An antiviral guanosine analog; has activity against multiple laboratory and clinical isolate strains of HSV-1, HSV-2, and VZV in a plaque reduction assay (IC50s = 0.4-3.1 μg/ml); inhibits HSV-1 and HSV-2 replication (IC99s = 0.4 and 0.7 μg/ml, respectively) and DNA synthesis (IC50 = 0.04 μg/ml) in MRC-5 cells; reduces the severity of skin lesions caused by HSV-1 in guinea pigs and mice; prevents development of HSV-2-induced genital lesions in guinea pigs infected intravaginally