PSI-7977 is an investigational nucleotide analog currently in Phase 2 for treatment of chronic HCV infection. PSI-7851 is a mixture of two similar molecules, PSI-7976 and PSI-7977, which exist in PSI-7851 as isomers of each other. Once inside a liver cell, both molecules are rapidly converted to the same active triphosphate. Given the ability to more easily manufacture PSI-7977, and potentially advantageous in vitro characteristics, PSI-7977 was selected for further clinical development. Antiviral agents.