siRNAs are being developed as a novel therapeutic modality; however, problems impeding their application in extrahepatic tissues persist, including inadequate stability in biological environments and inefficient drug delivery system to target tissues. Thus, technological improvements that enable robust silencing of target messenger RNA (mRNA) in extrahepatic tissues are necessary. We developed prodrug type covalently closed siRNA (circular siRNA) as a novel nucleic acid agent to knockdown target genes in extrahepatic tissues by systemic administration without drug delivery components. Circular siRNA, which is chemically synthesizable, can assume optimal structures for efficient knockdown using its cleavable linker; namely, circular and linear structure in extracellular and intracellular environment, respectively. In this study, we investigated circular siRNA physicochemical properties, knockdown mechanism, and characteristics in vitro, as well as pharmacokinetics, accumulation, knockdown activity, and safety in vivo. Our circular siRNA exhibited higher stability against serum and exonucleases, increased cellular uptake, and stronger knockdown activity without transfection reagent in vitro than linear siRNA. Furthermore, after systemic administration to mice, circular siRNA showed prolonged circulation and improved knockdown activity in the liver, kidney, and muscle, without causing adverse effects. Circular siRNA may represent an additional platform for RNAi therapeutics, providing alternate solutions for disease treatment.
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