Antiviral activities

The development of viral resistance towards antiviral agents enhances the need for new compounds active against viral infections, and therefore natural products may offer a new source of antiviral agents [124]. 

EO of Melaleuca alternifolia and eucalyptus exhibited a high level of antiviral activity against Herpes simplex virus type 1 (HSV-1) and Herpes simplex virus type 2 (HSV-2) in a viral suspension test [125]. Also, Santolina insularis EO 

Certain quassinoids display in vitro antiviral activity, namely against the oncogenic Rous sarcoma virus 79). This test is performed as follows Chick-embryo fibroblasts are infected by a known amount of the virus which transforms the morphology of the cells into foci. A number of quassinoids inhibit this transformation at concentration ranging from 0.15 to 1 pg/ml, without having toxic effects on normal cells. Some of the results are shown in Table 2. 

While this inhibition does not guarantee antileukemic activity, compounds which do not display this antitransforming behaviour are invariably inactive. 

Virus replication comprises numerous biochemieal transformations that might provide suitable targets for antiviral therapy. The antiviral effect of thiosemicarbazones was first demonstrated by Hamre et al. [53, 54], who showed that p-aminobenzaldehyde-3-thiosemicarbazone and several of its derivatives were active against vaccinia virus in mice. These studies were extended to include thiosemicarbazones of isatin, benzene, thiophene, pyridine, and quinoline derivatives, which also showed activity against vaccinia-induced encephalitis. The nature of the aldehyde/ketone moiety was not as significant as the presence of the thiosemicarbazide side chain the latter was deemed essential for antiviral activity. 

The structure/activity relationships for the methisazone, 3a, derivatives against adenoviruses and poxviruses have been shown to be similar [78]. Pearson and Zimmerman [79] demonstrated that all three types of polioviruses are inhibited by 2-acetylpyridine JV-dibutylthiosemicarbazone, which is similar to 3a, by blocking viral RNA synthesis. A 3-substituted triazinoindole derivative of isatin was effective against several strains of rhinovirus in tissue culture the mechanism of action is unknown [80]. 

In contrast to the effects obtained with viruses mentioned earlier, rous sarcoma virus (RSV) is inactivated by direct contact with 2 [81]. Evidence for the drug action by a chelate compound was obtained by using concentrations of 3a and copper(II) sulfate, neither of which individually affected enzyme activity or transforming abilities [82]. In a later study these workers showed that several metal complexes inhibit the RNA dependent DNA polymerases and the transforming ability of RSV, the most active compound being a 1 1 copper(II) 

A sulfate prepared by chlorosulfonation of GE-3-, a partially acetylated P—(I 6) glucan of the lichen Umbilicaria esculenta, inhibited the cytopathic effect of human inununodeficiency virus (HIV) and suppressed the HlV-antigen expression in Molt-4 (clone 8) cells (371). Emodin, 7-chloroemodin, 7-chloro-l-O-methylemodin and 5,7-dichloroemodin have exhibited antiviral activity 738, 72). Stiibler et al. 675,676) investigated the antiviral activity of the glucan lichenan. Seventeen depsides and depsidones have been examined for their inhibitory activity against HIV integrase 559). Yamamoto et al. 743) tested the extracts of numerous lichens on their inhibition of Epstein-Barr virus activation. 

Some cancers are believed to have a viral relationship. As such it is informative to look at the viral response to some of the platinum polymers. 

Tetramisole is an antihelmentic that acts on the cyclic nucleotide phosphodiesterases. It actually consists as a combination of optical isomers the most active one is levamisole. Levamisole was the first synthetic chemical that exhibited immunomodulatory properties. It appears to restore normal macrophase and T-lym-phocyte functions. 

Cisplatin polymer analogues, made through reaction of tetrachloroplatinate with tetramisole, were tested for their ability to inhibit EMC-D viruses that are responsible for the onset of juvenile diabetes symptoms in ICR Swiss male mice. Briefly, the mice were treated with 1, 5, and 10 mg/kg. Doses of 1 and 5 mg/kg decrease in the severity and incidence of virus-induced diabetes in comparison to untreated mice. In another series of tests, doses of 1 and 10 mg/kg were administered one day prior to injection of the virus, but here there was an increase in the severity and incidence of virus-induced diabetes. Other studies were imdertaken, showing that the polymer showed different activity profiles than the tetramisole (20) itself. 

The third experiment focused on treatment subsequent to viral infection. The polymer was 100 % effective in viral control with delivery of the polymer (15) one day after the mice were infected. In summary, the methotrexate polymer (15) is an effective antiviral agent against at least the EMC RNA virus. 

A number of platimun polyamines were tested for antiviral activity in tumor cells.For instance, the polymer from tetrachloroplatinate and 2,6-diamino-3-nitroso-pyridine, 20a, which exhibited a cell differential ratio of 3.4, was tested at a concentration of 2.2 pg/mL on L929 cells infected with Encephalomyocarditis, EMC, virus, strain MM. A virus reduction of about 25% was seen. This is considered to be a moderate antivirial response. 

---

Numerous applications have been reported. A derivative of the (alkyn-1-yl)nucleosides 295. which have anticancer and antiviral activities, has been synthesized by this reaction. They are also used as chain-terminating nucleosides for DN.A. sequencing[l98,199]. In this reaction, use of DMF as the solvent is most important for successful operation[200]. Only the alkenyl bromide moiety in 2-bromo-3-aceto.xycycloheptene (296) reacts with alkynes without attacking the allylic acetate moiety[201]. 

Plants and microorganisms produce unique and diverse chemical stmctures, some of which act as immunomodulators (18—28). Of specimens used in traditional medicine, approximately 450 plant species have shown antiviral activity out of 4000 plants screened (19). Several tannins (20) exhibit strong inhibition of tumor promotion experimentally. Pretreatment of mice with small amounts of tannins for several days strongly rejected transplanted tumors. This activity has been claimed to be effected through enhancement of host-mediated antitumor activity. 

Whereas over 200 plant constituents are reported to have antiviral activity, as determined by in vitro methods, only 31 compounds have shown antiviral activity in vivo (19). Immunotherapeutic activity has not been determined. 

Interferons (lENs) (52,53), a family of species-specific vertebrate proteins, confer nonspecific resistance to a broad range of viral infections, affect cell proliferation, and modulate immune responses. AH three principal interferons, a-interferon (lEN-a) produced by blood leucocytes, P-interferon (lEN-P) by fibroblasts, and y-interferon (lEN-y) by lymphocytes, also have antiviral activity. The abiUty of interferons to inhibit growth of transplantable and carcinogen-induced tumor led to research showing the direct antiproliferative and indirect immune-mediated antitumor activities (see Chemotherapeutics, anticancer). IENs have been found to be efficacious in certain malignancies and viral infections, eg, hairy cell leukemia (85% response) and basal cell carcinoma (86% response). However, the interferons do have adverse side effects (54). 

Antiviral Agents. Although a number of antibiotics have been shown to have some sort of antiviral activity, only vidarabine [5536-17-4] (adenine arabinoside) is used clinically against viral infections at this time. As the need for new antiviral agents (qv) increases and new screening procedures are developed, one would expect the discovery of other new effective antiviral antibiotics that could be used safely in human therapy. 

Rifampicin has also shown antiviral activity but at levels 500—1000 times greater than required for antibacterial activity (130,140—142). Rifampicin shows promise in the treatment of leprosy (130,143). A large number of rifampicinlike derivatives are potent inhibitors of reverse transcriptase (123,144-148). 

The antiviral activity of the ansamacroHdes does not result from inhibition of RNA polymerase but rather from the inhibition of the assembly of the vims particles (141,258). 

I eplanocins. Neplanocins A—D and E (37—41) are carbocycHc nucleoside antibiotic products oi Ampullariella regularis (1,4) that are stmcturaHy related to (36) in that they contain either a cyclopentene or epoxy cyclopentane ring (121,122). The chemical syntheses of (37—41) and the 3-deazaneplanocins have been reported (123—126). Compound (37), which is converted to its 5 -triphosphate, has potent antitumor and antiviral activities (127—129). It strongly inhibits SAM in ceUs and vimses (128—131) and is converted to the 3 -keto derivative by A-adenosyUiomocysteine hydrolase (132,133). 

Replacement of the sulfur atom in the thiosemicarba2one moiety by an oxygen atom leads to a loss of antiviral activity. Methisa2one has no significant effect on vaccinia vims DNA synthesis (14) but seems to inhibit late protein synthesis by a mechanism that remains to be elucidated. 

Amino-5-iodo-2, 5 -dideoxyuridine [56045-73-9] (13) C2H22IN2O4, was synthesized ia 1975 (27) and was found effective against herpes keratitis ia rabbits (28). This compound is markedly less cytotoxic than IdU, iadicating that it may have a safer and more specific mode of antiviral activity. A potential limitation of this group of nucleosides is their specificity, for they fail to inhibit all strains of herpes vimses. The specific antiviral activity of (13) is considered to be a result of the incorporation of the 5 -Ai-phosphate into both viral and host DNA in infected cells, but not into the DNA of normal cells. Phosphorylation of (13) occurs only in herpes vims-infected cells, brought about by a vims-induced thymidine kinase (29). 

The 3, 5 -cyclic phosphate of ara-C has shown significant antiviral activity in vivo with an efficacy greater than that exhibited by ara-C itself (35). It was speculated that the 3, 5 -cycHc phosphate moiety may inhibit the deamination of ara-C, thus causiag the iacreased in vivo potency. A number of derivatives of ara-C have been prepared ia an effort to improve on antiviral activity and to reduce the toxicity. One such derivative is 2 - uoro-5-iodo-l-p-D-arabiQofuranosylcytosiae [69123-90-6] (FIAC, 17), synthesized (36) ia 1979. It is active against certain DNA vimses. FIAC,... 

It is likely that ara-HxMP similarly exerts its antiviral activity in the form of the triphosphate, ara-HxTP, since ara-HxTP inhibits HSV-1 DNA polymerase (49). Another possible explanation of the antiviral activity of ara-HxTP is that it is metaboHcaHy converted to ara-AMP. In fact, it has been shown at Wellcome Research Laboratories that ara-HxMP is a substrate for adenylosuccinate synthetase, and that the resulting arabinofuranosyladenylosuccinate is cleaved to ara-AMP by adenylosuccinate lyase (1). The selective action of ara-A against HSV appears to be a consequence of the preferential inhibition of ara-ATP against HSV-1 and HSV-2 polymerases. Ara-ATP also inhibits normal cellular DNA polymerases, which may be the reason for its cellular toxicity. Also, it has been observed that ara-A is incorporated uniformly throughout the HSV-1 genome, which may result in defective viral DNA (50). 

Fig. 4. Aliphatic adenosine analogues with bioad-spectmm antiviral activity.

The antiviral activity of (5)-DHPA in vivo was assessed in mice inoculated intranasaHy with vesicular stomatitis vims ( 5)-DHPA significantly increased survival from the infection. (5)-DHPA did not significantly reduce DNA, RNA, or protein synthesis and is not a substrate for adenosine deaminase of either bacterial or mammalian origin. However, (5)-DHPA strongly inhibits deamination of adenosine and ara-A by adenosine deaminase. Its mode of action may be inhibition of Vadenosyl-L-homocysteine hydrolase (61). Inhibition of SAH hydrolase results in the accumulation of SAH, which is a product inhibitor of Vadenosylmethionine-dependent methylation reactions. Such methylations are required for the maturation of vital mRNA, and hence inhibitors of SAH hydrolase may be expected to block vims repHcation by interference with viral mRNA methylation. 

A large number of a-hydroxybenzylbenzimidazole [50-97-5] (HBB, 39), C24H22N2O, derivatives has been prepared and extensively studied as selective inhibitors of the RNA containing enterovimses (91). Although none of these derivatives have shown any antiviral activity in animals, l,2-bis(5-methoxy-2-benzimidazol-2-yl)-l,2-ethanediol [16656-27-2] (40), C2gH2gN404, was found to be active against an experimentally induced rhino vims infection in chimpanzees (92). However, the in vivo antiviral efficacy was accompanied by significant toxicity. 

The nucleotide form of ribavirin does not manifest its antiviral activity simply by lowering the GTP levels, but may indeed participate directly in binding to specific G proteins (124). Ribavirin has recently been studied as an inhibitor of vesicular stomatitis vims and La Crosse vims (125). Of the phosphorylated forms of the dmg, ribavirin-5 -diphosphate was by far the most potent inhibitor of viral repHcation for these two vimses. 

CS derivatives/salts have found limited use as detergents (25), antistatic coatings for photographic film (26), oil drilling fluids (25), thickeners in food, cosmetics, and pharmaceuticals (27). They have been recommended for use as cation exchangers (28,29). Also, sulfated polysaccharides have recendy shown interesting antiviral activity (30). 

ELEMENT PROFILE OF TABACO PLANTS AFTER THE TREATMENT WITH HETEROPOLY COMPOUNDS WITH ANTIVIRAL ACTIVITY INVESTIGATED BY EDXRF SPECTROMETRY... 

---