Marine-Derived Macrolides 1990–2020: An Overview of Chemical and Biological Diversity

Macrolides are a significant family of natural products with diverse structures and bioactivities. Considerable effort has been made in recent decades to isolate additional macrolides and characterize their chemical and bioactive properties. The majority of macrolides are obtained from marine organisms, including sponges, marine microorganisms and zooplankton, cnidarians, mollusks, red algae, bryozoans, and tunicates. Sponges, fungi and dinoflagellates are the main producers of macrolides. Marine macrolides possess a wide range of bioactive properties including cytotoxic, antibacterial, antifungal, antimitotic, antiviral, and other activities. Cytotoxicity is their most significant property, highlighting that marine macrolides still encompass many potential antitumor drug leads. This extensive review details the chemical and biological diversity of 505 macrolides derived from marine organisms which have been reported from 1990 to 2020.


Introduction
The term "macrolide" was coined by Woodward in 1957 [1] to describe antibiotics which typically consist of 14-, 15-or 16-membered macrolactam rings and feature double bonds and different saccharide and aminosaccharide functional groups. The naturally occurring 14-membered lactones erythromycin and clarithromycin, 15-membered macrolides azithromycin and spiramycin, and the 16-membered avermectin B1a are typical macrolide antibiotics in clinical use [2][3][4]. The 26-membered macrolide oligomycin A (an inhibitor of ATP synthase) [5,6] and the 36-membered macrocyclic lactone amphotericin B (an antifungal agent) are also used clinically [7,8]. In the last thirty years, many studies have described the molecular features, structures, and bioactivities of the intriguing macrolides obtained from plants, animals, and microbes in terrestrial and marine ecosystems [9][10][11][12]. Macrolides with larger macrocyclic rings have been reported, exemplified by the cytotoxic swinholide H, with its 40-membered lactone ring, obtained from the New Zealand deep-water marine sponge Lamellomorpha strongylata (La. strongylata) [13], and the novel 62-membered polyol symbiodinolide from the symbiotic dinoflagellate Symbiodinium sp. [14]. Macrolides, therefore, can be considered more broadly as a class of uncorrelated compounds containing a ring of twelve or more members.
This literature review from 1990 to 2020 highlights 505 new macrolides derived from marine organisms (65.8% of which are from sponges, fungi, and dinoflagellates) ( Figure 1). Compared with terrestrial environments, the oceans exhibit more wide-ranging hypersaline, hyperbaric, hypoxic, cryogenic, and oligotrophic conditions. Marine organisms must develop the capacity to produce diverse bioactive metabolites to survive in these complex and competitive ecosystems. Marine metabolites have huge potential as new drug leads, with nine approved pharmaceuticals and 31 compounds in clinical pharmaceutical trials [15]. Macrolides are a significant family of natural marine products ( Figure 2). The complex and competitive ecosystems. Marine metabolites have huge potential as new drug leads, with nine approved pharmaceuticals and 31 compounds in clinical pharmaceutical trials [15]. Macrolides are a significant family of natural marine products ( Figure  2). The marine macrolides reviewed herein display cytotoxic, antibacterial, antifungal, antimitotic, antiviral, antiplasmodial and other bioactivities, as listed in Table 1. This review discusses the isolation, structures, and chemical and bioactive diversity of marine macrolides from 309 publications.

Sponges
The Okinawan Theonella sp. (T. sp.) sponges produced a series of dimeric macrolides called swinholides A-G (1-7) and isoswinholide A (8) [16][17][18][19]. Four bistheonellide-related compounds-bistheonellide C (9), isobistheonellide A (10), and bistheonellic acids A (11) and B (12)-are also produced by Okinawan T. sp. sponges [20]. The structure of the macrolide miyakolide (13), which is weakly cytotoxic and obtained from Japanese sponge complex and competitive ecosystems. Marine metabolites have huge potential as new drug leads, with nine approved pharmaceuticals and 31 compounds in clinical pharmaceutical trials [15]. Macrolides are a significant family of natural marine products ( Figure  2). The marine macrolides reviewed herein display cytotoxic, antibacterial, antifungal, antimitotic, antiviral, antiplasmodial and other bioactivities, as listed in Table 1. This review discusses the isolation, structures, and chemical and bioactive diversity of marine macrolides from 309 publications.
A new 22-membered macrocyclic lactone named dictyostatin 1 (33) was isolated from a Republic of Maldives Spongia sponge and exhibited significant cytotoxicity towards murine P388 lymphocytic leukemia [32]. The relative stereochemistry of dictyostatin 1 was determined by Murata's method [33]. Two new 26-membered macrolides, reidispongiolides A (34) and B (35), have been produced by the marine sponge Reidispongia coerulea (R. coerulea) [34]. The relative and absolute stereochemistries of the C-23-C-35 portion of reidispongiolide A were determined by synthesis of an ozonolysis fragment of the natural product [35], which was later synthesized enantioselectively [36]. The relative stereochemistry of the C-7-C-15 fragment was reassigned through a series of diastereomers of a degradation fragment synthesis [37].
Cytotoxic superstolide A (36) and superstolide B (37) have been isolated from the deep-water marine sponge Neosiphonia superstes (N. superstes) [38,39]. Another cytotoxic macrolide, lasonolide A (38), was produced by the shallow-water Caribbean sponge Forcepia sp. [40]. Isohomohalichondrin B (39), belonging to the halichondrin family, was isolated from the New Zealand deep-water sponge Lissodendoryx sp. (Li. sp.) [41]. Phorboxazoles A (40) and B (41) have an unprecedented scaffold and were isolated from the Indian Ocean sponge Phorbas sp. (P. sp.), with complete stereochemistry and absolute configuration determined by spectroscopy and partial synthesis [42,43]. The structures and absolute A new 22-membered macrocyclic lactone named dictyostatin 1 (33) was isolated from a Republic of Maldives Spongia sponge and exhibited significant cytotoxicity towards murine P388 lymphocytic leukemia [32]. The relative stereochemistry of dictyostatin 1 was determined by Murata's method [33]. Two new 26-membered macrolides, reidispongiolides A (34) and B (35), have been produced by the marine sponge Reidispongia coerulea (R. coerulea) [34]. The relative and absolute stereochemistries of the C-23-C-35 portion of reidispongiolide A were determined by synthesis of an ozonolysis fragment of the natural product [35], which was later synthesized enantioselectively [36]. The relative stereochemistry of the C-7-C-15 fragment was reassigned through a series of diastereomers of a degradation fragment synthesis [37]. A new 22-membered macrocyclic lactone named dictyostatin 1 (33) was isolated from a Republic of Maldives Spongia sponge and exhibited significant cytotoxicity towards murine P388 lymphocytic leukemia [32]. The relative stereochemistry of dictyostatin 1 was determined by Murata's method [33]. Two new 26-membered macrolides, reidispongiolides A (34) and B (35), have been produced by the marine sponge Reidispongia coerulea (R. coerulea) [34]. The relative and absolute stereochemistries of the C-23-C-35 portion of reidispongiolide A were determined by synthesis of an ozonolysis fragment of the natural product [35], which was later synthesized enantioselectively [36]. The relative stereochemistry of the C-7-C-15 fragment was reassigned through a series of diastereomers of a degradation fragment synthesis [37].
Macrolide salicylihalamides A (59) and B (60) were isolated from the Haliclona sponge, representing a potentially important new class of antitumor leads [56]. The absolute configurations of salicylihalamides A and B have been revised by a reinterpretation of Mosher ester derivatives and enantioselective syntheses of both enantiomers [57][58][59]. Cytotoxic callipeltoside B (61) and C (62), two members of a novel class of marine glycoside macrolides, were isolated from the sponge Cal. sp. [60].
Macrolide salicylihalamides A (59) and B (60) were isolated from the Haliclona sponge, representing a potentially important new class of antitumor leads [56]. The absolute configurations of salicylihalamides A and B have been revised by a reinterpretation of Mosher ester derivatives and enantioselective syntheses of both enantiomers [57][58][59]. Cytotoxic callipeltoside B (61) and C (62), two members of a novel class of marine glycoside macrolides, were isolated from the sponge Cal. sp. [60].
Macrolide salicylihalamides A (59) and B (60) were isolated from the Haliclona sponge, representing a potentially important new class of antitumor leads [56]. The absolute configurations of salicylihalamides A and B have been revised by a reinterpretation of Mosher ester derivatives and enantioselective syntheses of both enantiomers [57][58][59]. Cytotoxic callipeltoside B (61) and C (62), two members of a novel class of marine glycoside macrolides, were isolated from the sponge Cal. sp. [60].
Macrolide salicylihalamides A (59) and B (60) were isolated from the Haliclona sponge, representing a potentially important new class of antitumor leads [56]. The absolute configurations of salicylihalamides A and B have been revised by a reinterpretation of Mosher ester derivatives and enantioselective syntheses of both enantiomers [57][58][59]. Cytotoxic callipeltoside B (61) and C (62), two members of a novel class of marine glycoside macrolides, were isolated from the sponge Cal. sp. [60].
Cytotoxic spongidepsin (87) has been isolated from the Vanuatu marine sponge Spongia sp. [71]. A new cytotoxic 20-membered macrolide, dactylolide (88), was isolated from a marine sponge of the genus Dactylospongia. This has been synthesized and the relative stereochemistry of the acyloxymethine and the absolute configuration of the whole molecule have been determined [72]. The Vanuatu marine sponge Ha. sp. was found to contain the cyclic metabolite haliclamide (89) [73].
A further collection led to the isolation of salarin C (145), which was considered to the precursor of salarins A and B [100]. Marine sponge Siliquariaspongia mirabilis contain an antitumor macrolide lactam named mirabilin (146) [101]. The nitrogenous bismacroli tausalarin C (147) was isolated from the Madagascar sponge F. sp. and was found to i hibit proliferation of K562 leukemia cells [102]. Muironolide A (148), containing a ra hexahydro-1H-isoindolone and trichlorocarbinol ester, was isolated from marine spon of the genus Phorbas [103].
Mar. Drugs 2021, 19, x FOR PEER REVIEW Three decalactones, xestodecalactones D-F (222-224), were purified from acetate extract of Corynespora cassiicola isolated from leaf tissues of the Chinese m medicinal plant Laguncularia racemose [140]. Seiricuprolide pestalotioprolides A ( B (226) (as the diacetate) were isolated from the fungus Pestalotiopsis spp., which ciated with mangrove twigs of Rhizophora mucronata [141]. Calcarides A-C (2 15G256α (230), and 15G256β (231) were obtained from crude extracts of the fun carisporium sp. KF525 isolated from German Wadden Sea water samples [142]. Thirteen new 12-membered macrolides, dendrodolides A-M (232-244), were ob tained from the fungus Dendrodochium sp. derived from sea cucumber Holothuria nobili Selenka in the South China Sea [143]. Dendrodolide K was obtained from a commerciall available substrate by a convergent strategy, and the dendrolides F, G, I, J, and L wer synthesized via a unified strategy employing ring-closing metathesis [144,145].  [146], its absolute configuration was corrected in a later study [147] The fungus Pen. sumatrense MA-92, associated with the mangrove Lumnitzera race Thirteen new 12-membered macrolides, dendrodolides A-M (232-244), were obtained from the fungus Dendrodochium sp. derived from sea cucumber Holothuria nobilis Selenka in the South China Sea [143]. Dendrodolide K was obtained from a commercially available substrate by a convergent strategy, and the dendrolides F, G, I, J, and L were synthesized via a unified strategy employing ring-closing metathesis [144,145]. Thirteen new 12-membered macrolides, dendrodolides A-M (232-244), were obtained from the fungus Dendrodochium sp. derived from sea cucumber Holothuria nobilis Selenka in the South China Sea [143]. Dendrodolide K was obtained from a commercially available substrate by a convergent strategy, and the dendrolides F, G, I, J, and L were synthesized via a unified strategy employing ring-closing metathesis [144,145].   [146], its absolute configuration was corrected in a later study [147].
Two 36-membered macrolides, bahamaolides A and B (295-296), were obtained from the culture of a marine actinomycete S. sp. isolated from a sediment sample collected at North Cat Cay in the Bahamas [170].
Two 36-membered macrolides, bahamaolides A and B (295-296), were obtained from the culture of a marine actinomycete S. sp. isolated from a sediment sample collected at North Cat Cay in the Bahamas [170]. Two 36-membered macrolides, bahamaolides A and B (295-296), were obtained from the culture of a marine actinomycete S. sp. isolated from a sediment sample collected at North Cat Cay in the Bahamas [170]. B. subtilis isolated from marine sediment collected at Gageocho (Republic of Korea) was a source of three new glycosylated methoxy-macrolactins (297-299) [171]. Three new 24-membered macrolactones, macrolactins X-Z (300-302), featuring an oxetane, an epoxide, and a tetrahydropyran ring, were isolated from an ethyl acetate extract of a marine B. sp. [172]. Cytotoxic juvenimicin C (303) was produced by a marine-derived actinomycete strain (CNJ-878) [173]. The M. strain FIM07-0019 isolated from shallow coastal waters near the island of Chiloe (Chile) produced a 20-membered macrolide, levantilide C (304) [174].
Mar. Drugs 2021, 19, x FOR PEER REVIEW
Two analogs of polycavernosolide A, polycavernosides C (405) and C2 (40 isolated from the red alga Gracilaria edulis (G. edulis) [255]. Manauealides A-C ( were isolated from extracts of red alga G. coronopifolia [256]. Anhydrodebromo toxin (410) and manauealide C were extracted from Hawaiian G. coronopifolia [257 tigation of Fijian red alga Callophycus serratus (C. serratus) led to the isolation of thr pene-benzoate natural products: bromophycolides A (411) and B (412), and a nonh Two analogs of polycavernosolide A, polycavernosides C (405) and C2 (406), were isolated from the red alga Gracilaria edulis (G. edulis) [255]. Manauealides A-C (407-409) were isolated from extracts of red alga G. coronopifolia [256]. Anhydrodebromoaplysiatoxin (410) and manauealide C were extracted from Hawaiian G. coronopifolia [257]. Investigation of Fijian red alga Callophycus serratus (C. serratus) led to the isolation of three diterpenebenzoate natural products: bromophycolides A (411) and B (412), and a nonhalogenated compound (413). Bromophycolides A and B exhibited moderate antibacterial and antifungal properties while bromophycolides A demonstrated potent anti-HIV and moderate cytotoxic activities [258]. Bromophycolides C-I (414-420) were also isolated from extracts of C. serratus. All the bromophycolides exhibited modest antineoplastic activity towards a range of human tumor cell lines while bromophycolides F and I showed weak antifungal activity [259]. antifungal properties while bromophycolides A demonstrated potent anti-HIV and moderate cytotoxic activities [258]. Bromophycolides C-I (414-420) were also isolated from extracts of C. serratus. All the bromophycolides exhibited modest antineoplastic activity towards a range of human tumor cell lines while bromophycolides F and I showed weak antifungal activity [259].
Further investigation of the C. serratus extract yielded a series of unusual antimalarial diterpene-benzoate macrolides, bromophycolides J-Q (421-428), with a range of moderate to strong antimicrobial and anticancer properties [260]. C. serratus was also a source of the diterpene-benzoate macrolides bromophycolides R-U (429-432). These demonstrated modest cytotoxicity toward selected human cancer cell lines while bromophycolide S was active (at submicromolar concentrations) against the human malaria parasite Plasmodium falciparum (Pla. falciparum) [261].
The α-pyrone macrolides neurymenolides A (433) and B (434) were obtained from the Fijian red alga Neurymenia fraxinifolia [262]. The brown alga Ecklonia stolonifera produced ecklonialactones C (435) and D (436) containing a 14-membered lactone moiety, and ecklonialactones E (437) and F (438), with a 16-membered moiety [263]. The absolute The α-pyrone macrolides neurymenolides A (433) and B (434) were obtained from the Fijian red alga Neurymenia fraxinifolia [262]. The brown alga Ecklonia stolonifera produced ecklonialactones C (435) and D (436) containing a 14-membered lactone moiety, and ecklonialactones E (437) and F (438), with a 16-membered moiety [263]. The absolute configurations of ecklonialactones A, B and E were determined from chiroptical data [264]. Eight oxylipins (439-446) with a macrolide scaffold and one cymathere-type oxylipin with an open ring were isolated from the brown alga Eisenia bicyclis. The absolute configurations of compounds 439-443 and 446 were determined by NMR spectroscopy with the relative stereochemistry at C-9 in 446 remaining unassigned [265]. The metamorphosisenhancing macrodiolide luminaolide (447) was isolated from the crustose coralline alga Hydrolithon reinboldii and its absolute relative configuration was determined by NMR spectroscopy with the relationships of the two side chains to the macrolide ring remaining unassigned [266,267]. configurations of ecklonialactones A, B and E were determined from chiroptical data [264]. Eight oxylipins (439-446) with a macrolide scaffold and one cymathere-type oxylipin with an open ring were isolated from the brown alga Eisenia bicyclis. The absolute configurations of compounds 439-443 and 446 were determined by NMR spectroscopy with the relative stereochemistry at C-9 in 446 remaining unassigned [265]. The metamorphosis-enhancing macrodiolide luminaolide (447) was isolated from the crustose coralline alga Hydrolithon reinboldii and its absolute relative configuration was determined by NMR spectroscopy with the relationships of the two side chains to the macrolide ring remaining unassigned [266,267].

Tunicates
Two 24-membered macrolide sulfates showing antineoplastic activity, iejimalides C (487) and D (488), were isolated from the Okinawan tunicate Eudistoma cf. rigida [298]. Two cytotoxic macrolides, lobatamides A (489) and B (490), were reported in the tunicate Aplidium lobatum [299]. A. lobatum from shallow waters in Australia, A. sp. from deep water, and an unidentified Philippine ascidian have been reported as sources of a series of macrolides, lobatamides C-F (491-494), demonstrating cytotoxicity towards human tumor cell lines [300]. The absolute stereochemistry of lobatamide C was determined by stereospecific synthesis [301]. The chlorinated macrolide haterumalide B (495) was obtained from an Okinawan ascidian L. sp. by bioassay-guided isolation and was shown to inhibit the first cleavage of fertilized sea urchin eggs at 0.01 μg/mL [302]. The Okinawan ascidian Didemnidae sp. was the source of the macrolides biselides A (496) and B (497) [303]. Further

Tunicates
Two 24-membered macrolide sulfates showing antineoplastic activity, iejimalides C (487) and D (488), were isolated from the Okinawan tunicate Eudistoma cf. rigida [298]. Two cytotoxic macrolides, lobatamides A (489) and B (490), were reported in the tunicate Aplidium lobatum [299]. A. lobatum from shallow waters in Australia, A. sp. from deep water, and an unidentified Philippine ascidian have been reported as sources of a series of macrolides, lobatamides C-F (491-494), demonstrating cytotoxicity towards human tumor cell lines [300]. The absolute stereochemistry of lobatamide C was determined by stereospecific synthesis [301]. The chlorinated macrolide haterumalide B (495) was obtained from an Okinawan ascidian L. sp. by bioassay-guided isolation and was shown to inhibit the first cleavage of fertilized sea urchin eggs at 0.01 µg/mL [302]. The Okinawan ascidian Didemnidae sp. was the source of the macrolides biselides A (496) and B (497) [303]. Further investigation of the D. sp. led to the isolation of biselides C (498), D (499) and E (500) which exhibited cytotoxicity against human cancer cells NCI-H460 and MDA-MB-231 [304]. Cytotoxic palmerolide A (501) was obtained from the Antarctic tunicate Synoicum adareanum [305] and its stereochemistry was revised and confirmed by synthesis [306,307]. investigation of the D. sp. led to the isolation of biselides C (498), D (499) and E (500) which exhibited cytotoxicity against human cancer cells NCI-H460 and MDA-MB-231 [304]. Cytotoxic palmerolide A (501) was obtained from the Antarctic tunicate Synoicum adareanum [305] and its stereochemistry was revised and confirmed by synthesis [306,307].

Bioactivities of Marine-Derived Macrolides
The biological activities of marine-derived macrolides have been studied extensive As listed in Table 1, marine macrolides harbor a broad range of bioactive properties cluding cytotoxicity, antibacteria, antifungi, antimitotic, antiviral, and other activiti with cytotoxicity being their most significant bioactivity. Glycosylated macrolides mandelalides A−D (502-505) were isolated from Lissoclinum ascidian collected in Algoa Bay near Port Elizabeth and the surrounding Nelson Mandela Metropole in South Africa [308].

Bioactivities of Marine-Derived Macrolides
The biological activities of marine-derived macrolides have been studied extensively. As listed in Table 1, marine macrolides harbor a broad range of bioactive properties including cytotoxicity, antibacteria, antifungi, antimitotic, antiviral, and other activities, with cytotoxicity being their most significant bioactivity.

Conclusions and Outlook
This review presents a summary of 505 marine-derived macrolides reported from 1990 to 2020 and highlights their chemical and biological diversity. As shown in Figure 1, sponges are the dominant producer of marine macrolides, yielding 173 of these 505 compounds (34.3%). Fungi and dinoflagellates are also important sources, producing 19.4% and 12.1%, respectively, of the macrolides reviewed. Marine animals (cnidarians, bryozoans, tunicates, and mollusks) produced significantly fewer macrolides with a combined percentage of 11.6%, while marine plants (red algae) yielded 9.5%. Marine microbes (including fungi, bacteria, cyanobacteria) produced 32.7% of 505 macrolides. Notably, macrolides obtained from sponges have fallen since 2010, while microbes, especially fungi, have grown to be important producers (Figure 2). This phenomenon suggests that biochemists are acknowledging that sampling slow-growing sessile organisms to identify natural products is not an eco-friendly practice. More attention is now being given to microbes due to their capacity for unlimited reproduction and the ease with which their genome can be mined for targeted metabolites. Marine macrolides have a broad range of properties, including cytotoxic, antifungal, antimitotic, and some other activities (Table 1). Cytotoxicity is their most significant bioactivity, highlighting that marine macrolides include many potential antitumor drug leads.
For macrolides with larger macrocyclic rings, such as reidispongiolides A and B [34], symbiodinolide [14] and zooxanthellatoxins A and B [232,233], the flexible ring structures make stereochemistry identification more difficult. Novel configuration determination technologies, such as sponge crystals [309], are needed to solve this problem. Although they possess diverse bioactivities, few marine macrolides have been developed into approved antitumor drugs or even for clinical trials during the last thirty years. Limited production from natural biomaterials and difficulties in synthesis may be hindering new drug discovery. High throughput screening and investigation of target prediction and additional bioactivity mechanisms must be employed to increase the successful discovery of lead compounds from marine macrolides. This should include mining for more structurally unusual macrolides with broader bioactivities.