Effects of ribonucleotide reductase inhibition on pyrimidine deoxynucleotide metabolism in acyclovir-treated cells infected with herpes simplex virus type 1

Antiviral Activity of Inhibitors of Pyrimidine De-Novo Biosynthesis

Evaluation of the elevation of host cell biosynthesis of deoxynucleoside triphosphates (dNTP's) induced by human cytomegalovirus (HCMV) infection as a target for antiviral therapeutics was carried out. The concentrations of all four intracellular dNTP's rose rapidly following HCMV infection, and were markedly above baseline by 8 h post infection (p.i.). All four deoxynucleoside triphosphates remained elevated above baseline for at least 72 h p.i. The effects of inhibitors of the de-novo pathway of pyrimidine biosynthesis on HCMV viral replication-were quantified by DNA dot blot. All pyrimidine biosynthesis inhibitors examined inhibited the HCMV DNA replication at concentrations that were non-toxic to the cell. These drugs were also more effective against HCMV, which is highly dependent on host denovo synthesis, than against HSV-1 which encodes enzymes capable of increasing the supply of dNTP's. The antiviral effect of brequinar, an inhibitor of one of the enzymes of the de-novo pathway (dihydroorotate dehydrogenase), was examined to determine if it coincided with a decrease in dNTP's. HCMV-infected fibroblasts and uninfected control cells were treated with a concentration of brequinar able to inhibit HCMV DNA levels 90%. It was found that brequinar markedly lowered the levels of dTTP found in treated cells compared to untreated cells in both HCMV-infected and uninfected cells.

The effects of interferon-alpha and acyclovir on herpes simplex virus type-1 ribonucleotide reductase

Herpes simplex virus-type 1 (HSV-1) encodes both the small (UL40) and large (UL39) subunits of the enzyme, ribonucleotide reductase. Treatment of HSV-1-infected cells with interferon-alpha (IFN-alpha) reduced the levels of both enzyme subunits. Reduced steady state levels of the large subunit were demonstrated by immunoblot using polyclonal antibody specific for the viral enzyme. Reduction in the amount of small subunit was shown by a reduction in the electron spin resonance signal derived from the iron-containing tyrosyl free-radical present in this subunit. Treatment of cells with 100 IU/ml of IFN-alpha decreased levels of both subunits resulting in a reduction in enzyme activity as measured by conversion of CDP to dCDP. The decrease in the amount of the large subunit was not due to a reduction in the level of its mRNA. The combination of IFN-alpha and ACV treatment of human cornea stromal cells did not result in a further reduction in amounts of ribonucleotide reductase relative to that detected with IFN-alpha alone. The IFN-alpha-induced reduction in ribonucleotide reductase activity is the likely cause of decreased levels of dGTP which we have previously demonstrated in IFN-alpha-treated, infected cells.

Anticytomegaloviral activity of methotrexate associated with preferential accumulation of drug by cytomegalovirus-infected cells

We extend the observation that inhibitors of pyrimidine biosynthesis are active against human cytomegalovirus by demonstrating that methotrexate (MTX) has preferential activity against cytomegalovirus replication. The 50% and 90% inhibitory concentrations of MTX for inhibition of cytomegaloviral DNA replication at 3 days postinfection in MRC-5 cells were 0.05 and 0.2 microM, respectively. No cell toxicity was observed in uninfected confluent cells at the highest concentration tested (1 microM). Under similar conditions (3 days of treatment with 0.2 microM MTX), intracellular dTTP pools were diminished in cytomegalovirus-infected cells (87% decrease relative to untreated infected cells, P < 0.001) but were not reduced in uninfected cells. A potential explanation for the preferential antiviral effect of MTX was that human cytomegalovirus-infected cells preferentially accumulated MTX. Increased intracellular accumulation and increased polyglutamation of MTX were observed in cytomegalovirus-infected cells compared with uninfected cells. Increased uptake of [3H]MTX by cytomegalovirus-infected cells was first observed at 48 h postinfection, with threefold-higher accumulation within infected cells. By 96 h, accumulation had increased to approximately fourfold in comparison with uninfected cells. The uptake of [3H]MTX was saturable and was blocked by addition of unlabelled MTX. Intracellular MTX in infected cells was almost entirely in the polyglutamated form, as demonstrated by thin-layer chromatography, whereas intracellular MTX was almost exclusively in the parent form in uninfected cells.

Specific inhibition of herpes virus replication by receptor-mediated entry of an antiviral peptide linked to Escherichia coli enterotoxin B subunit

Mimetic peptides capable of selectively disrupting protein-protein interactions represent potential therapeutic agents for inhibition of viral and cellular enzymes. This approach was first suggested by the observation that the peptide YAGAVVNDL, corresponding to the carboxyl-terminal 9 amino acids of the small subunit of ribonucleotide reductase of herpes simplex virus, specifically inhibited the viral enzyme in vitro. Evaluation and use of this peptide as a potential antiviral agent has, however, been thwarted by its failure to inhibit virus replication in vivo, presumably because the peptide is too large to enter eukaryotic cells unaided. Here, we show that the nontoxic B subunit of Escherichia coli heat-labile enterotoxin can be used as a recombinant carrier for the receptor-mediated delivery of YAGAVVNDL into virally infected cells. The resultant fusion protein specifically inhibited herpes simplex virus type 1 replication and ribonucleotide reductase activity in quiescent Vero cells. Preincubation of the fusion protein with soluble GM1 ganglioside abolished this antiviral effect, indicating that receptor-mediated binding to the target cell is necessary for its activity. This provides direct evidence of the usefulness of carrier-mediated delivery to evaluate the intracellular efficacy of a putative antiviral peptide.

Aciclovir. A reappraisal of its antiviral activity, pharmacokinetic properties and therapeutic efficacy

Aciclovir (acyclovir) is a nucleoside analogue with antiviral activity in vitro against the herpes simplex viruses (HSV), varicella zoster virus (VZV), Epstein-Barr virus (EBV), cytomegalovirus (CMV) and human herpesvirus 6 (HHV-6). Topical, oral or intravenous aciclovir is well established in the treatment of ophthalmic, mucocutaneous and other HSV infections, with intravenous aciclovir the accepted treatment of choice in herpes simplex encephalitis. The efficacy of aciclovir is increased with early (preferably during the prodromal period) initiation of treatment but, despite significant clinical benefit, viral latency is not eradicated, and pretreatment frequencies of recurrence usually continue after episodic acute treatment is completed. Intravenous administration has also shown benefit in the treatment of severe complications of HSV infection in pregnancy, and neonatal HSV infections. Recurrence of HSV has been completely prevented or significantly reduced during suppressive therapy with oral aciclovir in immunocompetent patients. Use of oral aciclovir is effective but controversial in the treatment of otherwise healthy individuals with varicella (chickenpox), and in some countries it has been recommended for use only in cases which may be potentially severe. The development of rash and pain associated with herpes zoster (shingles) is attenuated with oral or intravenous aciclovir therapy, ocular involvement is prevented, and post-herpetic neuralgia appears to be decreased. Similarly, in a few patients with zoster ophthalmicus, oral aciclovir has reduced the frequency and severity of long term ocular complications and post-herpetic neuralgia, and herpes zoster oticus is improved with intravenous aciclovir. Oral aciclovir has prevented recurrence of HSV genital or orofacial infections during suppressive therapy in > 70% of immunocompetent patients in most clinical trials. Suppression of latent HSV, VZV and CMV infections has been achieved in many immunocompromised patients receiving the oral or intravenous formulations. Aciclovir also appears to offer partial protection from invasive CMV disease in CMV-seropositive bone marrow transplant recipients. The few comparative trials published have shown aciclovir to be at least as effective as other investigated antivirals in the treatment of HSV infections in immunocompetent patients, and more effective than inosine pranobex in the prophylaxis of genital herpes. Similarly, in isolated clinical trials, oral aciclovir appears as effective as topical idoxuridine and oral brivudine in some parameters in immunocompetent patients with VZV infections, and the intravenous formulation appears at least as effective as oral brivudine and intravenous vidarabine in treating these infections in immunocompromised patients.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibitors of thymidylate synthase and dihydrofolate reductase potentiate the antiviral effect of acyclovir

In cells infected with herpes simplex virus type 1, intracellular dNTP pools increased markedly. Treatment of these cells with 3 microM acyclovir resulted in an additional expansion in pyrimidine deoxyribonucleoside triphosphate pools with dTTP increasing 32-fold and dCTP 8-fold. Both thymidine and deoxycytidine, however, compete with acyclovir for phosphorylation by the viral pyrimidine deoxyribonucleoside kinase and thus reduce the amount of drug that is anabolized to the active form. Theoretically, agents which inhibit thymidylate synthase or dihydrofolate reductase should reduce intracellular pools of thymidine, resulting in the potentiation of the antiviral effects of acyclovir. We explored this strategy by quantitating the synergy produced by combinations of acyclovir and other drugs using three-dimensional dose-response surface methodology (MacSynergy II). Significant synergy was seen with both 5-FdUrd and methotrexate whereas BrVdUrd, 5-CldUrd, 5-IdUrd, and 5-BrdUrd exhibited little to no synergistic activity. It is suggested that inhibitors of thymidylate synthase and dihydrofolate reductase warrant further exploration as potentiators of acyclovir.

2-Acetylpyridine 5-[(dimethylamino)thiocarbonyl]-thiocarbonohydrazone (1110U81) potently inhibits human cytomegalovirus replication and potentiates the antiviral effects of ganciclovir

We studied the effects of 2-acetylpyridine 5-[(dimethylamino)thiocarbonyl]-thiocarbonohydrazone (1110U81 or A1110U), a potent inhibitor of the ribonucleotide reductases encoded by herpes simplex virus types 1 and 2 and by varicella-zoster virus, against human cytomegalovirus (HCMV) replication in infected MRC-5 cells. We show that 1110U81 is a potent inhibitor of HCMV DNA replication (50% inhibitory concentration [IC50], 3.6 microM; IC90, 5.6 microM) and also potentiates the effects of ganciclovir (GCV) against HCMV. The IC90 of GCV is reduced from 65 microM when GCV alone is given to 2.8 microM when GCV is combined with 1110U81 at a molar ratio of 1:1.

Potential for combined therapy with 348U87, a ribonucleotide reductase inhibitor, and acyclovir as treatment for acyclovir-resistant herpes simplex virus infection

Inhibitors of the ribonucleotide reductase of herpes simplex viruses (HSV) potentiate the activity of acyclovir in vitro and in animal studies. In addition, the combination of the ribonucleotide reductase inhibitor 348U87 and acyclovir has synergistic therapeutic effects against infections in mice due to thymidine kinase-deficient, thymidine kinase-altered, and DNA polymerase mutants of HSV. We performed a pilot study of topical combination therapy with 348U87 (3%) and acyclovir (5%) cream for acyclovir-resistant, anogenital HSV infections in ten human immunodeficiency virus (HIV)-infected patients. Our results, with lack of complete reepitheliazation of lesions in all patients and poor virologic response, suggest that this therapy is unlikely to be useful for this indication.

Inactivators of herpes simplex virus ribonucleotide reductase: hematological profiles and in vivo potentiation of the antiviral activity of acyclovir

A1110U (BW 1110U81) is an inactivator of herpesvirus ribonucleotide reductases and a potentiator of the antiviral activity of acyclovir (ACV) (T. Spector, J. A. Harrington, R. W. Morrison, Jr., C. U. Lambe, D. J. Nelson, D. R. Averett, K. Biron, and P. A. Furman, Proc. Natl. Acad. Sci. USA 86:1051-1055, 1989) that was subsequently found to cause hematological toxicity at high oral doses in rats. Eleven structurally related inactivators of herpes simplex virus (HSV) ribonucleotide reductase were therefore tested in vivo for hematological toxicity and for potentiation of ACV. None of the novel ribonucleotide reductase inactivators was hematologically toxic to rats following oral dosing at 60 mg/kg/day for 30 days. Four of these inactivators statistically improved the antiviral topical potency of ACV on HSV type 1-infected nude mice. A promising compound, 2-acetylpyridine 5-[(2-chloroanilino)thiocarbonyl]thiocarbonohydrazone (BW 348U87), was studied more extensively in two in vivo models: dorsum-infected athymic nude mice and snout-infected hairless mice. BW 348U87 significantly potentiated the antiviral activity of ACV against all virus strains tested, i.e., wild-type (ACV-sensitive) HSV type 1 and HSV type 2 strains and three mutant (ACV-resistant) HSV type 1 strains. The latter included a virus expressing a DNA polymerase resistant to inhibition by ACV triphosphate, a virus deficient in thymidine kinase (the enzyme responsible for phosphorylating ACV), and a virus expressing an altered thymidine kinase, which catalyzes the normal phosphorylation of thymidine but not of ACV. BW 348U87 and ACV are currently being developed as a combination topical therapy for cutaneous herpes infections.

Acyclovir Treatment Stimulates dCMP Deaminase Activity in HSV-1-Infected Cells

Herpes simplex virus (HSV) infection induces enzymes involved in DNA precursor metabolism and DNA synthesis. The level of the DNA precursors dCTP, dTTP and dGTP increases after HSV infection, while the dATP level decreases. Inhibition of viral DNA synthesis by acyclovir (ACV) increases all four deoxyribonucleoside triphosphate levels, in particular that of dTTP. In the present work the dynamics of metabolism of pyrimidine deoxyribonucleotides and changes in the dTTP and dCTP pools in HSV-1-infected HL cells and in infected cells where viral DNA synthesis was inhibited by ACV were studied. In the absence of the drug the major part of the dCDP synthesized was incorporated into DNA. ACV treatment inhibited DNA synthesis completely but increased de novo dTMP synthesis. A continued ribonucleotide reductase activity in ACV-treated HSV-1-infected cells contributes to the increasing dTTP pool. The allosteric regulation of dCMP deaminase activity in HSV-1-infected cells was investigated. The great increase of de novo dTMP synthesis was shown to be due to stimulation of dCMP deaminase activity by an increasing dCTP pool. The high dTTP pool is suggested to be of secondary importance to the dCTP pool in the regulation of dCMP deaminase activity in HSV-1-infected cells.

A three-dimensional model to analyze drug-drug interactions

Nearly four generations of investigators have studied combined drug effects. Their methods of generating and analyzing data have changed dramatically over the years but the basic problem has not. This review examines the inherent difficulties in analyzing combined drug effects and evaluates modern methods of describing these interactions. Researchers have traditionally used two-dimensional (2-D) methods to approximate the actual three-dimensional (3-D) nature of drug interactions. We conclude that these 2-D methods are often inadequate when used to analyze synergistic and antagonistic drug interactions in antiviral and anticancer chemotherapy. We propose a direct and pragmatic 3-D approach to the problem, made possible by microcomputers and sophisticated graphics programs. This procedure directly elucidates the shape of the dose-response surface, identifies the regions of statistically significant synergy and antagonism, and quantitates these effects. It also greatly simplifies the problem since a 3-D surface presents complete drug interactions in a way that can be easily interpreted. We will show that understanding the shape of the resulting 3-D surface is essential to an understanding of complex drug interactions. This new method facilitates the rigorous analysis of drug-drug interactions and offers investigators powerful new tools to analyze combinations of antiviral and anticancer drugs.

Antiherpes virus activity and effect on deoxyribonucleoside triphosphate pools of (E)-5-(2-bromovinyl)-2'-deoxycytidine in combination with deaminase inhibitors

The antiviral activity and cytotoxicity of (E)-5-(2-bromovinyl)-2'-deoxycytidine (BrVdCyd) against herpes simplex virus type 1 (HSV-1), singly and in combination with deaminase inhibitors was determined using rabbit kidney (RK-13), HEP-2, BHK-21 and VERO cells. BrVdCyd was a potent inhibitor of HSV-1 replication with ED50 values of 0.30 to 1.20 microM depending on the cell line used. In the presence of tetrahydrouridine or tetrahydrodeoxyuridine (H4dUrd), potency of BrVdCyd increased approximately two fold (ED50: 0.54 microM) in HSV-infected VERO cells. The combination of BrVdCyd and H4dUrd was also effective in decreasing virus yield. Dihydrodeoxyuridine (H2dUrd) reversed the activity of BrVdCyd (ED50: 6 to 7 microM). The effect of (E)-5-(2-bromovinyl)-2'-deoxyuridine (BrVdUrd), BrVdCyd and BrVdCyd in combination with H4dUrd on deoxyribonucleoside triphosphate (dNTP) pools was assessed in VERO cells infected with a high multiplicity of infection (10 PFU/cell). Significant differences in dNTP poll sizes (pmol/10(6) cell) were observed with different treatments. BrVdUrd and BrVdCyd treatment resulted in marked expansion of the dTTP pool (greater than 1200 pmol) compared to HSV-infected VERO cells (303 pmol). Exposure to H4dUrd resulted in a 12-fold expansion of the dCTP pool (326 pmol) and barely detectable levels of dTTP (less than 1.0 pmol). BrVdCyd plus H4dUrd treatment resulted in a slight expansion of the dTTP pool (515 pmol). These results indicate: (i) H4dUrd inhibits de novo dCyd/dCMP deaminase pathway and (ii) exposure to BrVdCyd plus H4dUrd puts a strain on viral DNA synthesis to such an extent that even though dTTP is being formed from alternative pathways, its eventual utilization as a substrate is reduced and hence it builds up.

A herpes simplex virus ribonucleotide reductase deletion mutant is defective for productive acute and reactivatable latent infections of mice and for replication in mouse cells

Herpes simplex virus encodes a ribonucleotide reductase that is not essential for virus growth in dividing cells at 37 degrees. This enzyme has been proposed as a target for antiviral drugs; its utility in this regard could depend upon its importance in vivo. To test the requirement of viral ribonucleotide reductase in a mammalian host, we tested a mutant virus, lacking most of the gene encoding the ribonucleotide reductase large subunit, in a mouse eye model of pathogenesis and latency where the wild-type virus establishes reactivatable latent infections in trigeminal ganglia following corneal inoculation. The deletion mutant was severely impaired in its ability to replicate acutely in the eye and in the trigeminal ganglion and failed to establish reactivatable latent infections. In contrast, a recombinant virus in which the deleted sequences were restored was competent for both acute and latent infections. The defects of the deletion mutant in the mouse may be related to its severely impaired growth at 38 degrees in mouse cells relative to its growth in Vero cells. These results indicate that ribonucleotide reductase is critical for productive acute and reactivatable latent infections in mice and replication in mouse cells at 38 degrees and suggest that caution be exercised in extrapolating from studies conducted in mice to human infections when judging the utility of this enzyme as a target for antiviral chemotherapy.

Herpes simplex virus ribonucleotide reductase mutants are hypersensitive to acyclovir

Two mutants defective in herpes simplex virus-encoded ribonucleotide reductase activity exhibited the novel phenotype of hypersensitivity to acyclovir, aphidicolin, and to a lesser extent, phosphonoacetic acid. These results have implications for acyclovir resistance and the development of drugs that potentiate acyclovir action by inhibition of viral ribonucleotide reductase.

Acyclovir. An updated review of its antiviral activity, pharmacokinetic properties and therapeutic efficacy

Acyclovir (aciclovir) is a nucleoside antiviral drug with antiviral activity in vitro against members of the herpes group of DNA viruses. As an established treatment of herpes simplex infection, intravenous, oral and to a lesser extent topical formulations of acyclovir provide significant therapeutic benefit in genital herpes simplex and recurrent orofacial herpes simplex. The effect of acyclovir therapy is maximised by early initiation of treatment, especially in non-primary infection which tends to have a less protracted course than the primary episode. Long term prophylactic oral acyclovir, in patients with frequent episodes of genital herpes simplex, totally suppresses recurrences in the majority of subjects; as with other infections responding to acyclovir, viral latency is not eradicated and pretreatment frequencies of recurrence return after discontinuation of treatment. Caution should accompany the prophylactic use of acyclovir in the general population, due to the theoretical risk of the emergence of viral strains resistant to acyclovir and other agents whose mechanism of action is dependent on viral thymidine kinase. Intravenous acyclovir is the treatment of choice in biopsy-proven herpes simplex encephalitis in adults, and has also been successful in the treatment of disseminated herpes simplex in pregnancy and herpes neonatorium. Intravenous and oral acyclovir protect against dissemination and progression of varicella zoster virus infection, but do not protect against post-herpetic neuralgia. In immunocompromised patients, intravenous, oral and topical acyclovir shorten the clinical course of herpes simplex infections while prophylaxis with oral or intravenous dosage forms suppresses reactivation of infection during the period of drug administration. Ophthalmic application of 3% acyclovir ointment rapidly heals herpetic dendritic corneal ulcers and superficial herpetic keratitis. Thus, despite an inability to eradicate latent virus, acyclovir administered in therapeutic or prophylactic fashion is now the standard antiviral therapy in several manifestations of herpes simplex virus infection, and indeed represents a major advance in this regard. With the exception of varicella zoster virus infections, early optimism concerning the use of the drug in diseases due to other herpes viruses has generally not been supported in clinical investigations.

2-Acetylpyridine 5-[(dimethylamino)thiocarbonyl]-thiocarbonohydrazone (A1110U), a potent inactivator of ribonucleotide reductases of herpes simplex and varicella-zoster viruses and a potentiator of acyclovir

2-Acetylpyridine 5-[(dimethylamino)thiocarbonyl]thiocarbonohydrazone (A1110U) was found to be a potent inactivator of the ribonucleotide reductases (EC 1.17.4.1) encoded by herpes simplex virus types 1 and 2 and by varicella-zoster virus and to be a weaker inactivator of human ribonucleotide reductase. It also markedly potentiated the antiherpetic activity of acyclovir against these viruses in tissue culture. A1110U both decreased the dGTP pool that builds up when infected cells are treated with acyclovir and induced a large increase in the pool of acyclovir triphosphate. The resultant 100-fold increase in the ratio of the concentrations of acyclovir triphosphate to dGTP should facilitate the binding of the fraudulent nucleotide to its target enzyme, herpes virus-encoded DNA polymerase, and could account for the synergy between A1110U and acyclovir. A similar change in the acyclovir triphosphate-to-dGTP ratio was previously reported to be induced by another ribonucleotide reductase inhibitor, 2-acetylpyridine 4-(2-morpholinoethyl)thiosemicarbazone (A723U). However, A1110U is considerably more potent and may have better clinical potential. Synergistic toxic interactions between A1110U and acyclovir were not detected in uninfected cells.