RNAi Therapeutics Ebola Candidate Achieves Critical Milestone

Yesterday at the TIDES meeting, Tekmira reported new efficacy data for its Ebola RNAi Therapeutics development program.  The non-human primate data suggest that the main remaining hurdle before regulatory approval for this biodefense agent (on which any government stockpiling contract would hinge), safety in healthy human volunteer studies, is now more realistic than ever.     

Is second time's the charm?

In 2010, Tekmira and collaborators from the Geisbert lab and the US Army reported in The Lancet of a breakthrough in the post-exposure treatment of Ebola infection.  In the gold-standard rhesus monkey model for Ebola infection, their were able to demonstrate that SNALP-RNAi could effectively rescue Ebola-infected animals from near-certain death. 

The efficacy, however, depended on doses of 2.0mg/kg is likely too high a SNALP dose to be considered safe.  Based on the SNALP human clinical trial experience to date, first dose-limiting toxicities are observed at 1.0mg/kg (note: SNALP potencies are very comparable between humans and monkeys).  Moreover, at doses higher than 0.5mg/kg, sporadic signs of immune stimulations (flu-like symptoms) can be observed.  These caused Alnylam and Tekmira to adopt precautionary transient immune suppressions with corticosteroids in some of their other SNALP-based clinical studies.   

It therefore became important to take advantage of the improvements in SNALP potencies and develop more potent Ebola SNALP-RNAi candidates.  Fortunately, Tekmira was able to get the blessing from the US Department of Defense for this, and announced in April that it would receive up to ~$7M in additional funding on top of the up to $34M initially agreed upon in order to bring such a candidate to the end of phase I clinical studies. 

Importantly, since Ebola efficacy trials cannot be conducted in humans, the regulatory approval of TKM-Ebola will be according to the Animal Rule.  The Animal Rule states that for indications where human efficacy trials are not possible due to ethical considerations, it is sufficient to show efficacy in accepted animal models and then establish drug safety at corresponding doses in healthy human volunteer studies.  

With the new formulation, 12 out of 12 monkeys treated with the magic dose of 0.5mg/kg RNAi stayed alive in a post-exposure treatment setting, compared to only 1 out of 6 treated with placebo.  This means that should the human volunteer trials show good safety at the same 0.5mg/kg dose level, Tekmira will stand a good chance at gaining regulatory approval under the Animal Rule.  Of note, the first Ebola trial were likely conducted in the absence of corticosteroid pre-treatment and it is very likely, also in light of Alnylam's recent announcement of tapering off corticosteroids in the ALN-TTR02 trial, that the future TKM-EBOLA studies will also be conducted without them.

Since 0.5mg/kg is right on the border, however, there still remains an element of risk before victory can be declared on the safety front.  It is therefore good to hear that 4 out of 6 monkeys stayed alive at the 0.2mg/kg dose. This should provide a good safety cushion despite the aggressive daily treatments that will be required in this demanding setting.

Critical human safety data, ideally from repeat-dosing studies, should be forthcoming in 2014 as Tekmira has guided that it will commence clinical studies with the new candidate by the end of 2013.

Update 16/5/2013: One issue on the efficacy side of the equation that I forgot to mention in the original post is that the DoD might ask the company to test TKM-EBOLA with longer delays after the infection in monkeys.

SNALP Shows Good Potency with Subcutaneous Dosing

Also at the TIDES meeting, Tekmira presented efficacy data for SNALP when given via the subcutaneous route of administration.  To wit, SNALP has largely been administered intravenously which may not be suitable for a number of indications, especially the lower risk, chronic diseases.  Pfizer once famously criticized the state of RNAi delivery as being limited to intravenous options.

Comments like these (which are very relevant for partnering purposes) and trends in the industry to adopt subQ dosing, most notably Arrowhead’s DPCs and Alnylam’s GalNAc-conjugates, probably motivated Tekmira to explore this route of administration with SNALP further.

 

Based on an early ApoB patent application, Tekmira had clearly had some familiarity with subQ, and I had always wondered why the company did not pursue this further.  It could be that the more potent SNALP chemistries now allow for much more potent and robust subQ RNAi gene knockdown.

The new data show a very impressive 96% knockdown of transthyretin (TTR) with a single subQ administration of 1.0mg/kg SNALP in rodents.  This compares to a much more modest ~55% knockdown with ALN-TTRsc at 1.0mg/kg given on 5 consecutive days, the subQ GalNAc-siRNA candidate by Alnylam which just entered clinical development for the FAC form of transthyretin amyloidosis.

Of course, we need to learn more about the safety of subQ SNALP to know whether it really is a clinically viable strategy and superior to the competition.  GalNAc-siRNAs in particular look quite benign, albeit not very potent.  However, since peak plasma concentrations seem limiting for SNALP in terms of safety (infusion reactions can be managed by slowing the infusion), subQ, due to its slower systemic release might actually turn out to be safer, not just more convenient.  On the other hand, my impression is that subQ SNALP could cause the type of ‘nuisance side effect’ that is frequently seen with phosphorothioate antisense.

The GalNAc comparison is obvious since Alnylam developings both a SNALP- and a GalNAc-based TTR RNAi Therapeutics in parallel.  In terms of competition for potential partnering, however, the DPCs by Arrowhead Research would be the more appropriate and challenging comparison.  I look forward to watching the race unfold as healthy competition is propelling the RNAi Therapeutics field into a highly competitive industry.

Is Alnylam Foolishly Betting the Farm on GalNAcs?

It is remarkable that Alnylam seems to be embracing GalNAc subcutaneous delivery for all pipeline candidates, but for perhaps ALN-TTR02, that are most important to its 5x15^TM development and commercialization strategy: ALN-TTRsc (TTR amyloidosis), ALN-AT3 (hemophilia), and ALN-AS1 (acute intermittent porphyria).  Considering that gene knockdown has not been demonstrated in Man with GalNAc conjugates, and potency could be a critical issue determining whether this really is a subQ approach, this apparent high-risk strategy is uncharacteristic of a company with the laudable attitude of raising capital when the company does not need it.   

ALN-TTRsc: The Missing Clinical Trial

One possible reflection of the fact that management does not really feel as confident about the strategy as it may seem, is the missing entry of the ALN-TTRsc phase I clinical trial on clinicialtrials.gov (and other registries I looked at).

I like to look at clinical trial registries, esp. clinicaltrials.gov, also for the reason that information contained therein often provide interesting clues into otherwise undisclosed safety and efficacy issues.  For example, is it a single and/or multiple dose, what are the dosing ranges, etc. 

With ALN-TTRsc, I do not expect safety to be necessarily the dose limiting factor.  Rather, it may well be the ability to show robust efficacy with an injection volume of 1ml or less, the magic upper limit for subQ approaches.  It turns out that adequate efficacy was seen in non-human primates only starting at 2.5mg/kg which apparently corresponds to the 1ml volume in humans.  Moreover, the dose-response curve is relatively shallow making it difficult to extrapolate predicted doses from preclinical models into humans (being off by a factor of 2 in the ED50 could already prove disastrous).

Is the omission to list ALN-TTRsc on clinicaltrials.gov an attempt at hiding that the dose escalation schedule indicates that 2.5mg/kg is an optimistic guess? Of course, the secrecy could also be for competitive reasons in light of its race with the competing antisense approach by GSK-ISIS.  But for a company that so far has listed every clinical trial, including ALN-TTR01 and ALN-TTR02 all of which have been conducted exclusively outside the US, there will be a reason for it and doubts about GalNAc potency is a plausible one.

GalNAc Steals Spotlight from DPCs

Recently, Arrowhead scientists presented impressive subQ hepatic knockdown data with the DPC polyconjugate platform.  While the response to the presentation at the Oligonucleotide Society Meeting in Boston last year was tremendous, possibly the most enthusiastic one among all the presentations (and one of the reasons why I am bullish that there is wider industry demand for DPCs), Arrowhead has been getting no love from the financial markets, thus putting it at a disadvantage in any licensing negotiations.  One factor for that might be GalNAcs which take away in the typical investor’s mind the uniqueness about it being a subQ RNAi delivery approach.  So if Alnylam eventually realizes that it needs an alternative, it might get access to it on more affordable terms.  The phase I ALN-TTRsc results expected mid-year could be the critical event in that dynamic: poor results would benefit Arrowhead Research.

GalNAc Plus

Of course, the confidence in GalNAcs may rest in the knowledge that ALN-TTRsc is actually the pipeline candidate with the weakest potency, and if IT shows efficacy, it will be a stroll for all the others.  These apparently have proven to be more potent in preclinical studies.  

I consider this a possibility, but am not fully convinced that these conjugates can really be called GalNAc-siRNA conjugates.  This is because the TTR siRNA is a particularly potent one already and I am wondering how you would achieve a 10x increase in potency from ALN-TTRsc to ALN-PCSsc with stabilizing nucleic acid chemistry alone.  Moreover, it is a small miracle already that a simple siRNA with a targeting ligand can achieve ED50s in the low mg/kg, because the related cytoplasmic release would likely rely on spontaneous endosome rupture (miracle probably explained by the high volume ASGPR receptor-mediated trafficking).

Consequently, my feeling is that the GalNAc Plus conjugates involve additional functional moieties beyond stabilizing chemistries such as 2'-o-methyl and 2'-F…such as endosomolytic activities.  In a 2009 patent application by Manoharan et al. (WO 2009/126933) such endosomal release activities, melittin included, were indeed contemplated in the context of GalNAc-siRNAs.  In fact, the claims would cover GalNAc-targeted DPCs such as the one described in the Mirus/Arrowhead in the 2007 PNAS paper (Mirus cientists Publish Elegant Paper on Targeted siRNA Delivery to Hepatocytes).

The patent application, somewhat of an attempt at an early land-grap being devoid of any actual experimental data, is not very remarkable really as it is obvious to somebody skilled in the art that in order to significantly enhance GalNAc-siRNAs such functionalities will be highly desirable, so a lot of it would come down again to chemistry and figuring out which exact compositions are functional and safe.

In my mind, it is fair game to pursue promising (delivery) technologies, especially under the Research Exemption in the US.  However, as the Tekmira-Alnylam relationship has shown, it is foolish for two companies to collaborate on delivery when both are working on GalNAc conjugates internally.  At best, the technology is useless and the whole thing will be forgotten.  At worst, the technology is valuable and both parties will try to control it and not pay the other party their dues.   

Overall, I think that betting the farm on GalNAcs is taking on more risk than warranted considering the advancements and validation achieved with SNALP technology.  Also, if Alnylam considered the subcutaneous approach to be that much more attractive than intravenous, you would think DPCs are the logical fall-back, if not more desirable than simple GalNAcs.  But maybe GalNAc Plus and DPCs are not all that dissimilar and we are on the same page after all.


Note added in proof (March 27, 2013): the ALN-TTRsc clinicaltrials.gov entry has now appeared.  As expected, however, no insights were provided into the dosing range. Only interesting tidbit: the volume of the placebo injection will be matched to that of ALN-TTRsc.  Injection volumes will be critical.