Session Summary
Keynote presentation by Sudhir Agrawal, Founder & President at Arnay Sciences
Summarized by biotech researcher and writer Catarina Carrao
Dr. Sudhir Agrawal, Founder & President at Arnay Sciences explained that the antisense approach is conceptually simple and elegant, because to design an inhibitor of a specific mRNA, one needs only to know the sequence of the targeted mRNA and an appropriately modified complementary oligonucleotide. Of the many analogues of oligodeoxynucleotides explored as antisense agents until today, phosphorothioate analogues have been studied the most extensively and with most benefits. Advancement in nucleic acid-based drugs has been aided by the chemistry of oligonucleotides and nucleic acids in providing drug-like properties. The nucleic acid-based drug discovery approach is now being recognized as a major platform in addition to small molecules and peptide- or protein-based platforms. Advancement in nucleic acid-based drugs has been aided by studies of the chemistry of oligonucleotides and nucleic acids. Chemical structure–activity relationship studies, which included modification of the backbone, heterocyclic bases and sugars, or combinations thereof, provided essential knowledge to the use of oligonucleotides as antisense agents.
Dr. Agrawal emphasized that experience with a specific PS-ASO drug candidate, GEM91 in humans was very informative at the time for the development of the platform. Subcutaneous administration of GEM91 caused flu-like symptoms, prolongation of activated partial thromboplastin time (aPTT) and more importantly, rather than suppressing HIV-1, it increased HIV-1 RNA levels in blood. This was puzzling, but much later it became clear that PS-ASO containing the unmethylated CpG motif were activating the immune responses by binding to toll-like receptor 9 (TLR9), an innate immune receptor present in immune cells that recognizes DNA containing CpG dinucleotide motifs.
Next, the studies of stereo-pure PS-ASOs showed improved activity in cell culture and in in vivo studies, and revealed that the sequence composition of ASO and the particular placement of stereo-specific linkages at specific positions have an effect on the antisense activity. Also, studies on modified RNA helped in consolidating previous knowledge of PS-DNA effects. These have allowed the design of hybrid or Gapmer antisense oligonucleotides, with increased affinities, nucleolytic stability, reduced polyanionic related side effects, including compliment activation and prolongation of aPTT, and reduced interaction depending on the sequence. The modulation of PS content in gapmer antisense oligonucleotides allowed to modulate protein binding and other characteristics. Over 50 gapmer antisense oligonucleotides have advanced to clinical development, with 3 of them approved for use. Potential reasons for failure could involve off-target activity, or lack of efficient knockdown, among some of the causes.
The importance of oligonucleotide sequence and modification on interaction with pattern recognition receptors (PAPMS) gained more importance for the possible effects of these molecules on immune activation and gene regulation. For example, it is now known that immune-stimulatory dinucleotide at the 5’-end of oligodeoxynucleotide is critical for a TLR9-mediated response. Also, accessibility of 5’ end is important for immune activation. As such, there is now a toolbox of chemical modifications that allow immune-mediated stimulatory and inhibitory activity of oligodeoxynucleotides. This knowledge could have significant effects in intra-tumoral therapy due to the systemic responses seen. For example, it is now possible to modulate a tumour microenvironment by intra-tumoral administration of a novel TLR9 agonist, IMO-2125. On the other hand, TLR antagonists have also been developed, with positive results in different animal studies and on-going clinical trials for the treatment of psoriasis (IMO-3100, TLR7/9 antagonist). As such, Dr. Agrawal concluded that RNA therapeutics is finally taking its place as a main drug category alongside small molecules and proteins, with almost a dozen of RNA therapeutics based on multiple mechanisms approved, and over 100 therapeutic candidates in clinical trials for diverse disease indications.