Monday, December 17, 2007
SomaGenics Reports on the Use of shRNAs for the Treatment of HCV
Most shRNA approaches are based on DNA-directed expression of RNA hairpins in the nucleus that are then fed into the RNAi pathway. Somagenics’ approach is slightly different in that they are introducing the RNA directly into cells. Concentrating more on the science than IP issues here I only want to briefly remark that this probably overlaps quite a bit with the Hannon patents (licensed to both CytRx and Alnylam) on the use of shRNAs as RNAi triggers, both transcribed and synthetic.
The scope of the experiments were limited either to in vitro culture studies or its glorified in vivo counterpart, the hydrodynamic co-transfection experiment. Here, a large volume of RNA-containing liquid is injected into the tail vein of a mouse in a brief period of time, physically forcing the RNA into hepatocytes. It is of interest, that unlike the findings of the back-to-back papers of the Hannon and Rossi labs more than two years ago, hairpins (directed to the conserved HCV IRES) with minimal stems, 19 base-pairs, were generally more effective than those with extended 25 base-pair hairpins. While it has to be said that compared to the Hannon and Rossi papers which argued that the long stems that were processed by Dicer would increase RNAi efficacy, the sample size here was quite limited, it nevertheless shows that the processing of hairpins are often not as predictable as one would wish based on microRNA biology.
Johnston further reported that the shRNAs were at least as efficient as the corresponding siRNAs with some of the IC50s in the low picomolar range. I am curious how this will translate into in vivo uses for example because shRNAs, unlike classical siRNAs and Rossi-type two-stranded Dicer-substrates, contain the double-stranded RNA within one molecule which should make this structure thermodynamically quite stable. A potential disadvantage is that it is so much more expensive to synthetically make the ~42-55 nucleotide hairpin RNAs since cost and purity of manufacturing RNAs increases more than linearly after you reach a certain size, say 25 nucleotides. While these studies mostly used RNAs generated through biochemical synthesis in the test tube using recombinant phage RNA polymerases, for the clinic they are working together with Agilent to generate the “same” RNAs to scale. I say the “same” here since the phage polymerase leaves a triphosphate 5’ end while chemical synthesis does not. This is not a trivial issue here since 5’ modifications are known to influence RNAi processing. We will therefore have to wait how the transition from in vitro transcribed to synthetic shRNAs will affect the reproducibility of their data and potential concerns from the FDA. Similarly, some mention was made that shRNAs may tolerate RNA modifications less well than siRNAs, probably because they are subject to additional processing steps.
The use of more challenging animal models is also warranted before entering the clinic. It is therefore important for them to find suitable delivery solutions which they apparently have only started to. It was good to hear though hearing him mention Protiva/Tekmira’s SNALPs and Mirus Bio’s Dynamic PolyConjugates as probably the most advanced RNAi delivery platforms to the liver, reflecting my views on what’s out there in the literature. Overall, I like the fact that multiple RNAi approaches are in the pipeline to tackle HCV, namely siRNAs: Sirna/Merck; DNA-directed shRNAs: Tacere/Benitec and Nucleonics; and now synthetic shRNAs: SomaGenics. And maybe Tekmira/Protiva should harness the power of SNALP and officially nominate HCV as a clinical development program. I hope that the next 2 years should finally see programs moving into the clinic after HCV RNAi had been delayed in the wake of company-specific issues (Benitec; Merck/Sirna vs Protiva).
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