Antagomirs are one of a novel class of chemically engineered oligonucleotide

Oligonucleotide

An oligonucleotide is a short nucleic acid polymer, typically with fifty or fewer bases. Although they can be formed by bond cleavage of longer segments, they are now more commonly synthesized, in a sequence-specific manner, from individual nucleoside phosphoramidites...

s. Antagomirs are used to silence endogenous microRNA.

An antagomir is a small synthetic RNA that is perfectly complementary to the specific miRNA target with either mispairing at the cleavage site of Ago2 or some sort of base modification to inhibit Ago2 cleavage. Usually, antagomirs have some sort of modification, such as 2' methoxi groups and phosphothioates, to make it more resistant to degradation.
It is unclear how antagomirization (the process by which an antagomir inhibits miRNA activity) operates, but it is believed to inhibit by irreversibly binding the miRNA. Antagomirs are now used as a method to constitutively inhibit the activity of specific miRNAs. For example, antagomirs against miR-21

MIRN21

microRNA 21 also known as hsa-mir-21 or miRNA21 is a mammalian microRNA that is encoded by the MIR21 gene.MIRN21 was one of the first mammalian microRNAs identified. The mature miR-21 sequence is strongly conserved throughout evolution...

have been successfully used to inhibit fibrosis of heart and lung.