Mutually exclusive inhibition of herpesvirus DNA polymerase by aphidicolin, phosphonoformate, and acyclic nucleoside triphosphates

Distribution and effects of amino acid changes in drug-resistant α and β herpesviruses DNA polymerase

Emergence of drug-resistance to all FDA-approved antiherpesvirus agents is an increasing concern in immunocompromised patients. Herpesvirus DNA polymerase (DNApol) is currently the target of nucleos(t)ide analogue-based therapy. Mutations in DNApol that confer resistance arose in immunocompromised patients infected with herpes simplex virus 1 (HSV-1) and human cytomegalovirus (HCMV), and to lesser extent in herpes simplex virus 2 (HSV-2), varicella zoster virus (VZV) and human herpesvirus 6 (HHV-6). In this review, we present distinct drug-resistant mutational profiles of herpesvirus DNApol. The impact of specific DNApol amino acid changes on drug-resistance is discussed. The pattern of genetic variability related to drug-resistance differs among the herpesviruses. Two mutational profiles appeared: one favoring amino acid changes in the Palm and Finger domains of DNApol (in α-herpesviruses HSV-1, HSV-2 and VZV), and another with mutations preferentially in the 3′-5′ exonuclease domain (in β-herpesvirus HCMV and HHV-6). The mutational profile was also related to the class of compound to which drug-resistance emerged.

Interaction between ganciclovir and foscarnet as inhibitors of duck hepatitis B virus replication in vitro

Safe and effective treatments for chronic hepatitis B virus (HBV) infection have yet to be developed. Both ganciclovir (9-[1,3-dihydroxy-2-propoxymethyl]guanine) and foscarnet (trisodium phosphonoformate hexahydrate) are potent inhibitors of hepadnavirus replication when used individually in vitro and in vivo. However, the clinical usefulness of each drug is reduced by dose-limiting toxicity, especially during long-term monotherapy. Here we demonstrate additive inhibition of duck HBV DNA replication in cultures of primary duck hepatocytes congenitally infected with duck HBV by combinations of ganciclovir and foscarnet at low, clinically achievable concentrations. These results suggest that the effects of ganciclovir and foscarnet against HBV may be additive in vivo.

Resistance of herpesviruses to antiviral drugs

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Genetic characterization of the vaccinia virus DNA polymerase: identification of point mutations conferring altered drug sensitivities and reduced fidelity

We determined that 85 microM aphidicolin was sufficient to block macroscopic plaque formation by vaccinia virus and to cause a 10(4)-fold reduction in viral yield from a wild-type infection. A chemically mutagenized viral stock was passaged sequentially in the presence of drug, and plaque-purified viral stocks resistant to aphidicolin were isolated and characterized. By use of a marker rescue protocol, the lesion in each mutant was found to map within the same 500-bp fragment within the DNA polymerase gene. All of the mutants were found to contain a single nucleotide change in the same codon. In nine of these mutants, the alanine residue at position 498 was changed to a threonine, whereas a 10th mutant sustained a valine substitution at this position. Congenic viral strains which carried the Aphr lesion in an unmutagenized wild-type background were isolated. The Thr and Val mutations were found to confer equivalent levels of drug resistance. In the presence of drug, viral yields were 25% of control levels, and the levels of viral DNA synthesized were 30 to 50% of those seen in control infections. The two mutations also conferred an equivalent hypersensitivity to the cytosine analog 1-beta-D-arabinofuranosylcytosine (araC); strains carrying the Thr mutation were moderately hypersensitive to the pyrophosphate analog phosphonoacetic acid and the adenosine analog araA, whereas the Val mutation conferred acute hypersensitivity to these inhibitors. The Val mutation also conferred a mutator phenotype, leading to a 20- to 40-fold increase in the frequency of spontaneous mutations within the viral stock.

Foscarnet. A review of its antiviral activity, pharmacokinetic properties and therapeutic use in immunocompromised patients with cytomegalovirus retinitis

The pyrophosphate analogue, foscarnet, selectively inhibits the DNA polymerase of human herpes viruses, including cytomegalovirus, and the reverse transcriptase of HIV. Viral replication is therefore prevented, but resumes when the drug is cleared from infected cells. In vitro, the combination of foscarnet and zidovudine (azidothymidine) has an additive effect against cytomegalovirus and acts synergistically against HIV. An improvement in cytomegalovirus retinitis is obtained in over 85% of affected AIDS patients during foscarnet induction therapy, but relapse usually occurs within a month of ceasing treatment. There is a similar duration of remission during maintenance therapy given for 5 days each week, but this can be extended 4- to 5-fold with daily administration of higher doses. In allograft recipients, progression of retinitis can be halted by foscarnet until immune function recovers and eradicates the virus. The incidence of acute renal failure, which is common during foscarnet therapy, may be reduced by dosage adjustment and adequate prehydration. Anaemia, phlebitis, nausea and vomiting, and disturbances in serum calcium and phosphate levels, perhaps resulting from uptake of foscarnet into bone or chelation with ionised calcium, have also been associated with administration of the drug. Cytomegalovirus retinitis is difficult to treat, with few therapeutic options available. Although treatment with foscarnet produces some severe adverse effects, with care these can be minimised, and the drug produces clinical improvement in a large proportion of patients; this is a highly encouraging finding at this stage in its development. Preliminary comparative data indicate that foscarnet and ganciclovir are similarly effective, but foscarnet may have some theoretical advantages in AIDS patients since it can be used in combination with zidovudine without potentiating myelosuppression.

Increased efficacy of ganciclovir in combination with foscarnet against cytomegalovirus and herpes simplex virus type 2 in vitro and in vivo

In tissue culture, efficacy against either murine CMV or HSV-2 was increased 27-fold for the acyclic nucleoside ganciclovir and 3-fold for foscarnet (trisodium phosphonoformate) when the 2 drugs were combined; whereas against human CMV, efficacy was increased 3-fold for both drugs. In mice, efficacy was increased 2-fold for ganciclovir and 4- to 5-fold for foscarnet when used in combination against either murine CMV or HSV-2. These results suggest an additive interaction between the two drugs in vivo.

Combinations of 3'-azido-3'-deoxythymidine (zidovudine) and phosphonoformate (foscarnet) against human immunodeficiency virus type 1 and cytomegalovirus replication in vitro

Combinations of 3'-azido-3'-deoxythymidine and phosphonoformate produced a moderate synergistic inhibitory effect against human immunodeficiency virus type 1 in vitro at concentrations that are easily achieved in humans. The synergistic effect was more pronounced with increasing concentrations and was not secondary to toxic effects of the drugs. 3'-Azido-3'-deoxythymidine neither inhibited the replication of human cytomegalovirus in human embryonic lung fibroblasts nor interfered with the anticytomegalovirus effect of phosphonoformate. By using partially purified reverse transcriptase of human immunodeficiency virus type 1 and human cytomegalovirus DNA polymerase, various combinations of 3'-azido-3'-deoxythymidine-5'-triphosphate and phosphonoformate produced strong indications of additive interactions. The synergistic interactions in infected cells and the additive effects observed at the reverse transcriptase level indicate that mechanisms other than the reverse transcriptase may be of importance for the inhibition of human immunodeficiency virus replication by these two compounds. A concomitant treatment of cytomegalovirus infections, such as cytomegalovirus retinitis, with phosphonoformate in patients with acquired immunodeficiency syndrome receiving 3'-azido-3'-deoxythymidine may be appropriate, and this combination may also be useful in controlling human immunodeficiency virus infection.

Inhibition of human immunodeficiency virus reverse transcriptase by 2',3'-dideoxynucleoside triphosphates: template dependence, and combination with phosphonoformate

The 2',3'-dideoxynucleoside triphosphates (ddNTPs) are potent substrate analog inhibitors of human immunodeficiency virus (HIV) reverse transcriptase and have clinical utility in the treatment of acquired immunodeficiency syndrome. Several issues regarding the interaction of these compounds with HIV reverse transcriptase were examined. The potency of unsubstituted ddNTPs and the 3'-azido analog of dTTP (AZTTP) was influenced by the choice of template. Both compounds were more potent with the complementary homopolymer templates than with gapped duplex DNA, although the Km for the competing dNTP was similar with different templates. The Ki for AZTTP was greater than for the unsubstituted ddNTPs with either a homopolymer or a gapped duplex DNA template. HIV reverse transcriptase incorporated ddCMP and AZTMP into primed phage m13 DNA at sites specified for insertion of dCMP and dTMP, respectively. ddCTP was more efficiently utilized as a substrate than was AZTTP. Primer elongation due to base misincorporation was observed in the absence of one dNTP. The combined effect of ddNTPs and the pyrophosphate analog phosphonoformate (PFA) on HIV reverse transcriptase was also examined, and inhibition by PFA in combination with ddTTP or AZTTP was mutually exclusive.

Identification and some properties of a unique DNA polymerase from cells infected with human B-lymphotropic virus

A new DNA polymerase and DNase activity were identified from cells infected with human B-lymphotropic herpesvirus (HBLV). DNA polymerase associated with HBLV infection was similar in its sensitivity to inhibition by ppi analogs as other herpesvirus-specific DNA polymerases but was dissimilar in its inhibition by certain nucleoside triphosphates.

Antiviral effects of phosphonoformate (PFA, foscarnet sodium)

This article describes the antiviral properties of foscarnet (trisodium phosphonoformate) at the enzyme level as well as in cell cultures and in vivo. The mechanism of action against herpesvirus DNA polymerases and reverse transcriptases is outlined. Clinical studies using topical foscarnet against mucocutaneous herpes simplex virus infections are presented. The clinical use of intravenous foscarnet against severe viral infections caused by cytomegalovirus, hepatitis B virus and human immunodeficiency virus is discussed.

Identification of amino acids in herpes simplex virus DNA polymerase involved in substrate and drug recognition

Herpes simplex virus (HSV) encodes a DNA polymerase that is similar in several respects to the replicative mammalian DNA polymerase alpha. Recently, these and other DNA polymerases have been shown to share several regions of protein sequence similarity. Despite these similarities, antiviral drugs that mimic natural polymerase substrates specifically inhibit herpesvirus DNA polymerases. To study amino acids involved in substrate and drug recognition, we have characterized and mapped altered drug sensitivity markers of nine HSV pol mutants and sequenced the relevant portions of these mutants. The mutations were found to occur within four relatively small regions. One such region, which we designate region A, has sequence similarity only to DNA polymerases that are sensitive to certain antiviral drugs. The other three regions contain sequences that are similar among various DNA polymerases. The multiple mutations occurring within two of these regions make it likely that the regions interact directly with drugs and substrates. Our results lead us to favor a model in which protein folding allows interactions among the four regions to form the substrate and drug binding sites.

Interaction of Epstein-Barr virus DNA polymerase with aphidicolin, phosphonoformate and 5'-GMP

Epstein-Barr virus (EBV)-specified DNA polymerase was purified from P3HR-1 cells, a Burkitt lymphoma EBV producer cell line, treated with phorbol-12,13-dibutyrate (PDB) and n-butyrate. Its inhibition by aphidicolin, phosphonoformate (PFA) and 5'-GMP was examined. Aphidicolin could inhibit EBV DNA polymerase competitively with respect to dATP and dCTP and noncompetitively with respect to dGTP and dTTP; whereas 5'-GMP was a noncompetitive inhibitor with respect to all four dNTPs. Combinations of aphidicolin and PFA, or PFA and 5'-GMP, produced a mutually exclusive inhibition pattern of EBV DNA polymerase that suggested that the binding sites of these compounds on the enzyme molecule are kinetically overlapping.

alpha-Cl-alpha-Br-phosphonoacetic acid is a potent and selective inhibitor of Na+/Pi cotransport across renal cortical brush border membrane

We found that alpha-Cl-alpha-Br-phosphonoacetate (ClBrPAA) is a competitive, solute-specific inhibitor of Na+/Pi cotransport across renal cortical brush border membrane. Inhibition by ClBrPAA (Ki = 62 microM) is more than three times more effective than inhibition by phosphonoformate (PFA), the most potent Na+/Pi cotransport inhibitor known to date, and 26 times more effective than the parent compound, phosphonoacetate (PAA). These observations indicate that substitution of bromine and chlorine atoms at the alpha-carbon of PAA greatly enhances its efficacy as a competitive inhibitor of Na+/Pi cotransport. As ClBrPAA is much less inhibitory than PAA and PFA towards viral DNA polymerases and did not inhibit human alpha-DNA polymerase (ref. 10), the results also demonstrate that Na+/Pi cotransport inhibition can be dissociated from inhibition of DNA polymerases by phosphonocarboxylate compounds.

Delayed treatment with combinations of antiviral drugs in mice infected with herpes simplex virus and application of the median effect method of analysis

Mice were inoculated intracerebrally with a lethal dose of herpes simplex virus type 2. Three days later, the mice were treated intraperitoneally, twice daily for 4 days, with the following drugs alone or in combination: acyclovir (ACV), vidarabine (ara-A), 2'-fluoro-5-iodoaracytosine (FIAC), and 2'-fluoro-5-methylarauracil (FMAU). Despite delayed treatment, most of the animals receiving low doses of FMAU alone or in combination with ACV or ara-A survived. In contrast, significantly higher mortality rates were noted in mice receiving ara-A, ACV, or FIAC alone. The data were analyzed for quantitation of synergism, additivity, and antagonism of multiple drug effect by the median effect method. The median effective doses (in nanomoles per kilogram per day) calculated in this manner were: FMAU, 22.5; FIAC, 510; ara-A, 901; ACV, 7,587; ACV-ara-A (drug ratio, 1:1), 550; FIAC-ara-A (1:1), 376; FIAC-ACV (1:1), 133; FMAU-ACV (1:8), 60.3; and FMAU-ara-A (1:8), 65.2. Marked synergy was found throughout a wide range of effect levels with the five different combinations, with no increased toxicity over the single-drug treatments. Similar results were obtained when the data were analyzed by the isobologram method. Since many patients with severe herpetic infections, such as herpes encephalitis, have a poor prognosis despite single-drug therapy, the possible use of combinations including low doses of FMAU deserves further investigation.

Sequence and mapping analyses of the herpes simplex virus DNA polymerase gene predict a C-terminal substrate binding domain

The herpes simplex virus DNA polymerase provides an excellent model for studies of eukaryotic replicative polymerases. We report here the nucleotide sequence of the gene which encodes this enzyme. The gene includes a 3705-base-pair major open reading frame capable of encoding a Mr 136,519 polypeptide, in rough agreement with previous estimates of the size of the major polypeptide found in partially purified viral polymerase preparations. The predicted polymerase polypeptide shares extensive sequence homology with the Epstein-Barr virus open frame predicted to encode DNA polymerase and with a 13-amino acid segment of adenovirus 2 DNA polymerase. Mutations conferring altered sensitivity to antiviral deoxynucleoside triphosphate analogs, pyrophosphate analogs, or aphidicolin from eight different mutants map within the region encoding the carboxyl-terminal portion of the predicted polymerase polypeptide. Two of these are separated by a distance corresponding to at least 228 amino acids. We propose that this region of the gene encodes a polypeptide domain that contains the binding sites for deoxynucleoside triphosphates and pyrophosphate.

Interaction of DNA polymerase and nucleotide analog triphosphates

The properties of virus and host DNA polymerases are important factors in determining the selectivity of deoxynucleotide analogs used in antiviral chemotherapy. The high affinity of herpes DNA polymerase for nucleotide analogs may be particularly important in CMV and EBV-infected cells, since these viruses do not induce the synthesis of a virus-specified thymidine kinase. In general, the effect of nucleotide analog incorporation into DNA may be summarized as follows: analogs with modifications at the base moiety do not affect the rate of DNA chain elongation whereas those modified at the sugar moiety will inhibit the rate of chain elongation. ACGTP and DHPGTP competitively inhibit incorporation of dGTP into DNA; however, steric freedom of the acyclic phosphate may allow these nucleotides to bind virus enzyme in a conformation similar to that assumed by dGTP only at the transitional stage of the enzyme reaction. This may explain the high affinity of virus enzyme for these inhibitors. The interaction of aphidicolin with virus enzyme differs from that with host enzyme. These differences suggest new strategies for antiviral chemotherapy using aphidicolin derivatives.