4′-Thio-RNA: A Novel Class of Sugar-Modified β-RNA

Improving RNA Interference in Mammalian Cells by 4‘-Thio-Modified Small Interfering RNA (siRNA): Effect on siRNA Activity and Nuclease Stability When Used in Combination with 2‘-O-Alkyl Modifications

“…The 4‘-thioribonucleoside residues caused a slight increase in the thermal stability of siRNA duplexes (0.3 °C per residue) when present either in the sense or antisense strands. This is in agreement with the previously reported thermodynamic stability of 4‘-thio-RNA duplexes. − The 2‘- O -MOE modification is known to provide a significant enhancement in thermal stability to oligonucleotides. Indeed, siRNA- 20 with a 2‘- O -MOE-modified sense strand at the 3‘- and 5‘-termini exhibited improved thermal stability ( T m = 74.3 °C, Table ) relative to the unmodified siRNA- 12 ( T m = 72.8 °C, Table ).…”

“…This is in agreement with the previously reported thermodynamic stability of 4′-thio-RNA duplexes. [38][39][40][41][42] The 2′-O-MOE modification is known to provide a significant enhancement in thermal stability to oligonucleotides. Indeed, siRNA-20 with a 2′-O-MOE-modified sense strand at the 3′-and 5′-termini exhibited improved thermal stability (T m ) 74.3 °C, Table 1) relative to the unmodified siRNA-12 (T m ) 72.8 °C, Table 1).…”

“…This observation is in agreement with the known nuclease stability of 4′-thioribonucleotides. [38][39][40][41][42] The need to protect the ends of individual strands is obvious when one compares siRNA-18 and siRNA-19. siRNA-18, in which the unmodified 3′-and 5′-ends of the sense strand are exposed to serum nucleases, has a half-life of 2.5 h. When these exposed ends are blocked by incorporation of 4′-thioribose residues (siRNA-19), the plasma half-life increases to 14 h. It should however be noted that siRNA-18 is still 2.5 times more stable in mouse plasma than unmodified siRNA-12 probably due to stabilization of the duplex toward endonucleolytic cleavage.…”

“…The synthesis and characterization of 4′-thio-RNA was first reported by Imbach et. al [38][39][40][41] and more recently by Hoshika et al [42][43][44] They showed that phosphodiester 4′-thio-RNA oligomers were significantly more resistant to cleavage by serum nucleases than unmodified phosphodiester RNA. Furthermore, 4′-thio-RNA hybridizes with complementary RNA sequences to form stable duplexes with standard A-form helical geometry.…”