This phase 2b study will evaluate the efficacy and safety of 16 and 24 weeks of a 4-drug regimen with GS-9451 and Tegobuvir and 24 weeks of a 3-drug regimen of GS-9451 without Tegobuvir, all with Peginterferon Alfa-2a (Pegasys®) and Ribavirin (Copegus®).

Inclusion Criteria:

• Adult subjects 18 to 70 years of age
• Chronic HCV infection for at least 6 months prior to Baseline (Day 1)
• Liver biopsy results (performed no more than 2 years prior to Screening) indicating the absence of cirrhosis
• Monoinfection with HCV genotype 1a or 1b
• HCV treatment-naïve
• Body mass index (BMI) between 18 and 36 kg/m2
• Creatinine clearance ≥ 50 mL/min
• Subject agrees to use highly effective contraception methods if female of childbearing potential or sexually active male.
• Screening laboratory values within defined thresholds for alanine aminotransferase (ALT), aspartate aminotransferase (AST), leukopenia, neutropenia, anemia, thrombocytopenia, thyroid stimulating hormone (TSH), potassium, magnesium

Exclusion Criteria:

• Autoimmune disease
• Decompensated liver disease or cirrhosis
• Poorly controlled diabetes mellitus
• Severe psychiatric illness
• Severe chronic obstructive pulmonary disease (COPD)
• Serological evidence of co-infection with human immunodeficiency virus (HIV), hepatitis B virus (HBV), or another HCV genotype
• Suspicion of hepatocellular carcinoma or other malignancy (with exception of certain skin cancers)
• History of hemoglobinopathy
• Known retinal disease
• Subjects who are immunosuppressed
• Subjects with known, current use of amphetamines, cocaine, opiates (i.e., morphine, heroin), methadone, or ongoing alcohol abuse
• Subjects who are on or are expected to be on a potent cytochrome P450 (CYP) 3A4 or Pgp inhibitor, or a QT prolonging medication within 2 weeks of Baseline (Day 1) or during the study
• Subjects must have no history of clinically significant cardiac disease, including a family history of Long QT syndrome, and no relevant electrocardiogram (ECG) abnormalities at screening