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Many compounds of plant origin have been identified that inhibit different stages in the replication cycle of human immunodeficiency virus (HIV): 1) virus adsorption: chromone alkaloids (schumannificine), isoquinoline alkaloids (michellamines), sulphated polysaccharides and polyphenolics, flavonoids, coumarins (glycocoumarin, licopyranocoumarin) phenolics (caffeic acid derivatives, galloyl acid derivatives, catechinic acid derivatives), tannins and triterpenes (glycyrrhizin and analogues, soyasaponin and analogues); 2) virus-cell fusion: lectins (mannose- and N-acetylglucosamine-specific) and triterpenes (betulinic acid and analogues); 3) reverse transcription; alkaloids (benzophenanthridines, protoberberines, isoquinolines, quinolines), coumarins (calanolides and analogues), flavonoids, phloroglucinols, lactones (protolichesterinic acid), tannins, iridoids (fulvoplumierin) and triterpenes; 4) integration: coumarins (3-substituted-4-hydroxycoumarins), depsidones, O-caffeoyl derivatives, lignans (arctigenin and analogues) and phenolics (curcumin); 5) translation: single chain ribosome inactivating proteins (SCRIP's); 6) proteolytic cleavage (protease inhibition): saponins (ursolic and maslinic acids), xanthones (mangostin and analogues) and coumarins; 7) glycosylation: alkaloids including indolizidines (castanospermine and analogues), piperidines (1-deoxynojirimicin and analogues) and pyrrolizidines (australine and analogues); 8) assembly/release: naphthodianthrones (hypericin and pseudohypericin), photosensitisers (terthiophenes and furoisocoumarins) and phospholipids. The target of action of several anti-HIV substances including alkaloids (O-demethyl-buchenavianine, papaverine), polysaccharides (acemannan), lignans (intheriotherins, schisantherin), phenolics (gossypol, lignins, catechol dimers such as peltatols, naphthoquinones such as conocurvone) and saponins (celasdin B, Gleditsia and Gymnocladus saponins), has not been elucidated or does not fit in the proposed scheme. Only a very few of these plant-derived anti-HIV products have been used in a limited number of patients suffering from AIDS viz. glycyrrhizin, papaverine, trichosanthin, castanospermine, N-butyl-1-deoxynojirimicin and acemannan.
The mechanism of the antiviral activity of hypericin was characterized and compared with that of rose bengal. Both compounds inactivate enveloped (but not unenveloped) viruses upon illumination by visible light. Human immunodeficiency and vesicular stomatitis viruses were photodynamically inactivated by both dyes at nanomolar concentrations. Photodynamic inactivation of fusion (hemolysis) by vesicular stomatitis, influenza, and Sendai viruses was induced by both dyes under similar conditions (e.g., I50 = 20-50 nM for vesicular stomatitis virus), suggesting that loss of infectivity resulted from inactivation of fusion. Syncytium formation, between cells activated to express human immunodeficiency virus gp120 on their surfaces and CD4+ cells, was inhibited by illumination in the presence of 1 microM hypericin. Hypericin and rose bengal thus exert similar virucidal effects. Both presumably act by the same mechanism--namely, the inactivation of the viral fusion function by singlet oxygen produced upon illumination. The implications of this photodynamic antiviral action for the potential therapeutic usefulness of both hypericin and rose bengal are discussed.
Two aromatic polycyclic diones hypericin and pseudohypericin have potent antiretroviral activity; these substances occur in plants of the Hypericum family. Both compounds are highly effective in preventing viral-induced manifestations that follow infections with a variety of retroviruses in vivo and in vitro. Pseudohypericin and hypericin probably interfere with viral infection and/or spread by direct inactivation of the virus or by preventing virus shedding, budding, or assembly at the cell membrane. These compounds have no apparent activity against the transcription, translation, or transport of viral proteins to the cell membrane and also no direct effect on the polymerase. This property distinguishes their mode of action from that of the major antiretro-virus group of nucleoside analogues. Hypericin and pseudohypericin have low in vitro cytotoxic activity at concentrations sufficient to produce dramatic antiviral effects in murine tissue culture model systems that use radiation leukemia and Friend viruses. Administration of these compounds to mice at the low doses sufficient to prevent retroviral-induced disease appears devoid of undesirable side effects. This lack of toxicity at therapeutic doses extends to humans, as these compounds have been tested in patients as antidepressants with apparent salutary effects. Our observations to date suggest that pseudohypericin and hypericin could become therapeutic tools against retroviral-induced diseases such as acquired immunodeficiency syndrome (AIDS).
