In early 2013, Solstice Biologics was the first most notable
RNAi platform start-up after the industry had gone through the 2008-2011 RNAi Valley of Death. The idea was to develop new single molecule RNAi triggers that would have better
pharmacologic attributes than the highly negatively charged small
double-stranded RNAs, as well as increased stability and reduced immunogenicity.
Today, a publication by the Dowdy group (Meade et al 2014), the academic
birthplace of the technology, was published in Nature Biotechnology revealing
for the first time more detailed insights into the fundamental approach.
Accordingly, the charge and stability issues have been
addressed by esterifying the sugar-phosphate backbone with a biocleavable thioester,
turning the phosphate diester into a triester.
Despite some steric constraints due to the nature of the double helix,
the majority of phosphates could thus be triesterified thereby creating a more
or less neutral RNAi trigger molecule: siRNNs (small interfering ribonucleic neutrals).
Once in the cytoplasm of the target cell, the triesterified
RNAi triggers get converted by the ubiquitously expressed thioesterases into
canonical charged RNAi triggers which only then become competent for utilization by the
RNAi machinery.
Importantly, the molecules could be synthesized by methods
closely related to standard phosphoramidite-based synthesis using modified
phosphoramidites as the building blocks.
To this end, the Dowdy group and Solstice have created a library of
modified phosphoramidites, including those amenable to the conjugation to cell-targeting ligands and endosomal release functionalities.
The study validated the high stability and reduced
immunogenicity of the siRNNs and showed their increased binding affinity to plasma proteins such
as albumins. The latter is predicted to facilitate improved pharmacokinetics.
Unfortunately, the in vivo validation stopped at the stage of using GalNAcs as the
targeting ligand, because this attribute is predicted to be an advantage for
particularly the delivery outside the liver where we might not find receptors
with high uptake capacity similar to ASGPR on hepatocytes.
Another favorable attribute of the charge-neutral siRNNs, but which
remains to be demonstrated, is improved tissue penetration. Finally, it is possible that siRNNs have an
advantage in overcoming cell membranes as well which is consistent with the apparent
improved potency of GalNAc-siRNNs over standard siRNA-GalNAc conjugates (40 vs
55% knockdown in an experiment).
In many ways, siRNNs remind me of the self-delivering RNAi
trigger approach first pioneered by Dharmacon and later adopted by RXi
Pharmaceuticals. However, there are at
least two important differences: 1) self-delivering RNAi triggers still contain
negative charge; and 2) self-delivering RNAi triggers should be structurally
more flexible due to the shortened double-stranded region (~12 base-pairs vs 19
base-pairs) which, however, comes at the expense of impaired potency.
I greatly welcome this publication as it represents a
fundamentally differentiated approach to RNAi Therapeutics drug development and it
will be exciting to see where the Dowdy group and Solstice Biologics will take
this versatile platform for RNAi and potentially beyond.
