**9. Conclusions**

Last decade, there have been many works devoted to the antiviral potencies of sulphated polysaccharides. The formation of pathogen resistance to drugs on the pharmaceutical market requires new approaches to the treatment of viral diseases, including influenza. To do this, it is necessary to have drugs with different mechanisms of action, which in addition to antiviral effects, have anti-inflammatory, antioxidant, and immunomodulatory activity and to which viruses form resistance only on rare occasions. As presented in this review, the sulphated polysaccharides of brown, red, and green algae have such properties in relation to influenza infection (Figure 2).

**Figure 2.** Antiviral activity of algae metabolites.

The pharmaceutical market currently offers a carrageenan-based spray for local (intranasal) application. There are no other drugs based on polysaccharides, which is associated with difficulties in standardization. To standardize these compounds, the physicochemical parameters such as molecular weight, monosaccharide composition, degree of sulphation and other structural features of polysaccharides should be determined. One approach to solving this problem is to obtain structurally characterized and homogeneous samples of native polysaccharides with a low molecular weight or oligomeric fractions. At the same time, due to the wide spectrum of biological activity of polysaccharides and, most importantly, their ability to exert a virucidal effect, that prevent the penetration of influenza viruses into cells and suppress the replication of viral particles, these unique compounds can be used as the basis for the creation of a new generation of medicines. In addition, the almost complete absence of toxicity and pathogen resistance, relatively low cost, a significant yield of the final product, good solubility, significant reserves of natural sources, and the possibility of cultivation algae make polysaccharides promising candidates for the development of drugs with antiviral activity, in particular anti-influenza activity.

**Author Contributions:** N.B., T.Z., T.K., and I.M. prepared the text, L.F., S.K., O.M., and S.E. contributed the analysis of key parts of text related to it.

**Acknowledgments:** This work was supported by FEB RAS Grant № 18-5-084. We thank Professor Valentin Stonic, PIBOC FEB RAS, for the assistance.

**Conflicts of Interest:** The authors declare that there is no conflict of interest regarding the publication of this article.
