A highly sensitive and reproducible HCV RNA hybridization method valuable for antiviral drug discovery.

Real-time RT-PCR and Northern blot are employed for the measurement of HCV RNA but suffer from multiple purification steps, high cost, and relatively large variability. In this study, a hybridization method for HCV RNA detection is described. This method does not need RNA purification, and is sensitive enough to detect HCV RNA present in replicon cellular lysates harvested from a single well of a 96-well plate. Fixation of RNA by UV cross-linking is crucial for this sensitivity. A linear relationship exists between hybridization signal and cell density ranging from 10(5) to as few as 300 cells per well. The signal-to-background ratio is greater than 40 and the Z factor is above 0.7. Using several known anti-HCV agents, dose-response curves and EC(50) values generated from hybridization were similar to those obtained from a luciferase assay. This method has been successfully applied to replicons of different HCV subtypes and hepatitis B virus in our laboratory. In summary, this hybridization assay is sensitive, highly reproducible, easy to handle, and a valuable tool for antiviral drug discovery.

Selection of replicon variants resistant to ACH-806, a novel hepatitis C virus inhibitor with no cross-resistance to NS3 protease and NS5B polymerase inhibitors.

We have discovered a novel class of compounds active against hepatitis C virus (HCV), using a surrogate cellular system, HCV replicon cells. The leading compound in the series, ACH-806 (GS-9132), is a potent and specific inhibitor of HCV. The selection of resistance replicon variants against ACH-806 was performed to map the mutations conferring resistance to ACH-806 and to determine cross-resistance profiles with other classes of HCV inhibitors. Several clones emerged after the addition of ACH-806 to HCV replicon cells at frequencies and durations similar to that observed with NS3 protease inhibitors and NS5B polymerase inhibitors. Phenotypic analyses of these clones revealed that they are resistant to ACH-806 but remain sensitive to other classes of HCV inhibitors. Moreover, no significant change in the susceptibility to ACH-806 was found when the replicon cellular clones resistant to NS3 protease inhibitors and NS5B polymerase inhibitors were examined. Sequencing of the entire coding region of ACH-806-resistant replicon variants yielded several consensus mutations. Reverse genetics identified two single mutations in NS3, a cysteine-to-serine mutation at amino acid 16 and an alanine-to-valine mutation at amino acid 39, that are responsible for the resistance of the replicon variants to ACH-806. Both mutations are located at the N terminus of NS3 where extensive interactions with the central hydrophobic region of NS4A exist. These data provide evidence that ACH-806 inhibits HCV replication by a novel mechanism.

Discovery of novel dialkyl substituted thiophene inhibitors of HCV by in silico screening of the NS5B RdRp.

A novel 5,4-dialkyl substituted thiophene was discovered by in silico screening of the 3D polymerase crystal structure (1GX6) that demonstrated single digit micromolar HCV inhibition activity in the replicon assay and dose-dependent inhibition in the replicase complex assay. Subsequently, SAR was explored with a small set of dialkyl and tetrahydro-benzo thiophenes. Since these thiophenes inhibit synthesis of both, single- and double-stranded RNAs, their mechanism of action is distinct from other known HCV inhibitors.