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4'-ThioRNA Substitution and Sense-Strand Segmentation for the Design of Functional Fully Modified siRNAs Targeting
Yuhei Nogi1, Jun Tsukimoto1, Noriko Saito-Tarashima1
1Graduate School of Pharmaceutical Science, Tokushima University, Tokushima, Tokushima, Japan.
Chemistry (Weinheim an Der Bergstrasse, Germany)
|August 3, 2026
Summary
Chemically modified small interfering RNAs (siRNAs) show therapeutic promise. Modifying siRNAs with 4'-thioRNA is possible, but full modification of both strands impairs RNA interference (RNAi) activity.
Area of Science:
- Oligonucleotide chemistry
- Molecular biology
- Antiviral therapeutics
Background:
- Chemically modified small interfering RNAs (siRNAs) are effective therapeutic agents.
- The RNA interference (RNAi) machinery has specific constraints on siRNA structure.
- Optimizing chemical modifications is crucial for siRNA efficacy.
Purpose of the Study:
- Investigate the tolerance of 4 acronym{'}thioRNA substitutions in siRNAs targeting SARS-CoV-2.
- Determine the impact of modification extent and position on RNAi activity.
- Explore strategies to overcome activity loss in fully modified siRNAs.
Main Methods:
- Systematic investigation of 4 acronym{'}thioRNA substitution at various positions within siRNAs.
- Assessment of RNA interference activity using a SARS-CoV-2 model.
- Physicochemical analysis to understand the mechanism of activity loss.
- Sense-strand segmentation applied to fully modified siRNAs.
Main Results:
- Single 4 acronym{'}thioRNA substitutions were well-tolerated across multiple positions.
- Complete substitution of either the sense or antisense strand maintained robust RNAi activity.
- Full substitution of both strands significantly reduced RNAi activity.
- Sense-strand segmentation partially restored activity in fully modified siRNAs.
Conclusions:
- The extent and positioning of 4 acronym{'}thioRNA modifications critically influence siRNA activity.
- Activity loss in fully modified siRNAs is linked to RISC loading interference.
- Sense-strand segmentation offers a viable strategy for developing fully modified siRNAs.
- These findings broaden the scope of chemically modified siRNA design for therapeutic applications.
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