Acyclic guanosine analogs as substrates for varicella-zoster virus thymidine kinase

Novel Sensitivity of Acyclovir-Resistant Varicella–Zoster Virus to Anti-Herpetic Drugs

Acyclovir-resistant varicella–zoster virus (VZV) selected in the presence of acyclovir was 3–20 times more sensitive to nucleoside analogues with arabinosyl configuration than the parent virus. Their sensitivity in comparison with a thymidine kinase-deficient mutant to anti-herpetic drugs and sensitivity to phosphonoacetic acid suggested that recognition by altered mutants' DNA polymerase might result in their novel sensitivity to nucleoside analogues. Information on the unique sensitivity of these acyclovir-resistant mutants may be useful in designing chemotherapeutic agents against VZV.

Antiviral agents against equid alphaherpesviruses: Current status and perspectives

Equid herpesvirus infections cause respiratory, neurological and reproductive syndromes. Despite preventive and control measures and the availability of vaccines and immunostimulants, herpesvirus infections still constitute a major threat to equine health and for the equine industry worldwide. Antiviral drugs, particularly nucleoside analogues and foscarnet, are successfully used for the treatment of human alphaherpesvirus infections. In equine medicine, the use of antiviral medications in alphaherpesvirus infections would decrease the excretion of virus and diminish the risk of contagion and the convalescent time in affected horses, and would also improve the clinical outcome of equine herpesvirus myeloencephalopathy. The combined use of antiviral compounds, along with vaccines, immune modulators, and effective preventive and control measures, might be beneficial in diminishing the negative impact of alphaherpesvirus infections in horses. The purpose of this review is to analyse the available information regarding the use of antiviral agents against alphaherpesviruses, with particular emphasis on equine alphaherpesvirus infections.

β-l-1-[5-(E-2-bromovinyl)-2-(hydroxymethyl)-1,3-(dioxolan-4-yl)] uracil (l-BHDU) prevents varicella-zoster virus replication in a SCID-Hu mouse model and does not interfere with 5-fluorouracil catabolism

The alphaherpesvirus varicella-zoster virus (VZV) causes chickenpox and shingles. Current treatments are acyclovir (ACV) and its derivatives, foscarnet and brivudine (BVdU). Additional antiviral compounds with increased potency and specificity are needed to treat VZV, especially to treat post-herpetic neuralgia. We evaluated β-l-1-[5-(E-2-bromovinyl)-2-(hydroxymethyl)-1,3-(dioxolan-4-yl)] uracil (l-BHDU, 1) and 5'-O-valyl-l-BHDU (2) in three models of VZV replication: primary human foreskin fibroblasts (HFFs), skin organ culture (SOC) and in SCID-Hu mice with skin xenografts. The efficacy of l-BHDU in vivo and its drug-drug interactions were previously not known. In HFFs, 200μM l-BHDU was noncytotoxic over 3days, and l-BHDU treatment reduced VZV genome copy number and cell to cell spread. The EC50 in HFFs for l-BHDU and valyl-l-BHDU were 0.22 and 0.03μM, respectively. However, l-BHDU antagonized the activity of ACV, BVdU and foscarnet in cultured cells. Given its similar structure to BVdU, we asked if l-BHDU, like BVdU, inhibits 5-fluorouracil catabolism. BALB/c mice were treated with 5-FU alone or in combination with l-BHDU or BVdU. l-BHDU did not interfere with 5-FU catabolism. In SCID-Hu mice implanted with human skin xenografts, l-BHDU and valyl-l-BHDU were superior to ACV and valacyclovir. The maximum concentration (Cmax) levels of l-BHDU were determined in mouse and human tissues at 2h after dosing, and comparison of concentration ratios of tissue to plasma indicated saturation of uptake at the highest dose. For the first time, an l-nucleoside analog, l-BHDU, was found to be effective and well tolerated in mice.

Varicella-zoster virus thymidine kinase. Characterization and substrate specificity

The varicella-zoster virus (VZV) thymidine kinase (TK) EC 2.7.2.21) catalyzes the phosphorylation of many anti-VZV nucleosides. Purified, bacterially expressed VZV TK was characterized with regard to N-terminal amino acid sequence, pI value, pH optimum, metal ion requirement, phosphate donor and acceptor specificity, and inhibition by dTTP. Initial velocities of thymidine phosphorylation with variable MgATP concentrations fit a two-site model with apparent Km values for MgATP of 0.10 and 900 microM. dTTP was a noncompetitive inhibitor of thymidine phosphorylation but was competitive with MgATP. Phosphate donor and acceptor specificities of the bacterially expressed enzyme were indistinguishable from those of VZV TK purified from infected cells. Detailed studies of the nucleoside specificity with the bacterially expressed enzyme showed that, for a given sugar moiety, thymine nucleosides were the most efficient substrates followed by nucleosides of cytosine, uracil, adenine, and with some exceptions, guanine. For a given pyrimidine or purine (except guanine), 2'-deoxyribonucleosides were the most efficient substrates, followed by arabinosides, ribonucleosides, 2',3'-dideoxyribonucleosides, and the acyclic moiety of acyclovir.

Mutant varicella-zoster virus thymidine kinase: correlation of clinical resistance and enzyme impairment

Varicella-zoster virus (VZV) encodes a thymidine kinase (EC 2.7.2.21) which phosphorylates several antiviral nucleoside analogs, including acyclovir (ACV). A mutation in the VZV thymidine kinase coding sequence, resulting in an arginine-to-glutamine substitution at amino acid residue 130 (R130Q), is associated with clinical resistance to ACV. We have expressed the wild-type and the mutant enzymes in bacteria and have studied the kinetic characteristics of the purified enzymes. The arginine-to-glutamine substitution resulted in decreased catalytic activity and altered substrate specificity. The most striking effect was a decrease in the rates of nucleoside phosphorylation to less than 2% of the rates with the wild-type enzyme. This was accompanied by increased apparent Km values for thymidine and deoxycytidine. ACV was not detectably phosphorylated by the R130Q enzyme but still competed with thymidine for the enzyme. The inability of the R130Q enzyme to catalyze the phosphorylation of ACV correlates with resistance to ACV noted with a clinical isolate of VZV.

Comparison of the substrate specificities of human thymidine kinase 1 and 2 and deoxycytidine kinase toward antiviral and cytostatic nucleoside analogs

Deoxynucleoside kinases are required for the 5'-phosphorylation of deoxynucleoside analogs used in chemotherapy. Cytoplasmic thymidine kinase (TK1), deoxycytidine kinase (dCK) and mitochondrial thymidine kinase (TK2) were completely purified from human leukemic spleen and their capacities to phosphorylate 43 nucleoside analogs were compared. TK1 showed the most restricted substrate specificity but tolerated 3'-modifications of the sugar ring and some 5-substitutions of the pyrimidine ring. TK2 showed a much broader specificity and phosphorylated pyrimidine bases with bulky 5-substitutions, including cytosine analogs, while sugar analogs with substituents other than OH in the 2' and 3' positions were very poor substrates. dCK showed a very broad specificity phosphorylating several cytosine analogs with 2' and 3' modifications as well as acyclic sugar analogs. Purine deoxyribonucleosides were also efficiently phosphorylated by dCK but in this case sugar modifications led to drastically decreased activity.

Molecular forms in human serum of enzymes synthesizing DNA precursors and DNA

Both thymidine kinase (TK) and DNA polymerase (DNAp) are present in measurable amounts in human serum. Even though the use of TK as a clinical marker is rapidly increasing there has been no attempt to characterize the serum TK in a wider extent, i.e.; with respect to Mw or other biochemical parameters. Therefore sera with high TK or DNAp activities derived from patients with cytomegalovirus (CMV) infection, B12-deficiency and leukaemia were fractionated by gel exclusion chromatography. The TK activity eluted as two peaks, one major TK activity with an apparent molecular weight (Mw) or 730 kD and one minor TK activity corresponding to a Mw of 58 kD. The amount of TK activity at 58 kD varied between 7 and 23% of total activity, depending on the serum fractionated. The DNAp activity in sera from patients with malignant disease and B12 deficiency eluted as a single peak corresponding to a Mw of 240 kD. A DNAp with a different Mw (greater than 1000 kD) was recovered from 1 of 3 investigated immunosuppressed patients with CMV infection. A similar pattern of enzyme forms was observed when sera were separated by glycerol gradient centrifugation. The effect of high salt and various reaction solution components on the enzymes were studied. The only condition found that affected the molecular forms of TK was the state of reduction. Incubation of sera with high concentrations of dithioerythritol (DTE) (400 mM) prior to separation transferred all serum TK to the 58 kD form, it also converted most of the serum DNAp from the 240 kD form to a smaller form (56 kD) without affecting the total recovery of enzymatic activity. The reaction product from both TK forms was exclusively monophosphate and none of the TK forms could efficiently utilize cytidine triphosphate as phosphate donor. The substrate kinetics of the small serum TK fraction was identical with those of an enzyme with similar size purified from proliferating HeLa cells, indicating that both serum TK activities are forms of TK 1, the proliferation associated cellular isozyme.

Inhibition of varicella-zoster virus-induced DNA polymerase by a new guanosine analog, 9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine triphosphate

The triphosphates of the antiherpesvirus acyclic guanosine analogs 9-[4-hydroxy-2(hydroxymethyl)butyl] guanine (2HM-HBG), 9-(2-hydroxyethoxymethyl)guanine (acyclovir [ACV]), and 9-(3,4-dihydroxybutyl)guanine (buciclovir) were examined for their effects on partially purified varicella-zoster virus (VZV) DNA polymerase as well as cellular DNA polymerase alpha. The triphosphate of 2HM-HBG competitively inhibited the incorporation of dGMP into DNA catalyzed by the VZV DNA polymerase. 2HM-HBG-triphosphate (2HM-HBG-TP) had a higher affinity for the dGTP-binding site on the VZV DNA polymerase than did dGTP; apparent Km and Ki values of dGTP and 2HM-HBG-TP were 0.64 and 0.034 microM, respectively. ACV-triphosphate (ACV-TP) was found to be the most potent inhibitor of VZV DNA polymerase. ACV-TP had a 14 and 464 times better direct inhibitory effect than 2HM-HBG-TP and buciclovir-triphosphate, respectively. The cellular (human embryonic lung fibroblast) DNA polymerase alpha inhibition was related to viral polymerase inhibition as efficacy ratios: 2HM-HBG-TP had a ratio of more than 1,000, which appeared to be similar to that of ACV-TP.

Inhibiting effect of (RS)-9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine on varicella-zoster virus replication in cell culture

The activity and mode of action of the new nucleoside analog (RS)-9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine (2HM-HBG) against varicella-zoster virus (VZV) were determined. In cell culture, replication of different strains of VZV was inhibited to 50% by 0.4 to 0.7 microM 2HM-HBG, while 685 microM was required to inhibit 50% of the DNA synthesis in uninfected human lung fibroblasts. A thymidine kinase-negative VZV strain was not inhibited by 100 microM 2HM-HBG. Inhibition of VZV replication was not reversible after 7 to 14 days of incubation, depending on the multiplicity of VZV. 2HM-HBG was shown to be selectively phosphorylated by purified VZV thymidine kinase, with an inhibition constant of 32.5 microM. The antiviral activity of 2HM-HBG in cell culture was decreased by the addition of deoxythymidine and deoxycytidine but not by other ribo- or deoxyribonucleosides.