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.