Oligonucleotide N3′→P5′ phosphoramidates as potential therapeutic agents

Abstract

Uniformly modified nucleic acids analogues, oligonucleotide N3′→P5′ phosphoramidates, containing 3′-amino instead of 3′-hydroxyl nucleosides, were synthesized and studied. These compounds form very stable duplexes with complementary native phosphodiester DNA and exceptionally stable duplexes with RNA strands. Increases in duplex melting temperature, Δ T m, relatively to their phosphodiester counterparts, reaches 2.9–3.5°C per modified nucleoside. Moreover, the phosphoramidate compounds form extremely stable triple stranded complexes with single or double stranded DNA oligomers under near physiological salt and pH conditions. Melting temperatures of these triplexes usually exceed that of the isosequential phosphodiester counterparts by up to 35°C. For 11-15-mers 2′-deoxyphosphoramidates are structurally and functionally similar to the native RNA molecules and thus can be used as RNA decoys. They are resistant to enzymatic digestion by nucleases both in vitro and in vivo. Oligonucleotide phosphoramidates apparently are cell permeable, and they have a good bioavailability and biodistribution, while being non-toxic in mice at therapeutically relevant doses. Duplexes of the several studied phosphoramidates with complementary RNA strands apparently are not substrates for RNase H in vitro. Despite that, these compounds exerted high sequence-specific antisense activity in various cell lines and in SCID mice. The observed in vitro lack of RNase H recognition of the RNA:phosphoramidate duplexes may result in better specificity in biological activity of these compounds relative to RNase H inducing oligonucleotides. Experimental results also indicate that oligonucleotide phosphoramidates can be used as efficient and specific modulators of gene expression by an antigene mechanism of action. Finally, the oligo-2′-deoxyphosphoramidate double stranded complexes can structurally mimic native RNA complexes, which could be efficiently and specifically recognized by the RNA binding proteins, such as HIV-1 Rev and Tat.