9-([2-Hydroxy-1-(hydroxymethyl)ethoxy]methyl)guanine (BW B759U) is more potent and has a more prolonged inhibitory effect against Epstein-Barr virus (EBV) in vitro than does acyclovir (ACV). To assess the mechanism of this difference, we first compared the extent of phosphorylation of the two drugs in superinfected Raji cells. BW B759U is phosphorylated to levels 100-fold higher than is ACV. In addition, lower levels of phosphorylation of BW B759U and ACV were observed in uninfected Raji cells. Studies on the kinetics of formation of BW B759U triphosphate in superinfected Raji cells indicated that drug-phosphorylating activity was detected as early as 3 h after superinfection; this activity was steadily maintained for the first 7 h, followed by a burst of activity between 7 and 10 h and a doubling of phosphorylation between 10 and 25 h. During the superinfection cycle, the pool sizes of deoxyribonucleoside and ribonucleoside triphosphates were increased and reached their maxima at 10 h after infection. The maximal amount of triphosphorylated drug in a virus producer cell, P3HR-1 (LS), was obtained at 21 h after drug treatment. During long-term drug treatment, approximately 44 and 77% reduction in EBV genome copies per cell was observed on days 3 and 7, respectively. In a separate experiment, after treatment of P3HR-1 (LS) cells with BW B759U for 36 h, 4.2 pmol of BW B759U triphosphate per 10(6) cells was achieved. After the cells were released into drug-free medium, drug triphosphate was rapidly decreased to 11% of the original level in 1 day. Thereafter, the decrease was slow but steady, down to 0.22 pmol/10(6) P3HR-1 cells by 5 days. We calculated that 0.22 pmol of BW B759U triphosphate per 10(6) cells represents a cellular concentration of 0.22 microM, which is theoretically enough to inhibit EBV replication. This is based upon a comparison with the 50% effective dose of BW B759U (0.05 microM) for inhibition of genome replication and a Ki of 0.08 microM for BW B759U triphosphate inhibition of EBV DNA polymerase.