Antibodies (Ab), which are proteins also known as immunoglobulins (Ig), are large Y-shaped proteins produced from B cells, which are used by the immune system to identify and neutralize foreign objects such as viruses or bacteria. The antibodies are substances that recognize a specific part of foreign targets, called antigens, and specifically bind thereto to induce an antigen-antibody reaction. Due to the specific binding between the antigen and antibody, the antibodies are not only used to detect antigens, but are also used to diagnose and treat diseases. There has been a report on antibody dimerization as a method for improving the binding affinity for such target materials (Luo, Y. et al., mAbs. 2009, 1(5): 491-504). However, the antibodies not only show low expression levels and low solubility and must employ animal cell expression cell lines, but also show disadvantages of high purification costs, a reduced stability in reducing environments, etc. Therefore, efforts are actively being made to discover substances which can specifically bind to target materials in replacement of the antibodies.
Meanwhile, aptamers are specific types of single-stranded nucleic acids (DNA, RNA, or modified nucleic acids), which have stable three-dimensional structures themselves and can bind to target materials with high affinity and specificity. The aptamers can be obtained via Systematic Evolution of Ligands by Exponential Enrichment (SELEX) from a nucleic acid library with random sequences.
Such aptamers are considered as good alternatives for antibodies, and a large number of aptamers are known to bind to metal ions, small chemical molecules, proteins, and even cells specific enough that they have a dissociation constant at the level of nanomoles to picomoles. Also, the aptamers show advantageous characteristics over the antibodies as follows through specific experimentations. The first characteristic is that the aptamers can be obtained from a nucleic acid library such that the aptamers can target certain molecules (from small inorganic ions to cells). This characteristic enables the aptamers to overcome the limitation that the antibodies must be obtained from cells or animals. The second characteristic is that the aptamers selected from the library can be amplified via polymerase chain reaction (PCR) or transcribed in order to obtain a large amount of aptamers having high purity. The third characteristic is that functional groups on the aptamers can be easily modified if the aptamers are used for other purposes such as immobilization on a solid surface, etc., as the aptamers have relatively simple chemical structures. Lastly, the aptamers can be applied to chemical applications where slightly more extreme conditions are required (high temperatures or extreme pH) because the aptamers are much more stable than the antibodies. In addition, since the aptamers can be chemically produced from a large-scale synthesis, they are economically favorable and have a target affinity close to that of the antibodies. In contrast, the size of the aptamers is significantly smaller than that of the antibodies (about 1 to 2 nm).