Fomivirsen sodium is an antisense 21 mer phosphorothioate oligonucleotide. Fomivirsen is an antiviral agent that is used cytomegalovirus retinitis (CMV) research, incluiding in AIDs. Fomivirsen sodium binds to and degrades the mRNAs encoding CMV immediate-early 2 protein (required for viral replication), thus providing bioactive effects for CMV retinitis by inhibition of virus proliferation.

Antisense oligonucleotide acts a complementary nucleotide to the messenger RNA of the major immediate-early region proteins of human cytomegalovirus, it disrupts the replication of the virus through an antisense mechanism[1].Fomivirsen is 21-mer phosphorothioate ASO with CpG motif near its 5′ end, with the sequence 5′-GCG TTT GCT CTT CTT CTT GCG-3′, resulting in mRNA degradation by RNase H-mediated mechanism[2].Fomivirsen inhibits CMV replication in human retinal pigment epithelial (RPE) cells and in human fibroblast cells (MRC-5 cells) in a dose-dependent manner. The mean IC 50 are 0.03 μM for RPE cells and 0.2 μM for MRC5 cells[1].