Epstein-Barr virus induces a unique pyrimidine deoxynucleoside kinase activity in superinfected and virus-producer B cell lines

A phase 1/2 trial of arginine butyrate and ganciclovir in patients with Epstein-Barr virus-associated lymphoid malignancies

Malignancies associated with latent Epstein-Barr virus (EBV) are resistant to nucleoside-type antiviral agents because the viral enzyme target of these antiviral drugs, thymidine kinase (TK), is not expressed. Short-chain fatty acids, such as butyrate, induce EBV-TK expression in latently infected B cells. As butyrate has been shown to sensitize EBV(+) lymphoma cells in vitro to apoptosis induced by ganciclovir, arginine butyrate in combination with ganciclovir was administered in 15 patients with refractory EBV(+) lymphoid malignancies to evaluate the drug combination for toxicity, pharmacokinetics, and clinical responses. Ganciclovir was administered twice daily at standard doses, and arginine butyrate was administered by continuous infusion in an intrapatient dose escalation, from 500 mg/(kg/day) escalating to 2000 mg/(kg/day), as tolerated, for a 21-day cycle. The MTD for arginine butyrate in combination with ganciclovir was established as 1000 mg/(kg/day). Ten of 15 patients showed significant antitumor responses, with 4 CRs and 6 PRs within one treatment cycle. Complications from rapid tumor lysis occurred in 3 patients. Reversible somnolence or stupor occurred in 3 patients at arginine butyrate doses of greater than 1000 mg/(kg/day). The combination of arginine butyrate and ganciclovir was reasonably well-tolerated and appears to have significant biologic activity in vivo in EBV(+) lymphoid malignancies which are refractory to other regimens.

Epstein--Barr virus post-transplant lymphoproliferative disease and virus-specific therapy: pharmacological re-activation of viral target genes with arginine butyrate

Lymphoproliferative disorders associated with the Epstein-Barr virus (EBV) include non-Hodgkin's lymphoma, Hodgkin's lymphoma, and "post-transplant lymphoproliferative disorders" (PTLD), which occur with immunosuppression after marrow and organ transplantation. PTLD is characterized by actively proliferating, latently infected EBV(+) B-lymphocytes, and often manifests a rapidly progressive fatal clinical course if the immunosuppression cannot be reversed. Lung transplant recipients are a subset of patients at special risk for developing PTLD. The incidence of PTLD development in these patients has been estimated at 5--10%. Whereas immunologic and antiviral therapy have been moderately effective for treating EBV-associated infections in the lytic phase, they have been less useful in the more common latent phase of the disease. One common treatment for herpesvirus infections has targeted the virus-specific enzyme thymidine kinase (TK). The lack of viral TK expression in EBV(+) tumor cells, due to viral latency, makes anti-viral therapy alone ineffective as an anti-neoplastic therapy, however. We have developed a strategy for the treatment of EBV-associated lymphomas/PTLD using pharmacologic induction of the latent viral TK gene and enzyme in the tumor cells, followed by treatment with ganciclovir. Arginine butyrate selectively activates the EBV TK gene in latently EBV-infected human lymphoid cells and tumor cells. A Phase I/II trial has been initiated, employing an intra-patient dose escalation of arginine butyrate combined with ganciclovir. In six patients with EBV-associated lymphomas or PTLD, all of which were resistant to conventional radiation and/or chemotherapy, this combination produced complete clinical responses in four of six patients, with a partial response occurring in a fifth patient. Pathologic examination in two of three patients demonstrated complete necrosis of the EBV lymphoma, with no residual disease, following a single three-week course of the combination therapy. Possible side-effects of the therapy included nausea and reversible lethargy at the highest doses. One patient suffered acute liver failure, thought to be secondary to release of FasL from the necrotic tumor. Analysis of patient-derived tumor cells in culture demonstrated that arginine butyrate produced selective induction of the EBV TK gene, which then conferred sensitivity to ganciclovir, resulting in tumor apoptosis. Additional patient accrual is sought for further evaluation of this therapy.

Anti-Epstein-Barr virus (EBV) activity of beta-L-5-iododioxolane uracil is dependent on EBV thymidine kinase

beta-L-5-Iododioxolane uracil was shown to have potent anti-Epstein-Barr virus (EBV) activity (50% effective concentration = 0.03 microM) with low cytotoxicity (50% cytotoxic concentration = 1,000 microM). It exerts its antiviral activity by suppressing replicative EBV DNA and viral protein synthesis. This compound is phosphorylated in cells where the EBV is replicating but not in cells where the EBV is latent. EBV-specific thymidine kinase could phosphorylate beta-L-5-iododioxolane uracil to the monophosphate metabolite. The K(m) of beta-L-5-iododioxolane uracil with EBV thymidine kinase was estimated to be 5.5 microM, which is similar to that obtained with thymidine but about fivefold higher than that obtained with 2' fluoro-5-methyl-beta-L-arabinofuranosyl uracil, the first L-nucleoside analogue discovered to have anti-EBV activity. The relative V(max) is seven times higher than that of thymidine. The anti-EBV activity of beta-L-5-iododioxolane uracil and its intracellular phosphorylation could be inhibited by 5'-ethynylthymidine, a potent EBV thymidine kinase inhibitor. The present study suggests that beta-L-5-iododioxolane uracil exerts its action after phosphorylation; therefore, EBV thymidine kinase is critical for the antiviral action of this drug.

The Epstein-Barr virus thymidine kinase does not phosphorylate ganciclovir or acyclovir and demonstrates a narrow substrate specificity compared to the herpes simplex virus type 1 thymidine kinase

The Epstein-Barr virus (EBV) thymidine kinase (TK) was expressed in mammalian 143B TK- cells to investigate its substrate specificity. The herpes simplex virus type 1 (HSV-1) TK was similarly expressed for comparison. Both viral TKs conferred a TK+ phenotype on 143B TK- cells. The nucleoside analog ganciclovir (GCV) did not affect the growth of 143B EBV TK or 143B TK- cells but effectively killed 143B HSV-1 TK cells. Furthermore, lysates of 143B EBV TK cells could not phosphorylate GCV, which was confirmed by high-performance liquid chromatography. EBV TK, HSV-1 TK, and EBV TK N-, a truncated EBV TK missing 243 N-terminal amino acids, were purified as fusion proteins expressed in bacteria, and all had TK activity. In addition, EBV TK was observed to have a thymidylate kinase activity but could not phosphorylate GCV, acyclovir, or 2'-deoxycytidine. In competition assays, only nucleoside analogs of thymidine significantly inhibited thymidine phosphorylation by EBV TK, with the following rank order: 5-bromodeoxyuridine > zidovudine > stavudine > sorivudine. These results demonstrate that EBV TK substrate specificity is narrower than those of alphaherpesvirus TKs and that thymidine analogs may be the most suitable nucleoside antivirals to target the enzyme. Clinical implications for gammaherpesviruses are discussed.

Arginine butyrate-induced susceptibility to ganciclovir in an Epstein-Barr-virus-associated lymphoma

Lymphoproliferative disorders associated with Epstein-Barr virus (EBV) infections can occur in the setting of immunosuppression. In some patients, the lymphoproliferative disorder can resemble an aggressive monoclonal non-Hodgkins lymphoma (NHL). These NHL are poorly responsive to conventional therapy. Similarly, antiviral therapy with synthetic nucleosides such as ganciclovir are ineffective because the genes that render the virus susceptible to therapy are not expressed in EBV+ lymphomas. Using a cell line derived from a lung transplant recipient with an EBV+ immunoblastic NHL, we studied the ability of arginine butyrate to induce the expression of EBV thymidine kinase. Arginine butyrate was not only effective in inducing EBV thymidine kinase transcription, but also acted synergistically with the antiviral agent ganciclovir to inhibit cell proliferation and decrease cell viability. Based on these findings, the patient from whom the cell line was derived was treated with arginine butyrate/ganciclovir as well as conventional cytotoxic chemotherapy. No additional toxicity was observed with the arginine butyrate/ganciclovir therapy. Histologic examination of the tumor showed substantial necrosis. These observations suggest the feasibility of arginine butyrate induction of ganciclovir susceptibility in patients with EBV-associated lymphomas.

Substrate specificity of Epstein-Barr virus thymidine kinase

Purified recombinant protein encoded by the BXLF-I open reading frame of the Epstein-Barr virus genome has thymidine kinase activity. The substrate behaviors of various nucleosides toward this enzyme were tested. Halogenated deoxyuridines, zidovudine, and bromovinyldeoxyuridine are efficient substrates, while acyclovir and dihydroxypropylmethylguanine are relatively poor substrates for the Epstein-Barr virus thymidine kinase.

Use of antigen expressed in bacteria for detection of EBV-specific thymidine kinase antibodies in sera from patients with nasopharyngeal carcinoma

Two cDNA clones covering the N- and C-terminal portions of the EBV BXLF1 open reading frame were selected from a cDNA library derived from P3HR1 cells. The two clones were ligated, the N-terminal untranslated region truncated, and the product inserted into an E. coli expression vector, pET3CP*. The fusion protein was expressed under control of the T7 phage phi 10 gene promoter and shown to possess thymidine kinase activity. The protein was then used as an antigen to detect antibody reactivities in serum samples of nasopharyngeal carcinoma patients and healthy blood donors. Using a 1:400 dilution of serum samples in Western blot analyses, it was possible to differentiate the reactivities of serum IgA of NPC patients and healthy donors. The prevalence of positive reactivity to EBV TK in NPC was around 84%. The test was compared to others used for early diagnosis of NPC and was able to detect some patients who were negative in those tests.

Cloning and expression of a cDNA encoding the Epstein-Barr virus thymidine kinase gene

A clone of the Epstein-Barr virus (EBV) thymidine kinase (TK) gene was derived from a cDNA library of P3HR1 cells. The gene product was expressed as a fusion protein in a procaryotic system by using T7 RNA polymerase. The recombinant TK showed a molecular mass of 67 kDa and was biologically active. Antiserum raised in mice immunized with partially purified TK recognized an antigen present in EBV-superinfected Raji cells using an indirect immunofluorescence assay.

Murine cytomegalovirus DNA polymerase: purification, characterization and role in the antiviral activity of acyclovir

Murine cytomegalovirus (MCMV) neither induces a viral thymidine kinase (TK) nor enhances the activity of a cellular TK. Nevertheless, MCMV is highly susceptible to 9-(2-hydroxyethoxymethyl)guanine (acyclovir, ACV). The cellular TK is neither responsible for phosphorylation of ACV nor its anti-MCMV activity. This is clear from the findings that little ACV triphosphate is formed in MCMV-infected mouse embryo fibroblasts (MEF) and that the replication of MCMV is inhibited equally well by ACV in TK+ and TK- cells. Even if trace amounts of ACV triphosphate would be formed by enzymes other than TK, and ACV triphosphate would be responsible for the anti-MCMV activity of ACV, then the MCMV DNA polymerase ought to be highly sensitive to ACV triphosphate. To examine this possibility, the MCMV DNA polymerase was partially purified and characterized. The apparent Ki value of the MCMV DNA polymerase for ACV triphosphate indicates that the sensitivity of the MCMV DNA polymerase to ACV triphosphate is equivalent to that of the HSV DNA polymerase. Therefore, the trace amounts of ACV triphosphate that are formed in MCMV-infected MEF seem to be insufficient to inhibit MCMV DNA polymerase and may not play a key role in the anti-MCMV activity of ACV.

Characterization of the Epstein-Barr virus-encoded thymidine kinase expressed in heterologous eucaryotic and procaryotic systems

The establishment of mammalian and procaryotic systems which express the Epstein-Barr virus (EBV) thymidine kinase (TK) has been reported previously (E. Littler, J. Zeuthen, A. A. McBride, E. Trøst-Sørensen, K. L. Powell, J. E. Walsh-Arrand, and J. R. Arrand, EMBO J. 5:1959-1966, 1986). The EBV TK activity expressed in both of these systems was characterized by in vitro assays and found to resemble that of the herpes simplex virus TK both in its broad range of nucleoside and nucleotide utilization and also in its ability to accept antiviral nucleoside analogs as substrates. Further results are presented which suggest that these in vitro systems may prove suitable for studying the potential anti-EBV activity of other candidate antiviral compounds.

Differential effects of acyclovir and 9-(1,3-dihydroxy-2-propoxymethyl)guanine on herpes simplex virus and Epstein-Barr virus in a dually infected human lymphoblastoid cell line

We investigated the effects of acyclovir and 9-(1,3-dihydroxy-2-propoxymethyl)guanine (DHPG) on a lymphoblastoid cell line dually infected with Epstein-Barr virus and herpes simplex virus (HSV) type 1. The numbers of Epstein-Barr virus genomes were reduced during 70 days of treatment with either drug. Both drugs suppressed HSV replication in a dose-related manner. In the continued presence of the drugs, HSV developed resistance, rapidly to acyclovir and much more slowly to 30 microM DHPG. Analysis of HSV glycoprotein C production and viral DNA showed that treatment with 100 microM DHPG eliminated HSV production, curing the cell line of HSV persistent infection.

Metabolic activation of 9([2-hydroxy-1-(hydroxymethyl)ethoxy]methyl)guanine in human lymphoblastoid cell lines infected with Epstein-Barr virus

9-([2-Hydroxy-1-(hydroxymethyl)ethoxy]methyl)guanine (BW B759U) is more potent and has a more prolonged inhibitory effect against Epstein-Barr virus (EBV) in vitro than does acyclovir (ACV). To assess the mechanism of this difference, we first compared the extent of phosphorylation of the two drugs in superinfected Raji cells. BW B759U is phosphorylated to levels 100-fold higher than is ACV. In addition, lower levels of phosphorylation of BW B759U and ACV were observed in uninfected Raji cells. Studies on the kinetics of formation of BW B759U triphosphate in superinfected Raji cells indicated that drug-phosphorylating activity was detected as early as 3 h after superinfection; this activity was steadily maintained for the first 7 h, followed by a burst of activity between 7 and 10 h and a doubling of phosphorylation between 10 and 25 h. During the superinfection cycle, the pool sizes of deoxyribonucleoside and ribonucleoside triphosphates were increased and reached their maxima at 10 h after infection. The maximal amount of triphosphorylated drug in a virus producer cell, P3HR-1 (LS), was obtained at 21 h after drug treatment. During long-term drug treatment, approximately 44 and 77% reduction in EBV genome copies per cell was observed on days 3 and 7, respectively. In a separate experiment, after treatment of P3HR-1 (LS) cells with BW B759U for 36 h, 4.2 pmol of BW B759U triphosphate per 10(6) cells was achieved. After the cells were released into drug-free medium, drug triphosphate was rapidly decreased to 11% of the original level in 1 day. Thereafter, the decrease was slow but steady, down to 0.22 pmol/10(6) P3HR-1 cells by 5 days. We calculated that 0.22 pmol of BW B759U triphosphate per 10(6) cells represents a cellular concentration of 0.22 microM, which is theoretically enough to inhibit EBV replication. This is based upon a comparison with the 50% effective dose of BW B759U (0.05 microM) for inhibition of genome replication and a Ki of 0.08 microM for BW B759U triphosphate inhibition of EBV DNA polymerase.

Characterization of an Epstein-Barr virus-induced thymidine kinase

Previous work from our laboratory suggested that the selective inhibition of Epstein-Barr virus (EBV) replication by 1-beta-D-arabinofuranosylthymine in human lymphoid cell lines involved the induction of a new thymidine kinase (TK) able to phosphorylate the thymidine analog. We further characterized this enzyme induced in various EBV-positive cell lines after viral genome activation with a combination of sodium butyrate and 12-O-tetradecanoylphorbol-13-acetate. The following results confirmed the existence of an EBV-specific deoxypyrimidine kinase: induction of EBV-related TK was connected with the appearance of viral early antigens in EBV-carrying cells; unexpected behaviors of the enzyme activity upon different fractionating treatments led to the conclusion that EBV-induced TK was extracted as a complex molecular form, larger than other known cellular or viral isozymes; enzymatic properties distinguished EBV-induced TK from host lymphoid cell isozymes but made it resemble other herpesvirus-specific deoxypyrimidine kinases, i.e., by partial inhibition by dTTP or ammonium sulfate, insensitiveness to dCTP, and nonstringent specificity for normal TK substrates. Genetic evidence is required to definitively ensure that EBV-specific TK actually is virus coded in EBV-transformed human lymphoid cells.