Efficacies of antiherpesvirus nucleosides against two strains of herpes simplex virus type 1 in Vero and human embryo lung fibroblast cells

Analysis of antiviral drug properties of thymidine kinase of herpes B virus using recombinant herpes simplex virus 1

IMPORTANCE

Zoonotic infection of humans with herpes B virus (BV) causes severe neurological diseases. Acyclovir (ACV) and ganciclovir (GCV), most frequently used as anti-herpes drugs, are recommended for prophylaxis and therapy in human BV infection. In this study, we examined the property of BV thymidine kinase (TK) against anti-herpes drugs using a recombinant herpes simplex virus type 1 (HSV-1) carrying BV TK gene. We found that HSV-1 carrying BV TK was similarly sensitive to GCV as HSV-1 carrying varicella zoster virus TK. In addition, we demonstrated that BV TK was not mutated in the GCV- and ACV-resistant HSV-1 carrying BV TK, suggesting that ACV- or GCV-resistant BV might be rare during treatment with these antiviral drugs. These data can provide a new insight into the properties of BV TK in terms of the development of drug resistance.

Characteristics of Helicase-primase Inhibitor Amenamevir-resistant Herpes Simplex Virus

The antiherpetic drug amenamevir (AMNV) inhibits the helicase-primase complex of herpes simplex virus type 1 (HSV-1), HSV-2 and varicella-zoster virus directly as well as inhibiting the replication of these viruses. Although several mutated HSV viruses resistant to helicase-primase inhibitors have been reported, the mutations contributing to the resistance remain unclear as recombinant viruses containing a single mutation have not been analyzed. We obtained AMNV-resistant viruses with amino acid substitutions by several passages under AMNV-treatment. Twenty HSV-1 and 19 HSV-2 mutants with mutation(s) in UL5 helicase and/or UL52 primase, but not in co-factor UL8, were isolated. The mutations in UL5 were located downstream of motif IV, with UL5 K356N in HSV-1 and K355N in HSV-2, in particular, identified as having the highest frequency: 9/20 and 9/19, respectively. We generated recombinant AMNV-resistant HSV-1 with a single amino acid substitution using BAC mutagenesis. As a result, G352C in UL5 helicase and F360C/V and N902T in UL52 primase were identified as novel mutations. The virus with K356N in UL5 showed 10-fold higher AMNV resistance than did other mutants, and showed equivalent viral growth in vitro and virulence in vivo as the parent HSV-1, although other mutants showed attenuated virulence. All recombinant viruses were susceptible to the other antiherpetic drugs, acyclovir and foscarnet. In conclusion, based on BAC mutagenesis, this study identified for the first time mutations in UL5 and UL52 that contributed to AMNV resistance, and found that a mutant with the most frequent K356N mutation in HSV-1 maintained viral growth and virulence equivalent to the parent virus.

Identification of the Components in a Vaccinium oldhamii Extract Showing Inhibitory Activity against Influenza Virus Adsorption

We previously reported that extracts from plants of the Ericaceae genus Vaccinium, commonly known as the kind of blueberry, inhibited the early steps of influenza virus (IFV) infection to host cells, and that the activity was correlated with the total polyphenol content. Particularly potent inhibitory activity was observed for Vaccinium oldhamii. In this study, we identified the active components in Vaccinium oldhamii involved in the inhibition of IFV infection. We sequentially fractionated the Vaccinium oldhamii extract using a synthetic adsorbent resin column. High inhibitory activity was observed for the fractions eluted with 30%, 40%, and 50% ethanol, and three peaks (peak A, B, and C) considered to represent polyphenols were identified in the fractions by HPLC analysis. Among these peaks, high inhibitory activity was detected for peak A and B, but not for peak C. These peaks were analyzed by LC/MS, which revealed that peak A contained procyanidin B2 and ferulic acid derivatives, whereas peak B contained two ferulic acid O-hexosides, and peak C contained quercetin-3-O-rhamnoside and quercetin-O-pentoside-O-rhamnoside. It is already known that these polyphenols have anti-IFV activity, but we speculate that ferulic acid derivatives are the major contributors to the inhibition of the early steps of IFV replication, such as either adsorption or entry, observed for Vaccinium oldhamii.

Human Brain Tumour Cell Lines as Cell Substrate to Demonstrate Sensitivity/Resistance of Herpes Simplex Virus Types 1 and 2 to Nucleoside Analogues

Herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) grow in vitro in a broad range of human and animal cells. Here we describe the activity of several nucleoside and nucleotide analogues against HSV-1 and HSV-2 in human brain-tumour-derived cells. Of the compounds tested, ( E)-5-(2′-bromovinyl)-2′-deoxyuridine (BVDU) proved to be the most potent inhibitor of the wild-type, thymidine-kinase-positive (TK+) HSV-1. In contrast to BVDU, acyclovir and ganciclovir, which were virtually inactive against TK− HSV-1, the two acyclic nucleoside phosphonates ( S)-9-(3-hydroxy-2-phosphonylmethoxypropyl)adenine (HPMPA) and ( S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine (HPMPC) were equally active against TK+ and TK− HSV-1 strains. Depending on the brain tumour cell line used, differences in virus-drug sensitivity varied up to 30-fold. Upon passage of the virus in the brain tumour cell lines in the presence of the compounds, virus resistance developed rapidly to BVDU, acyclovir and foscarnet but not to HPMPC. Furthermore, the HSV-1 mutant that had become resistant to BVDU, acyclovir or foscarnet remained as sensitive to HPMPC and HPMPA as did the wild-type HSV-1 (KOS strain).

Protection from lethal herpes simplex virus type 1 infection by vaccination with a UL41-deficient recombinant strain

The UL41 gene of herpes simplex virus type 1 (HSV-1) encodes a virion host shut off protein which is involved in immune evasion.　The growth and virulence of HSV-1 is markedly reduced by the deletion of UL41.　In this report, the UL41-deleted recombinant HSV-1 strain VR∆41 was evaluated as a prophylactic live attenuated vaccine against lethal HSV-1 infection in a mouse model.　Intraperitoneal (i.p.) inoculation with the VR∆41 strain clearly inhibited lethal wild-type HSV-1 (VR-3 strain) infection after both i.p. and intracerebral (i.c.) inoculations.　Vaccination with the VR∆41 strain was safer than VR-3 vaccination and was able to protect against a wild-type challenge to the same degree as VR-3 vaccination.　In contrast, i.p. inoculation with ultraviolet-irradiated VR-3 induced resistance against i.p. infection, but not against i.c.

INFECTION

Although replication of the VR∆41 strain in mice was greatly reduced compared to that of the VR-3 strain, VR∆41 strain maintained the ability to spread to the central nervous system (CNS) from a peripheral inoculation site. These results indicated that the VR∆41 strain evoked a potent immune reaction through viral protein expression within CNS without the induction of lethal encephalitis.　The entry of antigens into the CNS was essential for the establishment of protective immunity against the lethal HSV encephalitis.　We concluded that only a live attenuated vaccine is able to afford a prophylactic effect against CNS infection with HSV.　In order to fulfill this requirement, UL41-deleted viruses provide a strong candidate for use as a recombinant live vaccine.

Structural basis for the interaction of CCR5 with a small molecule, functionally selective CCR5 agonist

The chemokine receptor CCR5 is an attractive target for HIV-1 drug development, as individuals whose cells lack surface CCR5 expression are highly resistant to HIV-1 infection. CCR5 ligands, such as CCL5/RANTES, effectively inhibit HIV-1 infection by competing for binding opportunities to the CCR5 and inducing its internalization. However, the inherent proinflammatory activity of the chemotactic response of CCR5 ligands has limited their clinical use. In this study, we found that a novel small molecule, functionally selective CCR5 agonist, 2,2-dichloro-1-(triphenylphosphonio)vinyl formamide perchlorate (YM-370749), down-modulates CCR5 from the cell surface without inducing a chemotactic response and inhibits HIV-1 replication. In molecular docking studies of YM-370749 and a three-dimensional model of CCR5 based on the rhodopsin crystal structure as well as binding and functional studies using various CCR5 mutants, the amino acid residues necessary for interaction with YM-370749 were marked. These results provide a structural basis for understanding the activation mechanism of CCR5 and for designing functionally selective agonists as a novel class of anti-HIV-1 agents.

Differential mutation patterns in thymidine kinase and DNA polymerase genes of herpes simplex virus type 1 clones passaged in the presence of acyclovir or penciclovir

A total of 21 clones of acyclovir (ACV)-resistant (ACV(r)) herpes simplex virus type 1 (HSV-1) and 23 clones of penciclovir (PCV)-resistant (PCV(r)) HSV-1, emerging during serial passages in the presence of ACV or PCV, were isolated under conditions excluding contamination of resistant mutants in the starting virus culture, and their mutations in the thymidine kinase (TK) and DNA polymerase (DNA Pol) genes were analyzed comparatively. Mutations in the TK genes from ACV(r) mutants consisted of 50% single nucleotide substitutions and 50% frameshift mutations, while the corresponding figures for the PCV(r) mutants were 4 and 96%, respectively (P < 0.001). Eight of the 21 ACV(r) clones, but none of the 23 PCV(r) clones, had mutations in DNA Pol. Only nucleotide substitution(s) could be detected in the DNA Pol gene, as the gene is essential for virus replication. Therefore, the results for the DNA Pol mutants are concordant with those for the TK mutants in that a single nucleotide substitution was commonly observed in the ACV(r), but not in the PCV(r), mutants. These results clearly point to differential mutation patterns between ACV(r) and PCV(r) HSV-1 clones.

Genotypic and phenotypic characterization of the thymidine kinase of ACV-resistant HSV-1 derived from an acyclovir-sensitive herpes simplex virus type 1 strain

Twenty-four strains of acyclovir (ACV)-resistant (ACV(r)) herpes simplex virus type 1 (HSV-1) were generated from the HSV-1 TAS strain by exposure to ACV, and the genotype and phenotype of the thymidine kinase (TK) from these mutants were analyzed. The TK polypeptide of the ACV(r) HSV-1 strains was examined by Western blot using an anti-HSV-1 TK rabbit serum. The sensitivity of each strain to ACV, foscarnet and cidofovir (CDV) was also determined. A single guanine (G) insertion or a single cytosine (C) deletion was detected in 12 of the 24 ACV(r) strains at the G or C homopolymer stretches within the TK gene. Genotypic analysis predicted that two thirds of the ACV(r) HSV-1 strains expressed truncated TK polypeptides, while one third expressed viral TK polypeptide with a single amino acid substitution at various sites. Western blot abnormalities in the viral TK polypeptides were identified in 21 ACV(r) strains. There was an inverse correlation between the susceptibility of the HSV-1 mutant strains to ACV and that to CDV. Nucleotide sequencing of the TK gene and Western blot analysis of the viral TK polypeptides are considered to be one of the methods for predicting virus sensitivity to ACV and CDV.

Importance of C-terminus of herpes simplex virus type 1 thymidine kinase for maintaining thymidine kinase and acyclovir-phosphorylation activities

We previously isolated an acyclovir (ACV)-resistant herpes simplex virus type 1 (HSV-1), strain TAR, from a child with Wiskott-Aldrich syndrome. An acyclovir-sensitive HSV-1, strain TAS, had been isolated from the same patient before the isolation of HSV-1 TAR. The TK protein of ACV-sensitive HSV-1 TAS was composed of 376 amino acids, while that of HSV-1 TAR was composed of 407 amino acids with altered amino acid residue between positions 355-407. The elongation of TK was caused by a single nucleotide deletion of cytosine from a homopolymer stretch of 4 cytosine residues between positions 1061-1064. There was no viral TK activity in HSV-1 TAR-infected Vero cells, indicating the importance of the C-terminal portion of TK protein from positions 355-376. Recombinant TK polypeptides with amino acid deletions at the C-terminus were prepared, and TK and ACV-phosphorylation activities were examined. Deletion of 5 and 6 amino acids from the C-terminus of the TK polypeptide of HSV-1 TAS resulted in a reduction of TK activity by approximately 75% and 100%, respectively. These mutant TK polypeptides did not phosphorylate ACV. These results indicate that amino acid residues from positions 371-376 in the C-terminal portion of HSV-1 TK protein are essential for keeping TK and ACV-phosphorylation activities.

Participation of type I interferon in the decreased virulence of the UL13 gene-deleted mutant of herpes simplex virus type 1

We isolated a UL13 gene-deleted mutant of the herpes simplex virus type 1 (HSV-1) strain VR3 (VRDelta13) and its revertant virus (VRDelta13R). This deletion mutant still had virus host shutoff (vhs) activity, although a previous report had suggested the possibility of a functional relation between the UL13 product, that is protein kinase (PK), and vhs activity. We compared the in vivo growth of these viruses in BALB/c mice. VRDelta13 was cleared in the early period of intraperitoneal infection. VRDelta13 had a higher sensitivity to the mouse type I interferon (IFN) and showed a higher level of IFN induction in the study period of infection than did VR3 and VRDelta13R. These results suggest that a nonspecific antiviral response (i.e., the IFN system) may contribute to this rapid inhibition of viral replication in vivo.

Rapid phenotypic characterization method for herpes simplex virus and Varicella-Zoster virus thymidine kinases to screen for acyclovir-resistant viral infection

A rapid phenotypic screening method for herpes simplex virus (HSV) and varicella-zoster virus (VZV) thymidine kinase (TK) genes was developed for monitoring acyclovir-resistant viruses. This method determines the biochemical phenotype of the TK polypeptide, which is synthesized in vitro from viral DNA using a procedure as follows. The TK gene of each sample virus strain is amplified and isolated under the control of a T7 promoter by PCR. The PCR products are transcribed with T7 RNA polymerase and translated in a rabbit reticulocyte lysate. Using this method, enzymatic characteristics and the size of the TK polypeptides encoding HSV and VZV DNA were defined in less than 2 days without virus isolation. The assay should be a powerful tool in monitoring drug-resistant viruses, especially in cases in which virus isolation is difficult.

Susceptibility of protein kinase (ORF47)-deficient varicella-zoster virus strains to anti-herpesvirus nucleosides

To clarify whether varicella-zoster virus (VZV) protein kinase (PK; ORF47) takes part in phosphorylation of anti-herpesvirus nucleosides, thymidine kinase (TK) deficient, and PK/TK double deficient recombinant VZV strains were isolated and their susceptibility, and that of wild type and PK-deficient strains to various nucleoside analogs was evaluated. The PK-deficient VZV strains showed a sensitivity equal to that of the wild type strain against all compounds tested, including ganciclovir. This indicates that PK is not involved in phosphorylation of the tested nucleosides in VZV-infected cells.

Recurrent aciclovir-resistant herpes simplex in a child with Wiskott-Aldrich syndrome

A boy with Wiskott-Aldrich syndrome suffered from thymidine kinase (TK)-altered and aciclovir-resistant herpes simplex virus type 1 (HSV-1) skin infections. He presented with severe herpes simplex around the left eye in March 1993 at the age of 8 years. HSV-1 strain TAS was isolated and was shown to be susceptible to aciclovir (50% inhibitory concentration (IC50) 0.23 microg/mL). He was treated with intravenous (i.v.) high dose aciclovir, 2 mg/kg per h, which produced an improvement. About 1 year later (May 1994), a severe herpes simplex infection appeared on his face, arm, genitalia, back and foot. Treatment with i.v. aciclovir, 2 mg/kg per h, was initiated, but the skin lesions did not improve. HSV-1 strain TAR was isolated and was shown to be resistant to aciclovir (IC50 36 microg/mL). HSV-1 TAR and TAS were susceptible to vidarabine (IC50 4. 4 and 2.9 microg/mL, respectively). The skin lesions were treated with i.v. vidarabine, 15-20 mg/kg per day, and healed satisfactorily. However, in March 1995, the patient again experienced a severe herpes simplex infection around the left eye. HSV-1 strain R95 was isolated and was shown to be resistant to aciclovir (IC50 36 microg/mL). Diminished sensitivity of HSV-1 TAR and R95 to aciclovir was associated with reduced viral TK activity and loss of aciclovir phosphorylation activity.

Metabolic fate of [3H]1-beta-D-arabinofuranosyl-5-[(E)-2-bromovinyl]uracil in herpes simplex virus type 1-infected cells

The metabolic fate of 1-beta-D-arabinofuranosyl-5-[(E)-2-bromovinyl]uracil (BV-araU) in herpes simplex virus type 1-infected cells was studied using tritium-labeled BV-araU. [3H]BV-araU was selectively taken-up by infected cells. Approximately 10% of the total uptake of [3H]BV-araU was recovered from the acid-insoluble fraction at any time post-infection. Both cellular uptake of [3H]BV-araU and its incorporation into the acid-insoluble fraction increased with increasing incubation time through 8 hr post-infection. Uptake of [3H]BV-araU and its incorporation into the acid-insoluble fraction also increased proportionally to the duration of exposure to [3H]BV-araU. An alkaline sucrose gradient sedimentation analysis revealed that the radioactive DNA obtained from cells pulse-labeled with [3H]BV-araU were small DNA fragments which remained at the top following a chasing period in isotope-free medium, whereas that pulse-labeled with [3H]thymidine was chased to a fraction of high molecular weight DNA. Nuclease P1 digestion reduced 99% of the [3H]BV-araU-labeled DNA extracted from infected cells to a low molecular weight. Following digestion of [3H]BV-araU-labeled DNA with micrococcal nuclease and spleen exonuclease, all of the radioactivity was recovered as [3H]BV-araU 3'-monophosphate. Thus, BV-araU strongly inhibits the elongation of viral DNA strands as demonstrated by the alkaline sucrose gradient sedimentation analysis, whereas at least a portion of the [3H]BV-araU is incorporated inside viral DNA strands in infected cells.

Evaluation of antiherpetic compounds using a gastric cancer cell line: pronounced activity of BVDU against herpes simplex virus replication

We developed a rapid and simple method for the screening of antiviral agents against herpes simplex virus (HSV) in a model of gastrointestinal herpetic infection in vitro. This method was based on inhibition of HSV-induced cytopathogenicity in gastric adenocarcinoma MKN-28 cells, as monitored by an MTT colorimetric assay. From the various compounds that were evaluated for their activity against HSV-1 and HSV-2, brivudine (BVDU) emerged as the most effective. When the 50% effective concentration (EC50) values of the antiherpes agents in MKN-28 cells were compared with those in human embryo lung MRC-5 cells, all compounds, except for BVDU, showed higher EC50 values in MKN-28 cells. For BVDU the EC50 values in MKN-28 cells were 0.8 (HSV-1) and 0.036 (HSV-2) times the EC50 values in MRC-5 cells. Thus BVDU was 27.5 times more active against HSV-2 in MKN-28 cells than in MRC-5 cells. The MKN-28 gastric cancer cells may be useful for the rapid screening of anti-HSV agents and, in particular, those that may be useful in therapy of gastrointestinal HSV infections in gastrointestinal herpetic infection.

Analysis of phosphorylation pathways of antiherpesvirus nucleosides by varicella-zoster virus-specific enzymes

The inhibitory activities of acyclovir (ACV), 1-beta-D-arabinofuranosyl-E-5-(2-bromovinyl)uracil (BV-araU), ganciclovir (GCV), 9-(2-deoxy-2-hydroxymethyl-beta-D-erythro-oxetanosyl)guanine (OXT-G), and (+)-9-[(1R,2R,3S)-2,3-bis(hydroxymethyl)Cyclobutyl]guanine (cOXT-G) on the replication of wild-type and thymidine kinase (TK)-negative strains of herpes simplex virus types 1 and 2 and varicella-zoster virus (VZV) and the wild-type strain of human cytomegalovirus were tested to clarity whether the phosphorylation of these compounds is catalyzed by viral TK or other enzymes. ACV and BV-araU had little effect on the replication of TK-negative virus strains. On the other hand, GCV, OXT-G, and cOXT-G inhibited the replication of TK-negative VZV at concentrations 10 times higher than those at which they inhibited wild-type VZV, indicating that a kinase other than TK phosphorylates GCV and OXT-G in VZV-infected cells. GCV phosphorylation activity was not detected in VZV-infected cell lysates; therefore, this activity was evaluated in COS 1 cells expressing viral TK and viral protein kinase (PK). The COS 1 cells expressing VZV TK were shown to be susceptible to all compounds tested. In contrast, VZV Pk-expressing COS 1 cells were susceptible to only GCV, OXT-G, and cOXT-G. These results suggest that VZV PK phosphorylates some nucleoside analogs, for example, GCV, OXT-G, and cOXT-G. This phosphorylation pathway may be important in the anti-VZV activities of some nucleoside analogs.

Analysis of the relationship between cellular thymidine kinase activity and virulence of thymidine kinase-negative herpes simplex virus types 1 and 2

The virulence of thymidine kinase-negative herpes simplex virus type 1 (HSV-1; VRTK- strain) and type 2 (HSV-2; UWTK- strain) was studied in comparison with that of their parental strains (VR-3 and UW-268, respectively) in an encephalitis model of adult (4-week-old) and newborn (3-day-old) mice. Viral thymidine kinase (TK) activity was essential for the maximum expression of virulence of HSV-1, because the 50% lethal dose (LD50) of VRTK- was 60 times higher than that of VR-3 in the brains of newborn mice expressing high levels of cellular TK activity. However, the UWTK- strain showed that replication of the UWTK- strain was completely supported by cellular TK activity. This difference in the role of viral and cellular TKs for virus growth between HSV-1 and HSV-2 was¿ confirmed with the one-step growth of virus strains in L-M and L-M(TK-) cells.

A sensitive assay system screening antiviral compounds against herpes simplex virus type 1 and type 2

A highly sensitive and accurate assay system was developed for in vitro evaluation of anti-herpes simplex virus (anti-HSV) agents using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and human embryonic lung fibroblast (MRC-5) cells. This assay system was found to be highly sensitive for both HSV-1 and -2. Confluent MRC-5 cells were infected with either HSV-1 KOS strain or HSV-2 G strain of 25 TCID50 in the presence of various concentrations of test compounds. The optical density of formazan was used to determine cell viability. The EC50 values of acyclovir and several other anti-HSV agents were found to be similar to those obtained by the plaque reduction method. These results indicate that this MTT assay is useful for screening anti-HSV-1 and -2 agents.

Antiviral activity of 1-beta-D-arabinofuranosyl-E-5-(2-bromovinyl)uracil against thymidine kinase negative strains of varicella-zoster virus

Mechanism of antiviral activity of 1-beta-D-arabinofuranosyl-E-5-(2-bromovinyl)uracil (BV-araU) against the YSR strain of varicella-zoster virus (VZV), which is a mutant derived from the wild YS strain and is completely deficient in viral thymidine kinase (TK), was searched in comparison with antiviral activity of other thymidine analogues, guanosine analogue and thymidylate synthase (TS) inhibitor in human embryo lung fibroblast cells. Thymidine analogues, such as BV-araU,5-iododeoxyuridine (IUDR), 1-beta-D-arabinofuranosylthymine (araT), and guanosine analogue, such as 9-(2-hydroxyethoxymethyl)guanine (ACV), showed higher antiviral activity to the YS strain than to the YSR strain. Though, BV-araU also had the antiviral activity of a microgram level against the YSR strain. In contrast to these results, TS inhibitor, 5-fluorodeoxyuridine (FUDR), had higher antiviral activity to the YSR strain than to the YS strain. Highly synergistic antiviral activities of FUDR to the YS strain and the YSR strain were observed in combination with IUDR, araT, or ACV. However, weakly synergistic or additive inhibition to the YSR strain was shown in combination of BV-araU and FUDR, in spite of highly synergistic effect of this combination to the YS strain. The viral and cellular TS activity was partially inhibited by BV-araU monophosphate, but not by BV-araU. These results indicate that BV-araU is converted into BV-araU monophosphate by cellular TK, and the inhibition of TS activity by BV-araU monophosphate in the YSR strain-infected cells results in the suppression of viral replication.

Mechanism of inhibition of DNA synthesis by 1-beta-D-arabinofuranosyl-E-5-(2-bromovinyl)uracil

The mechanism of the inhibitory action of 1-beta-D-arabinofuranosyl-E-5-(2-bromovinyl) uracil triphosphate (BV-araUTP) on DNA synthesis by Escherichia coli DNA polymerase I Klenow fragment was studied. Acting as a chain terminator, BV-araUTP inhibited DNA synthesis by Klenow fragment more effectively than 2',3'-dideoxythymidine triphosphate (ddTTP). However, the incorporation sites of BV-araU monophosphate were restricted at consecutive dTMP sequence whereas ddTMP was incorporated at every dTMP site.

Analysis of toxic and mutagenic activities of antiherpesvirus nucleosides against HeLa cells and herpes simplex virus type 1

The toxic and mutagenic activities of five antiherpesvirus agents to HeLa cells and herpes simplex virus type 1 (HSV-1) were investigated. 5-Iodo-2'-deoxyuridine (IDU) and 9-beta-D-arabinofuranosyl-adenine (araA) showed very potent inhibitory effects on cell growth and the cloning efficiency of HeLa cells, whereas 1-beta-D-arabinofuranosyl-E-5-(2-bromovinyl)uracil (BV-araU), E-5-(2-bromovinyl)-2'-deoxyuridine (BVDU) and 9-(2-hydroxyethoxymethyl)guanine (ACV) showed less inhibitory effect. 50% inhibitory doses of BV-araU and BVDU for cell growth were 657 and 253 micrograms/ml, respectively. Although the growth inhibitory activity of BVDU was very weak, as above, the mutagenic activity of this drug to the cells, estimated by induction of colchicine-resistant mutants, was observed to be 4 micrograms/ml, which was a markedly smaller dose than the inhibitory dose for cell growth, and the highest frequency of mutation of the cells was shown at 100 micrograms/ml of BVDU. This activity was more potent than that of IDU. No mutagenic activity of BV-araU, araA and ACV to cells was observed within the concentration range of 1-800 micrograms/ml. IDU showed high mutagenic activity to HSV-1 growing in human embryo lung fibroblasts, and IDU-resistant mutants were induced at a high frequency. BVDU also induced a small amount of BVDU-resistant mutant virus, although this drug induced many mutant cells. No mutagenic activity of BV-araU, araA and ACV to HSV-1 was observed.

Random mutagenesis of the thymidine kinase gene of varicella-zoster virus

To understand the relationship between the primary structure and function of varicella-zoster virus thymidine kinase (VZV TK; EC 2.7.1.21), we established rapid screening and phenotypic selection of mutant VZV TK genes in TK-deficient Escherichia coli C600 by using a constitutive pKK223-3 expression plasmid. In this screening system, mutant TK genes generated by random mutagenesis were identified by the sensitivity of E. coli-expressing VZV TKs to 5-bromo-2'-deoxyuridine and 1-beta-D-arabinofuranosyl-E-5-(2-bromovinyl) uracil. Twenty-four mutant clones with amino acid substitutions were isolated, and their nucleotide sequence and enzymatic activities were determined. Of the 24 clones, 20 had single amino acid substitutions, 2 clones had double amino acid substitutions, and 1 clone had triple amino acid substitutions. In 17 cases of single amino acid substitution, six mutations led to lost enzyme activity, and four of these six mutations centered in the ATP-binding site. The other 11 mutations resulted in reduction of both TK and thymidylate kinase activities or only thymidylate kinase activity and were located in scattered positions in the VZV TK gene, although 5 mutations showed a tendency to cluster in the region between positions 251 and 260.

Comparative activity of various compounds against clinical strains of herpes simplex virus

The following compounds were evaluated for their inhibitory activity against clinical strains of herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) in both primary rabbit kidney (PRK) and HeLa cell cultures: (S)-9-(3-hydroxy-2-phosphonylmethoxypropyl)adenine (HPMPA), 9-(2-phosphonylmethoxyethyl)adenine (PMEA), (S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine (HPMPC), (RS)-9-(3-hydroxy-2-phosphonylmethoxypropyl)-2,6-diaminopurine (HPMPDAP), 5-(5-bromothien-2-yl)-2'-deoxyuridine (BTDU), 5-(5-chlorothien-2-yl)-2'-deoxyuridine (CTDU), 9-(2-deoxy-2-hydroxymethyl-beta-D-erythro-oxetanosyl)guanine (OXT-G), pentosan polysulfate, heparin, dextran sulfate (MW 10,000), acyclovir, 9-(2-hydroxyethoxymethyl)guanine (ACV), (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU), 1-beta-D-arabinofuranosyl-(E)-5-(2-bromovinyl)-uracil (BVaraU), vidarabine (9-beta-D-arabinofuranosyladenine) (ara-A) and phosphonoformate (PFA). The most potent inhibitors of HSV-1 were (in order of decreasing activity in PRK cells) BVDU, ACV, BVaraU and OXT-G, their mean 50% inhibitory concentration (IC50) ranging from 0.02 micrograms/ml to 0.9 micrograms/ml. Then followed BTDU and CTDU (IC50 1-2 micrograms/ml), the sulfated polysaccharides (IC50 1.3-5.8 micrograms/ml), the phosphonylmethoxyalkyl derivatives (IC50 5.6-25 micrograms/ml),ara-A (IC50 11 micrograms/ml) and PFA (IC50 38.5 micrograms/ml). Except for BVDU, BVaraU, BTDU and CTDU, the compounds did not discriminate between HSV-2 and HSV-1. All the compounds studied could be considered specific anti-HSV agents. Their selectivity indexes varied from 3 (PFA) to 6400 (BVDU).

Different antiviral potencies of BV-araU and related nucleoside analogues against herpes simplex virus type 1 in human cell lines and Vero cells

Antiviral potencies against herpes simplex virus type 1 (HSV-1) of 1-beta-D-arabinofuranosyl-E-5-(2-bromovinyl)uracil (BV-araU) and ten other nucleoside analogues in human embryonic lung fibroblast (HEL) cells were compared with those in Vero cells. 5-Halogenovinylarabinosyluracils, highly active in HEL cells, were inactive against all three laboratory-stocked strains of HSV-1 but exerted moderate antiviral effects on three clinical isolates in Vero cells. The reduction in anti-HSV-1 potencies of other representative nucleoside analogues in Vero cells was much less than those of 5-halogenovinylarabinosyluracils. However, significant antiviral potencies of BV-araU against laboratory strains were observed in other human and monkey fibroblast cells including an immortalized cell line. Significant enhancement of antiviral activity of BV-araU against a laboratory strain and a clinical isolate was demonstrated in Vero cells by the addition of 0.1 microM aminopterin or FUdR, an inhibitor of thymidylate synthesis. The potentiated anti-HSV-1 activity in Vero cells was comparable to the potency in HEL cells without the inhibitor. These results suggested that high activity of thymidylate synthesis and a large thymidylate pool size in Vero cells seem to be related to loss of anti-HSV-1 potency of BV-araU. Original tissue type, species, and the immortality may not be responsible for the reduced antiviral activity of BV-araU in Vero cells.

Antiviral activity of oxetanocins against varicella-zoster virus

Several new nucleosides with an oxetanosyl-N-glycoside group, named oxetanocins, were evaluated for their antiviral activities against varicella-zoster virus (VZV) in human embryo lung cells. 9-(2-deoxy-2-hydroxy-methyl-beta-D-erythro-oxetanosyl)guanine (OXT-G) and 9-(2-deoxy-2-hydroxymethyl-beta-D-erythro-oxetanosyl)-2- aminoadenine were effective against not only thymidine kinase-positive (TK+) VZV (YS strain) but also thymidine kinase-negative (TK-) VZV (YSR strain), whereas carbocyclic OXT-G was effective against TK+ VZV but not against TK- VZV. [3H]OXT-G was incorporated into TK+ VZV-infected cells and TK- VZV-infected cells more than into mock-infected cells and was converted into the triphosphate form.

Antiviral potencies of BV-araU and related nucleoside analogues against varicella-zoster virus in different cell lines

1-beta-D-Arabinofuranosyl-E-5-(2-bromovinyl)uracil (BV-araU) and nine other antiherpesviral nucleoside analogues were compared for their potencies against four strains of varicella-zoster virus (VZV) on three different cell lines: HEL cells, Vero cells, and MS cells established from a human malignant schwannoma. In contrast to the activity against herpes simplex virus type 1 previously reported, BV-araU showed extremely marked antiviral activity against VZV even on Vero cells. ED50, 50% plaque reduction dose, of BV-araU for VZV was 0.20-3.1 and 0.14-0.63 ng/ml on Vero cells and on HEL cells, respectively. Potency of BV-araU on MS cells was similar to that on these cell lines. There was not significant variation in anti-VZV activities of other nucleoside analogues on these three different cell lines except a few combinations of VZV strain and test compound.

Animal models for antiviral chemotherapy

Traditionally animal models have formed a vital part of the preclinical evaluation of new forms of antiviral therapy. A variety of models used in the past or potentially useful in the future are considered in this short review. Several valuable and complex questions concerning virus-drug interactions in vivo have been successfully addressed by means of animal models. Better understanding of drug modes of action and virus pathogenesis in the models enable even more accurate predictions to be made for the outcome of antiviral therapy in man. The complexity of virus infections in man is such that animals are likely to remain an important part in drug evaluation for many years. To this end, new developments such as improved techniques in the production of transgenic animals are opening up a variety of completely novel methods for studying inhibitors of a wider group of viruses in vivo including the human immunodeficiency virus. However, the correct interpretation of animal data requires the critical evaluation of animal models. This review will identify several important difficulties which confront those working on antiviral chemotherapy in animals and which must continue to be addressed if confidence in animal data is to be maintained.

Effects of various nucleosides on antiviral activity and metabolism of 1-beta-D-arabinofuranosyl-E-5-(2-bromovinyl)uracil against herpes simplex virus types 1 and 2

Uptake of 1-beta-D-arabinofuranosyl-E-5-(2-bromovinyl)uracil (BV-araU) into herpes simplex virus type 1 (HSV-1)- and 2 (HSV-2)-infected cells was elevated about 190 to 40 times, compared with that into mock-infected human embryo lung fibroblast cells. Uptake was not enhanced by infection with thymidine kinase-negative HSV-1 and HSV-2 mutants, however. In HSV-1-infected cells, 9.7% of BV-araU was phosphorylated to BV-araU triphosphate, but only 1.1% was phosphorylated in HSV-2-infected cells. The antiviral effect, uptake, and turnover of BV-araU were inhibited significantly by thymidine (dThd), moderately by deoxyuridine, and not at all by deoxycytidine. On the other hand, the antiviral activity of acyclovir (ACV) was inhibited only by dThd. The effect of BV-araU was influenced by dThd and dThd phosphates (mono-, di-, and triphosphates), and the effect of ACV was influenced only by dThd, which competitively inhibited the phosphorylation of ACV to ACV monophosphate. The combination of 5-fluorodeoxyuridine (FUdR)-BV-araU or FUdR-ACV had a synergistic effect on HSV-1 and HSV-2 replication. The effect of FUdR on the turnover of BV-araU in HSV-1- and HSV-2-infected cells was analyzed by high-performance liquid chromatography. In HSV-1-infected cells, 86% of the BV-araU was phosphorylated to BV-araU triphosphate, but no effect was observed in HSV-2-infected cells, in which 98% of the BV-araU remained as BV-araU monophosphate.