The Biological and Chemical Diversity of Tetramic Acid Compounds from Marine-Derived Microorganisms

Tetramic acid (pyrrolidine-2,4-dione) compounds, isolated from a variety of marine and terrestrial organisms, have attracted considerable attention for their diverse, challenging structural complexity and promising bioactivities. In the past decade, marine-derived microorganisms have become great repositories of novel tetramic acids. Here, we discuss the biological activities of 277 tetramic acids of eight classifications (simple 3-acyl tetramic acids, 3-oligoenoyltetramic acids, 3-decalinoyltetramic acid, 3-spirotetramic acids, macrocyclic tetramic acids, N-acylated tetramic acids, α-cyclopiazonic acid-type tetramic acids, and other tetramic acids) from marine-derived microbes, including fungi, actinobacteria, bacteria, and cyanobacteria, as reported in 195 research studies up to 2019.


Introduction
Secondary metabolites bearing a tetramic acid (pyrrolidine-2, 4-dione) motif have been isolated from various terrestrial and marine species, such as bacteria, actinobacteria, cyanobacteria, fungi, and sponges. The tetramic acid scaffold can be modified by unusual and intricate substituents to form complex, diverse chemical structures with multiple stereogenic centers. Intriguingly, an increasing number of tetramic acid products have shown a remarkable diversity of bioactivities, including antitumor, antibacterial, antifungal, and antiviral activities [1][2][3][4][5]. Due to their intricate structures and potent biological activity, natural tetramic acids have attracted a great deal of attention for their biosynthesis mechanisms, medicinal potential, and chemical synthesis in the biological, chemical, and pharmaceutical fields. Up to 2013, there were several reviews covering numerous aspects of naturally occurring tetramate products, such as isolation, biological activity, and synthesis, published by Royles [1], Ghisalberti [2], Gossauer [3], Schobert and Schlenk [4], and Ju et al. [5]. Many reviews have discussed the biosynthetic mechanisms of the PKS-NRPS biosynthesis pathways of tetramic acids in detail [2,[5][6][7][8][9].
Marine natural products (MNPs) are considered an unexploited treasure trove of new bioactive NPs for the 21st century. Among them, marine microorganism-derived NPs have become the primary source of new MNPs, from less than 20% of newly discovered MNPs in 2006 to 57% in 2017 (based on a summary of a series of reviews "Marine Natural Products" published by Blunt and his colleagues during 2008-2019 [10,11]. While there have been no special reviews about tetramic acid compounds from marine microbes, especially in the past six years, numerous examples of new tetramate molecules from colleagues during 2008-2019 [10,11]. While there have been no special reviews about tetramic acid compounds from marine microbes, especially in the past six years, numerous examples of new tetramate molecules from marine-derived microorganisms, and their related bioactivities, have been reported (up to 94 articles, 48% of the total 195 research articles from 1970-2019). In the current review, we focus our attention on the isolation, structural features, and biological activities of natural tetramate products isolated from marine-derived microorganisms (fungi, actinobacteria, bacteria, and cyanobacteria) reported up to September 2019. Notably, three broad groups of compounds (cytochalasins, 4-O-substituted derivatives (i.e., tetronates), and 2-pyridones), from putative tetramic acid-related biosynthesis pathways have been covered in numerous reviews [4,[6][7][8][12][13][14][15][16] and are excluded from this review.
A total of 195 research papers describing 277 tetramate compounds from marine-derived microbes were analyzed for this review (Supplementary Table S1). The assignments of a given compound to a certain category were based on their particular structural features and biogenetic pathways. The compounds were characterized into eight groups of chemical structures, as shown in Figure 1: simple 3-acyl-tetramic acids (3-ATAs), 3-oligoenoyltetramic acids (3-OTAs), 3decalinoyltetramic acids (3-DTAs), 3-spirotetramic acids , macrocyclic tetramic acids (MTAs), N-acylated tetramic acids, α-cyclopiazonic acid (CPA)-type tetramic acids, and other tetramic acids. Furthermore, the macrocyclic tetramic acids were distributed into two subcategories: polycyclic tetramate macrolactams (PTMs) from marine actinobacteria and bacteria, and pyrrocidine tetramate alkaloids (PTAs) from marine fungi. The pie chart in Figure 1 provides deeper insight into the diversity and complexity of TAs from marine-derived microbes, revealing the complexity and diversity of molecules characterized as the dominating compounds. MTAs (21.0%) comprised the largest proportion of TAs from marine microbes, followed by N-acylated TAs (16.0%), 3-DTAs (13.0%), 3-STAs (12%), CPA-type TAs (9%), 3-ATAs (9%), and 3-OTAs (5%). As is known, structures can determine properties; thus, the complex and diverse structures of TAs will lead to the diversity of their bioactivities. Therefore, this review aims to give an overview of the naturally occurring tetramate products from marine-derived microbes and their biological activities, as reported in the literature until September 2019, to illustrate their biodiversity, chemical diversity, and bioactive diversity. The origins of the strains and the diversity and biological properties of the compounds, as well as the relevant publication details are also summarized (Supplementary Table S1).  Some examples of typical molecules belonging to these classes are illustrated: simple 3-acyl-tetramic acids (penicillenol A 1 ), 3-oligoenoyltetramic acids (tirandamycin A), 3-decalinoyltetramic acids (equisetin), 3-spirotetramic acids (pseurotin A), macrocyclic tetramic acids (from left to right, ikarugamycin, GKK1032A 2 ), N-acylated tetramic acids (symplostatin 4), α-cyclopiazonic acid (CPA)-type tetramic acids (α-cyclopiazonic acid), and other tetramic acids (vermelhotin). The main characteristics of each chemical class are highlighted in red.

3-Oligoenoyltetramic Acids
To date, only 13 members of 3-oligoenoyltetramic acids (3-OTAs) ( Figure 3) possessing a 1-oxopentadienyl substituent at C-3 in the tetramate ring have been discovered from marine microorganisms-three of them from fungi and ten of them from actinobacteria Streptomyces.

3-Oligoenoyltetramic Acids
To date, only 13 members of 3-oligoenoyltetramic acids (3-OTAs) ( Figure 3) possessing a 1oxopentadienyl substituent at C-3 in the tetramate ring have been discovered from marine microorganisms-three of them from fungi and ten of them from actinobacteria Streptomyces.

3-Decalinoyltetramic Acids
The class of 3-decalinoyltetramic acids (3-DTAs) derived from microorganisms features a tetramate unit at position N-1 connected to H or CH 3 , and C-3 connected to "decalin" with multiple chiral centers. Up to 35 members of 3-DTAs ( Figure 4) have been uncovered from marine fungi and actinobacteria.
Altercrasins and fusarisetins are 3-DTA derivatives with a unique structure, some of which exhibited unusual bioactivities. Altercrasin A (66), a novel decalin derivative with spirotetramic acid, was reported from a strain of Alternaria sp. OUPS-117D-1, originally associated with the sea urchin The typical 3-DTA, equisetin (40) and epi-equisetin (41), were isolated initially from terrestrial Fusarium genera and displayed various biological activities, such as antimicrobial, anti-HIV, cytotoxicity, and phytotoxicity activities [1]. These two molecules were also later isolated from the marine-derived fungi Fusarium sp. 152 and F. equiseti D39 [47] and displayed potent anti-phytopathogenic bacterial and fungal activities [47]. Notably, equisetin (40) exhibited potent anti-methicillin-resistant Staphylococcus aureus (MRSA) activity (MIC = 1 µg/mL, equivalent to vancomycin) and antimicrobial activities against Pseudomonas syringae and Rhizoctonia cerealis (MIC = 1.1 and 8.4 µM, respectively) superior to the positive control and could be exploited as a potential antimicrobial drug candidate [48].
Screening for antitumor agents, the marine-sediment-derived fungus Aspergillus sydowii D2-6 was found to produce two new 3-STAs, named azaspirofurans A and B (84 and 85), which feature a new furan ring instead of the long linear side chain of pseurotin [87]. In vitro cytotoxicity experiments have demonstrated that 85 has moderate cytotoxicity toward A549 HTCL (IC 50 = 10 µM) [87]. Six years later, azaspirofuran B (85) and pseurotin F1 (86) were re-obtained from a marine jellyfish-derived fungus A. fumigates [88]. In recent years, seven known 3-STAs, 75, 76, 79, 83-86, were isolated from the marine Aspergillus fumigatus MR2012, associated with Red Sea sediment, using zebrafish embryos and larvae in an attempt to discover promising compounds from marine microorganisms that may have in vivo antiseizure activity [89]. Based on a series of experiments (including the larval zebrafish pentylenetetrazole seizure experiment, electrophysiological analysis, and ADMET assessment) among them, 79 and 84 were demonstrated to be lead antiseizure compounds and possible new antiseizure therapeutics [89]. A new pseurotin derivative, pseurotin G' (87), together with 11-O-methyl pseurotin A (76), was discovered from the co-culture of the fungus A. fumigatus MR2012 and the bacterium Streptomyces leeuwenhoekii C34 [90].
A subgroup of the fungal 3-STA derivatives with a 6-membered carbocyclic motif, spirostaphylotrichin X (104), and three related analogues, spirostaphylotrichins A and R as well as triticone E (105, 106, and 107), were identified as metabolites of the marine alga-derived fungus Cochliobolus lunatus SCSIO41401 [98]. Compounds 105-107 showed weak or inactive anti-influenza virus (IAV) activity, while 104 displayed a noticeable inhibitory effect against multiple IAVs (IC 50 = 1.2-5.5 µM) by inhibiting polymerase PB2 protein activity and interfering with the production of its progeny's viral RNA, thus representing a new type of potential lead compound for anti-IAV therapeutics [98]. Another analogue, triticone D (108), was isolated from the marine sediment-derived Westerdykella dispersa, and found to lack antibacterial and cytotoxic properties [99].
In subsequent years, many ikarugamycin-related structures have been continually isolated from marine-derived actinomycetes. Butremycin (110), a 3-hydroxylated ikarugamycin, was reported in 2014 from the new Ghanaian mangrove river-sediment-derived actinomycete Micromonospora sp. K310, representing the first example of a microbial producer of ikarugamycins other than the Streptomyces species; however, it only displayed fragile antibacterial activity (MIC ≥ 50 µg/mL) [114].
The activation of the silent PTM gene clusters of the Streptomyces pactum SCSIO 02999 by genome-mining led to the production of six new PTMs, 5/5/6-PTMs (pactamide A, B, D, and F) (134, 135, 137, and 139 suggesting that the presence of a double bond in the A ring of the 5/5/6 ring system significantly decreased their cytotoxicity [120]. Alteramides are a family of PTMs containing a 5/5 ring system fused to the macrolactam. Alteramides A and B (140 and 141) were obtained from the marine-sponge-associated bacterium Alteromonas sp. by the Kobayashi group in 1992 [121]. Their corresponding isomers, 6-epi-alteramides A and B (142 and 143), were sourced from the coral-associated Pseudoalteromonas sp. OT59 [122] by microbial MALDI-imaging mass spectrometry coupled with a molecular network strategy and were used to revise the original stereochemistry of alteramides, which were originally isolated from the Streptomyces albus J1074 in 2014 [123]. Alteramides 142 and 143 were responsible for the observed antifungal activity of this strain when grown in the dark, although they were inactivated by light through photoinduced intramolecular [4+4] cycloaddition to generate the hexacyclic products 140a and 141a [121,122]. Further, 140 exhibited in vitro cytotoxicity against P388, L1210, and KB cells, with IC 50 values of 0.1, 1.7, and 5.0 µg/mL, respectively [121], while 141 showed no cytotoxicity, indicating that the presence of the C-25-hydroxyl group led to the abolishment of antiproliferative activity [121].
Ypaoamide (186), a lipopeptide with a feeding deterrent, was isolated from a marine cyanobacterial assemblage composed of Schizothrix calcicola and L. majuscula in 1996 [144]. Recently, biochemical studies on marine Okeania sp. collected in Okinawa produced two new analogues, ypaoamides B and C (187 and 188), which stimulated glucose uptake in a dose-dependent and insulin-independent manner in cultured L6 myotubes [145]. Furthermore, the effect of 188 on glucose uptake was found to occur by activation of the AMP-activated protein kinase (AMPK) pathway regulating cellular metabolism, suggested to be a potential therapeutic candidate for the treatment of Type 2 diabetes mellitus (T2DM) [145].
Two chlorine-containing lipopeptides, malyngamides A and B (190 and 191), were described as the constituents of shallow water varieties of marine M. producens collected at Kahala Beach, Hawaii by Moore et al. in 1978 [147,148]. The same workers subsequently isolated seven closely related nontoxic compounds, pukeleimides A-G (192)(193)(194)(195)(196)(197)(198), lacking the fatty acid side chain and chlorine atoms of the 190 analogues from the same strain [149,150]. Malyngamide Q and R (201 and 202) [151] and isomalyngamides A and B (199 and 200) [152], a new subtype of malyngamide with different geometrical stereochemistry at C-6 (Z-chloromethylene), were purified from marine L. majuscula from Madagascan and Hawaiian waters, respectively. A bioassay-directed fractionation of the active fractions of a strain of M. producens derived from the Red Sea resulted in the isolation of a new malyngamide 4 (203), along with malyngamides A and B. Compounds 203 and 191 revealed moderate cytotoxicity against three HTCLs (A549, HT29, and MDA-MB-231,) (IC 50 = 40-60 µM) [153]. (Z)-malyngamides 199 and 200 showed lethal toxicity to crayfish at less than 0.5 mg/kg [152]. Subsequently, a new (Z)-malyngamide, named isomalyngamide A-1 (204), along with 199, was obtained from a Taiwanese L. majuscula [154]. Compounds 204 and 199 displayed potential in suppressing breast cancer cell (MDA-MB-231) migration, with nanomolar IC 50 values of 337 and 60 nM, and blocked cell proliferation, with micromolar IC 50 values of 12.7, and 2.8 µM, by inactivating the expression of focal adhesion kinase (FAK), p-FAK, Akt, and p-Akt through the β-1 integrin-mediated antimetastatic pathway [154]. It was indicated that the C-12' enol methyl ether group of 204 was essential for its cytotoxicity against breast HTCLs [154]. A new malyngamide (205)-the first report of a malyngamide with a hydroxy group at C-7 of the fatty acid portion-as well as 199 and 200, were discovered in Hawaiian M. producens [155]. The bioactivity of 205, showing very weak cytotoxicity against L1210 and lethal toxicity to shrimp, was approximately 10-100 times weaker than that of 199 and 200, suggesting that the methoxy group at C-7 of the fatty acid section was the important pharmacophore of the malyngamide [155].
In 2009, the cyanobacterial linear lipodepsipeptide symplostatin 4 (Sym4) (209) [159] and gallinamide A (209) [160], containing a methylmethoxypyrrolinone (MMP) moiety, were independently isolated from Symploca sp. and Schizothrix sp., respectively. Subsequently, the total syntheses of both compounds revealed that they are indeed identical [160,161]. Subsequent biological evaluations of 209 and three synthetically generated N-terminal diastereoisomers demonstrated their potent antimalarial properties: potent antimalarial activities against the Plasmodium falciparum 3D7 strain (IC 50 = 37-104 nM), similar to that of the positive control, chloroquine (IC 50 = 17.8 nM) [160]. Compound 209 was also moderately activated in inhibiting the chloroquine-resistant strain of P. falciparum W2 (IC 50 = 8.4 µM) [162]. Compound 209 also displayed moderate cytotoxicity against mammalian Vero cells (IC 50 = 10.4 µM), HeLa cervical carcinoma cells (IC 50 = 12 µM), and HT-29 colon adenocarcinoma cells (IC 50 = 53 µM); surprisingly, the lack of cytotoxicity toward NCIH460 lung tumors or neuro-2αmouse neuroblastoma cell lines at 16.9 µM indicated that 209 could be considered as a promising lead antimalarial hit [159,162]. Furthermore, compound 209 was demonstrated to induce the G2 cell cycle arrest at a high micromolar concentration, which is related to microtubule-disrupting effects [162]. Notably, compound 209 did not cause the lysis of red blood cells (RBCs), even at high concentrations (>25 mM) [161], indicating that its antiparasitic effect was not due to the permeabilization of the RBC membrane. Compound 209 also potently and selectively inhibited the human cysteine protease cathepsin L (IC 50 = 5.0 nM) through a covalent and irreversible mechanism [163]. The sym4-treatment of P. falciparum-infected RBCs led to the generation of a swollen food vacuole phenotype and a reduction in parasitemia at an EC 50 of 0.7 µM [164]. Further studies of 209 and its derivatives revealed that 209 acts as a nanomolar inhibitor of the P. falciparum falcipains (FPs) in infected RBCs by inhibiting the hemoglobin degradation pathway and indicating its unusual MMP unit as the critical pharmacophores [164].

Other Tetramic Acids
All of the smaller subgroups of TAs, whose numbers were less than 13, were called "other tetramic acids", which include 42 compounds (236-277 in Figure 10).
Recently, deep-sea-sediment-derived fungi have been demonstrated to be the source of the chemical diversity of TAs. The genera of Cladosporium from deep-sea sediments are sources of many subclasses of tetramic acid derivatives with different C-3 substituent groups, including C3-acyl-linear side chains ( Notably, some of these compounds have special structures. For example, 248-250, 252, and 253 have a quaternary (C-3) center carrying a trans-hexylenic alcohol side chain and a six-membered lactone ring [31,91]. The pharmacological results showed that only compound 247 had cytotoxic activities against four HTCLs (MCF-7, HeLa, HCT-116, and HL-60), with IC 50 values of 18.7, 19.1, 17.9, and 9.1 µM [198]. However, cladosporiumins I' -J' (252-253), and other cladosporiumins displayed weak or no cytotoxicity against the four breast HTCLs [31,91].
Epolactaene (271), with a long-chain-substituted γ-lactam group, was discovered from the marine fungus Penicillium sp. BM 1689-P and displayed neuritogenic properties by arresting the cell cycle at the G0/G1 phase and inducing the outgrowth of neurites in human neuroblastoma SH-SY5Y cells [207], selectively inhibiting the activities of mammalian DNA polymerases α and β as well as human DNA topoisomerase II [208], and could combine with Hsp60 as a Michael acceptor to inhibit Hsp60 chaperone activity [209,210]. Pulchellalactam (272) was reported from the marine-derived-fungus Corollospora pulchella and was used as a selective inhibitor of the CD45 protein, tyrosine phosphatase [211].
Andrimid (275), moiramides B-C (276-277), and their precursor, moiramide A, were produced by the bacterium Pseudomonas fluorescens, isolated from marine tunicates [214]. In contrast to moiramide A and 277, both 275 and 276 showed antibacterial activity, highlighting that the intact succinimide moiety is the critical pharmacophore [214]. Compound 275 was active in inhibiting MRSA (MIC: 2 µg/mL) and VRE (32 µg/mL), while 276, the congener with the shortest polyene chain, was more potent in its inhibition against both MRSA (0.5 µg/mL) and VRE (4 µg/mL) [214]. Further studies revealed that 275 and 276 had broad-spectrum antibacterial activity as a class of a new potent bacterial acetyl-CoA carboxylase inhibitor, targeting its fatty acid biosynthesis [215] and highlighting the fatty acid side chain and the pyrrolidinedione moiety as the most important pharmacophores [216].
The number of different chemical classes of TAs displaying each bioactivity is shown in Figure  15. The bioactivities of the compounds were evaluated for different targets, ranging from a specific cellular mechanism to the entire organism. For example, the inhibitory activity of special protease was shown to target enzymatic processes when antiprotozoal, lethality-toxicity, phytotoxicity, and antimicrobial activity were tested against whole organisms. Further, cytotoxicity was based on the cell line level, and some research is related to their specific cellular and molecular mechanisms; antiinflammatory and antioxidant activities are mainly assessed on the basis of specific cellular mechanisms, which may also be included in cytotoxicity and other activities. The present analysis confirms the preceding observations (i.e., that cytotoxicity is the most common bioactivity, followed by antibacterial and antifungal activity). Some activities were displayed only for certain compounds: phytotoxicity involved only 3-D TAs; lethality-toxicity involved only N-acylated TAs and CPA-type TAs; anti-inflammatory activity was observed for 3-STAs, N-acylated TAs, and other TAs; and antioxidant activity was observed for 3-STAs, MTAs, and other TAs. For the chemical classes, no specific activities were observed for one chemical class concerning different types. Four chemical classes (3-STAs, 3-DTAs, N-acylated TAs, and MTAs) seem to present a relatively more extensive set of activities. Figure 13. The TAs from marine microorganisms were divided by their sources (habitats); 277 TAs were isolated from 120 species of microorganisms in 120 habitats.
In the bioassay of the 261 tetramic acids (94.2% compounds) from marine microorganisms, 77.4% of compounds (202) displayed various activities (n = 327) and, on average, exhibited 1.62 activities per bioactive-TA. This result is because some compounds presented various activities and were counted in more than one category. The ten major bioactivities are listed in Figure 14 (cytotoxicity, antibacterial, antifungal, antiviral, antiprotozoal, lethality-toxicity, phytotoxicity, anti-inflammatory, and antioxidant activities, as well as special protease enzyme inhibition activities). Cytotoxicity (40%) was the most significant pharmacological activity, with up to 132 compounds among the 327 listed compounds, which inhibited the proliferation of different tumor cell lines in vitro, followed by anti-infective/antimicrobial activities (30%), including antibacterial activities for 57 compounds (17%), antifungal activities for 30 compounds (9%), and antiviral activities for 14 compounds (4%). This result is consistent with the focus of medical research, as tumors and infectious diseases remain the primary threat to human health in modern society. Other selected major activities included lethality-toxicity for 18 compounds (5%), special protease inhibition for 15 compounds (5%), and antiprotozoal activity for 10 compounds (3%).  The number of different chemical classes of TAs displaying each bioactivity is shown in Figure 15. The bioactivities of the compounds were evaluated for different targets, ranging from a specific cellular mechanism to the entire organism. For example, the inhibitory activity of special protease was shown to target enzymatic processes when antiprotozoal, lethality-toxicity, phytotoxicity, and antimicrobial activity were tested against whole organisms. Further, cytotoxicity was based on the cell line level, and some research is related to their specific cellular and molecular mechanisms; anti-inflammatory and antioxidant activities are mainly assessed on the basis of specific cellular mechanisms, which may also be included in cytotoxicity and other activities. The present analysis confirms the preceding observations (i.e., that cytotoxicity is the most common bioactivity, followed by antibacterial and antifungal activity). Some activities were displayed only for certain compounds: phytotoxicity involved only 3-D TAs; lethality-toxicity involved only N-acylated TAs and CPA-type TAs; anti-inflammatory activity was observed for 3-STAs, N-acylated TAs, and other TAs; and antioxidant activity was observed for 3-STAs, MTAs, and other TAs. For the chemical classes, no specific activities were observed for one chemical class concerning different types. Four chemical classes (3-STAs, 3-DTAs, N-acylated TAs, and MTAs) seem to present a relatively more extensive set of activities.  The number of compounds is symbolized by the disc diameters for each bioactivity and each chemical class. The colors correspond to the different categories of the activity targets. Gray represents a mixed target; yellow mainly represents a cell line target, blue primarily represents the specific cellular mechanism, green represents the enzyme target, and purple represents the entire organism target.

Conclusions and Outlooks
This review has provided a comprehensive overview of 277 tetramic acid products from 120 marine-derived microbes (containing fungi, actinobacteria, bacteria, and cyanobacteria), presented by their structural characteristics and covering up to September 2019, with 195 research publications related to tetramic acids and their bioactivities. Marine fungi are the dominant source of the rapidly Figure 15. Classification of the 202 bioactive TAs according to their activities and chemical classes. The number of compounds is symbolized by the disc diameters for each bioactivity and each chemical class. The colors correspond to the different categories of the activity targets. Gray represents a mixed target; yellow mainly represents a cell line target, blue primarily represents the specific cellular mechanism, green represents the enzyme target, and purple represents the entire organism target.

Conclusions and Outlooks
This review has provided a comprehensive overview of 277 tetramic acid products from 120 marine-derived microbes (containing fungi, actinobacteria, bacteria, and cyanobacteria), presented by their structural characteristics and covering up to September 2019, with 195 research publications related to tetramic acids and their bioactivities. Marine fungi are the dominant source of the rapidly increasing numbers of TAs, of which the Aspergillus, Penicillium, Cladosporium species are the predominant marine microbe sources of TAs. Most TAs (77.4%) displayed various pharmacological activities, especially cytotoxicity (40%). Interestingly, deep-sea sediment-derived fungi have become an essential source of the unique structure of bioactive tetramic acids.
As microbial-derived compounds will almost certainly dominate the MNP field in the coming sesquidecade [11], the tetramic acid compounds from marine-derived microorganisms will reveal increasingly greater biological and chemical diversity. Because of the relative ease of collecting marine microbes, a wide variety of approaches for natural product discovery (including metagenomics and genome mining approaches, the heterologous expression method, the OSMAC approach, and chemical epigenetic modification) can be used, as well as advanced and combinational methods for metabolite identification, and several public, private, and commercial databases for rapid dereplication. The various pharmacological properties displayed by TAs with intriguing structures provide medicinal chemists with a variety of potential lead compounds for the development of marine drugs.
Author Contributions: S.C. and M.J. conceived and designed the format of the manuscript. S.C. and M.J. analyzed the data, and drafted and edited the manuscript. S.C. and M.J. drew the chemical structure of compounds. S.C., L.L., and J.L. reviewed the manuscript. All the authors contributed to the critical reading and discussion of the manuscript. All authors have read and agreed to the published version of the manuscript.