Compounds of Formula (I) are disclosed, wherein R1, R2, R3, R4, R5, and R18 are defined herein. The compounds encompassed by Formula (I) include compounds which are HCV NS5B inhibitors and other compounds which can be metabolized in vivo to HCV NS5B inhibitors. The compounds and their pharmaceutically acceptable salts are useful for the prophylaxis or treatment of infection by HCV and the prophylaxis, treatment, or delay in the onset of disease caused by HCV. The compounds and their salts can be employed as ingredients in pharmaceutical compositions, optionally in combination with other antivirals, immunomodulators, antibiotics or vaccines.
