Tapping the RNA world for therapeutics

Abstract

A recent revolution in RNA biology has led to the identification of new RNA classes with unanticipated functions, new types of RNA modifications, an unexpected multiplicity of alternative transcripts and widespread transcription of extragenic regions. This development in basic RNA biology has spawned a corresponding revolution in RNA-based strategies to generate new types of therapeutics. Here, I review RNA-based drug design and discuss barriers to broader applications and possible ways to overcome them. Because they target nucleic acids rather than proteins, RNA-based drugs promise to greatly extend the domain of 'druggable' targets beyond what can be achieved with small molecules and biologics.

Fig. 1 |. Antisense mechanisms of RNA-based drugs.

Most RNA-based drugs take advantage of complementary base-pairing of RNA analogs. They are generally extensively modified to improve their resistance to RNases and innate immune sensors and to potentially increase binding affinity to their nucleic acid or protein targets, which mediate their intracellular activity. These drugs bind to complementary sequences in endogenous genomic DNA or its RNA transcripts. The major classes are ASOs (usually single-stranded with a central DNA gapmer region) (a), double-stranded siRNAs (b), miRNA mimics (c) or antagomirs (d) and, most recently, single guide RNAs (sgRNAs) for gene editing using CRISPR-Cas (e). Although the targets are depicted in the diagram as pre-mRNAs, mRNAs or genomic DNA, in principle, these strategies could also be used to target ncRNAs. Credit: Debbie Maizels/Springer Nature

Fig. 2 |. Additional RNA-based drug mechanisms.

a, Sense-RNA drugs can be used for transient in vivo transcription of mRNAs to replace mutated proteins or for vaccination without the risk of genomic alteration. b, Aptamers take advantage of selection for high-affinity binding to molecular ligands, often in the nanomolar or subnanomolar range. They can be thought of as nucleic acid antibodies and have many of the advantages of conventional protein antibodies. They can be agonists or antagonists, linked for bifunctional targeting and conjugated to other RNAs, small-molecule drugs, toxins or peptides. However, unless modified, they are rapidly excreted and do not activate immune functions via binding to Fc receptors. Credit: Debbie Maizels/Springer Nature

Fig. 3 |. RNA-based drug delivery strategies.

Intracellular delivery is the most challenging obstacle to RNA-based drug development. Most siRNA drugs need to act in the cytoplasm, where their target RNAs are located. ASOs, acting on pre-mRNA, and sgRNAs, acting on genomic DNA, have the additional challenge of getting into the nucleus. Drug development is moving away from using particles or complexes, which have a tendency to get trapped in the liver, are complicated to manufacture and have side effects. Instead, conjugation of nucleic acid analogs to sugars, lipids, peptide or nucleic acid ligands that bind to the cell membrane or surface receptors is used. Binding activates cellular uptake and provides the opportunity for cell-specific delivery. Credit: Debbie Maizels/Springer Nature