An aptamer is a single-stranded oligonucleotide that binds to and inactivates a specific target, usually a protein. Several properties of aptamers make them attractive therapeutic agents similar to, but potentially better than, antibodies. They act in the same way as MAbs by folding into a three-dimensional structure based on their nucleic acid sequence to bind to their target. The smaller size of aptamers compared with MAbs might enable them to easily reach targets that are not readily exposed on the cell surface.
Because their small size results in rapid renal clearance (half-life of a few minutes) minutes, aptamers must be modified, e.g., by PEGylation. An aptamer drug on the market is pegaptanib sodium (Macugen®), a pegylated aptamer that adopts a three-dimensional conformation that enables it to bind to extracellular VEGF. In age-related macular degeneration (AMD), an isoform of VEGF is overexpressed, promoting growth of new blood vessels. Macugen specifically binds and inhibits the isoform of VEGF, leading to reduced pathologic vessel growth. It is delivered locally to the eye by intraocular injection for the treatment of wet AMD.