N-[[P(S),2'R]-2'-deoxy-3'-O-[(1,1-dimethylethyl)dimethylsilyl]-2'-fluoro-2'-methyl-P-phenyl-5'-uridylyl]-L-alanine-d5 1-Methylethyl Ester is an intermediate in synthesizing PSI-7977-D5 (P839643), an isotope labelled analog of PSI-7977. PSI-7977 is a prodrug that is metabolized to the active antiviral agent 2'-deoxy-2'-α-fluoro-β-C-methyluridine-5'-monophosphate and is currently being investigated in phase 3 clinical trials for the treatment of hepatitis C. Studies have profiled PSI-7977 as a nucleotide inhibitor of hepatitis C virus, exerting selective inhibitory effects towards HCV NS5B polymerase.