Antisense oligonucleotides: Efficient synthesis of 2′-O-methoxyethyl phosphorothioate oligonucleotides using 4,5-dicyanoimidazole. Are these oligonucleotides comparable to those synthesized using 1H-tetrazole as coupling activator?

Abstract Multiple 2′-O-methoxyethyl modified phosphorothioate oligonucleotides of 18–20-mer in length were synthesized at various scales using 4,5-dicyanoimidazole (DCI) as coupling activator. Extensive synthetic, analytical (using ion-pair LC–MS), and in vivo pharmacological, toxicological studies showed that oligonucleotides made with DCI and 1 H -tetrazole are chemically and biologically equivalent. This extensive study will help the oligonucleotide therapeutic industry to move from using a potentially explosive activator (1 H -tetrazole) to a safe activator (DCI).