Targeting RNA: A Transformative Therapeutic Strategy

Abstract

The therapeutic pathways that modulate transcription mechanisms currently include gene knockdown and splicing modulation. However, additional mechanisms may come into play as more understanding of molecular biology and disease etiology emerge. Building on advances in chemistry and delivery technology, oligonucleotide therapeutics is emerging as an established, validated class of drugs that can modulate a multitude of genetic targets. These targets include over 10,000 proteins in the human genome that have hitherto been considered undruggable by small molecules and protein therapeutics. The approval of five oligonucleotides within the last 2 years elicited unprecedented excitement in the field. However, there are remaining challenges to overcome and significant room for future innovation to fully realize the potential of oligonucleotide therapeutics. In this review, we focus on the translational strategies encompassing preclinical evaluation and clinical development in the context of approved oligonucleotide therapeutics. Translational approaches with respect to pharmacology, pharmacokinetics, cardiac safety evaluation, and dose selection that are specific to this class of drugs are reviewed with examples. The mechanism of action, chemical evolution, and intracellular delivery of oligonucleotide therapies are only briefly reviewed to provide a general background for this class of drugs.

Figure 1

Selected key milestones in the development of oligonucleotide therapeutics. Purple box: milestones in biology; green box: milestones in chemistry; orange box: clinical milestones. 2ʹ‐F, 2ʹ‐fluoro; PS, phosphorothioate; 2ʹ‐MOE, 2ʹ‐O‐methoxyethyl; 2ʹ‐O‐Me, 2ʹ‐O‐methyl; ASO, antisense oligonucleotide; GalNAc, N‐acetylgalactosamine; IT, intrathecal; RNAi, interference RNA; siRNA, short interference RNA.

Figure 2

Schematic illustration of three common mechanisms adopted by the approved ASOs and siRNA. (a) An ASO with a central “gap” of DNA bases (gapmer ASO) binds to target mRNA by Watson‐Crick hybridization; RNase‐H1 recognizes an RNA–DNA heteroduplex, cleaving the target RNA strand selectively while leaving ASO strand intact to bind to additional target RNA. (b) An siRNA is recognized by the RISC complex, where the sense strand is degraded and removed, and the antisense strand is left bound to Ago2 protein to form a ribonucleoprotein complex. The Ago2 complex facilitates hybridization of the antisense strand to the target RNA, cleaving the target RNA selectively while leaving the antisense stand intact to bind to additional target RNA. (c) An ASO modified to remove any potential to form RNA–DNA hybrids (non‐DNA‐like ASO) acts as a steric blocker to alter RNA maturation process, including modulation of splicing. Ago2, argonaute‐2; ASO, antisense oligonucleotide; mRNA, messenger RNA; siRNA, short interference RNA; RISC, RNA‐induced silencing complex.

Figure 3

Common chemical modifications for the ASOs and siRNAs approved and in the clinic. The modifications utilized in the approved ASOs and siRNA (fomivirsen, mipomersen, eteplirsen, nusinersen, inotersen, and patisiran) are PS, 2ʹ‐MOE, 2ʹ‐O‐Me, 2ʹ‐F, and PMO. Fomivirsen: PS DNA, no sugar modification; mipomersen and inotersen: PS and 2ʹ‐MOE modified gapmer ASOs; nusinersen: PS and 2ʹ‐MOE fully modified ASO; patisiran: PS, 2ʹ‐F and 2ʹ‐O‐Me modified siRNA; eteplirsen: PMO. 2ʹ‐F, 2ʹ‐fluoro; 2ʹ‐O‐Me, 2ʹ‐O‐methyl; 2ʹ‐MOE, 2ʹ‐O‐methoxyethyl; ASO, antisense oligonucleotide; cEt, constrained ethyl; LNA, locked nucleic acid; PMO, phosphorodiamidate morpholino oligomer; PS, phosphorothioate; siRNA, short interference RNA.