The development of interferon-free regimens for the treatment of chronic HCV infection constitutes a preferred option that is expected in the future to provide patients with improved efficacy, better tolerability, and reduced risk for emergence of drug-resistant virus. We have pursued non-nucleoside NS5B polymerase allosteric inhibitors as combination partners with other direct acting antivirals (DAAs) having a complementary mechanism of action. Herein, we describe the discovery of a potent follow-up compound (BI 207524, 27) to the first thumb pocket 1 NS5B inhibitor to demonstrate antiviral activity in genotype 1 HCV infected patients, BILB 1941 (1). Cell-based replicon potency was significantly improved through electronic modulation of the pKa of the carboxylic acid function of the lead molecule. Subsequent ADME-PK optimization lead to 27, a predicted low clearance compound in man. The preclinical profile of inhibitor 27 is discussed, as well as the identification of a genotoxic metabolite that led to the discontinuation of the development of this compound.