Antiviral activity of Australian tea tree oil and eucalyptus oil against herpes simplex virus in cell culture

The antiviral effect of Australian tea tree oil (TTO) and eucalyptus oil (EUO) against herpes simplex virus was examined. Cytotoxicity of TTO and EUO was evaluated in a standard neutral red dye uptake assay. Toxicity of TTO and EUO was moderate for RC-37 cells and approached 50% (TC50) at concentrations of 0.006% and 0.03%, respectively. Antiviral activity of TTO and EUO against herpes simplex virus type 1 (HSV-1) and herpes simplex virus type 2 (HSV-2) was tested in vitro on RC-37 cells using a plaque reduction assay. The 50% inhibitory concentration (IC50) of TTO for herpes simplex virus plaque formation was 0.0009% and 0.0008% and the IC50 of EUO was determined at 0.009% and 0.008% for HSV-1 and HSV-2, respectively. Australian tea tree oil exhibited high levels of virucidal activity against HSV-1 and HSV-2 in viral suspension tests. At noncytotoxic concentrations of TTO plaque formation was reduced by 98.2% and 93.0% for HSV-1 and HSV-2, respectively. Noncytotoxic concentrations of EUO reduced virus titers by 57.9% for HSV-1 and 75.4% for HSV-2. Virus titers were reduced significantly with TTO, whereas EUO exhibited distinct but less antiviral activity. In order to determine the mode of antiviral action of both essential oils, either cells were pretreated before viral infection or viruses were incubated with TTO or EUO before infection, during adsorption or after penetration into the host cells. Plaque formation was clearly reduced, when herpes simplex virus was pretreated with the essential oils prior to adsorption. These results indicate that TTO and EUO affect the virus before or during adsorption, but not after penetration into the host cell. Thus TTO and EUO are capable to exert a direct antiviral effect on HSV. Although the active antiherpes components of Australian tea tree and eucalyptus oil are not yet known, their possible application as antiviral agents in recurrent herpes infection is promising.     

Synergistic antiviral activity of acyclovir and vidarabine against herpes simplex virus types 1 and 2 and varicella-zoster virus

Acyclovir and vidarabine both exhibit anti-herpetic activity. Because different mechanisms of action of vidarabine and acyclovir have been reported, we analyzed their combined anti-herpetic activity on plaque formation of herpes simplex virus (HSV)-1, HSV-2, and varicella-zoster virus (VZV) by isobolograms. The results indicate that acyclovir and vidarabine have a synergistic effect on wild type HSV-1, HSV-2, and VZV. The susceptibility of thymidine kinase-deficient HSV-1 to vidarabine was not affected by the presence of acyclovir, suggesting that phosphorylation of acyclovir is essential for synergism. The combined anti-HSV activity of acyclovir and vidarabine against phosphonoacetic acid-resistant HSV-1 with DNA polymerase mutation did not show synergism in contrast to that against wild-type herpesviruses. Alteration of the substrate specificity of viral DNA polymerase to acyclovir and vidarabine annihilated the synergism. Thus, the nature of their binding sites on DNA polymerase is important to the synergistic anti-herpesvirus activity of acyclovir and vidarabine.     

Inhibitory effect of cycloSaligenyl-nucleoside monophosphates (cycloSal-NMP) of acyclic nucleoside analogues on HSV-1 and EBV

The in vitro antiviral activity of a new series of cycloSal-pro-nucleotides derived from the acyclic nucleoside analogues aciclovir and penciclovir against herpes simplex virus type 1 (HSV-1), thymidine kinase deficient (TK-) HSV-1, and Epstein-Barr virus (EBV) was evaluated. Using the XTT-based tetrazolium reduction assay EZ4U, the cycloSal derivatives were examined for their antiviral and cytotoxic effects in HSV-1 as well as HSV-1-TK--infected Vero cells. The anti-EBV activity was assessed by means of an EBV DNA hybridization assay using a digoxigenin-labeled probe specific for the Bam H1-W-fragment of the EBV genome and by measuring viral capsid antigen (VCA) expression in P3HR-1 cells by indirect immunofluorescence. Among the new cycloSal-phosphotriesters the three aciclovir monophosphates proved to be potent and selective inhibitors of HSV-1 replication, EBV DNA synthesis and EB-VCA expression. Of interest is the retention of activity of the aciclovir monophosphates in HSV-1-TK--infected cells. Particularly 3-methyl-cycloSal-aciclovir monophosphate retained the same effectiveness, as compared to the wild type virus strain. In contrast to the aciclovir pro-nucleotides the penciclovir cycloSal-phosphotriesters exhibited at best only a marginal antiviral effect on HSV and EBV replication.     

Virucidal activity of a beta-triketone-rich essential oil of Leptospermum scoparium (manuka oil) against HSV-1 and HSV-2 in cell culture

The inhibitory activity of manuka oil against Herpes simplex virus type 1 (HSV-1) and Herpes simplex virus type 2 (HSV-2) was tested in vitro on RC-37 cells (monkey kidney cells) using a plaque reduction assay. In order to determine the mode of antiviral action of the essential oil, manuka oil was added at different times to the cells or viruses during the infection cycle. Both HSV types were significantly inhibited when the viruses were pretreated with manuka oil 1 h prior to cell infection. At non-cytotoxic concentrations of the essential oil, plaque formation was significantly reduced by 99.5 % and 98.9 % for HSV-1 and HSV-2, respectively. The 50 % inhibitory concentration (IC (50)) of manuka oil for virus plaque formation was determined at 0.0001 % v/v ( = 0.96 microg/mL) and 0.00006 % v/v ( = 0.58 microg/mL) for HSV-1 and HSV-2, respectively. On the other hand, pretreatment of host cells with the essential oil before viral infection did not affect plaque formation. After virus penetration into the host cells only replication of HSV-1 particle was significantly inhibited to about 41 % by manuka oil. Flavesone and leptospermone, two characteristic ss-triketones of manuka oil, inhibited the virulence of HSV-1 in the same manner as the essential oil itself. When added at non-cytotoxic concentrations to the virus 1 h prior to cell infection, plaque formation was reduced by 99.1 % and 79.7 % for flavesone and leptospermone, respectively.     

Synthesis and biological activity of dansyl thymidines

The synthesis of thymidines selectively dansylated in the 3'- and 5'-positions is reported. The biological investigation showed that these fluorescent nucleosides behave as competitive inhibitors of thymidine kinase (TK) from herpes simplex virus (HSV) and are endowed with a certain degree of antiviral activity against HSV-2 and HSV-1.     

Inactivation of HSV-1 and HSV-2 and prevention of cell-to-cell virus spread by Santolina insularis essential oil

The essential oil obtained in toto from Santolina insularis was investigated for its antiviral activity on herpes simplex type 1 (HSV-1) and type 2 (HSV-2) in vitro. The IC(50) values, determined by plaque reduction assays, were 0.88 and 0.7 microg/ml for HSV-1 and HSV-2, respectively, while the CC(50) determined by the MTT test on Vero cells was 112 microg/ml, indicating a CC(50)/IC(50) ratio of 127 for HSV-1 and 160 for HSV-2. Results obtained by plaque reduction assays also indicated that the antiviral activity of S. insularis was principally due to direct virucidal effects. Antiviral activity against HSV-1 and HSV-2 was not observed in a post-attachment assay, and attachment assays indicated that virus adsorption was not inhibited. Up to 80% inhibition of HSV-1 was achieved at the concentration of 40 microg/ml by yield reduction assay. Furthermore, reduction of plaque formation assays also showed that S. insularis essential oil inhibits cell-to-cell transmission of both HSV-1 and HSV-2.     

Efficacy of 2'-nor-cyclicGMP in treatment of experimental herpes virus infections

9-[(2-Hydroxy-1,3,2-dioxaphosphorinan-5-yl)oxymethyl]guanine P-oxide (2'-nor-cGMP), the cyclic phosphate of 2'-nor-deoxyguanosine (2'-NDG) was synthesized by phosphorylation of 2'-NDG and evaluated for antiherpetic activity in cell cultures and in animal protection studies. 2'-nor-cGMP was effective in cell culture against both thymidine kinase deficient and wild-type herpes simplex virus type 1 strains and also against herpes simplex virus type 2. The anti-herpes activity of 2'-nor-cGMP against thymidine kinase deficient HSV-1 was confirmed by animal protection studies. Also, in comparative cell culture protection studies, the ED50 (microM) of 2'-nor-cGMP was approximately 10-fold lower than that of 2'-NDG against three strains of varicella zoster virus. In addition, 2'-nor-cGMP was effective orally in preventing HSV-1 orofacial infection and HSV-2 genital infection of mice. Topical therapeutic applications of 2'-nor-cGMP prevented orofacial HSV-1 lesion development in mice and development of HSV-2 genital lesions in guinea pigs. Subcutaneous application of 2'-nor-cGMP to intracerebral HSV-1 challenged weanling mice significantly prolonged survival. These studies indicate that 2'-nor-cGMP is not dependent on viral thymidine kinase for its antiviral activity and is highly effective in preventing experimental HSV infections.     

Synthesis and antiviral evaluation of carbocyclic analogues of 2-amino-6-substituted-purine 3'-deoxyribofuranosides

Carbocyclic analogues of 2-amino-6-substituted-purine 3'-deoxyribofuranosides were synthesized by beginning with (+/-)-(1 alpha,3 alpha,4 beta)-3-amino-4-hydroxycyclopentanemethanol and 2-amino-4,6-dichloropyrimidine. The route parallels the earlier syntheses of the corresponding ribofuranoside and 2'-deoxyribofuranoside analogues. The 2-amino-6-chloropurine, guanine, and 2,6-diaminopurine derivatives and the analogous 8-azapurines were prepared. The analogue (3'-CDG) of 3'-deoxyguanosine is active in vitro against a strain of type 1 herpes simplex virus (HSV-1) that induces thymidine kinase and is modestly active against a thymidine kinase inducing strain of type 2 HSV. 3'-CDG is not active against a strain of HSV-1 that lacks the thymidine kinase inducing capacity, whereas the carbocyclic analogue of 2-amino-6-chloropurine 3'-deoxyribofuranoside is active against that strain. The carbocyclic analogue of 2,6-diaminopurine 3'-deoxyribofuranoside displayed modest activity in vitro against influenza virus.     

Antiviral activity of cyclosaligenyl prodrugs of the nucleoside analogue bromovinyldeoxyuridine against herpes viruses

A series of 42 lipophilic bromovinyldeoxyuridine monophosphates (BVDUMPs) are presented as potential prodrugs of the antiviral agent (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU). The 5'-cycloSal-masking group technique has been applied to this cyclic nucleoside analogue to achieve delivery of the monophosphate of BVDU inside the target cells. The new substances have been tested for their antiviral activity against herpes simplex virus types 1 and 2 (HSV-1 and -2), thymidine kinase-deficient (TK(-)) HSV-1, varicella-zoster virus (VZV), human cytomegalovirus (HCMV) and Epstein-Barr virus (EBV). The XTT-based tetrazolium reduction assay EZ4U (for HSV), the plaque inhibition test (for VZV and HCMV) and a DNA hybridisation assay (for EBV) were used to assess antiviral activity. The results indicate that cycloSal-BVDUMP triesters proved to be potent and selective inhibitors of HSV-1 comparable with aciclovir. VZV replication was inhibited by very low concentrations, and two substances had a slightly better anti-VZV activity than the parent compound BVDU. No antiviral effect could be demonstrated against TK(-)-HSV-1, HSV-2 and HCMV, most likely owing to the lack of phosphorylation to BVDU diphosphate. Most remarkably, several cycloSal-BVDUMP triesters yielded promising anti-EBV activity whereas the parent compound BVDU was entirely inactive.     

Efficacy of ASP2151, a helicase-primase inhibitor, against thymidine kinase-deficient herpes simplex virus type 2 infection in vitro and in vivo

ASP2151 was developed as a novel inhibitor of herpes simplex virus (HSV) and varicella-zoster virus helicase-primase. The anti-HSV activity of ASP2151 toward a clinical HSV isolate with acyclovir (ACV)-resistant/thymidine kinase (TK)-deficiency was characterized in vitro and in vivo using a plaque reduction assay and the ear pinna infection in mice. The IC₅₀ ranged from 0.018 to 0.024 μg/ml, indicating the susceptibility of TK-deficient HSV-2 was similar to that of wild-type HSV-2 strains. Anti-HSV activity of ASP2151 in vivo was evaluated in mice infected with wild-type HSV-2 and TK-deficient HSV-2. ASP2151 significantly reduced the copy numbers of wild-type HSV-2 and TK-deficient HSV-2 at the inoculation ear pinna, while valacyclovir significantly reduced the copy number of wild type HSV-2 but not that of TK-deficient HSV-2 in the inoculated ear pinna. Thus, ASP 2151 showed therapeutic efficacy in mice infected with both wild-type and TK-deficient HSV-2. In conclusion, ASP2151 is a promising novel herpes helicase-primase inhibitor that indicates the feasibility of ASP2151 for clinical application for the treatment of HSV infections, including ACV-resistant/TK-deficient HSV infection.     

Broad-spectrum antiviral activity of the acyclic guanosine phosphonate (R,S)-HPMPG

(R,S)-9-(3-hydroxy-2-phosphonomethoxypropyl)guanine [(R,S)-HPMPG] exhibits broad spectrum antiviral activity with an ED50 of less than 1 microM against herpes simplex virus (HSV) types 1 and 2, varicella zoster virus, human cytomegalovirus (HCMV) and vaccinia in plaque reduction assays. Wild type HSV-2 and its thymidine kinase deficient variant are equally sensitive to (R,S)-HPMPG. (R,S)-HPMPG is 100-fold more potent than acyclovir (ED50 = 0.45 microM vs. 44 microM, respectively) against HCMV in cell culture, and 10-fold more active than acyclovir in extending survival time in mice intraperitoneally infected with 70 LD50 HSV-1. However, (R,S)-HPMPG is toxic when administered repeatedly at 44 mg/kg/day in uninfected adult mice. The diphosphoryl derivative of HPMPG was enzymatically synthesized and is a competitive inhibitor of HSV-1 DNA polymerase relative to dGTP (K1 = 0.03 microM). HPMPG-PP is 70-fold less active at inhibiting HeLa DNA polymerase alpha than HSV-1 DNA polymerase. At concentrations between 0.3 and 1.5 microM (R,S)-HPMPG inhibited HSV-1 DNA replication greater than or equal to 50% in infected cells as measured by nucleic acid hybridization. Consistent with inhibition of viral DNA synthesis, 6 to 30 microM (R,S)-HPMPG reduces late viral polypeptide synthesis in HSV-1 infected cells. These data indicate that (R,S)-HPMPG is a thymidine kinase independent broad spectrum antiviral drug which is capable of inhibiting viral DNA polymerase.     

Potential prophylactic and therapeutic vaccines for HSV infections

The human herpesviruses herpes simplex virus types 1 and 2 (HSV-1 and HSV-2) can cause severe recurrent disease in humans and establish lifelong infection in their hosts. Several antiviral therapies are available to control disease and spread, but these are not completely effective and do not affect latent virus. The need for vaccines for HSV is urgent, both for controlling initial infection and spread of disease as well as to limit recurrences. Several approaches including subunit vaccines, peptide vaccines, live virus vectors and DNA vaccine technology have been used in developing both prophylactic and therapeutic vaccines for HSV and these are reviewed here.     

Distinct thymidine kinases encoded by cowpox virus and herpes simplex virus contribute significantly to the differential antiviral activity of nucleoside analogs

Orthopoxviruses and herpesviruses are both large enveloped DNA viruses, yet these virus families exhibit very different susceptibilities to antiviral drugs. We investigated the activation of nucleoside analogs by the types I and II thymidine kinase (TK) homologs expressed by herpes simplex virus type 1 (HSV-1) and cowpox virus (CV). Antiviral activity against TK(-) and TK(+) strains of HSV-1 and CV was determined, and the ratio of the EC(50) values was used as a measurement of TK dependence. As to HSV-1, most of the selected compounds were markedly less effective against the TK(-) strains, suggesting that this enzyme was required for the activation of these nucleoside analogs. This differs from the results for CV where only idoxuridine and bromodeoxyuridine appeared to be activated, putatively by the type II TK expressed by this virus. These data confirm that the type II TK encoded by CV exhibits a more limited substrate specificity than the type I TK encoded by HSV-1. These data suggest that the inefficient activation of nucleoside analogs by the orthopoxvirus TK significantly limits their activity. Additional screening against orthopoxviruses will be required to identify nucleoside analogs that are efficiently activated by their type II TK.     

Prophylaxis against herpesvirus infections in transplant recipients

Herpesvirus infections are important after stem cell and organ transplant. During the last decades several antiviral agents have been introduced with efficacy against herpesviruses. These agents are the nucleoside analogues aciclovir, valaciclovir, famciclovir, and ganciclovir; the nucleotide analogue cidofovir; and the pyrophosphate analogue foscarnet. Several studies have been performed with antiviral agents with the aim to reduce morbidity and mortality associated with herpesvirus infections in transplant recipients. Aciclovir and valaciclovir have been examined in randomised, controlled trials in both solid organ and stem cell transplant patients, and were shown to be very effective for the prevention of herpes simplex virus (HSV) and varicella-zoster virus infections. In addition, these drugs were shown to reduce cytomegalovirus (CMV) infection and improve survival in allogenic stem cell transplant patients and to reduce CMV infection, CMV disease (aciclovir and valaciclovir), and acute rejection (valaciclovir) in renal transplant patients. Ganciclovir is very effective for the prevention of CMV infection and disease in both stem cell and solid organ transplant recipients. It can also be used in preemptive strategies in which the aim is to prevent CMV disease in patients who have ongoing CMV infection documented by antigenaemia or detection of CMV DNA. The latter strategy has the advantage of reducing the exposure to the drug and thereby the risk for toxicity. Foscarnet has also been shown to be effective as preemptive therapy for CMV in allogenic stem cell transplant patients and as therapy for aciclovir-resistant HSV infections. Finally cidofovir is an interesting agent with broad spectrum antiherpesvirus efficacy. However, because of the drug's toxicity profile, further studies are needed.     

A wide range of medium-sized, highly cationic, alpha-helical peptides show antiviral activity against herpes simplex virus

Ten highly cationic, alpha-helical peptides were synthesized and tested for antiviral activity against herpes simplex virus 1 and 2 (HSV-1 and HSV-2). Several of the peptides were found to exhibit antiviral activity. The peptides affinity for heparan sulfate (HS) increased with the number of cationic residues. Net charge could be decisive for the anti-HSV-1 activity, while secondary structure of the peptides seems more important for the anti-HSV-2 activity. The peptides were able to inhibit the entry of HSV-1 into the host cell, probably by blocking HS at the cell surface. HSV plaque formation was inhibited in a dose-dependent manner when cells were exposed to the peptides prior to the addition of virus. Lower inhibition activity was observed when the virus was allowed to attach to the cell surface before the addition of peptide. However, the plaque size was smaller compared to the untreated control, indicating that the peptides may also interfere with cell-to-cell spread of the virus. The two most potent antiviral peptides exhibited synergy with acyclovir against HSV.     

Famciclovir, from the bench to the patient--a comprehensive review of preclinical data

Famciclovir is converted rapidly and efficiently after oral administration to the selective antiviral compound, penciclovir. In cell culture, penciclovir is a potent inhibitor of herpes simplex virus (HSV) types 1 and 2, varicella-zoster virus (VZV), Epstein-Barr virus (EBV) and hepatitis B virus (HBV). Phosphorylation of penciclovir and aciclovir in uninfected cells is limited, and penciclovir, like aciclovir, has minimal effect on replicating cells in culture as expected for a selective antiviral agent. Mode of action studies with VZV and HSV have shown that the phosphorylation of penciclovir in infected cells is far more efficient than for aciclovir. This compensates for differences observed between penciclovir triphosphate and aciclovir triphosphate in the inhibition of HSV and VZV DNA polymerases. Because HBV is not known to encode a thymidine kinase, a different rationale for the selective inhibition of this virus by penciclovir is required. Recent data indicate that the DNA polymerase of HBV is far more sensitive to inhibition by penciclovir triphosphate than cellular DNA polymerases, suggesting that for this virus, selectivity operates at the DNA polymerase. Penciclovir triphosphate is more stable within infected cells than aciclovir triphosphate, and consequently penciclovir has more prolonged antiviral activity than aciclovir. Similarly, famciclovir is more effective than aciclovir or valaciclovir in suppressing HSV replication when given at a lower dosing frequency in certain animal models. These preclinical properties have helped to provide the foundation for the famciclovir clinical programme.     

Isolation and characterization of an anti-HSV polysaccharide from Prunella vulgaris

A water soluble substance was isolated from a Chinese herb, Prunella vulgaris, by hot water extraction, ethanol precipitation and gel permeation column chromatography. Chemical tests showed that the substance was an anionic polysaccharide. Using a plaque reduction assay, the polysaccharide at 100 microg/ml was active against the herpes simplex virus types 1 and 2 (HSV-1 and HSV-2), but was inactive against cytomegalovirus, the human influenza virus types A and B, the poliovirus type 1 or the vesicular stomatitis virus. The 50% plaque reduction dose of the polysaccharide for HSV-1 and HSV-2 was 10 microg/ml. Clinical isolates and known acyclovir-resistant (TK-deficient or polymerase-defective) strains of HSV-1 and HSV-2 were similarly inhibited by the polysaccharide. Pre-incubation of HSV-1 with the polysaccharide at 4, 25 or 37 degrees C completely abrogated the infectivity of HSV-1, but pre-treatment of Vero cells with the polysaccharide did not protect cells from infection by the virus. The addition of the polysaccharide at 0, 2, 5.5 and 8 h post-infection of Vero cells with HSV-1 at a multiplicity of infection (MOI) of five reduced the 20 h-yield of intracellular infectious virus by 100, 99, 99 and 94%, respectively. In contrast, a similar addition of heparin showed 85, 63, 53 and 3% reduction of intracellular virus yield, respectively. These results suggest that the polysaccharide may inhibit HSV by competing for cell receptors as well as by some unknown mechanisms after the virus has penetrated the cells. The Prunella polysaccharide was not cytotoxic to mammalian cells up to the highest concentration tested, 0.5 mg/ml and did not show any anti-coagulant activity. In conclusion, the polysaccharide isolated from P. vulgaris has specific activity against HSV and its mode of action appears to be different from other anionic carbohydrates, such as heparin.     

In vitro antiviral activity of aphidicolin and its derivates. Synergistic effects of aphidicolin with other antiviral drugs

Twenty derivatives of aphidicolin were tested against HSV (herpes simplex virus), HCMV (human cytomegalovirus) and adenovirus in vitro. In addition, the antiviral activity of aphidicolin (CAS 38966-21-1) in combination with aciclovir (CAS 59277-89-3) or cidofovir (CAS 113852-37-2) against HSV was determined. The antiviral effects were evaluated using plaque reduction assay in Vero cells or human Foreskin Fibroblasts (HFF) for HSV and HCMV, respectively. Combination indexes were calculated using the method of Chou and Talalay. Two derivatives (K14254 and K14266) that are considered to be prodrugs of aphidicolin were shown to inhibit HCMV and HSV replication comparably to aphidicolin. None of the tested substances inhibited adenovirus replication. Aphidicolin acted synergistically with aciclovir in a 1:1 molar ratio and with cidofovir in different ratios. Aphidicolin and its two antiviral active derivatives might represent useful additional tools for antiviral therapy of HSV and HCMV infections, especially in combination with clinically used drugs.     

New advances in anti-HSV chemotherapy

Treatment of human herpes simplex virus (HSV) diseases represents an important goal, as herpetic infections are not controlled by vaccination. Many therapeutic agents have been developed and used for HSV infections and several alternative natural compounds are under investigation. Most of the drugs clinically employed against HSV types 1 and 2 are represented by guanosine nucleoside analogues, such as aciclovir and aciclovir-like drugs. The emergence of aciclovir-resistant virus strains provided a stimulus for increased search of new effective agents. Alternative drugs are other nucleoside analogues, such as the vidarabine, brivudin, and cidofovir, or pyrophosphate analogues such as foscarnet, that showed efficacy for HSV infections refractory to aciclovir. However, the risk of adverse effects reported for available anti-herpetic compounds and the frequent development of drug-resistant strains of HSV following therapeutic treatment generate the need for new antiviral agents. In the last years, several studies have been carried out on the anti-HSV activity of different components of innate host defences such as cationic antimicrobial peptides. The antiviral activity of these peptides often appears to be related to the viral adsorption and entry process or is a result of a direct effect on the viral envelope. Other natural compounds, extracts from medicinal plants employed in ethnomedicine and displaying marked anti-herpetic activity, are at present under investigation to determine the scientific evidence and rationale for their clinical use. This review discusses the anti-HSV activity of compounds licensed for clinical use and promising natural molecules.     

Anti-herpesvirus activity of citrusinine-I, a new acridone alkaloid, and related compounds

Citrusinine-I, a new acridone alkaloid isolated from the root bark of the citrus plant (Rutaceae), exhibited potent activity against herpes simplex virus (HSV) type 1 and type 2 at low concentrations relative to their cytotoxicity; 50% effective concentrations (ED50) of citrusinine-I were 0.56 micrograms/ml and 0.74 micrograms/ml against HSV-1 and HSV-2, respectively. Inhibitory action was also demonstrated against cytomegalovirus (CMV) and thymidine kinase-deficient or DNA polymerase mutants of HSV-2. The compound markedly suppressed HSV-2 and CMV DNA synthesis at concentrations which did not inhibit the synthesis of virus-induced early polypeptides. However, citrusinine-I had no inhibitory activity against HSV and CMV DNA polymerases in cell-free extracts. Although the target of this inhibitor remains to be elucidated, the most plausible candidate is a virus-coded ribonucleotide reductase. Citrusinine-1, when combined with acyclovir or ganciclovir, synergistically potentiated the antiherpetic activity of these agents. Based on a comparative study of the antiherpetic activity of citrusinine-1 and 28 related compounds, a structure-activity relationship could be established.