5-Ethyl-, 5-vinyl-, 5-propyl-, and 5-allyl-2'-deoxyuridine (dUrd) had antiviral activity against herpes simplex type 1 and type 2 grown in HeLa TK-cells, in the order 5-vinyl-dUrd, 5-ethyl-dUrd, 5-propyl-dUrd, 5-allyl-dUrd, but they were inactive against a TK-mutant of herpes simplex type 1. The antiviral activity of these compounds could be partially reversed by thymidine. Except for 5-vinyl-dUrd, they were not toxic to WI-38 and HeLa TK-cells at a concentration of 25 ,uM. All four analogues inhibited the growth of herpes simplex type 1-transformed HeLa TK-cells at a concentration of 1 ,uM.Herpesviruses are responsible for many diseases in man, and recent evidence also suggests a strong correlation between herpes infections and the incidence of certain human malignancies (4). One characteristic of herpes simplex type I (HSV-1) and type II (HSV-2) is that they can induce virus-specific thymidine kinases (TK) in some human cells after infection. These viral TK's are different from the cytoplasmic and mitochondrial TK's of human cells not only with respect to their physical and chemical properties, such as electrophoretic mobility, activation energy, and salt sensitivity, but also in substrate specificity (Y. C. Cheng, submitted for publication; L. S. Lee and Y. C. Cheng, submitted for publication). This fact suggested to us that specific anti-HSV-1 and HSV-2 agents that specifically utilize viral TK might be developed.The approach taken in this laboratory is to seek analogues of thymidine that could act as alternative substrates for viral TK Chem., in press). A previous study on the binding specificity of some pyrimidine 2'-deoxyribonucleosides showed that 5-ethyl-, 5-propyl-, and 5-allyl-deoxyuridine (dUrd) had good binding affinity to HSV-1-and HSV-2-specific TK's (Km of thymidine to Ki of analogue less than 0.2) but much poorer affinity for either human cytosol or mitochondrial TK's (Km of thymidine to K, of analogue larger than 0.005) and that 5-vinyl-dUrd had a high binding affinity to both viral and human mitochondrial TK's. For instance, the Ki of 5-ethyl-dUrd for human cytosol, human mitochondrial, HSV-1 (strain KOS), and HSV-2 (strain 333) TK's was 82, 305, 0.7, and 0.3 ,uM, respectively. TheKi of 5-vinyl-dUrd for human cytosol, human mitochondrial, HSV-1 (strain KOS), and HSV-2 (strain 333) TK's was 35, 1.7, 0.5, and 0.5 ,iM, respectively (Cheng and Lee, submitted for publication). This communication reports the results of a comparative study on the antiviral and cytotoxic effects of all four analogues and suggests that selective affinity to various TK's is responsible for their differential activity. Some studies on the biological activity of 5-ethyl-, 5-vinyl-, and 5-allyl-dUrd have been reported previously by other investigators (2, 7, 9-11).MATERIALS AND METHODS Cells and viruses. The stock viruses, HSV-1 (strains KOS and B2006) and HSV-2 (strain 333), were kindly given to us by W. Munyon of this institute (1, 3). Both the KOS and 333 strains of herpes simplex viruses but not the B2006 strain cou...
