Unique metabolism of a novel antiviral L-nucleoside analog, 2'-fluoro-5-methyl-beta-L-arabinofuranosyluracil: a substrate for both thymidine kinase and deoxycytidine kinase.

2'-Fluoro-5-methyl-beta-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.