The first report studying the antiviral properties from the blue-green alga Spirulina was published by Hayashi et al. in 1993. In vitro, they demonstrated that a hot water soluble extract prepared from S. platensis inhibited the replication of herpes simplex virus type 1 (HSV-1) in HeLa cells, in a dose-dependent concentration, from 0.08 to 50 mg/mL. The extract did not have a virucidal effect, and the viral inhibition was due to an interference with the penetration event.11
After this report, a new antiviral research era started. Hayashi and Hayashi, 1996, by a bioactivity-directed fractionation of the hot water extract from S. platensis, isolated a novel sulfated polysaccharide chelating calcium ion, termed calcium spirulan (Ca-SP) as an antiviral principle. Ca-SP was obtained by gel filtration on Sepharose 6B of the hot water extract of Spirulina treated with 10% trichloroacetic acid, and a further purification step was done by DEAE cellulose chromatography. Ca-SP did not show antiviral effect against noneveloped viruses such as poliovirus and coxsack-ievirus. But it showed to be a very effective antiviral agent against enveloped viruses such as HSV-1, human cytomegalovirus (HCMV), measlesvirus, mumpsvirus, influenza A virus and HIV-1, with high therapeutic index (IC50/EC50, concentration required to reduce cell growth by 50% /concentration required to reduce virus replication by 50%) of 8,587, 578, 371, 274, 574, and 1,261, respectively. The antiviral activity of Ca-SP is due to a selective inhibition of the penetration event of HSV-1 into the host cell, effect previously observed with the aqueous extract of Spirulina.12 Ca-SP besides its antiviral activity, has shown an antitumor effect. It has successfully inhibited experimental lung metastasis. This effect is probably associated to its binding properties, especially to rhamnose receptors located on the cellular surface. It has been demonstrated that rhamnose is a major component in Ca-SP, and lung-metastasizing tumor cells from different types of primary tumor sites, present specific receptors for rhamnose.13
Other papers have confirmed and extended the studies about the antiviral properties of S. platensis on HIV-1 replication. Ayehunie et al., 1996, by using an aqueous extract of S. platensis and different human cells, T cell lines, and peripheral blood mononuclear cells (PMBC) infected with HIV-1, found that the extract at a concentration of 5-10 ^g/mL reduced viral production and syncytium formation by 50%.
Same result was found for Rausher murine leukemia virus (RLV) at a concentration of 9-30 ^g/mL. The inhibitory effect of the Spirulina extract was associated directly to an inactivation of HIV-1 virions.14 A further study with this extract and HIV-1 on T cell lines, PMBC, and Langerhans cells showed that the extract inhibited 50% of the viral infection at concentrations of 0.3-1.2 ^g/mL, and the therapeutic indices ranged between 200 and 6000. It was also confirmed that the inactivation of HIV-1 was a result of a direct virus inactivation. The antiviral activity was associated to the polysaccharide fraction and also to a fraction depleted of polysaccharides and tannins.15
It is known that Spirulina contains 2-5% of sulfolipids.16 Sulfolipids from S. platensis have proved to be effective against HIV. Sulfolipids inhibit efficiently and selectively only the DNApolymerase activity of the HIV-RT, requiring a concentration of 24 nM for a 50% inhibition. Both the sulfonic acid moiety and the fatty acid ester side chain have a substantial effect in potentiating the extent of inhibition.17
A protein-bound pigment allophycocyanin purified from S. platensis, has shown an antiviral activity against enterovirus 71, in both rhabdomyosarcoma cells and African green monkey kidney cells. The allophycocianin inhibit 50% of enterovirus 71-induced cytophtic effect, viral plaque formation, and viral-induced apoptosis at concentrations of 0.056-0.101 ^M. It has been shown to be more effective in preventing enterovirus infection when it was added to the cells before viral infection, than after, suggesting that it may interfere with a very early stage of viral replication such as virus adsorption and penetration.18
Several polysaccharide fractions isolated from S. platensis have showed a broad-spectrum antiviral activity, characterized by a strong inhibition in vitro of human viruses such as: HCMV, HSV-1, HHV-6 and HIV-1. The ongoing biochemical analysis of these preparations (intracellular and extracellular polysaccharides) indicates the presence of spirulan-like substances, in addition to a small group of other uncharac-terized polysaccharides and possibly protein components. The highest inhibition was observed with the extracellular fractions, which presented a therapeutic index of209.2 for HCMV. The extracellular fraction was isolated from culture supernatant by centri-fugation of culture broth, liophylization and dialysis against deionized and ultrapure water. Apparently, this is the first description of antiviral activity of substances isolated from the extracellular fraction of Spirulina.19
These findings demonstrate that Spirulina besides the Ca-SP contains other compounds with antiviral properties.
Other Spirulina species, such as S. maxima has also shown antiviral activity against human and animal herpesviruses such as: HSV-2, HCMV, and suid herpesvirus 1 or pseudorabies virus (SuHV-1). However, this antiviral activity was not observed against other enveloped viruses such as: two measles strains (Edmonston-Zagreb vaccine strain and subacute sclerosing panencephalitis Halle strain) and vesicular stomatitis virus (VSV). Similarly to S. platensis the S. maxima extract does not have a virucidal effect on herpesvirus, both extracts inhibit herpesvirus infection by blocking the adsorption and penetration events of t7he viral replication cycle. According to the antiviral results obtained with the extracts from S. maxima using solvents with different polarity, the antiviral effect is related to the presence of highly polar compounds.20
Table 11.1 presents a summary of the in vitro antiviral studies of Spirulina.
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