The esteemed editors and reviewers of
CELL got it wrong this time.
Usually, when a high-impact journal like
CELL decides to publish back-to-back papers on a given topic, it believes that
they mark a turning point of some sort that will be cited all over.
In this case (
Lima et al.;
Yu et al.), the turning point would be nanoparticle, or more precisely
SNALP-formulated RNAi delivery ‘out’, and unformulated, single-stranded RNAi ‘in’.
For those new to my blog: it is the double-strand feature
that is a defining property of the RNAi mechanism. While a single-stranded intermediate is
generated in the process, numerous studies, including indeed the one by Lima et al., show that
these are contrived, and consequently about 100-fold less potent ‘inducers’ of RNAi
gene silencing.
It is therefore surprising that
CELL would publish a confirmation of this. What is new though is that the
in vivo ssRNAi data involved
unformulated ssRNAi application, whereas previous
in vivo ssRNAi work by e.g. Merck involved LNP-mediated delivery (
Haringsma et al., 2012; also covered on this blog
here). However, as detailed below, the efficacy was not
impressive. To me, the main point of interest related to chemistry and how this
sheds light on the basic RNAi mechanism, which actually made this paper enjoyable to read. For example, the metabolically stable 5'-(E)-vinylphosphonate
modification and the positive effect of 2’F on Ago binding. Nevertheless, such biochemical detail is not
the groundbreaking stuff that lands you a paper in
CELL, but more something for the dedicated aficionado.
Does ISIS feel
threatened by SNALP delivery?
The complexity of SNALP delivery,
by which actually the difficulty of re-engineering SNALP
technology without access to Tekmira’s trade secrets and know-how is meant, is held
against the technology also in a commercially competitive sense. If patient
outcomes is the main goal, as long as you master complexity, isn’t that a good
thing, especially in terms of the all-important length of market exclusivity (note that the main cost of SNALP delivery is still the siRNA ingredient)?
A
common criticism of my writings is that I connect all things to Tekmira’s SNALP technology. But
read the Lima et al. paper and see
for yourself how ISIS equates formulated RNAi delivery with liposomal delivery
(start of the abstract e.g.):
‘The therapeutic utility of siRNAs is limited by the
requirement for complex formulations to deliver them to tissues. If potent
single-stranded RNAs could be identified, they would provide a simpler path to
pharmacological agents. Here, we describe single-stranded siRNAs (ss-siRNAs)
that silence gene expression in animals absent lipid formulation.’
Or
'However, in their current state, the therapeutic utility of siRNA is limited by the requirement for complex lipid formulations to deliver siRNA to peripheral tissues (Vaishnaw et al., 2010).'
It looks like 1-billion market cap ISIS feels threatened
by $40M market cap Tekmira’s SNALP technology after all and is a very interested participant in
the
frivolous patent infringement lawsuit against Tekmira (frivolous for the reason alone that Alnylam expressly congratulated Tekmira on the BMS deal).
SNALP requires 1000-10.000-fold less oligonucleotides
The assessment that the ssRNAi work by ISIS
does not mark a turning point in systemic RNAi delivery is based on simple
math. 1000 to 10.000-fold higher amounts
of oligonucleotides were required to achieve equivalent knockdowns in
mice: conservative 50microgram/kg/month for SNALP vs 50mg/kg/twice a week for ISIS ssRNAi.
As with any drug, large doses increase the risk of causing toxicities. In this case, it is particularly the
accumulations of large amounts of phosphorothioated oligonucleotides in the liver and kidney that
causes such concern. Of course, SNALP LNP
delivery is not entirely without its safety issues. For example, in the clinic it still involves
the use of transient immune suppression which may e.g. be prohibitive to their use
in millions of patients with less severe forms of hypercholesterolemia.
Although assuming for a moment that the amount of required
oligonucleotides should be irrelevant as long as it was safe, antisense
technologies still suffer from poor cost of goods. Last week for example, the CEO of another antisense
company, Sarepta Therapeutics (formerly known as AVI Biopharma), wrote in an unsettling
Open Letter to the Duchenne Muscular Dystrophy community that the company
essentially cannot afford the large, almost nutritional amounts of
oligonucleotides that are required for attempting a therapeutic splice correction. Closer to home, instead of acknowledging
their current cost of goods, ISIS Pharmaceuticals is only providing estimates
for their future oligo manufacturing cost goals.
Extrahepatic tissues, which ones please?
Another claim by
ISIS
related to their ssRNAi tech was that it would be applicable beyond the liver ('
broadly distributed and active in multiple organs'),
also following systemic administration.
However, once again, this was directly contradicted by their own data which
showed a maximally 35% knockdown (the type of from 100 to 65, not from 100 to
35 mind you) for such a tissue (the kidney) when administering…100mg/kg of oligonucleotides in mice.
The accompanying Huntington’s Disease paper achieved
ssRNAi-mediated knockdown in the brain.
This, however, was observed following non-systemic, intraventricular
infusion of large amounts of oligonucleotides in small mice brains.
Alnylam was right in
terminating ssRNAi collaboration
Taken together, the publications explain why Alnylam decided two years ago to terminate their ssRNAi collaboration with ISIS. In addition to using Tekmira’s intravenously infused SNALP LNPs,
Alnylam has been working on GalNAc-siRNA conjugates as a subcutaneously deliverable
alternative for gene knockdown in the liver.
Although the gene knockdown achievable with that technology still pales
in comparison to SNALP LNP, it is about 10-fold more potent than ISIS’ ssRNAi, which goes to show that despite the
disadvantages in cellular delivery of unformulated, rigid dsRNAs, their
dramatically increased potency more than compensates for it.
At least Lima
and colleagues and I agree on this point (opening statement of the
introduction): 'RNA interference (RNAi)
is a mechanism by which double-stranded
RNA triggers the loss of homologous sequence (Fire et al., 1998).' [Emphasis
mine]