Medicinal significance of naturally occurring cyclotetrapeptides.

Bioactive natural products are serendipitous drug candidates, which stimulate synthetic approaches for improving and supporting drug discovery and development. Therefore, the search for bioactive metabolites from different natural sources continues to play an important role in fashioning new medicinal agents. Several cyclic peptides were produced by organisms, such as β-defensins, gramicidin S, and tyrocidine A, and exhibited a wide range of bioactivities, such as antiviral activity against HIV-1, influenza A viruses, or antibacterial activity. Cyclic tetrapeptides are a class of natural products that were found to have a broad range of biological activities, promising pharmacokinetic properties, as well as interesting conformational dynamics and ability of slow inter-conversion to several different structures. Cyclooligopeptides, particularly medium ring-sized peptides, were obtained from marine microorganisms and exhibited a wide range of pharmacological properties, including antimicrobial and anti-dinoflagellate activities, cytotoxicity, and inhibitory activity against enzyme sortase B. Most of the naturally occurring cyclotetrapeptides are obtained from fungi. Some natural cyclic tetrapeptides were found to inhibit histone deacetylase (HDAC), which regulate the expression of genes. These compounds are very useful as cancer therapeutics. Various analogues of the natural cyclotetrapeptides were successfully synthesized to find novel lead compounds for pharmacological and biotechnological applications. Therefore, in this review, previously reported novel natural cyclotetrapeptides are briefly discussed, along with their important biological activities as drug candidates, together with their promising therapeutic properties. Moreover, their future perspective in drug discovery as potential therapeutic agents will be determined.