The benzimidazole HCV translation inhibitor 3, which had recently been co-crystallized with the subdomain IIa RNA target, was included in the screen as a positive control. in non-coding functional RNAs in conjunction with the key roles they play in biological processes has spurred the interest in RNA as a potential drug target.1C3 The area of antiinfective therapy attests to the utility of RNA as a target since it is well established that many chemical classes of antibiotics selectively SU10944 act on bacterial ribosomal RNA (rRNA).4, 5 While attractive RNA targets outside the bacterial ribosome have been identified, including regulatory domains in genomes of pathogenic viruses, drug discovery for these targets faces formidable SU10944 challenges.6 A prime obstacle to drug discovery for RNA is the scarcity of synthetic small molecules that exhibit bias for binding to RNA as well as drug-like properties but which are not encumbered by the complexity of natural products.7 Further challenges arise from the difficulty to establish meaningful screening assays that report on functional consequences of ligand binding to an RNA target rather than returning binding affinities only. The triggering of a unique event, often related to conformational changes, in a specific target induced by ligand binding is tied to the achievement of selectivity over other cellular nucleic acids.8 Within the bacterial ribosome, the decoding site is a well-characterized RNA target of aminoglycoside antibiotics, which, upon binding, change the conformation of flexible adenine residues involved in mRNA decoding and thereby increase the error rate in protein synthesis.9, 10 Outside the ribosome, such conformational RNA targets for small molecule ligands had been largely elusive. We have recently shown that a benzimidazole inhibitor of hepatitis C virus (HCV) blocks translation of the viral RNA by conformational induction at a target site in the 5 untranslated region (UTR) of the HCV genome.11 The 5 UTR contains an internal ribosome entry site (IRES) which recruits host cell ribosomes directly at the start codon of the viral genome, independent of initiation factors and ribosomal scanning.12C14 The target site of the benzimidazole translation inhibitor is located in the IRES subdomain IIa which folds into a sharply bent motif that is required to accurately position the highly conserved hairpin loop IIb at the interface between the small (40S) and large (60S) ribosomal subunits (Fig. 1).15C17 Crystal structure analysis of the subdomain IIa revealed an L-shaped RNA fold that is stabilized by magnesium ions (Fig. 1b).18 We had previously suggested that specific ligand binding to the IIa RNA might affect the native conformation of the bent subdomain and prevent the correct positioning of the hairpin loop IIb at the ribosome, ultimately blocking viral translation. Subsequently we demonstrated that the benzimidazole translation inhibitor 2 of the HCV replicon acts by GPIIIa capture of an RNA conformation with a widened interhelical angle in the subdomain IIa.11 Compound 2 emerged from optimization of the simple benzimidazole derivative 1 which was SU10944 originally discovered at Isis Pharmaceuticals as a hit in a mass spectrometry-based affinity screen for IRES RNA-binding ligands (Fig. 1c).19 Open in a separate window Figure 1 The subdomain IIa RNA target in the HCV IRES. (a) Secondary structure of the IRES element in the HCV 5 UTR (nucleotides 1C341 of HCV genotype 1b). The boxed region indicates the subdomain IIa whose sequence is shown. (b) Crystal structure of the subdomain IIa RNA.18 Mg2+ ions are shown as green spheres. (c) HCV translation inhibitors that bind at the subdomain IIa RNA.11, 19, 20, 22 (d) Crystal structure of the subdomain IIa RNA in complex with the benzimidazole inhibitor 3.21 Crystal structure analysis of subdomain IIa RNA bound with the closely related ligand 320 demonstrated that the benzimidazole inhibitors capture a dramatic conformational change in the RNA target, leading to an entirely straight conformation in the complex (Fig. 1d) and providing further support for a conformational mechanism of IRES inhibition.21 These structural data along with our previous discovery of another class of IRES inhibitors, 3,5-diamino piperidine (DAP) ligands such as compound 4, which act by arresting the RNA fold in a bent state,22 support the notion of the subdomain IIa as a conformational switch that provides an attractive target for small molecule HCV.