Thymidine kinase inhibitor of formula I ##STR1## in which R is C.sub.1-6 alkyl or AR(CH.sub.2).sub.n --, wherein Ar represents phenyl optionally substituted with one to five same or different halogen, C.sub.1-6 alkyl or C.sub.1-6 alkyloxy; and n equals one to six.A further aspect of the present invention provides a method of inhibiting viral thymidine kinase. Yet another aspect of the invention relates to treating herpes simplex viral infections in mammals. Yet another aspect of the invention provides a pharmaceutical formulation.