The past few years have witnessed important breakthroughs in the identification of compounds that specifically bind and regulate RNAs and in optimizing them for therapeutic use. Here, we review successful and unsuccessful approaches in screening for RNA-targeted small molecules. We discuss advantages and disadvantages of the different screening techniques and variables that affect the outcome of RNA-screening projects. We also highlight key challenges that hamper the development of quality RNA ligands, especially the still-low availability of RNA-specific compound libraries and the poor understanding of RNA structural dynamics. We conclude that the development of new RNA-targeting drugs would greatly benefit from integration of the power of high-throughput screening technologies with improved biochemical, structural, and computational characterization of RNA targets.
The unique challenges of discovering drugs against RNAAs a messenger of genetic information, a catalyzer of biological reactions like splicing or protein synthesis, a transcriptional and translational regulator, and a modulator of metabolic proteins, RNA participates in fundamental physiopathological processes in cells [1]. Intervening in RNA with small molecules could thus be a powerful way to treat human diseases [1]. RNA-binding molecules have played important roles in medicine since the 1940s, when aminoglycoside antibiotics, which target rRNA, were discovered through unbiased phenotypic and genetic screens [2]. However, aminoglycosides tend to bind RNA promiscuously, which limits their applicability [3], so to achieve selective and potent targeting of RNAs it is important to enlarge the chemical space of RNA-binding compounds.The recent discovery of many new, vast classes of RNAs, such as riboswitches and long noncoding RNAs (lncRNAs), some of which adopt well-defined functional structures, has now expanded the universe of potentially druggable RNAs [4][5][6]. However, the development of drugs against RNA presents unique challenges (Box 1). Drug discovery focused on RNA targets consequently lags behind drug discovery aimed at protein targets [2] . Besides the structural and chemical properties of protein versus RNA ligands, which have been reviewed elsewhere [2,7], the design and implementation of screening strategies optimized for RNA are crucial factors contributing to successful RNA drug discovery (Figure 1).Screening design: the choice of a suitable RNA target RNA-focused screens seek to identify compounds that target RNA either directly or within ribonucleoprotein (RNP) (see Glossary) complexes. RNA-specific drugs have the potential to provide new therapeutic opportunities for diseases where protein targeting is difficult (i.e., genetic disorders associated with aberrant transcripts) or for infectious diseases. However, targeting RNA directly may be challenging if transcripts are expressed at low levels or lack structured regions. RNPs, which are being rapidly discovered thanks to proteomics and crosslinking and immunoprecipitation (CLIP)-b...