Antiviral activity of arabinosylthymine in herpesviral replication: mechanism of action in vivo and in vitro

1-beta-d-Arabinofuranosylthymine (ara-T), a metabolite of the sponge Tethya crypta, has shown selective activity against herpes simplex virus (HSV) replication (G. A. Gentry and J. F. Aswell, Virology 65:294-296, 1975). Analysis of HSV-infected and uninfected cell lysates by CsCl isopycnic centrifugation showed that ara-T blocked the incorporation of [(3)H]hypoxanthine into viral deoxyribonucleic acid and, to a large extent, into host deoxyribonucleic acid of infected (but not uninfected) cells. Additional experiments with [gamma-(32)P]adenosine 5'-triphosphate as a radiophosphate donor demonstrated that ara-T is phosphorylated by extracts of HSV-infected BHK cells and not by those of uninfected cells. At an ara-T concentration that almost completely inhibited the growth of LM cells, which had been transformed to a pyrimidine deoxyribonucleoside kinase(+) (dPyK(+)) phenotype by ultraviolet-inactivated HSV-1, the growth of uninfected LM cells was not affected. These results indicate that the viral dPyK is responsible for the selective antiviral activity of ara-T. This conclusion was further supported by experiments that showed that the replication of a variety of dPyK(-) mutants of HSV-1 and HSV-2 were not affected by ara-T and that ara-T inhibited the phosphorylation of deoxycytidine and deoxythymidine by HSV-1 dPyK, but not by host deoxycytidine and deoxythymidine kinases, respectively. Ara-T also selectively inhibited the replication of equine herpesvirus type 1 (EHV-1) in vitro and was effective against EHV-1 infection in vivo in hamsters. Further, EHV-1 was inhibited by ara-T and by bromodeoxyuridine in LM cells lacking a cytosol thymidine kinase, suggesting that EHV-1 induces a dPyK. Finally, spectrophotometric assay for thymine suggested that ara-T is not a substrate for nucleoside phosphorylase of hamster liver, and a microbiological assay indicated that substantial amounts of ara-T were excreted in the urine of uninfected hamsters that had received a single injection of 5 mg of ara-T, the amount given in each injection in the in vivo experiments with EHV-1.

Herpesvirus-induced availability of host DNA to exogenous endonuclease

Deoxyribonuclease I penetrates herpesvirus-infected L cells and degrades host DNA without interfering with viral multiplication. The DNA of uninfected control monolayers of L cells, disrupted by scraping, is similarly attacked. In contrast, undisturbed L cells are not affected. This suggests that altered cell membrane permeability allows the nuclease to enter the cell, thereby permitting access of exogenous deoxyribonuclease to cellular DNA.