The present invention provides double-Gap modified oligonucleotides and their applications in anti-
hepatitis B and
hepatitis D viruses. Specifically, the present invention provides a compound, or a pharmaceutically acceptable salt,
hydrate or solvate thereof, wherein (a) the compound is a double-Gap modified
oligonucleotide; and wherein the
oligonucleotide has a sequence as shown in SEQ ID NO.2, SEQ ID NO.3, SEQ ID NO.4 or SEQ ID NO.5; (b) the double-Gap modification refers to that each
nucleoside portion in the
oligonucleotide is modified in a manner as shown in Formula I; and (c) the internucleoside bond of the oligonucleotide is an unmodified or modified internucleoside bond, wherein P1, P2, P3, S1 and S2 and specific modifications are as described in the text. The double-Gap modified oligonucleotide of the present invention has significantly improved antiviral activity. P1-S1-P2-S2-P3(I).