Tekmira today reported very encouraging results from their first clinical study with SNALP siRNA delivery targeting apolipoprotein B for the treatment of hypercholesterolemia (PRO-040201, aka SNALP-ApoB). While the ~20% ApoB/LDL-c knockdown following single dose administration and evidence for flu-like toxicities at the highest tested dose level indicate that the present formulation is not suitable as a therapeutic, the company is rightly optimistic that based on these results, follow-up formulations that it intends to enter into the clinic later this year will be able to provide significant improvements in the absolute dose and therapeutic index. This is because the new knowledge can now be applied to the rapid progress that has been made in finding much more potent SNALP/LNP formulations (see recent PNAS paper and ILS2009 presentation) since the present formulation was locked down 2 years ago. In addition, much has been learned since on how to predict and mitigate SNALP siRNA-triggered immunostimulation in humans (extensive review). I therefore agree that it was probably the scientifically and financially best decision by the company to stop this trial at this stage and focus their resources on the next formulation.
When I first adopted SNALP/liposomal delivery as my pet systemic siRNA delivery approach 2-3 years ago, my main outstanding nightmare stemmed from an unexpected early-stage clinical disaster during a phase I trial by the same company (Protiva branch) almost a decade ago employing related stabilized cationic liposomes for the delivery of plasmids (SPLPs). In that trial, severe flu-like symptoms were observed at the very early and low dosages and the was trial terminated and the drug never heard of again. Instead of giving up, this has stimulated them to undertake almost heroic and industry-leading work aimed at mitigating immunostimulatory effects of liposomal nucleic acid delivery. It is thus quite remarkable that after just seven years after stumbling on siRNAs as more promising payloads for the technology, Tekmira has now demonstrated unambiguously functional liposomal siRNA delivery following systemic (intravenous) administration, with first, transient flu-like symptoms observed at what should be much higher dosages compared to the SPLP trial.
It is worth bearing in mind that the decision to stop the trial was not because the one case of flu-like adverse event was deemed a serious one, but because they felt that at this point they had already learnt what they wanted to learn and given the superior performance of the next-generation formulations it would therefore not have been ethical to further expose patients to a drug that will not be developed for commercialization (all my interpretation). It also might have risked tainting the asset with little to gain. To put the toxicity into context: in the most recent published clinical results on ISIS’ mipomersen targeting the same gene, 70% of patients receiving drug exhibited flu-like symptoms at similar 20% ApoB/LDLc knockdowns.
Equally encouraging is the fact that the knockdown, which based on the natural intra-person lipid variability and the pharmacokinetics of the reported knockdown (fast onset as expected for an RNAi drug) was almost certainly due to drug, occurred at dosages where the company expected to start seeing efficacy based on pre-clinical animal studies. One cannot underestimate the importance of being able to predict efficacious dose (and toxicity) based on these models, particularly at the early stages of evaluating such a novel technology where very little human pharmacological experience has been obtained.
The trial design involved cohorts of 4 patients for each dose level: one treated with placebo and three with study drug. While 8 cohorts had originally been planned, the trial was stopped in approximately the 6th or 7th cohort. According to the company, at this point the dose had been escalated to somewhere in the 0.6mg/kg ballpark and is thus consistent with IC50s for SNALP-siRNAs in non-human primates of ~1mg/kg at that time. Considering that according to a presentation at the International Liposome Society a month ago by Pieter Cullis, scientific founder of Tekmira, SNALP-like formulations have now reached IC50 potencies in non-human primates of ~0.03mg/kg, i.e. around 1.5 magnitudes better, there is every reason to believe that significant improvements in absolute dose and therapeutic index can be achieved with the follow-on SNALP-ApoB candidate. Strengthening this claim, similarly potent formulations have just recently surfaced from a study supposedly (Alnylam IR: I still cannot find the paper) published in PNAS by Alnylam and collaborators at the MIT on lipidoid-containing SNALP-like particles (allow a delay of at least a year from experimental finding to publication).
While all this sounds good, a critical question to ask is what gives them also the confidence that the new formulations (= more potent liposomes + less immunologically active siRNAs through modification) will not perform worse immunologically than the first one, thereby eliminating some of the benefit from the improvements in SNALP potency. The ability to answer this question would hinge on the ability to sensitively detect immunostimulation and translate such findings pre-clinical models into humans. The JCI paper last year by Tekmira scientists on the application of SNALP for distant tumors in mice showed that Tekmira indeed understands how to detect such responses even in the absence of cytokines in the serum (tissue-based PCR assay for IFIT1 mRNA). And from all the presentations I have seen, another important piece of the puzzle in being able to translate from animals into humans is the good correlation between immune responses seen in in vitro human PBMC immuno-assays and responses in animals. In combination, these elements allow for efficiently modifying siRNAs until predicted immune-related tox in humans have been minimized. Through such knowledge, Tekmira was able to provide a good explanation for why some non-human primates exhibited liver toxicity in the 2006 Nature study (the first demonstration of systemic RNAi in non-human primates) even in the absence of outward immune activation and remedy it through siRNA modification. It is interesting, however, that in the present clinical trial the presumed immune stimulation appeared to have (thankfully) been uncoupled from liver enzyme elevations as an absence of liver toxicities was noted. Although the reasons behind these are unclear, this uncoupling lends further credence to the notion that also efficacy and liver toxicity can be uncoupled: the mechanism that allows for functional entry of the liposome itself does not cause hepato-cytotoxicity.
According to a conversation with the company that I had following yesterday’s release of top-line results, it also appears that they have developed what appears to be a separate assay that they have not disclosed yet, and that should be even better in predicting immune responses in humans. Although, in the absence of seeing the data, I will have to take the company by its word here, as a proven leader in liposomal siRNA delivery, I’d like to give them the benefit of the doubt.
The company indicated that the new formulation will go into the clinic later this year, although discussions with the FDA have yet to determine the exact regulatory requirements for this (e.g. an abbreviated IND package would make sense IMO). Like the first trial, the second one is likely to be a single-dose one, and a multi-dose trial in humans would similarly require additional pre-clinical data. All this makes it clear that the main value of this program to the company is that ApoB is an exceptionally good target to most rapidly develop insights into the entire SNALP-siRNA delivery platform. The market reaction tomorrow, however, will show whether this is shared by investors who may be disappointed by the delay on ApoB and may have also hoped that SNALP-ApoB could find a development partner soon. However, I would think that most invested in Tekmira as a leader in systemic RNAi delivery and are equally encouraged that the company is now able to build real data which have for the first time demonstrated unambiguous systemic RNAi gene silencing in Man. In addition to investor perceptions, the financial repercussions of these results also depend on what value such pioneering work will fetch from potential partners that are interested in the broad development of the RNAi Therapeutics platform.
Whether Tekmira’s enthusiasm is shared by current partners will be seen by their clinical actions. As such, Alnylam until recently reiterated that it was still on track to file an IND for the liver-directed SNALP-TTR. Since Alnylam apparently was aware of the status of SNALP-ApoB in November already (see RNAi Clinical Experience chart in this slide presentation), Alnylam also seems to be comfortable that the more potent SNALP formulations will have clinical applicability given Tekmira's results. I look forward to learning about the TTR trial design, including whether multi-dose will be attempted, and there should be results by the end of this year. In addition, the fact that we have just learned that Roche entered IND-enabling studies, most likely with a SNALP product, further validates Tekmira’s decision. The fact, however, that the IND submission of SNALP-PLK1 has been moved into the second half of this year represents a slight delay, and it remains unclear whether this might be related to today’s news.
In any case, today makes me feel like we have witnessed possibly the beginning of the most important line of RNAi Therapeutics clinical research thus far. Many more data-points on clinical SNALP-siRNA delivery should accumulate over the next two years.
Disclosure: I have financial interests in the company, largely by holding TKM shares.
Note added in proof: Scientific papers by Tekmira and Alnylam on the development of significantly improved SNALP formulations:
1) By combinatorial chemistry screen (December 2009 PNAS paper)
2) By rational design of lipids (January 2010 Nature Biotech paper)