Interaction of herpes simplex virus-induced DNA polymerase with 9-(1,3-dihydroxy-2-propoxymethyl)guanine triphosphate

The triphosphate of 9-(1,3-dihydroxy-2-propoxymethyl)guanine (DHPG) competitively inhibits incorporation of dGTP into DNA catalyzed by DNA polymerases specified by both type 1 and type 2 herpes simplex virus. K1 values were estimated to be 33 nM for type 1 and 46 nM for type 2-specified DNA polymerase. DHPG acted as an alternate substrate to dGTP for the virus-specified DNA polymerase. Incorporation of DHPG into DNA resulted in the slowing down of the rate of DNA synthesis. The position of DHPG incorporation was analyzed, and it was found to enter both internal and terminal linkages. DNA which contained DHPG at termini was found to competitively inhibit utilization of activated DNA as primer. DNA polymerase alpha and DNA polymerases from several phosphonoformic acid-resistant herpes simplex virus type 1 strains were examined for sensitivity to 9-(1,3-dihydroxy-2-propoxymethyl)guanine triphosphate. A lack of correlation between the in vivo sensitivities of the virus mutants and the K1 values of the DNA polymerases was noted.     

Phosphorylation of acyclovir [9-(2-hydroxyethoxymethyl)guanine] in Epstein-Barr virus-infected lymphoblastoid cell lines.

The extent of phosphorylation of 9-(2-hydroxyethoxymethyl)guanine (acyclovir [ACV]) in fresh peripheral leukocytes, in Epstein-Barr virus (EBV)-infected lymphoblastoid cell lines, and in herpes simplex virus type 1-infected lymphoblastoid (P3HR-1) and monkey kidney (Vero) cells was determined by high-pressure liquid chromatography, Mono-, di-, and triphosphorylated derivatives of [8-14C]ACV were detected at low levels at various times after superinfection of Raji cells with EBV. The extent of phosphorylation appeared to be related to the concentration of ACV in the medium. Small amounts of ACV mono-, di-, and triphosphates were formed in fresh peripheral leukocyte preparations from EBV- seropositive and -seronegative donors. Comparable ACV monophosphate levels were detected in EBV-negative BJAB and the EBV-positive BJAB/GC cell lines; however, no di- or triphosphate derivatives were detected. Comparable ACV-monophosphate levels were detected in both P3HR-1 and HSV-infected P3HR-1 cell lines; however, larger amounts of ACV di- and triphosphorylated derivatives were detected in the HSV-infected P3HR-1 cells. ACV was converted to the triphosphate to a greater extent in HSV-infected Vero cells than in mock-infected Vero cells or in HSV-infected P3HR-1 cells. ACV or its phosphorylated derivatives were converted to guanine nucleotides to a greater extent in lymphoblastoid cells than in fibroblasts (Vero). In conclusion, neither the productive replication of EBV nor the presence of latent viral DNA is required for ACV monophosphate formation in B lymphoblastoid cells. ACV triphosphate, however, was detected only in cells infected productively with EBV.     

Inhibition of herpes simplex virus-induced DNA polymerase activity and viral DNA replication by 9-(2-hydroxyethoxymethyl)guanine and its triphosphate.

The effect of the nucleoside analog 9-(2-hydroxyethoxymethyl)guanine (acycloguanosine) on herpes simplex virus type 1 DNA synthesis was examined. Acycloguanosine inhibited herpesvirus DNA synthesis in virus-infected cells. The synthesis of host cell DNA was only partially inhibited in actively growing cells at acycloguanosine concentrations several hundred-fold greater than the 50% effective dose for herpes simplex virus type 1. Studies using partially purified enzymes revealed that the triphosphate of this compound inhibited the virus-induced DNA polymerases (DNA nucleotidyltransferases) to a greater degree than the DNA polymerase of the host cell, that the inhibition was dependent upon the base composition of the template, and that the triphosphate was a better substrate for the virus-induced polymerases than for the alpha cellular DNA polymerases.     

Metabolism and mode of action of (R)-9-(3,4-dihydroxybutyl)guanine in herpes simplex virus-infected vero cells

The metabolism and mode of action of the anti-herpes compound buciclovir [R)-9-(3,4-dihydroxybutyl)-guanine, BCV) has been studied in herpes simplex virus-infected and uninfected Vero cells. In uninfected cells, a low and constant concentration of intracellular BCV was found, while in herpes simplex virus-infected cells, an increasing concentration of BCV phosphates was found due to metabolic trapping. The major phosphorylation product was BCV triphosphate (BCVTP) which was 92% of the total amount of BCV phosphates. BCV phosphates were accumulated to the same extent in cells infected with either a herpes simplex virus type 1 or a herpes simplex virus type 2 strain while thymidine kinase-deficient mutants of herpes simplex virus type 1 were 10 times less efficient in accumulating BCV phosphates. In uninfected Vero cells, the concentration of the phosphorylated forms of BCV was less than 1% of that found in herpes simplex virus-infected cells. The BCVTP formed in herpes simplex virus-infected cells was highly stable, as 80% of the amount of BCVTP was still present even 17 h after removal of extracellular BCV. BCV was a good substrate for herpes simplex virus type 1- and type 2-induced thymidine kinases but not for the cellular cytosol or mitochondrial thymidine kinases. BCV monophosphate could be phosphorylated by cellular guanylate kinase to BCV diphosphate. BCVTP was a selective and competitive inhibitor to deoxyguanosine triphosphate of the purified herpes simplex virus type 1- and type 2-induced DNA polymerases. BCVTP could neither act as an alternative substrate in the herpes simplex virus type 2 or cellular DNA polymerase reactions, nor could [3H]BCV monophosphate be detected in DNA formed by herpes simplex virus type 2 DNA polymerase, or be detected in nucleic acids extracted from herpes simplex virus type 1-infected cells. These data indicate that BCVTP may inhibit the herpes simplex virus-induced DNA polymerase without being incorporated into DNA.     

Prolonged inhibitory effect of 9-(1,3-dihydroxy-2-propoxymethyl)guanine against replication of Epstein-Barr virus.

The effects of 9-(1,3-dihydroxy-2-propoxymethyl)guanine (DHPG), a new antiviral drug, and acyclovir (ACV) [9-(2-hydroxyethoxymethyl)guanine] on the replication of Epstein-Barr virus (EBV) were compared. Both drugs inhibited EBV DNA replication in P3HR-1 cells and superinfected Raji cells, but neither inhibited replication of the plasmid form of the EBV genome in latently infected Raji cells. However, DHPG had a more prolonged inhibitory effect than ACV. Although the effect of the drugs is prompt, the kinetics of inhibition of EBV replication indicated that a drug exposure of 14 days was needed to reduce the EBV genome copy number to the residual plasmid level (30 copies per cell). The inhibitory effect of ACV was readily reversed within 11 days after removal of the drug, in contrast to the more prolonged effect exerted by DHPG, which persisted for more than 21 days. The 50% inhibitory doses for cell growth of ACV and DHPG were estimated to be 250 and 200 microM, respectively. The viral 50% and 90% effective doses of inhibition were, respectively, 0.3 and 9 microM for ACV and 0.05 and 3 microM for DHPG. The therapeutic indices (50% inhibitory dose/50% effective dose) for ACV and DHPG were 833 and 4,000, respectively. Synthesis of EBV-associated polypeptides was also affected. In superinfected Raji cells, ACV (100 microM) and DHPG (30 microM) inhibited synthesis of polypeptides with molecular weights of 145,000 and 140,000; in addition, synthesis of polypeptides with molecular weights of 110,000 and 85,000 was markedly reduced by DHPG but not by ACV. However, after drug removal, the inhibitory effect of ACV on polypeptide synthesis was abolished in contrast to the more persistent effect of DHPG.     

Interaction of 9-(1,3-dihydroxy-2-propoxymethyl)guanine with cytosol and mitochondrial deoxyguanosine kinases: possible role in anti-cytomegalovirus activity

Summary The acyclic nucleoside 9-(1,3-dihydroxy-2-propoxymethyl)guanine (DHPG) is a potent inhibitor of human cytomegalovirus in vitro and in vivo. In order to investigate the phosphorylation of DHPG to the monophosphate and identify the enzyme responsible, attempts were made to isolate DHPG kinase from calf thymus and from human cytomegalovirus-infected lung cells. From calf thymus, a mitochondrial deoxyguanosine kinase was partially purified which co-migrated with DHPG phosphorylating activity on DEAE-cellulose, and had the same mobility by electrophoresis. DHPG triphosphate and DHPG kinase were elevated in cytomegalovirus-infected cells, but not enough enzyme activity was recovered to identify the kinase. However, DHPG was found to inhibit a cytosol deoxyguanosine kinase induced in these infected cells. The role of mitochondrial and cytosol deoxyguanosine kinases is discussed relative to the anti-cytomegalovirus activity of DHPG.     

Inhibition of purified human and herpes simplex virus-induced DNA polymerases by 9-(2-hydroxyethoxymethyl)guanine triphosphate. Effects on primer-template function

The inhibition of highly purified herpes simplex virus (HSV)-induced and host cell DNA polymerases by the triphosphate form of 9-(2-hydroxyethoxymethyl)guanine (acyclovir; acycloguanosine) was examined. Acyclovir triphosphate (acyclo-GTP) competitively inhibited the incorporation of dGMP into DNA, catalyzed by HSV DNA polymerase; apparent Km and Ki values of dGTP and acyclo-GTP were 0.15 microM and 0.003 microM, respectively. HeLa DNA polymerase alpha was also competitively inhibited; Km and Ki values of dGTP and acyclo-GTP were 1.2 microM and 0.18 microM, respectively. In contrast, HeLa DNA polymerase beta was insensitive to the analogue. The "limited" DNA synthesis observed when dGTP was omitted from HSV or alpha DNA polymerase reactions was inhibited by acyclo-GTP in a concentration-dependent manner. Prior incubation of activated DNA, acyclo-GTP, and DNA polymerase (alpha or HSV resulted in a marked decrease in the utilization of the primer-template in subsequent DNA polymerase reactions. This decreased ability of preincubated primer-templates to support DNA synthesis was dependent on acyclo-GTP, enzyme concentration, and the time of prior incubation. Acyclo-GMP-terminated DNA was found to inhibit HSV DNA polymerase-catalyzed DNA synthesis. Kinetic experiments with variable concentrations of activated DNA and fixed concentrations of acyclo-GMP-terminated DNA revealed a noncompetitive inhibition of HSV-1 DNA polymerase. The apparent Km of 3'-hydroxyl termini was 1.1 X 10(-7) M, the Kii and Kis of acyclo-GMP termini in activated DNA were 8.8 X 10(-8) M and 2.1 X 10(-9) M, respectively. Finally, 14C-labeled acyclo-GMP residues incorporated into activated DNA by HSV-1 DNA polymerase could not be excised by the polymerase-associated 3',5'-exonuclease activity.     

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.     

Cellular metabolism of 2',3'-dideoxycytidine, a compound active against human immunodeficiency virus in vitro

The nucleoside analog 2',3'-dideoxycytidine (ddCyd) has been shown to inhibit the infectivity and cytopathic effect of human immunodeficiency virus on human OKT4+ lymphocytes in vitro. Metabolism of ddCyd by human T-lymphoblastic cells (Molt 4) negative for human immunodeficiency virus and OKT4 was examined. Molt 4 cells accumulated ddCyd and its phosphorylated derivatives into acid-soluble and acid-insoluble material in a dose-dependent manner. For each concentration tested, 2',3'-dideoxycytidine triphosphate represented 40% of the total acid-soluble pool of ddCyd metabolites. Uptake of 5 microM ddCyd was linear for 4 h after addition of drug. Efflux of ddCyd metabolites from cells followed a biphasic course with an initial retention half-life of 2.6 h for 2',3'-dideoxycytidine triphosphate. DNA, but not RNA, of cells incubated with [3H]ddCyd became radiolabeled. Nuclease and phosphatase treatment of DNA followed by reverse-phase high pressure liquid chromatography showed that the nucleoside was incorporated into DNA in its original form. ddCyd was not susceptible to deamination by human Cyd-dCyd deaminase. It was a poor substrate for human cytoplasmic and mitochondrial dCyd kinases, with Km values of 180 +/- 30 and 120 +/- 20 microM, respectively. DNA polymerases alpha, beta, and gamma varied in their sensitivity to inhibition by ddCTP with Ki values of 110 +/- 40, 2.6 +/- 0.3, and 0.016 +/- 0.008 microM, respectively; however, inhibition was competitive with dCTP in each case.     

Potent synergistic inhibition of herpes simplex virus-2 by 9-[(1, 3-dihydroxy-2-propoxy)methyl]guanine in combination with recombinant interferons

DHPG, an acyclic guanine nucleoside with the structure 9-[(1,3-dihydroxy-2-propoxy)methyl]guanine], showed potent synergism with recombinant alpha or beta interferons and modest synergism with gamma interferon in inhibiting the replication of herpes simplex virus type 2 in vitro. The most potent direct anti-herpes viral synergism was obtained by combination of DHPG and recombinant human interferon-beta-ser17; when combined, doses of each near their separate effective dose50's resulted in almost complete elimination of production of infectious virus within a single viral replication cycle. The anti-herpes viral activity of DHPG-interferon combinations was significantly greater than that obtained with acyclovir-interferon combinations.     

Selective inhibition of proliferation in v-abl- and bcr-abl-transformed cells by a nucleoside analog.

The nucleoside analog acyclovir (9-[2-hydroxy-ethoxy)methyl]guanine or acycloguanosine; ACV) inhibited the in vitro transformation of NIH 3T3 cells by Abelson murine leukemia virus and the proliferation of abl- and bcr-abl-transformed hemopoietic murine cell lines. This effect is selective since ACV at the same concentration had no effect on the src and Ha-ras transformation of NIH 3T3 cells or on the proliferation of hemopoietic cells transformed by those oncogenes. The inhibitory effect on proliferation of abl-transformed cells correlated with the extent of ACV triphosphate formation and incorporation into cellular DNA that was greater than that in normal or other oncogene-transformed cells. The increased ACV triphosphate formation might be due to a higher level of 5'-nucleotidase, the enzyme responsible for trace levels of ACV phosphorylation in uninfected cells.     

Herpes simplex virus type 1 and human DNA polymerase interactions with 2'-deoxyguanosine 5'-triphosphate analogues. Kinetics of incorporation into DNA and induction of inhibition

The ability of herpes simplex virus type 1 (HSV-1) DNA polymerase, HeLa polymerase alpha, and HeLa polymerase beta to utilize several dGTP analogues has been investigated using a defined synthetic template primer. The relative efficiencies of the triphosphates of 9-[(2-hydroxyethoxy)methyl]guanine (acyclovir triphosphate, ACVTP), 9-[(1,3-dihydroxy-2-propoxy)methyl] guanine (ganciclovir triphosphate, DHPGTP), and 2',3'-dideoxyguanosine (ddGTP) as substrates for the three polymerases were: HSV-1 polymerase, dGTP greater than ACVTP approximately equal to DHPGTP greater than ddGTP; polymerase alpha, dGTP greater than ACVTP approximately equal to DHPGTP much greater than ddGTP; polymerase beta, ddGTP greater than dGTP much greater than ACVTP approximately equal to DHPGTP. The potent inhibition of HSV-1 polymerase by ACVTP has been shown previously to be due to the formation of a dead-end complex upon binding of the next 2'-deoxynucleoside 5'-triphosphate encoded by the template after incorporation of acyclovir monophosphate into the 3' end of the primer (Reardon, J. E., and Spector, T. (1989) J. Biol. Chem. 264, 7405-7411). This mechanism was shown here to be a general mechanism for inhibition of polymerases by the obligate chain terminators, ACVTP and ddGTP. The ACVTP-induced inhibition was 30-fold more potent with HSV-1 polymerase than with polymerase alpha. This difference may contribute to the antiviral selectivity of this nucleotide analogue. The effect of ganciclovir monophosphate incorporation (a nonobligate chain terminator) on subsequent primer extension was also evaluated. With HSV-1 polymerase and polymerase alpha, although there was a considerable reduction in the efficiency of utilization of the 3'-DHPGMP-terminal primer, contrasting kinetic behavior was observed. With HSV-1 polymerase, insertion of DHPGTP resulted in a significant reduction in Vmax for subsequent nucleotide incorporations. In contrast, with polymerase alpha, a relatively small decrease in Vmax was accompanied by increased Km values for subsequent nucleotide incorporations.     

Inhibition of cellular DNA polymerase alpha and human cytomegalovirus-induced DNA polymerase by the triphosphates of 9-(2-hydroxyethoxymethyl)guanine and 9-(1,3-dihydroxy-2-propoxymethyl)guanine.

The triphosphates of 9-(2-hydroxyethoxymethyl)guanine and 9-(1,3-dihydroxy-2-propoxymethyl)guanine were examined for their inhibitory effect on highly purified cellular DNA polymerase alpha and human cytomegalovirus (Towne strain)-induced DNA polymerase. These two nucleoside triphosphates competitively inhibited the incorporation of dGMP into DNA catalyzed by the DNA polymerases. The virus-induced DNA polymerase had greater binding affinity for the triphosphate of 9-(2-hydroxyethoxymethyl)guanine (Ki, 8 nM) than for the triphosphate of 9-(1,3-dihydroxy-2-propoxymethyl)guanine (Ki, 22 nM), although the nucleoside of the latter compound was strikingly more effective against human cytomegalovirus replication in cell cultures than the nucleoside of the former. The Ki values of these two nucleoside triphosphates for alpha polymerase were 96 and 146 nM, respectively, and were 7- to 12-fold higher than those for the virus-induced enzyme. These data indicated that virus-induced DNA polymerase was more sensitive to inhibition by these two nucleoside triphosphates than was the cellular alpha enzyme.     

Thymidine kinase not required for antiviral activity of acyclovir against mouse cytomegalovirus.

Previous studies of herpesvirus infections have indicated that a virus-specified thymidine kinase is required for the initial phosphorylation of acyclovir [acycloguanosine or 9-(2-hydroxyethoxymethyl)guanine] in the formation of acycloguanosine triphosphate. The latter compound accumulates in infected cells and competitively inhibits the viral DNA polymerase. We found that mouse cytomegalovirus, which does not express a thymidine kinase, was sensitive to the antiviral effects of acyclovir at a 50% inhibitory dose of approximately 0.23 microM. Acyclovir was equally effective against mouse cytomegalovirus in normal 3T3 cells and in 3T3 cells deficient in cellular thymidine kinase. Furthermore, the activity of acyclovir could not be reversed by excess thymidine, which easily reversed the antiviral activity of acyclovir against herpes simplex virus. Using a high-pressure liquid chromatography technique that easily detected acycloguanosine triphosphate in cells infected with herpes simplex virus, we could not detect acycloguanosine triphosphate in mouse cytomegalovirus-infected cells. These experiments demonstrated that the activity of acyclovir against mouse cytomegalovirus is not dependent on a thymidine phosphorylation pathway. Additional experiments are underway to determine whether acycloguanosine triphosphate is produced by another pathway in concentrations sufficient to inhibit mouse cytomegalovirus DNA polymerase.     

Design and characterization of N2-arylaminopurines which selectively inhibit replicative DNA synthesis and replication-specific DNA polymerases: guanine derivatives active on mammalian DNA polymerase alpha and bacterial DNA polymerase III

The 2-amino substituted derivatives of guanine, N2-(p-n-butylphenyl)guanine (BuPG) and N2-(3',4'-trimethylenephenyl) guanine (TMPG), were synthesized and found to selectively inhibit, respectively, HeLa cell DNA polymerase alpha (po1 alpha) and B. subtilis DNA polymerase III (po1 III). Both purines, like their corresponding uracil analogs, BuAu and TMAU (2,9), were specifically competitive with dGTP in their inhibitory action on their target polymerases. BuPG, the pol alpha-specific purine, was also toxic for HeLa cells in vivo, selectively inhibiting DNA synthesis. These N2-substituted purines, in contrast to the 6-substituted uracils, provide a structural basis for the synthesis of nucleosides and nucleotides with considerable potential as probes for the analysis of the structure of specific replicative DNA polymerases and their function in cellular DNA metabolism.     

Bcl-2 blocks apoptosis caused by pierisin-1, a guanine-specific ADP-ribosylating toxin from the cabbage butterfly

Pierisin-1, a 98-kDa protein that induces apoptosis in mammalian cell lines, is capable of being incorporated into cells where it ADP-ribosylates guanine residues in DNA. To investigate the apoptotic pathway induced by this unique protein, the bcl-2 gene was transfected into HeLa cells. Cy2-fluorescent pierisin-1 was incorporated into the resultant cells expressing Bcl-2 protein and ADP-ribosylated dG was detected to almost the same extent as in parent cells. However, bcl-2-transfected HeLa cells did not display apoptotic morphological changes, PARP cleavage, and DNA fragmentation, indicating acquisition of resistance. In parent HeLa cells, activation of caspase-9 and release of cytochrome c were observed after 8h treatment with 0.5ng/ml pierisin-1. Caspase substrate assays revealed further cleavage of Ac-DEVD-pNA, Ac-VDVAD-pNA, and Ac-VEID-pNA, suggesting activation of caspase-2, -3, and -6 in pierisin-1-treated HeLa cells. The caspase-3 inhibitor, Ac-DEVD-CHO, was also found to inhibit apoptosis. In contrast, this caspase activation was not observed in bcl-2-transfected HeLa cells. Our results thus indicate that pierisin-1-induced apoptosis is mediated primarily via a mitochondrial pathway involving Bcl-2 and caspases.     

Phosphorylation of the antiviral precursor 9-(1,3-dihydroxy-2-propoxymethyl)guanine monophosphate by guanylate kinase isozymes

Guanylate kinase was purified from human erythrocytes by affinity chromatography using GMP-agarose, and the four isozymes which are present were separated by chromatofocusing. The kinetic properties of each isozyme were analyzed with respect to the natural substrates GMP and dGMP, and the 5'-monophosphate derivatives of the antiviral nucleoside analogs 9-(1,3-dihydroxy-2-propoxymethyl)guanine (DHPG) and 9-(2-hydroxyethoxymethyl)guanine (ACV, Acyclovir). The analysis of substrate kinetics yielded Km values for DHPG 5'-monophosphate which were similar with all isozymes (42-54 microM), and about 3-fold higher than the Km values obtained for GMP. Km values obtained with ACV 5'-monophosphate were 10-20-fold higher than the GMP values and varied nearly 4-fold among isozymes (209-753 microM). GMP produced the highest enzyme velocities with all isozymes, followed by dGMP, DHPG 5'-monophosphate, and ACV 5'-monophosphate, in that order. Differences in maximal velocities among isozymes were generally small. DHPG 5'-monophosphate inhibited the isozymes by a simple competitive mechanism with respect to GMP. In contrast, ACV 5'-monophosphate acted as an apparent hyperbolic mixed-type inhibitor. Similar patterns of inhibition were obtained with all isozymes. It is probable that differences is the reactivity of DHPG 5'-monophosphate and ACV 5'-monophosphate with individual guanylate kinase isozymes do not contribute significantly to differences in their antiviral effects.     

Increased cytotoxicity of 2',2'-difluoro-2'-deoxycytidine in human leukemic cell-lines after a preincubation with cyclopentenyl cytosine

The in vitro modulating effect of Cyclopentenyl cytosine (CPEC) on the metabolism of gemcitabine was studied in lymphocytic and myeloid leukemic cell-lines. In MOLT-3 cells, that were pretreated with CPEC, the incorporation of 2',2'-difluoro-2'-deoxycytidine triphosphate (dFdCTP) into DNA was significantly increased by 57-99% in comparison with cells that were only treated with gemcitabine. The increased incorporation of dFdCTP into DNA in CPEC pretreated cells was paralleled by an increase in apoptotic and necrotic cells of 17-34%. In HL-60 cells that were preincubated with CPEC, increased concentrations of the mono-/di- and triphosphate form of gemcitabine were observed, as well as an increased incorporation of dFdCTP into DNA (+773%). This increased incorporation was paralleled by a significant increase in apoptosis and necrosis. We conclude that CPEC enhances the incorporation of dFdCTP into DNA and thus increases the cytotoxicity of gemcitabine in lymphocytic and myeloid leukemic cell-lines.     

Novel 2'',5''-oligoadenylates synthesized in interferon-treated, vaccinia virus-infected cells.

2-5A[ppp(A2'p)nA] and related materials accumulate to greater than micromolar concentrations in vaccinia virus-infected HeLa cells and in interferon-treated, vaccinia-infected HeLa, CV1, and L929 cells even when virus replication is not inhibited. A variably complex mixture of authentic 2-5A (n = 2 to 4), nonphosphorylated cores [(A2'p)nA; n = 2 to 5], and additional compounds of unknown structure was observed.