o Since the chemical synthesis of oligonucleotides (oligos) in the late 1970s, the pharmaceutical industry and academia have made enormous efforts to develop antisense drugs because antisense provides a vast potential for therapeutic applications.
 

o Initially, synthetic antisense oligos of natural nucleotide compositions (first-generation antisense) were employed, but it was soon realized that the antisense oligos were unduly unstable to nucleases which are abundant in biological fluids. For example, first-generation antisense molecules in serum are completely degraded in less than 30 minutes.

 

o To improve the stability of antisense molecules, antisense molecules were chemically modified in a number of ways (second/Third-generation antisense). Salient members among the chemically modified antisense oligos are those ones with phosphorothioates (PS) and methylphosphonate (MP) modifications.

 

o Second/Third-generation antisense demonstrated some improvement in stability. However, nearly all of the chemically-modified antisense oligos exhibited various side effects such as non-specific binding to irrelevant sequences, systemic complement activation, prolongation of blood coagulation, and so on. In addition, antisense oligos also required laborious target site selection (antisense design) and incurred high production costs. These problems need to be solved before antisense can became a practical therapeutic technique.