The chemotherapeutic exploitation of virus-specified enzymes

Abstract

The virus-specified thymidine kinase which is induced following infection with herpes simplex virus types 1 and 2 can phosphorylate 9-(2-hydroxyethoxymethyl)guanine (acyclovir, acycloguanosine) and a number of other acyclic analogs of purine nucleosides which are not substrates for the cellular thymidine kinase. Purine deoxyribonucleosides or arabinosides are not good substrates for this virus-coded enzyme; acyclic derivatives of guanine are better substrates than those of pyrimidines or adenine. The potent and selective anti-herpetic activity of acyclovir is due to its conversion to a monophosphate by the viral-specified thymidine kinase, the subsequent conversion to a triphosphate by cellular enzymes, and the inhibition of the herpes virus-coded DNA polymerase by the triphosphate. Acyclovir triphosphate is both an inhibitor and substrate for DNA polymerases. The virus-coded DNA polymerases induced by herpes viruses incorporate acyclovir more rapidly and are more sensitive to inhibition and inactivation than are the cellular α DNA polymerases. The ability of mouse LM cells, thymidine-deficient LM cells (LTK −), and LTK − cells transformed to the LTK + phenotype by HSV-1 or HSV-2 genetic information (LTK −C139, LH 7 and D 21) to anabolize acyclovir has been investigated. The transformed cells converted acyclovir to its mono-, di- and triphosphates, and showed greater sensitivity to growth inhibition by acyclovir than did the untransformed LTK − cells or the original LM line which contains only cellular thymidine kinase. Evidence has been obtained for the inhibition by acyclovir of the elongation of newly synthesized DNA in LH 7 cells, and for the incorporation of acyclovir into the DNA as a chain-terminating event.