A robust method using reducing inorganic salts for preventing the desulfurization of phosphorothioate oligonucleotides during the cleavage and deprotection step

The phosphorothioate (PS) linkage has been widely utilized in oligonucleotide therapeutics because it dramatically enhances the nuclease resistance of oligonu-cleotides. Although PS-modified oligonucleotides are easily synthesized using the phosphoramidite method, they are partially desulfurized during the cleavage and deprotection (C&D) step. The separation and purification of these desulfurized oligonucleotides are quite difficult because they have physical properties similar to those of their intact counterparts. Therefore, the suppression of desulfurization is important for oligonucleotide manufacturing. In this study, we developed a robust C&D method that avoids desulfurization by adding reducing inorganic salts such as NaI and Na2S2O3 to the reaction solutions. This new C&D method has potential application for manufacturing high-quality oligonucleotide therapeutics.