2'-fluoro-5-iodo-aracytosine, a potent and selective anti-herpesvirus agent

In vitro metabolism of 1-beta-D-arabinofuranosylcytosine and 1-beta-2'-fluoroarabino-5-iodocytosine in normal and herpes simplex type 1 virus-infected cells

Phosphorylation of 1-beta-D-2'-F-arabino-5-iodocytosine (FIAC), a newly synthesized pyrimidine nucleoside with potent antiherpesvirus activity, was compared with that of its parent compound, 1-beta-D-arabinofuranosylcytosine (ara-C). While ara-C was phosphorylated extensively by homogenates of normal, rapidly proliferating mouse tissues, FIAC was a poor substrate for the nucleoside kinase occurring in such normal tissues. With cell homogenates of noninfected Vero cells, thymidine (TdR) was phosphorylated about fifty and twenty times more efficiently than FIAC and ara-C, while infection of Vero cells with Herpes Simplex Virus Type 1 (HSV-1) resulted in a 23-fold increase of TdR- and a 1270-fold increase of FIAC phosphorylation. In contrast, phosphorylation of ara-C was increased only by a factor of 2.6. While the reaction products obtained with homogenates of normal mouse tissues were 5'-mono-, di- and triphosphates of ara-C and FIAC, the reaction products with noninfected and infected Vero cell homogenates were predominantly monophosphates. In contrast, TdR was efficiently phosphorylated to its 5'-mono-, di- and triphosphates by such homogenates. In intact HSV-1-infected Vero cells. FIAC was rapidly taken up and phosphorylated to FIACMP and to an as yet unidentified metabolite. In contrast, TdR was taken up and phosphorylated to 5'-mono-, di- and triphosphates and ara-C was taken up moderately but metabolized poorly to its 5'-mono-, di- and triphosphates. Thus, in normal tissues, FIAC was a poorer substrate than ara-C for nucleoside kinases, but in intact HSV- 1-infected Vero cells FIAC was efficiently phosphorylated and thus behaved like a TdR analog, except that it was phosphorylated only to the 5'-monophosphate and a hitherto unidentified metabolite. The greatly increased phosphorylation of FIAC by HSV-1-infected Vero cells probably accounts, at least in part, for its great selectivity of action.

In vitro susceptibility of varicella-zoster virus to E-5-(2-bromovinyl)-2'-deoxyuridine and related compounds

The in vitro susceptibility of eight strains of varicella-zoster virus (VZV) to E-5-(2-bromovinyl)-2'-deoxyuridine (BVDU) was examined in human embryonic fibroblasts by the following techniques: inhibition of focus formation by either cell-free VZV (4-day assay) or cell-associated VZV (2-day assay), inhibition of viral antigen formation (2-day assay), and inhibition of viral cytopathogenicity (15-day assay). The 50% inhibitory dose (ID50) of BVDU ranged from 0.001 microgram/ml (2-day assay) to 0.01 microgram/ml (15-day assay). BVDU appeared highly selective in its anti-VZV activity since even at concentrations as high as 100 micrograms/ml, BVDU did not markedly affect the viability of the host cells. The ID50 of BVDU for VZV was comparable to that of IVDU (E-5-(2-iodovinyl)-2'-deoxyuridine). Both drugs inhibited the replication of VZV at a much lower concentration than did other antiviral compounds such as iododeoxyuridine, ethyldeoxyuridine, arabinosylcytosine, arabinosyladenine, phosphonoacetic acid, iododeoxycytidine, and acycloguanosine. BVDU and IVDU were virtually inactive against a thymidine kinase-deficient VZV mutant, suggesting that phosphorylation by the viral enzyme is responsible, at least in part, for the selective anti-VZV activity of the compounds.

Differential activity of potential antiviral nucleoside analogs on herpes simplex virus-induced and human cellular thymidine kinases

Potential antiviral nucleoside analogs 1-beta-D-arabinofuranosylthymine, the 1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-nucleosides of -5-methyluracil, -5-iodouracil, -5-methylcytosine, -5-iodocytosine, and -E-5-(2-bromovinyl)uracil, E-5-(2-bromovinyl)-2'-deoxyuridine, E-5-(2-bromovinyl)-1-beta-D-arabinofuranosyluracil, and 9-(2-hydroxyethyoxymethyl)guanine were studied to compare their phosphorylation rates relative to thymidine by purified thymidine kinases from human and herpes simplex virus sources. Most of these analogs are capable of being phosphorylated by both human and viral enzymes. On the assumption that inhibition constants (Ki) reflect binding affinity, Ki values were determined for these analogs with the same thymidine kinases. In general, these analogs have a greater affinity for the viral enzymes. The amount of the analogs phosphorylated to the monophosphate form, which is presumably necessary to produce cytotoxic effects, was determined by the combined effects of phosphorylation rates and binding affinities. All of these analogs act as preferential substrates for the viral thymidine kinases at low concentrations, which may be one of the main reasons for their selective antiviral action.

Selective inhibition of herpesvirus deoxyribonucleic acid synthesis by acycloguanosine, 2'-fluoro-5-iodo-aracytosine, and (E)-5-(2-bromovinyl)-2'-deoxyuridine

The selectivity of inhibition of herpesvirus deoxyribonucleic acid synthesis by acycloguanosine, 2'-fluoro-5-iodo-aracytosine, and (E)-5-(2-bromovinyl)-2'-deoxyuridine was determined by isopycnic banding of (32)P-labeled deoxyribonucleic acid from herpesvirus-infected and uninfected cells.

On the mechanism of selective inhibition of herpesvirus replication by (E)-5-(2-bromovinyl)-2'-deoxyuridine

Bromovinyldeoxyuridine (BVdUrd) is a potent antiherpesvirus compound with low cytotoxicity. To gain an insight into its selectivity and mechanism of inhibition, we chemically synthesized the 5'-triphosphate of BVdUrd, BVdUTP, and tested its effect on the activities of DNA polymerases [DNA nucleotidyltransferase (DNA directed), EC 2.7.7.7] of two herpesviruses--i.e., herpes simplex virus type 1 (HSV-1) and Epstein-Barr virus (EBV)--as well as cellular DNA polymerases alpha, beta, and gamma. The effects on the DNA polymerases were determined under assay conditions optimal for the individual polymerases. We found that the BVdUTP was considerably more inhibityory to the utilization of dTTP by the HSV-1 DNA polymerase then by the cellular DNA polymerases. For instance, as little as 1 microM BVdUTP inhibited the utilization of dTTP by HSV-1 DNA polymerase 50%, whereas the same concentration inhibited the DNA polymerase alpha and the DNA polymerase beta activities only 9% and 3%, respectively. The BVdUTP inhibited DNA synthesis by competing with the natural substrate, dTTP. The Km for dTTP and the Ki for the BVdUTP of the HSV-1 DNA polymerase were 0.66 and 0.25 microM, respectively. Kinetic analyses with the DNA polymerases alpha and beta and the EBV DNA polymerase also reflected a similar difference in sensitivity between the HSV-1 enzyme and other enzymes. Increasing the concentration of either the DNA template or the enzyme in the reaction mixture did not bring about a significant change in the extent of inhibition. Preincubation of the inhibitor with the enzyme was not necessary for inhibition. Studies on time course of inhibition revealed that the compound is inhibitory even after the initiation of DNA synthesis. These studies indicate that the ability of BVdUTP to preferentially inhibit the HSV-1 DNA polymerase may contribute towards its selective inhibition of the viral DNA replication in infected cells.