Antiviral agents as adjuncts in cancer chemotherapy

Antiviral Evaluation of Herbal Drugs

The viral infection and resistance to the existing antiviral drugs are alarming, which is a serious public health concern. Medicinal plants are valuable resources for treatment of viral infections and can be used for the management of infections like herpes simplex virus (HSV), human immunodeficiency virus (HIV), influenza, etc. The antiviral screening of plant extracts should be highly selective, specific, and sensitive for bioactivity guided isolation of the active compounds from the plant extracts. The antiviral screening system should be validated for accuracy, reproducibility, simplicity, and cost effectiveness. This chapter highlights on various aspects for screening and evaluation of antiviral natural components including factors affecting antiviral in vivo studies, host cells, organisms, and culture media followed by different virus-specific assays for antiviral screening of natural products.

The potential of nucleotide analogs as inhibitors of retroviruses and tumors

The biologically active form of most purine or pyrimidine analogs is the nucleoside 5'-mono, di- or triphosphate. The nucleoside form is most often administered because of the ease with which it penetrates cells by facilitated transport. However, many nucleoside derivatives fail to exhibit significant antiviral or antitumor activity because they are not phosphorylated by cellular enzymes to the active nucleotide form. In this review, the potential use of suitable nucleotide analogs as selective inhibitors of ribonucleotide reductase and viral reverse transcriptase is considered. Masked nucleotides such as phosphoramidates or methyl phosphates could be employed to allow transport across cellular membranes. Furthermore, phosphonocarboxamide, phosphonoformate or sulfamidophosphoramidate may mimic nucleotide di- and triphosphates. Tumor cells and virally infected cells are often more permeable to nucleotides and their analogs than normal cells, which could provide a therapeutic advantage. There could be considerable therapeutic potential for nucleotide analogs that can penetrate the tumor cell membranes and that are resistant to enzymatic hydrolysis and are non-incorporable into DNA or RNA.

Novel carbocyclic nucleoside analogs suppress glomerular mesangial cells proliferation and matrix protein accumulation through ROS-dependent mechanism in the diabetic milieu

The synthesis of a series of novel 3,4-cis- and 3,4-trans-substituted carbocyclic nucleoside analogs from protected uracil and thymine is described. The key reaction in the followed synthetic protocols utilized the Mitsunobu reaction to couple 3,4-substituted cyclopentanols to (3)N-benzoyl uracil or (3)N-benzoyl thymine. These molecules were evaluated with regard to their ability to treat diabetic nephropathy. Our results show that two analogs significantly reduced high-glucose induced glomerular mesangial cells proliferation and matrix protein accumulation in vitro and, more interestingly, exhibited an anti-oxidative effect suggesting that the activity may be mediated through ROS-dependent mechanism.

Chapter 3 Antiviral drugs: general considerations

The development of an antiviral drug as well as of other drugs is a long process. In most programmes the screening and evaluation start using inhibition of virus multiplication in cell cultures, but in some instances the screening starts in animal models of different viral diseases. In these cases, the mechanism of action has to be analyzed after the in vivo effect has been found. It is not possible to specify precisely the time and resources required in a newly started project to find a compound active against a virus infection but 5-10 years is a reasonable estimation. For some viruses such as herpesviruses, where a number of active inhibitors are already known, the task is simpler than it is to find inhibitors of a virus such as influenza against which only a few active inhibitors have been reported. Evaluation of clinical efficacy in humans is a large and difficult part of the development of an antiviral drug. The number of uncontrolled clinical studies claiming efficacy of different drugs against viral diseases is depressingly large. It is essential to perform double-blind, placebo-controlled and statistically well evaluated studies to be able to judge the clinical efficacy of an antiviral drug. As the knowledge of the detailed natural history and molecular biology of viral diseases and viruses themselves increases, one will obviously have better opportunities to find new drugs. Methods such as X-ray diffraction measurement and NMR determinations will probably lead to a detailed understanding of the structures and interactions taking place at the active site of viral enzymes and their cellular counterparts.

Structure-activity relationships and conformational features of antiherpetic pyrimidine and purine nucleoside analogues. A review

A rational approach to the design of antiherpetic nucleoside analogues is based in part on the broad specificity of virus-coded thymidine kinases. Herpes virus thymidine kinase 'activates' many 5-substituted 2'-deoxyuridines, analogues of thymidine (e.g., idoxuridine, trifluridine, edoxudine, brivudine), 5-substituted arabinofuranosyluracil derivatives (e.g., 5-Et-Ara-U, BV-Ara-U, Cl-Ara-U), acyclonucleosides of guanine (e.g., aciclovir, ganciclovir, penciclovir), and purine nucleosides with the pentafuranosyl ring replaced by a cyclobutane ring (e.g., cyclobut-G, cyclobut-A). Activation involves selective, and frequently regiospecific, phosphorylation of these analogues to the 5'-monophosphates. These are further phosphorylated by cellular enzymes to the 5'-triphosphates, which are usually competitive inhibitors of the viral-coded DNA polymerases. Some analogues are also incorporated into viral, and to a lesser extent cellular, DNA. A recent, unusual, exception is human cytomegalovirus, which does not code for a thymidine kinase, but for a protein with the sequence characteristics of protein kinase and which phosphorylates ganciclovir to its 5'-monophosphate. The interaction of the analogues with cellular catabolic enzymes such as uridine and thymidine nucleoside phosphorylases is also discussed, as is the relationship between physicochemical properties (configuration, conformation, electronic and hydrophobic parameters) and antiviral activities, with particular reference to those drugs that are licensed, or under consideration, for clinical use.

Quantum chemical study of the electronic and conformational characteristics of adenosine and 8-substituted derivatives: functional implications in the mechanism of reaction of adenosine deaminase

A quantum chemical study of 10 substrates of adenosine deaminase is performed. The conformational preference around the glycosidic bond of several 8-substituted derivatives of adenosine is studied using semiempirical modified neglect of diatomic overlap (MNDO) and Austin model 1 (AM1) methods. All the compounds studied show preference for the anti conformation; the syn - anti energetic differences calculated are small and in excellent agreement with experimental data. A relationship between the ab initio molecular electrostatic potential minimum energy of N3 and the syn - anti energetic difference is found. A highly significant relationship is also found between the ab initio net charge over the purine and pyrimidine rings and the logarithm of the maximum rate of deamination (log Vm) of the nucleosides by adenosine deaminase. In contrast, no significant relationship is found between the anti preference of 8-substituted derivatives of adenosine and their log Vm of deamination.

Selective inhibition of the cytopathic effect of type A influenza viruses by oligodeoxynucleotides covalently linked to an intercalating agent

Oligodeoxynucleotides covalently linked to an acridine derivative were targeted to part of the 3'-terminal sequence which is common to the eight RNAs of type A influenza viruses. The cytopathic effect of the virus on MDCK cells in culture was strongly decreased by a heptanucleotide covalently attached to the acridine ring. Control experiments using other oligonucleotide sequences showed that the effect was specific for the complementary sequence of the 3'-terminal region of the viral RNAs. The RNA transcriptase reaction of a type A virus was also selectively inhibited in vitro by the heptanucleotide-acridine conjugate. A type B influenza virus was used as a control. The common sequence at the 3' end of its eight viral RNAs is different from that of type A viruses. Three mismatches were expected with the heptanucleotide which was fully complementary to type A viral RNAs. This heptanucleotide had no effect on the cytopathic effect of a type B influenza virus. These results demonstrate that viral RNAs are specific targets for the oligonucleotide-acridine conjugate that inhibits the cytopathic effect of type A influenza viruses.

Selective inhibition of virus multiplication by new acylated 1,2,4-triazole derivatives

Six out of 99 new acylated 1,2,4-triazole derivatives specifically inhibited rubella virus replication in RK 13 cell cultures. These are the following: 3-methylthio-5-(2-chlorobenzamido)-1H-1,2,4-triazole; 3-methylthio-5-(2-bromobenzamido)-1H-1,2,4-triazole; 3-methylthio-5-(2-methylbenzamido)-1H-1,2,4-triazole; 3-methylthio-5-(2-nitrobenzamido)-1H-1,2,4-triazole; 3-methylthio-5-(2-methylthiobenzamido)-1H-1,2,4-triazole and 3-ethylthio-5-(2-methylbenzamido)-1H-1,2,4-triazole. The compounds did not directly interfere with the infectivity of the rubella virus particles and the antiviral effect was demonstrable only within cells infected with rubella virus. The active compounds did not inhibit the replication of herpes simplex virus type 1, influenza virus and adenovirus in cell culture systems. Structure-activity relationships are discussed.

Conjugates between monoclonal antibodies to HBsAg and cytosine arabinoside

Chemotherapeutic agents such as cytosine arabinoside (ARA-C) and adenine arabinoside (ARA-A) have been shown to eliminate or suppress replication of some DNA viruses. These effects were however associated with systemic side-effects to the treated hosts. We are currently exploring a strategy to construct conjugates between these antiviral agents and monoclonal antibodies directed against viral surface proteins. Such conjugates should enable specific delivery of the antiviral agents to their preferred site of action. In the present study, monoclonal antibodies to hepatitis B virus surface antigen (HBsAg) of the IgG2a and IgM isotypes were conjugated to ARA-C via a dextran bridge. The use of dextran enables the binding of a high number of drug molecules to the antibody with minimal loss of activity. Briefly, partially oxidized dextran was coupled to ARA-C and then to monoclonal anti-HBs. Following the conjugation process, the IgG2a anti-HBs-(dex)-ARA-C conjugate retained its capacity to bind to HBsAg fixed to a solid matrix as compared to non-conjugated homologous antibodies. Conjugates between ARA-C and IgM anti-HBs lost a significant degree of their binding activity to purified HBsAg. However, conjugates containing anti-HBs of both isotypes bound specifically to PLC/PRF/5 human hepatoma cells that express HBsAg on their cell surface. Conjugates containing non relevant monoclonal antibodies did not bind to target cells. Pharmacologic activity of the various compounds was assessed by an [3H]thymidine incorporation assay in hepatoma cells in culture. IgM and IgG2a containing conjugates caused suppression of [3H]thymidine incorporation into PLC/PRF/5 cells. This effect was more pronounced for conjugates containing monoclonal IgM anti-HBs.(ABSTRACT TRUNCATED AT 250 WORDS)

An overview of the role of nucleosides in chemotherapy

A wide variety of nucleosides have been synthesized which interfere with the functions of natural nucleosides at many different loci. These analogues may act as substrates or inhibitors of nucleoside-metabolizing enzymes or may bind to cell membrane receptors. Because of quantitative and qualitative differences in the enzymes and receptors of various tissues and species, the nucleoside analogues have found important uses as antitumor, antiviral, antiparasitic and immunomodulating agents.

Resistance of herpes simplex virus to 9-[[2-hydroxy-1-(hydroxymethyl)ethoxy]methyl]guanine: physical mapping of drug synergism within the viral DNA polymerase locus

A herpes simplex virus type 2 (HSV-2) mutant TS6 (strain HG52) induces a heat-labile viral DNA polymerase at the nonpermissive temperature and is markedly resistant to 9-[[2-hydroxy-1-(hydroxymethyl)ethoxy]methyl]-guanine [2'-nor-2'-deoxyguanosine; 2'NDG]. This antiviral drug requires HSV thymidine kinase for phosphorylation to an active inhibitor (2'NDG-triphosphate), and thymidine kinase-deficient mutants of HSV exhibit varying degrees of resistance to 2'NDG, with the HSV type 1 (HSV-1) B2006 mutant (Kit) being markedly resistant. The ts6 mutation and the 2'ndgR-1 mutation within the viral DNA polymerase locus have been physically mapped by marker rescue and generation of HSV-1/HSV-2 intertypic recombinants. The physical map limits for the ts6 mutation and 2'ndgR-1 mutation are closely linked within a 2.2-kilobase-pair region of DNA sequences and are physically separate from the paaR-1 and acvR-1 mutations. Resistance to 2'NDG by HSV-2 ts6 can be overcome in the presence of combinations of 2'NDG and phosphonoacetic acid, indicating drug synergism within the viral DNA polymerase locus. These physical mapping studies expand the limits of DNA sequences defining an active center in the viral polymerase to 3.5 kilobase pairs, indicating that regions spanning the entire polymerase polypeptide may contribute to a specialized surface able to interact with nucleotides of different structure.

Evaluation of prodrugs of 9-beta-D-arabinofuranosyladenine for therapeutic efficacy in the topical treatment of genital herpesvirus infections in guinea pigs

Prodrugs of the antiviral agent 9-beta-D-arabinofuranosyladenine (araA), which were more effective than the parent compound in penetrating vaginal membranes in vitro, were synthesized and examined for efficacy in the topical treatment of genital infections with herpes simplex virus type 2 in female guinea pigs. Treatment with 10% araA-5'-monophosphate or 10% araA-5'-monovalerate twice a day for 7 days, starting 6 h after intravaginal inoculation with virus, completely aborted the primary infection. When initiation of treatment was delayed until 24 h postinfection, araA-5'-monophosphate and araA-5'-monovalerate were no longer effective in reducing the mean lesion scores or mean vaginal virus titers. Treatment with 5% acyclovir, starting at 24 h postinfection, failed to prevent genital lesion development but significantly reduced the peak mean lesion score (approximately 50%). Topical therapy with 10% araA-2',3'-diacetate, initiated at 24 h postinfection, was as effective as, if not more effective than, acyclovir in reducing the severity of herpes genitalis in guinea pigs. Treatment with 10% araA-2',3'-dipropionate or 10% araA-2',3'-dibutyrate was without benefit. Among a series of 5'-monoesters of araA, araA-5'-monobutyrate appeared to be the most effective but was less active than araA-2',3'-diacetate. These data indicate that araA-2',3'-diacetate may be an effective antiviral agent for topical use against genital herpesvirus infections.

Comparison of the efficacy of vidarabine, its carbocyclic analog (cyclaradine), and cyclaradine-5'-methoxyacetate in the treatment of herpes simplex virus type 1 encephalitis in mice

The relative therapeutic effects of vidarabine (9-beta-D-arabinofuranosyladenine), cyclaradine (the adenosine deaminase-resistant carbocyclic analog of vidarabine), and cyclaradine-5'-methoxyacetate in the parenteral treatment of systemic herpes simplex virus type 1 infections in Swiss mice were determined. Among control mice inoculated intraperitoneally with virus, a mortality rate of 95% was observed. The intraperitoneal administration of nontoxic doses of vidarabine (125 to 250 mg/kg per day) or cyclaradine (113 to 450 mg/kg per day), by daily injections for 7 days beginning 4 h after virus inoculation, reduced mortality to 0 to 10%. Among control animals inoculated intracerebrally with 32 50% lethal doses of virus, 100% mortality was observed, with a mean survival time of 4.6 days. Treatment with either drug at equimolar dose levels ranging from ca. 32 to 750 mg/kg per day produced significant (P less than 0.0005), dose-dependent increases in the mean survival time of animals dying of herpesvirus encephalitis. Mice inoculated intracerebrally with 10 50% lethal doses of virus exhibited 97% mortality and a mean survival time of 5.5 to 6.4 days. Treatment with vidarabine, cyclaradine, or cyclaradine-5'-methoxyacetate significantly increased the mean survival time of dying animals and, at doses ranging from 250 to 750 mg/kg per day, produced significant increases in survival. The three drugs displayed equivalent antiviral efficacy in vivo. Drug toxicity (measured by weight loss) was not detected in mice treated with cyclaradine or cyclaradine-5'-methoxyacetate at 750 mg/kg per day, whereas severe toxicity (weight loss of greater than or equal to 3 g) was observed in mice treated with vidarabine at an equivalent dose level. Thus, cyclaradine or its 5'-methoxyacetic acid ester may possess some advantage over vidarabine in the treatment of severe herpesvirus infections and should therefore be considered for clinical trials in humans.

Carbocyclic arabinofuranosyladenine (cyclaradine): efficacy against genital herpes in guinea pigs

Carbocyclic arabinofuranosyladenine (cyclaradine), a novel nucleoside analog with such desired features as hydrolytic and enzymatic stability, adenosine deaminase resistance, and low systemic toxicity, inhibited the replication of herpes simplex virus types 1 and 2. The 5'-methoxyacetate prodrug form exhibited significant efficacy in the topical treatment of genital infections by herpes simplex virus type 2.

Different in vitro effects of dual combinations of anti-herpes simplex virus compounds

Five promising antivirals have been tested individually and in pairs on herpes simplex virus (HSV) types 1 (Strain F) and 2 (Strain G) in Vero cells. These are: 9-(2-hydroxyethoxymethyl)guanine (acyclovir, ACV), 9-beta-D-arabinofuranosyladenine (ara-A), 5-trifluorothymidine (TFT), E-5-(2-bromovinyl)-2'-deoxyuridine (BVDU), and phosphonoformate (PFA). Various types of interactions depending on virus type were observed: synergistic [ara-A/TFT; ara-A/BVDU (G); BVDU/PFA (F)]; additive [ACV/ara-A; ACV/TFT; ACV/BVDU; ACV/PFA (G); BVDU/TFT; PFA/ara-A; PFA/TFT]; and sub-additive [ACV/PFA (F); ara-A/BVDU (F) and BVDU/PFA (G)]. Neither antagonism nor interference was noted for any combinations.

Combined interaction of antiherpes substances and interferon beta on the multiplication of herpes simplex virus

The inhibition of herpes simplex virus type 1 (strain McIntyre) by two anti-herpes substances applied in combination was studied by means of a CPE inhibition method In Vero cell microcultures. The following substances were studied: AraA monophosphate, thymine arabinoside, ethyldeoxyuridine, acycloguanosine (acyclovir). 3-fluorothymidine, phosphonoformic acid. Most substance pairs showed a moderate degree of synergism. Two additive interactions and one case of antagonism was observed. Human fibroblast interferon, combined with the antiherpes substances, showed an additive interaction with AraAMP, but synergism with the others.

Inhibition of reverse transcriptase activity of avian myeloblastosis virus by pyrophosphate analogues

Several pyrophosphate analogues have been studied for their effects on avian myeloblastosis virus reverse transcriptase and on cellular DNA polymerase alpha. Examination of structure-activity relationships for these compounds revealed that two acidic groups connected by a short bridge were necessary, but not sufficient, for inhibition of the enzyme activities. Foscarnet sodium (trisodium phosphonoformate) was the most potent inhibitor of reverse transcriptase, giving non-competitive inhibition of reactions primed by (rA)n . (dT)12-18, (rC)n . (dG)12-18, (dC)n . (dG)12-18, and activated DNA. Carbonyldiphosphonate and 2-hydroxyphosphonoacetate also caused non-competitive inhibition patterns, whereas hypophosphate and imidodiphosphonate inhibited AMV reverse transcriptase in a competitive, non-linear manner. The reverse transcriptase reactions directed by (rA)n . (dT)12-18 and activated DNA were most affected by the non-competitive inhibitors. Hypophosphate and imidodiphosphonate inhibited preferentially reactions primed by (dC)n . (dG)12-18 and activated DNA. In all cases the (rC)n . (dG)12-18 directed reaction was the least affected.

Lack of efficacy of 2-deoxy-D-glucose in the treatment of experimental herpes genitalis in guinea pigs

Topical treatment of herpes genitalis in female guinea pigs with 2-deoxy-D-glucose in either agarose gels or miconazole nitrate ointments failed to prevent the development of genital lesions or to reduce the mean titers of recoverable virus in vaginal swabs from infected animals. In contrast, phosphonoacetic acid was therapeutically effective.

Inhibition of lymphocyte-mediated cytolysis and cyclic AMP phosphodiesterase by erythro-9-(2-hydroxy-3-nonyl)adenine

The adenosine deaminase (ADA) inhibitor erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA), at low concentrations (less than 10 microM), enhances the inhibitory activity of adenosine against lymphocyte-mediated cytolysis (LMC) without itself being inhibitory. At higher concentrations, EHNA alone is inhibitory to LMC with an IC50 of 160 microM. This inhibition is reversible upon washout, appears to affect an early stage of the lytic process, and does not appear to involve changes in basal levels of cyclic AMP (cAMP), ribonucleoside 5'-triphosphate pool sizes, S-adenosylhomocysteine levels, or protein carboxymethylation. EHNA does enhance the cAMP response of cytolytic lymphocytes (CL) to activators of adenylate cyclase such as prostaglandin E1. EHNA inhibits lymphocyte high-affinity cAMP phosphodiesterase at immunosuppressive levels, exhibiting hyperbolic mixed-type inhibition (Ki = 83 microM, alpha = 0.47, beta = 0.18). Whereas inhibition of intralymphocytic ADA is complete at low concentrations (less than 25 microM) of EHNA, inhibition of LMC and intralymphocytic cAMP phosphodiesterase increases linearly with EHNA concentration to at least 200 microM. The presence of 200 microM EHNA during the centrifugation of mixtures of CL and EL4 leukemia target cells leads to increased CL cAMP levels. 2'-Deoxycoformycin, a more potent ADA inhibitor than EHNA, is not inhibitory to LMC and shows none of these cAMP-related effects. These results suggest that CL-target cell contact stimulates adenylate cyclase in the CL and that EHNA inhibits LMC due to its enhancement of this target cell-stimulated elevation of cAMP.

Comparative effects of 5'-amino-5'-deoxythymidine, a new antiviral agent, and 5-iodo-2'-deoxyuridine on the immune response of mice. No immunosuppression with 5'-amino-5'-deoxythymidine

Immunological function was analyzed in mice that received daily inoculations of 5'-amino-5'-deoxythymidine (1000 mg/kg/day, i.p.) or 5-iodo-2'-deoxyuridine (100 mg/kg/day, i.p.) following antigenic stimulation. 5' Amino-5'-deoxythymidine did not suppress the development of: (1) delayed type hypersensitivity response to sheep red blood cells, (2) cell-mediated cytotoxicity response to allogeneic tumor cells, and (3) IgM and IgG antibody response to sheep red blood cells. In contrast 5-iodo-2'-deoxyuridine suppressed the development of all of these responses.