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.

The antisense oligonucleotide fomivirsen serves as a complementary sequence for the messenger RNA (mRNA) of major immediate-early proteins in human cytomegalovirus, thereby hindering the virus's replication via an antisense mechanism[1]. As a 21-mer phosphorothioate antisense oligonucleotide (ASO) featuring a CpG motif near its 5′ end (sequence: 5′-GCG TTT GCT CTT CTT CTT GCG-3′), fomivirsen leads to mRNA degradation through an RNase H-mediated process[2]. Demonstrated to suppress cytomegalovirus (CMV) replication in both human retinal pigment epithelial (RPE) cells and human fibroblast (MRC-5) cells in a concentration-dependent fashion, fomivirsen presents a mean inhibitory concentration (IC 50) of 0.03 μM for RPE cells and 0.2 μM for MRC-5 cells[1].