IP issues could have prompted Arrowhead to put the galactose on the ‘less precious’ peptides.  However, an alternative explanation could be that galactose conjugated to nucleic acid might be more prone to immune stimulations.  Due to the apparent dose-relatedness of the reactions, it will be important that future subQ candidates (both Alnylam's GalNAc and Arrowhead's subQ DPC which have galactose and the RNAi trigger on the same molecule) have therapeutic knockdown potencies at 2.5mg/kg and below.  

A very perplexing observation, albeit not discussed at all, is the fact that elevated immune cytokine levels were reported in the placebo cohort, but none in the ALN-TTRsc cohorts (see slide 15 of the presentation).  I really don’t know what to make of this. 


Summary

Despite some of the blemishes above, the results are a decent start for subcutaneously administered RNAi Therapeutics for liver-expressed gene expression.  In addition to the safety profile, this thus renders also the last argument for phosphorothioate-based RNaseH antisense technology in such indications obsolete, namely the ‘patient-friendly’ route of administration.  Patients with TTR amyloidosis and their relatives can now look forward to two attractive RNAi options, and I expect there to be continued RNAi improvements for this indication.


For those interested in Tekmira, the results are positive in the sense that they increase the value of ALN-TTR02 given the modest potency of ALN-TTRsc, and support SNALP to be the clinically validated gold standard in RNAi delivery.  For those interested in Arrowhead Research, the results are positive as they underline the value of more potent subQ RNAi delivery technologies and because they lower the safety risk around the galactose chemistry.

12 comments:

Latebloomer said...

Dirk,

Regarding Arrowhead's subcutaneous version of DPCs...

Tettrazini pointed me to the following article, which came out shortly after Arrowhead's NR about their new subcutaneous version of DPC:

http://www.arrowheadresearch.com/sites/default/files/news/Gene_Silen_News_111612_reprint.pdf

Because it discusses both ARC520 AND Arrowhead's latest work (at the time) on subcutaneous delivery, it wasn't exactly clear to us, but it seemed that this article pointed to the subcutaneous version also being made of two molecules.

Either way, with this article in mind, I'd appreciate your thoughts on the possibility that the subcutaneous version might also be using 2 molecules.

Here, for example, among other things, it talks about the fact that the two molecule version allows for not just ease of manufacturing, but also for high biodegradability, which better allows for multi-dosing:

“DPCs haven’t been standing still,” he said. “The technology
has been growing.”

Part of that growth includes the discovery that the DPC
and the siRNA payload, which is cholesterol conjugated, do
not need to be linked so long as they are both decorated with
targeting ligands to the same cell type.

“We found that we could actually target the delivery polymer
separately from the siRNA and still get very efficient delivery,”
Lewis told Gene Silencing News, noting that a paper
better describing this process in currently in press.

“This was a key finding because it made manufacturing a little more straightforward,” eliminating the step of linking the siRNA and the polymer, he noted. “More importantly, it allowed us to explore different types of polymers that weren’t amenable to siRNA attachment.”

Among the polymers considered were peptides synthesized to have the same endosomolytic properties as the polymers in the first-generation DPCs, Lewis said.

Not only could these peptides be cost-effectively scaled up, but they are highly biodegradable. “That really allows you to think about multi-dosing … without accumulation of your
delivery vehicle and possibly cumulative toxicity,” he said.

~~~~~

As far as your post today...

If Arrowhead's subcutaneous version does use two molecules afterall, then maybe, as you surmised might be the case for the two molecule IV version, it could differ from the ALNY subcutaneous version in the same way - by separating the galactose from the nucleic acid (and, in that way, perhaps be safer/more tolerable).

Linda

Anonymous said...

ISR were reported only in multi dose cohorts so I suspect they occurred during the first five days when they were giving daily injections. This is speculation. Company hasn't given details regarding how many ISR and when they occured in each subject.

Dirk Haussecker said...

Both excellent comments.

Linda, I do not entirely exclude subQ DPCs for liver indications to be 2 molecule. Based on the 2-molecule IV experience, however, I would expect higher doses to be necessary for such an subQ approach which might cross the 1ml magic injection volume.

The second comment also makes a good point (ISRs and when). However, keep in mind that many more doses were administered in the multi-dose part compared to the single-dose part of the trial (12 vs 90). Don't forget, too, that there was a local bruise and tenderness each in the single dose presumably caused by the high-volume injection.

Anonymous said...

Will you comment the DIA this week

Anonymous said...

Though TTRsc is not very potent, it still looks better than ISIS-TTRRx which is in P3 and with whom they are going to compete in the near future. ISIS-TTRRx knocks down TTR by only 70% at a 300mg weekly dose based upon their P1 data. ALN-TTRsc knocks down TTR by >80% at a dose of 5mg/kg/week or 400mg/week with a superior safety profile. So Alnylam is doing fairly good with their first GalNAc compound in the clinic at this time.

Dirk Haussecker said...

Agree with the comparison with phosphorotioate-based ISIS-TTRRx (ISIS/GSK), but the clinical data are making it quite clear already that RNaseH antisense will not be best-in-class for liver-directed gene knockdown drugs where there is direct competition with good RNAi candidates.

Taking a step back, however, it is quite interesting that I often focus on the competitive drug profiles instead of on the feasibility of oligoRx per se. The industry has come a long way.

tettrazini said...

Are there any realistic prospects for oral administration of RNAi drugs?

Dirk Haussecker said...

Oral RNAi..sure. ISIS has e.g. shown that you can get oligonucleotides into the circulation this way. With a more potent mechanism of action, e.g. small-conjugate RNAi, oral RNAi may not only be technically feasible, but also economical.

tettrazini said...

Dirk, on your HBV blog you wrote, "In addition, although the required degree and kinetics of the knockdown according to the immune reactivation hypothesis is unknown, it is likely that it takes rapid changes to shake up the immune system so that it recognizes HBV as a foreign agent."

Is that an intuitive conclusion and are there any reasons to believe that the knockdown you discussed with ASOs is not rapid enough for a functional cure?

Dirk Haussecker said...

It's largely intuition. Since HBV has managed to hide itself so well from the immune system and now passes as a 'self' it would seem to require a high activation energy, some kind of shock, to move it from the tolerogenic camp into the 'foreign' camp. A really slow, watching-grass-grow-type of decline may not be sensed as a significant enough change to alter the immune status. Biological circuits in general respond better to rapid changes.

Anonymous said...

Dirk,

So it sounds like your intuition may also be wrong since it is not based on any scientific evidence. It is more like an unproven religious belief. So functional cure for HBV is all hype right now.

Anonymous said...

To anonymous above,

Dirk's statements are not like "an unproven religious belief". They are, infact, aligned and consistent with the general observations regarding chronic HBV infection, the somewhat immunologically tolerant milieu of the liver, and biological systems in general. You are knocking down a straw man by discounting his comments simply because he confesses "intuition".

Your response might be characterized as more like "ignorant"

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