Preclinical Characterization andIn VivoEfficacy of GSK8853, a Small-Molecule Inhibitor of the Hepatitis C Virus NS4B Protein

Abstract

ABSTRACTThe hepatitis C virus (HCV) NS4B protein is an antiviral therapeutic target for which small-molecule inhibitors have not been shown to exhibitin vivoefficacy. We describe here thein vitroandin vivoantiviral activity of GSK8853, an imidazo[1,2-a]pyrimidine inhibitor that binds NS4B protein. GSK8853 was active against multiple HCV genotypes and developedin vitroresistance mutations in both genotype 1a and genotype 1b replicons localized to the region of NS4B encoding amino acids 94 to 105. A 20-dayin vitrotreatment of replicons with GSK8853 resulted in a 2-log drop in replicon RNA levels, with no resistance mutation breakthrough. Chimeric replicons containing NS4B sequences matching known virus isolates showed similar responses to a compound with genotype 1a sequences but altered efficacy with genotype 1b sequences, likely corresponding to the presence of known resistance polymorphs in those isolates.In vivoefficacy was tested in a humanized-mouse model of HCV infection, and the results showed a 3-log drop in viral RNA loads over a 7-day period. Analysis of the virus remaining at the end ofin vivotreatment revealed resistance mutations encoding amino acid changes that had not been identified byin vitrostudies, including NS4B N56I and N99H. Our findings provide anin vivoproof of concept for HCV inhibitors targeting NS4B and demonstrate both the promise and potential pitfalls of developing NS4B inhibitors.