Synthesis of scpBNA-C, -A, and -G Monomers and Evaluation of the Binding Affinities of scpBNA-Modified Oligonucleotides toward Complementary ssRNA and ssDNA.

We previously reported the synthesis and evaluation of 2'-O,4'-C-spirocyclopropylene-bridged nucleic acid (scpBNA) bearing a thymine (T) nucleobase. Oligonucleotides (ONs) modified with scpBNA-T exhibited strong binding affinity to complementary single-stranded RNA (ssRNA) and high enzymatic stability. These biophysical properties suggest that scpBNAs are well suited for use in antisense strategies. Herein, we describe the synthesis of scpBNA monomers bearing 5-methylcytosine (C), adenine (A), and guanine (G) nucleobases for use in a variety of sequences. The prepared scpBNA monomers were incorporated into ONs at various positions. The scpBNA-modified ONs exhibited excellent duplex-forming ability with the complementary ssRNA comparable to ONs modified with 2'-O,4'-C-methylene-bridged nucleic acid (2',4'-BNA/LNA). Moreover, ON modified with scpBNA-C, -A, and -G showed higher enzymatic stability than the corresponding 2',4'-BNA/LNA-modified ON. These results demonstrated a promising role for the incorporation of scpBNA monomers into therapeutic antisense ONs.