Antisense oligonucleotides: the promise and the problems

Welcome, everyone. My name is Dr. Kendall S. Frazier. I'm a Director of Pathology at GlaxoSmithKline in King of Prussia, Pennsylvania in the United States. Today, I'd like to talk about a new and expanding field of drug development that involves antisense oligonucleotides. I'll talk about their promise and their problems.

So antisense oligonucleotide therapies have been around since approximately 1993. The first generation phosphorothioate oligoantinucleotides were much less stable and had poor efficacy as compared to the products that we have now. The original ASO molecules, ASO meaning antisense oligonucleotides, did not effectively enter cellular compartments. They had poor solubility and rapid degradation by exonucleases, which led to the development and clinical introduction of the so called second generation ASOs, where they were substitutions of the phosphate backbone. If you look at this slide to the right, you see at the two prime position methoxyethyl or also O-methyl substitutions were created which made more stable molecules. They had very predictable toxicity, but they also had proinflammatory effects. From 2002 to the present, we were using primarily the second generation oligonucleotides as therapies. However, in about 2005, the third generation oligonucleotides were developed. These included the locked nucleic acids and the short interfering RNAs and aptamers. Short interfering RNAs or the siRNAs are really not true antisense oligonucleotides, they use a separate form of inhibition of RNA. However, they've become more widely available and are actually more efficacious. The LNA's are in use by many companies at present but we still have a large number of phosphorothioate second generation or generation 2.5 oligonucleotides active in clinical trials. In 2011 we started seeing a new generation, some people call these third generation plus or fourth generation antisense oligonucleotides. They include constrained ethyl phosphorothioates and bicyclic nucleic acids with N-methyl substitutions. And finally we have Gal-Nac conjugated oligonucleotides. Those are oligos linked with N-acetylgalactosamine. Using these types of conjugated exogenous ligands, the ASO or siRNA multivalent molecule binds with high affinity to the asialoglycoprotein receptor and these are expressed on liver cells and get rapidly incorporated into the cytoplasm of the hepatocyte. There are also tricyclic DNA oligonucleotides which are just now coming on the scene and have shown some really exciting efficacy.