In-cell click labelling of small molecules to determine subcellular localisation.

UNLABELLED: Small molecule fluorometric boron dipyrromethene probes were developed to bind hepatitis C virus-encoded NS5A protein and aid subcellular distribution studies. These molecules did not co-locate with NS5A, therefore alternative 'silent' azide reporters were used to obtain a more relevant picture of their distribution. Following pre-incubation with replicon cells, click chemistry was used to append a fluorophore to the azide that confirmed the co-localisation of the small molecule with the NS5A protein, thus providing greater insight into the antiviral mode of action of this chemotype. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s12154-010-0047-1) contains supplementary material, which is available to authorized users.

Colony-forming assays reveal enhanced suppression of hepatitis C virus replication using combinations of direct-acting antivirals.

The current standard of care for patients infected with hepatitis C virus (HCV) is not effective universally and is associated with severe side effects. Direct-acting antiviral molecules have potential to transform treatment of HCV-infected individuals but emergence of drug-resistant virus will be problematic. It is anticipated that, to limit the emergence of drug-resistant virus, future HCV therapies must consist of multiple direct-acting antivirals. In the present study, cell culture-based colony-forming assays were used to demonstrate enhanced suppression of HCV RNA replication following simultaneous treatment of HCV replicon-containing cells with two direct-acting antivirals. Specifically, combinations of NS5Ai and Filibuvir (small molecule inhibitors of HCV-encoded NS5A and NS5B proteins respectively) were able to suppress colony formation fully at concentrations that individually they could not. HCV replicon RNA isolated from colonies that emerged following treatment with suboptimal concentrations of NS5Ai were found to encode resistance substitutions in the NS5A gene, which rendered them insensitive to subsequent high doses of NS5Ai. Furthermore, both NS5Ai and Filibuvir were effective at suppressing colony formation in combination with BILN 2061, an inhibitor of HCV-encoded NS3. Collectively, these data underscore the increased inhibitory capacity of direct-acting antivirals to suppress HCV RNA replication when present in combination.

Small molecules targeting hepatitis C virus-encoded NS5A cause subcellular redistribution of their target: insights into compound modes of action.

The current standard of care for hepatitis C virus (HCV)-infected patients consists of lengthy treatment with interferon and ribavirin. To increase the effectiveness of HCV therapy, future regimens will incorporate multiple direct-acting antiviral (DAA) drugs. Recently, the HCV-encoded NS5A protein has emerged as a promising DAA target. Compounds targeting NS5A exhibit remarkable potency in vitro and demonstrate early clinical promise, suggesting that NS5A inhibitors could feature in future DAA combination therapies. Since the mechanisms through which these molecules operate are unknown, we have used NS5A inhibitors as tools to investigate their modes of action. Analysis of replicon-containing cells revealed dramatic phenotypic alterations in NS5A localization following treatment with NS5A inhibitors; NS5A was redistributed from the endoplasmic reticulum to lipid droplets. The NS5A relocalization did not occur in cells treated with other classes of HCV inhibitors, and NS5A-targeting molecules did not cause similar alterations in the localization of other HCV-encoded proteins. Time course analysis of the redistribution of NS5A revealed that the transfer of protein to lipid droplets was concomitant with the onset of inhibition, as judged by the kinetic profiles for these compounds. Furthermore, analysis of the kinetic profile of inhibition for a panel of test molecules permitted the separation of compounds into different kinetic classes based on their modes of action. Results from this approach suggested that NS5A inhibitors perturbed the function of new replication complexes, rather than acting on preformed complexes. Taken together, our data reveal novel biological consequences of NS5A inhibition, which may help enable the development of future assay platforms for the identification of new and/or different NS5A inhibitors.