Marine Cyclic Peptides: Antimicrobial Activity and Synthetic Strategies

Oceans are a rich source of structurally unique bioactive compounds from the perspective of potential therapeutic agents. Marine peptides are a particularly interesting group of secondary metabolites because of their chemistry and wide range of biological activities. Among them, cyclic peptides exhibit a broad spectrum of antimicrobial activities, including against bacteria, protozoa, fungi, and viruses. Moreover, there are several examples of marine cyclic peptides revealing interesting antimicrobial activities against numerous drug-resistant bacteria and fungi, making these compounds a very promising resource in the search for novel antimicrobial agents to revert multidrug-resistance. This review summarizes 174 marine cyclic peptides with antibacterial, antifungal, antiparasitic, or antiviral properties. These natural products were categorized according to their sources—sponges, mollusks, crustaceans, crabs, marine bacteria, and fungi—and chemical structure—cyclic peptides and depsipeptides. The antimicrobial activities, including against drug-resistant microorganisms, unusual structural characteristics, and hits more advanced in (pre)clinical studies, are highlighted. Nocathiacins I–III (91–93), unnarmicins A (114) and C (115), sclerotides A (160) and B (161), and plitidepsin (174) can be highlighted considering not only their high antimicrobial potency in vitro, but also for their promising in vivo results. Marine cyclic peptides are also interesting models for molecular modifications and/or total synthesis to obtain more potent compounds, with improved properties and in higher quantity. Solid-phase Fmoc- and Boc-protection chemistry is the major synthetic strategy to obtain marine cyclic peptides with antimicrobial properties, and key examples are presented guiding microbiologist and medicinal chemists to the discovery of new antimicrobial drug candidates from marine sources.

Stellatolides, a new cyclodepsipeptide family from the sponge Ecionemia acervus: isolation, solid-phase total synthesis, and full structural assignment of stellatolide A

The marine environment is a rich source of metabolites with potential therapeutic properties and applications for humans. Here we describe the first isolation, solid-phase total synthesis, and full structural assignment of a new class of cyclodepsipeptides from the Madagascan sponge Ecionemia acervus that shows in vitro cytotoxic activities at submicromolar concentrations. Seven structures belonging to a new family of compounds, given the general name stellatolides, were characterized. The sequence and stereochemistry of all the amino acids in these molecules were established by a combination of spectroscopic analysis, chemical degradation, and derivatization studies. Furthermore, the complete structure of stellatolide A was confirmed by an efficient solid-phase method for the first total synthesis and the full structural assignment of this molecule, including the asymmetric synthesis of the unique β-hydroxy acid moiety (Z)-3-hydroxy-6,8-dimethylnon-4-enoic acid.

Bioactive natural products from Papua New Guinea marine sponges

The discovery of novel natural products for drug development relies heavily upon a rich biodiversity, of which the marine environment is an obvious example. Marine natural product research has spawned several drugs and many other candidates, some of which are the focus of current clinical trials. The sponge megadiversity of Papua New Guinea is a rich but underexplored source of bioactive natural products. Here, we review some of the many natural products derived from PNG sponges with an emphasis on those with interesting biological activity and, therefore, drug potential. Many bioactive natural products discussed here appear to be derived from non-ribosomal peptide and polyketide biosynthesis pathways, strongly suggesting a microbial origin of these compounds. With this in mind, we also explore the notion of sponge-symbiont biosynthesis of these bioactive compounds and present examples to support the working hypothesis.