**2. Pharmacology of antisense drugs**

The antisense oligonucleotides have the potential to manipulate the gene expression which prompted the field toward the therapeutic application and value of oligonucleotides as potential drugs and their targets [4]. The direct route to target RNA in a selective way is a well-established platform for drug discovery. The well-defined mechanisms, uncomplicated and easy to design, bring antisense oligonucleotides as a promising candidate for therapeutic development. The therapeutic potential of antisense drugs for the treatment of several diseases is already translated from bench to bedside, and many antisense drugs have entered into clinical trials for the treatment. The first patent on antisense therapy was granted to Molecular Biosystems company in 1991 for developing the antisense compounds. The first FDA-approved antisense product drug was afovirsen developed by Ionis Pharmaceuticals in 1992 which was a phosphorothioate oligonucleotide that targeted mRNA sequence of the E2 gene, which is associated with human papillomavirus transcription and replication. Later oblimersen, a phosphorothioate oligonucleotide, was designed to target the Bcl-2 protein for the treatment of melanoma and certain leukemias. Unfortunately, both the drugs failed in the clinical trial programs due to lack of efficacy and failure to demonstrate overall survival benefits and dose-limiting toxicity. Currently, several gene therapy- and antisense therapy-based clinical trials are ongoing. The major challenge of antisense drugs is effective and safe delivery to the target. The advancement toward antisense-based drug delivery is in progress. Several chemical modifications, novel chemistries, better formulation, and design of oligonucleotide not only have improved the potency and tolerability of antisense drug but also have enhanced the drug distribution to the targeted RNA inside the cells [5, 6]. The clinical application of antisense drugs requires safe and efficient carrier system, and currently, the viral and non-viral vectors are the most common methods used to deliver the antisense drugs specifically to the target tissues and cells. The viral vector-based delivery is most advantageous due to their high transfection efficiency [7]. Also, the new chemistries and better antisense oligonucleotide designs further improve the unwanted side effects, safety, and tolerability. From the last three decades, several antisense drugs have entered

into clinical trials and market for the treatment of a broad variety of diseases, and numerous oligonucleotides are under clinical development [6, 8–10]. The firstgeneration antisense drug, fomivirsen, targeting cytomegalovirus, was approved for the treatment of cytomegalovirus retinitis [11]. Many second-generation drugs are under development and are showing encouraging activity in the clinic. Now oligonucleotide therapy has come a long way and has been established as promising therapeutic tool. During this period, several clinical trials have been performed on thousands of participants for several diseases and only six molecules provided the clear clinical benefit in rigorously controlled trials [10]. As of now, there are six FDA-approved drugs based on oligonucleotide therapy: (1) fomivirsen for treatment of CMV retinitis in AIDS patients, (2) mipomersen for treatment of familial hypercholesterolemia, (3) defibrotide for treatment of veno-occlusive disease in the liver, (4) eteplirsen for the treatment of Duchenne muscular dystrophy, (5) pegaptanib for the treatment of neovascular age-related macular degeneration, and (6) nusinersen for the management of spinal muscular atrophy [10, 12]. In conclusion oligonucleotide-based antisense therapy has provided solutions to untreatable diseases. Future inventions in this technology will help in establishing the better and affordable cure to many more diseases.
