Pages

Thursday, March 8, 2018

Commonly Used RNAi Trigger Modification Can Integrate into Genome and Cellular Transcripts


To endow RNAi molecules with drug-like properties, they need to be modified (for stabilization, immuno-silence, RISC incorporation).  Modifications commonly used these days include 2’-O-methyl and 2’-fluoro (2’-F) modification of the ribose sugar ring and phosphorothioation of the phosphate connecting the constituent nucleotide monomers.

A study by Ionis and AstraZeneca scientists now shows (Saleh et al 2018) that 2’-F nucleosides are readily incorporated into RNA polymerase transcripts and the genome in tissue culture cells. By contrast, 2’-MOE nucleosides, a modification that Ionis chiefly uses for its antisense oligos, was highly refractory to such incorporation under the same conditions.

So regardless of the political motivation behind this publication- Ionis likes to paint the 2’-F modification used by competitor Alnylam as dangerous, whereas Alnylam likes to say same about Ionis’ phosphorothioates- the fact that turnover products from RNAi trigger degradation may be used in this way raises genotoxicity concerns that need to be taken seriously.

Even if minor degrees of 2’-F incorporations into transcripts and genomic and mitochondrial DNA turned out to be harmless, not undertaking the appropriate studies could catch companies in the space on the backfoot when regulators suddenly demand them.

It is possible that RNAi bellwether Alnylam indeed has responded to this concern as they have taken to minimizing the 2’-F content in their latest generation of GalNAc-conjugates while increasing 2’-O-methylation.  Although Alnylam justified this change with wanting to further increase the stability and thus longevity of the gene silencing, in light of twice annual administrations already possible with the old format (see inclisiran for PCSK9 lowering) and increasing 2'-O-methyl content making it harder to find intrinsically potent molecules, this move had me wondering whether it had actually to do with toxicity concerns instead.  This paper would support this notion.

3 comments:

Anonymous said...

Does Arrowhead Pharmaceutical use either of these? If not what do they use?

Dirk Haussecker said...

You can assume that Arrowhead is using these also for their latest conjugate platform.

Anonymous said...

Dirk, is Arrowhead's TRiM likely to bypass this being potentially toxic if there are no issues with off targeting? TRiM appears very accurate and does not need endosomal escape due to this accuracy. Is this likely to translate to safety in this case?

By Dirk Haussecker. All rights reserved.

Disclaimer: This blog is not intended for distribution to or use by any person or entity who is a citizen or resident of, or located in any locality, state, country or other jurisdiction where such distribution, publication, availability or use would be contrary to law or regulation or which would subject the author or any of his collaborators and contributors to any registration or licensing requirement within such jurisdiction. This blog expresses only my opinions, they may be flawed and are for entertainment purposes only. Opinions expressed are a direct result of information which may or may not be accurate, and I do not assume any responsibility for material errors or to provide updates should circumstances change. Opinions expressed in this blog may have been disseminated before to others. This blog should not be taken as investment, legal or tax advice. The investments referred to herein may not be suitable for you. Investments particularly in the field of RNAi Therapeutics and biotechnology carry a high risk of total loss. You, the reader must make your own investment decisions in consultation with your professional advisors in light of your specific circumstances. I reserve the right to buy, sell, or short any security including those that may or may not be discussed on my blog.