Chimeric 2′-O-methyl oligoribonucleotides (2′-OMe ORNs) containing internucleotide linkages which were modified with phosphonoacetate (PACE) or thiophosphonoacetate (thioPACE) were prepared by solid-phase synthesis. The modified 2′-OMe ORNs contained a central phosphate or phosphorothioate sequence with up to 4 PACE or thioPACE modifications, respectively, at either end of the ORN in a “gapmer” motif. Both PACE and thioPACE 2′-OMe ORNs formed stable duplexes with complementary RNA. The majority of these duplexes had higher thermal melting temperatures than an unmodified RNA:RNA duplex. The modified 2′-OMe ORNs were effective passenger strands with complementary, unmodified siRNAs, for inducing siRNA activity in a dual luciferase assay in the presence of a lipid transfecting agent. As single strands, thioPACE 2′-OMe ORNs were efficiently taken up by HeLa cells in the absence of a lipid transfecting agent. Furthermore, thioPACE modifications greatly improved the potency of a 2′-OMe phosphorothioate ORN as an inhibitor of microRNA-122 in Huh7 cells, without lipid transfection.
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