Marine Natural Products as Anticancer Agents 2.0

Global cancer incidence and death are expected to increase to 28 [...].

Thus, the efforts to discover new chemical entities from natural origins that can inspire the development of new anticancer drugs, including synthesis and/or semi-synthesis approaches, are of the utmost relevance.
Sala and collaborators [6] isolated two novel free porphyrins, isabellin A and B, and the known compounds corallistin D and deuteroporphyrin IX from the marine sponge Isabella sp., previously collected in the hard substrate off Zuytdorp, Western Australia. The new free porphyrin, isabellin A, exhibited a marked cytotoxic effect on the NS-1 malignant myeloma cell line, similar to the positive control sparsomycin. Mohamed and co-authors [7] isolated five oxygenated cembrenoids, sarcoconvolutum A-E, from the soft coral Sarcophyton sp. collected in the Egyptian Red Sea. The cembrene diterpene sarcoconvolutum E exhibited the highest cytotoxicity on A549 (IC 50 : 49.70 µg/mL) and HSC-2 (IC 50 : 53.17 µg/mL) malignant cells derived from lung adenocarcinoma and squamous cell carcinoma of the oral cavity, respectively.
On the other hand, Patel and co-workers [8] explored the chemical structure of clavatadines, previously isolated from the marine sponge Suberea clavate, as a scaffold for developing new anticancer drugs. The authors synthesized a library of thirty-two pirocyclic clavatadine analogs, displaying a CC 50 range between 0.4 and 12.3 µM on A-375 melanoma cells. The highest selectivity was exhibited by analogs 29 and 37, which activate caspases 3/7, key biomarkers of apoptosis. Lu and collaborators [9] also designed and semisynthesized a series of fifteen derivatives of brefeldin A, a macrolactone biosynthesized by the marine fungus Penicillium sp. (HS-N-29), previously isolated from the medicinal mangrove Acanthus ilicifolius. Analog 7 (brefeldin A 7-O-2-chloro-4,5-difluorobenzoate) exhibited the smallest IC 50 value (0.84 µM) on the human chronic myelogenous leukemia K562 cancer cell line. The mechanism of action underlying its cytotoxicity seems to be related to the blockage of the cell cycle, induction of apoptosis, inhibition of BCR-ABL phosphorylation, and downregulation of the expression of signaling molecules of the AKT pathway, including mTOR and p70S6K. Suksamai and collaborators [10] synthesized 5-O-(N-Boc-L-alanine)-renieramycin T (OBA-RT) from renieramycin T, an alkaloid commonly found in sponges and nudibranchs. OBA-RT can suppress cancer stem cell (CSC) activity and induce apoptosis in malignant lung cells, inhibiting Ak, which plays a key role in regulating CSC maintenance.
Alves and colleagues [11] studied the cytotoxic properties of three bromoditerpenes isolated from the red seaweed Sphaerococcus coronopifolius, named 12R-hydroxy-bromosphaerol, 12S-hydroxy-bromosphaerol, and bromosphaerol. Bromo groups, in their structure, characterize these terpenes due to bromine substitution, a rare chemical feature among secondary metabolites. The compounds displayed increased hydrogen peroxide production and induced apoptosis of MCF-7 cells derived from breast adenocarcinoma.
The anticancer activities of peptides (1-12), extracted and purified from the brown seaweed Laminaria japonica (LJPs), were evaluated through in vitro and in vivo models of liver cancer [12]. The LJP-1 peptide exhibited the highest anticancer activity, decreasing tumor growth in in vivo models. According to the evidence reported by Wu and coworkers [12], its mechanism of action seems to be related to the induction of cell death by caspase-dependent apoptosis, partly by inhibiting PI3K and MAPK signaling pathways and regulating the expression of cell cycle checkpoint proteins.
The development of resistance to anticancer drugs seriously threatens cancer treatment success. Distinct strategies have been designed to overcome these challenges, including drug synergism. Su and collaborators [13] explored the potential of a novel resolvin, 7S,15R-dihydroxy-16S,17S-epoxy-docosapentaenoic acid (diHEP-DPA), synthesized by cyanobacterial lipoxygenase from docosa-hexaenoic acid (DHA) to overcome the chemoresistance to 5-fluorouracil (5-FU) of colorectal cancer cells (CRCs) using in vitro and in vivo models. The new resolvin suppressed key events related to chemoresistance to 5-FU resulting from prolonged exposure, such as the enrichment of CSCs, the infiltration of tumor-associated macrophages, and the epithelial-mesenchymal transition in colorectal tu-mors. This evidence suggests that the therapeutic application of diHEP-DPA in combination with 5-FU-based chemotherapeutics against colorectal cancer is potentially effective.
Angiogenesis is the biological process responsible for forming new blood vessels, which play a key role in tumor growth, maintenance, and metastasis, being a valid target for tumor therapeutics. Dong and collaborators [14] evaluated the anti-angiogenesis activity of bis(2,3,6-tribromo-4,5-dihydroxybenzyl) ether (BTDE), a bromophenol previously isolated from the red seaweed Rhodomela confervoides. BTDE inhibited migration, invasion, tube formation, and the activity of matrix metalloproteinase 9 of human umbilical vein endothelial cells. It also prevented the migration, invasion, and vasculogenic mimicry formation of A549 lung adenocarcinoma cells. In vivo, BTDE blocked intersegmental vessel formation in zebrafish embryos.
Altogether, the eleven scientific papers published in this Special Issue provide an exciting overview of marine natural products as potential anticancer agents.
As Guest Editors, we acknowledge the efforts provided by all researchers and reviewers that participated in this Special Issue, the editorial board, and the Marine Drugs editorial office for their support and assistance. Special thanks go to Dr. Grace Qu.

Conflicts of Interest:
The authors declare no conflict of interest.