Unique Metabolism of a Novel Antiviral l-Nucleoside Analog, 2′-Fluoro-5-Methyl-β-l-Arabinofuranosyluracil: a Substrate for Both Thymidine Kinase and Deoxycytidine Kinase

2′-Fluoro-5-methyl-β-l-arabinofuranosyluracil (l-FMAU) is the first l-nucleoside analog with low cytotoxicity discovered to have potent antiviral activities against both hepatitis B virus and Epstein-Barr virus but not human immunodeficiency virus. This spectrum of activity is different from those of the other l-nucleoside analogs examined. l-FMAU enters cells through equilibrative-sensitive and -insensitive nucleoside transport as well as through nonfacilitated passive diffusion. l-FMAU is phosphorylated stepwise in cells to its mono-, di-, and triphosphate forms. In the present study the enzymes responsible for the first step of l-FMAU phosphorylation were identified. This is the first thymidine analog shown to be a substrate not only for cytosolic thymidine kinase and mitochondrial deoxypyrimidine kinase but also for deoxycytidine kinase. This finding suggests that the antiviral activity of l-FMAU will not be limited by the loss or alteration of any of these deoxynucleoside kinases.