Marine Indole Alkaloids—Isolation, Structure and Bioactivities

Indole alkaloids are heterocyclic natural products with extensive pharmacological activities. As an important source of lead compounds, many clinical drugs have been derived from natural indole compounds. Marine indole alkaloids, from unique marine environments with high pressure, high salt and low temperature, exhibit structural diversity with various bioactivities, which attracts the attention of drug researchers. This article is a continuation of the previous two comprehensive reviews and covers the literature on marine indole alkaloids published from 2015 to 2021, with 472 new or structure-revised compounds categorized by sources into marine microorganisms, invertebrates, and plant-derived. The structures and bioactivities demonstrated in this article will benefit the synthesis and pharmacological activity study for marine indole alkaloids on their way to clinical drugs.


Introduction
Marine natural products have incomparable skeleton diversity and novelty relative to terrestrial source ones. They often exhibit superexcellent physiological activities and occupy an important position in today's pharmaceutical industry as a continuously rich source of potential drugs [1][2][3][4][5]. The diversity of their structure enables them to have a broader range of pharmacological activities and action mechanisms, such as neuroprotection, analgesia, smoking cessation, antibacterial, antiviral, antitumor, antihypotension, and antihyperlipidemia [6].
The indole nucleus is one of the most crucial ring systems in nature. It has been termed a "privileged structure" in respect of pharmaceutical development. Viibryd (vilazodone, neurological disorders), decapeptyl (triptorelin, hormonal disorders), symdeko (tezacaftor and lvacaftor, genetic disorders), cialis (tadalafil, sexual health), cubicin (daptomycin, anti-bacterial), zepatier (elbasvir and grazoprevir, infectious diseases), tagrisso (osimertinib, oncology), sutent (sunitinib, oncology), zoladex (goserelin, oncology), alecensa (alectinib, oncology) and lupron (leuprolide, oncology) are all indole-containing top 200 small molecule pharmaceuticals by retail sales in 2018, which were summarized by Njarðarson Group (The University of Arizona, https://njardarson.lab.arizona.edu, 30 October 2021). Due to the high market occupancy and diverse physiological activities, indole alkaloids are now a research hotspot for pharmacologists. In recent years, pharmacological activities of indole alkaloids have been reviewed, including indole alkaloids with anti-diabetic activity [7], anti-malarial potential [8], anti-depression and anti-anxiety activity [9], antitumor and anti-drug-resistant cancer activity [10,11], and immune-regulatory activity [12]. This review focuses on marine indole alkaloids discovered since 2015, when the last comprehensive review, covering the time from 2003 to 2015, was reported by Netz and Opatz [13]. In this review, the newly isolated and structure-revised indole alkaloids from 2005 to 2021, 472 in total, are reported by the classification of sources. All the chemical structures are drawn in this review, and the bioactivities are discussed. The Isonaseseazine B (1), an antimicrobial diketopiperazine dimer, was isolated from Streptomyces sp. SMA-1 by bioassay-guided separation ( Figure 1). Streptomyces sp. SMA-1 was one of the 613 actinobacterial strains isolated from the sediments collected from the Yellow Sea, China [14]. Indolepyrazines A (2) and B (3) were isolated from Acinetobacter sp. ZZ1275, and they showed antimicrobial activities against methicillin-resistant Staphylococcus aureus (MRSA), Escherichia coli (E. coli), and Candida albicans with minimum inhibitory concentration (MIC) values of 12 µg/mL, 8-10 µg/mL, and 12-14 µg/mL, respectively. Indolepyrazine A (2) is the first indole-pyrazine-oxindole alkaloid, and both 2 and 3 are the first reported natural products isolated from marine-derived Acinetobacter species [15]. Streptoprenylindoles A-C (4-6) were acquired from Streptomyces sp. ZZ820. Streptoprenylindoles A and B were enantiomers that were separated by the preparation of Mosher's method. No inhibiting activities of the streptoprenylindoles were reported for the tested MRSA and E. coli [16]. 3-hydroxy-N-methyl-2-oxindole (7)(8) were obtained from marine Salinispora arenicola strain from sediments of Brazil, and they showed no antibacterial activity against Gram-positive (Enterococcus faecalis and Staphylococcus aureus) and Gram-negative (E. coli) bacteria strains [17]. Two new chlorinated bisindole alkaloids, dionemycin (9) and 6-OMe-7 ,7"-dichorochromopyrrolic acid (10) were isolated from the deep-sea derived Streptomyces sp. SCSIO 11791. In vitro antibacterial and cytotoxic assays revealed that compound 9 shows anti-staphylococcal activity with a MIC range of 1-2 µg/mL against six clinic strains of MRSA isolated from human and pig. The cytotoxicity of the trichloro-bisindole 9 was evaluated on human cancer cell lines NCI-H460, MDA-MB-231, HCT-116, HepG-2, and noncancerous MCF10A with IC 50 values ranging from 3.1 to 11.2 µM. Structure-activity relationship analysis of compounds 9, 10, and seven known analogs showed C-6" chlorine as an essential pharmacophore in their cytotoxic activities [18].

Marine-Sourced Fungi
Marine fungi are important components of marine microorganisms, and they are the main source of marine natural products. Among them, Cephalosporin C is the brightest star molecule as the first marine antibiotic [44,45]. In this part, 257 new indole alkaloids were summarized, including 93 from sediment-derived fungi, 62 from coral-derived fungi, 19 from bivalve-mollusk-derived fungi, 20 from Mangrove-sourced fungi, 16 from marine alga endophytic fungi, and 20 from sponge-sourced fungi.
Seven new quinazoline-containing indole alkaloids named aspertoryadins A-G (229, 230, 232, 233, 231, 234 and 235) were isolated from the marine-derived fungus Aspergillus sp. HNMF114, which was separated from the bivalve mollusk Sanguinolaria chinensis ( Figure 25). Compound 229 bears an aminosulfonyl group in the structure, which is rarely encountered in natural products. Compounds 234 and 235 exhibited quorum sensing inhibitory activity against Chromobacterium violaceum CV026 with MIC values of 32, 32 and 16 µg/well, respectively [95]. A continuous work by feeding tryptophan to the marine-derived fungus Aspergillus sp. HNMF114 was carried out, another three new quinazoline-containing indole alkaloids aspertoryadins H-J (236-238) were obtained. The biological activity of these compounds against the insect ryanodine receptor (RyR) was tested using HEK cells stably expressing RyR from Spodoptera frugiperda (sfRyR) or RyR1 from rabbit (rRyR1) and R-CEPIA1er. Alkaloids 236-238 only showed a weak activation effect on sfRyR, which reduced the [Ca 2+ ] ER by less than 7% [96]. Seven new quinazoline-containing indole alkaloids named aspertoryadins A−G (229,  230, 232, 233, 231, 234 and 235) were isolated from the marine-derived fungus Aspergillus sp. HNMF114, which was separated from the bivalve mollusk Sanguinolaria chinensis (Figure 25). Compound 229 bears an aminosulfonyl group in the structure, which is rarely encountered in natural products. Compounds 234 and 235 exhibited quorum sensing inhibitory activity against Chromobacterium violaceum CV026 with MIC values of 32, 32 and 16 μg/well, respectively [95]. A continuous work by feeding tryptophan to the marinederived fungus Aspergillus sp. HNMF114 was carried out, another three new quinazolinecontaining indole alkaloids aspertoryadins H-J (236-238) were obtained. The biological activity of these compounds against the insect ryanodine receptor (RyR) was tested using HEK cells stably expressing RyR from Spodoptera frugiperda (sfRyR) or RyR1 from rabbit (rRyR1) and R-CEPIA1er. Alkaloids 236-238 only showed a weak activation effect on sfRyR, which reduced the [Ca 2+ ]ER by less than 7% [96].
Mar. Drugs 2021, 19, x FOR PEER REVIEW 31 of 56 model and cytotoxicity for HepG-2 human liver carcinoma cells. As a result, compound 314 exhibited proangiogenic activity in a PTK787-induced vascular injury zebrafish model in a dose-dependent manner [129]. Asperginine (315), an alkaloid possessing a rare skeleton, was isolated from the cultural broth of the marine fungus Aspergillus sp., and it has no cytotoxicity against prostate cancer PC3 and human HCT-116 ( Figure 33) [130]. Compounds 316-318 were isolated from the culture broth of a marine gut fungus Aspergillus sp. DX4H, and only showed weak inhibitory activity at 20 μg/mL against PC3 cell line [131]. Two new dioxopiperazine alkaloids (319 and 320) were isolated from Antarctic marine-derived Aspergillus sp. SF-5976. Compound 320 decreased PGE2 production in RAW 264.7 and BV2 cells, and 319 only showed inhibitory effects in BV2 cells and the same situation on LPS-stimulated NO production in RAW 264.7 and BV2 cells [132]. Quellenin (321) was isolated from deep-sea fungus Aspergillus sp. YK-76. It showed weak inhibition against the growth of S. parasitica with inhibition zones of 19.9 mm at the dosage of 200 μg/disc [133]. Asperginine (315), an alkaloid possessing a rare skeleton, was isolated from the cultural broth of the marine fungus Aspergillus sp., and it has no cytotoxicity against prostate cancer PC3 and human HCT-116 ( Figure 33) [129]. Compounds 316-318 were isolated from the culture broth of a marine gut fungus Aspergillus sp. DX4H, and only showed weak inhibitory activity at 20 µg/mL against PC3 cell line [130]. Two new dioxopiperazine alkaloids (319 and 320) were isolated from Antarctic marine-derived Aspergillus sp. SF-5976. Compound 320 decreased PGE2 production in RAW 264.7 and BV2 cells, and 319 only showed inhibitory effects in BV2 cells and the same situation on LPS-stimulated NO production in RAW 264.7 and BV2 cells [131]. Quellenin (321) was isolated from deep-sea fungus Aspergillus sp. YK-76. It showed weak inhibition against the growth of S. parasitica with inhibition zones of 19.9 mm at the dosage of 200 µg/disc [132].

Sponges
Sponges are the simplest multicellular animals in the world, and they settle across the bottom of the sea with more than 10,000 species. There are 133 new indole related compounds isolated from invertebrates recently, and 109 are from 33 species of sponges.

Sponges
Sponges are the simplest multicellular animals in the world, and they settle across the bottom of the sea with more than 10,000 species. There are 133 new indole related compounds isolated from invertebrates recently, and 109 are from 33 species of sponges.
A novel pyridinium, tricepyridinium (355), and a novel benzoxazine-indole hybrid (356, racemic mixture) were obtained from the culture of an E. coli clone incorporating metagenomic libraries from the marine sponge Discodermia calyx. Compound 356 was speculated to be formed through a nonenzymatic process during the isolation procedure. The synthesized tricepyridinium bromide showed antimicrobial activity against Bacillus cereus, MSSA and Candida albicans with MIC values of 0.78, 1.56 and 12.5 µg/mL, but not against E. coli. In addition, tricepyridinium bromide had cytotoxicity to P388 cells with an IC 50 value of 0.53 ± 0.07 µg/mL. Compound 356 exhibited no antibacterial activity against the tested Bacillus cereus [145,146].   Two new indole alkaloids (357 and 358) were obtained from Spongia sp. collected by SCUBA in the South Sea of Korea ( Figure 37). They did not display any significant inhibitory activity on farnesoid X-activated receptor (FXR) up to 100 μM, and they were not cytotoxic to CV-1 cells up to 200 μM on MTT assay, either [148]. 1-(1H-indol-3-yloxy) propan-2-ol (359) was isolated from the Red Sea sponges Haliclona sp. and showed weak cytotoxic activities against the tested HepG-2, Daoy and HeLa by MTT assay [149]. Two bisindole alkaloids tethered by a guanidino ethylthiopyrazine moiety, dragmacidins G (360) and H (361), were isolated from Lipastrotethya sp. marine sponge. Dragmacidin G (360), and dragmacidin H (361), showed cytotoxicity against HeLa cells with IC50 values of 4.2 and 4.6 μM, respectively [150]. Chemical investigation of a specimen of Jaspis splendens collected from the Great Barrier Reef resulted in the isolation of a new bisindole alkaloid, splendamide (362), and 6-bromo-1H-indole-3-carboximidamide (363) are reported for the first time as naturally occurring metabolites. They were subjected to an unbiased phenotypic assay on hONS cells as a model of Parkinson's disease followed by cluster Two new indole alkaloids (357 and 358) were obtained from Spongia sp. collected by SCUBA in the South Sea of Korea ( Figure 37). They did not display any significant inhibitory activity on farnesoid X-activated receptor (FXR) up to 100 µM, and they were not cytotoxic to CV-1 cells up to 200 µM on MTT assay, either [147]. 1-(1H-indol-3-yloxy) propan-2-ol (359) was isolated from the Red Sea sponges Haliclona sp. and showed weak cytotoxic activities against the tested HepG-2, Daoy and HeLa by MTT assay [148]. Two bisindole alkaloids tethered by a guanidino ethylthiopyrazine moiety, dragmacidins G (360) and H (361), were isolated from Lipastrotethya sp. marine sponge. Dragmacidin G (360), and dragmacidin H (361), showed cytotoxicity against HeLa cells with IC 50 values of 4.2 and 4.6 µM, respectively [149]. Chemical investigation of a specimen of Jaspis splendens collected from the Great Barrier Reef resulted in the isolation of a new bisindole alkaloid, splendamide (362), and 6-bromo-1H-indole-3-carboximidamide (363) are reported for the first time as naturally occurring metabolites. They were subjected to an unbiased phenotypic assay on hONS cells as a model of Parkinson's disease followed by cluster analysis of cytological effects and showed similar biological activity in cluster B. under a Pearson's correlation of 0.91 [150]. A new acrylic jasplakinolide congener (364) and another structure-revised acyclic derivative (365) were isolated from the Indonesian marine sponge Jaspis splendens, and the jasplakinolides inhibited the growth of mouse lymphoma (L5178Y) cells in vitro with IC 50 values in the low micromolar to the nanomolar range [151]. A new cyclic peptide, jamaicensamide A (366), composed of six amino acids, including thiazolehomologated amino acid, was isolated from the Bahamian sponge Plakina jamaicensis collected from Plana Cay, and no bioactivity have been evaluated due to the insufficient quantities [152]. A novel brominated marine indole (367) was isolated from the boreal sponge Geodia barretti collected off the Norwegian coast. Compound 367 was inactive (IC 50 > 690 µM) on electric eel AChE even with a structural resemblance with other known natural AChE inhibitors and showed somewhat higher inhibitory potential towards BChE (IC 50 = 222 µM) [153]. Geobarrettin A-C (368-370) were isolated from the sub-Arctic sponge Geodia barrette by UPLC-qTOF-MS-based dereplication study. Both 369 and 370 reduced DC secretion of IL-12p40, but 370 concomitantly increased IL-10 production. Maturing DCs treated with 369 or 370 before co-culturing with allogeneic CD4+ T cells decreased T cell secretion of IFN-γ, indicating a reduction in Th1 differentiation [154].
Antibacterial-guided fractionation of an extract from a deep-water Topsentia sp. marine sponge led to the isolation of two new indole alkaloids, tulongicin A (371) and dihydrospongotine C (372) (Figure 38). Antibacterial, anti-HIV activity and cytotoxicity were evaluated for compounds 371 and 372. They showed strong antibacterial effects toward S. aureus with 1.2 and 3.7 µg/mL MICs. However, only weak to no inhibition toward E. coli at the maximum concentration tested (100 µg/mL) was reported. Both compounds inhibited HIV infection in HIV infectivity assays against the CCR5-tropic primary isolate YU2 and the CXCR4-tropic strain HxB2 with the IC 50 values ranging from 2.7 to 4.5 µM. They were inactive (IC 50 > 10 µM) in cytotoxicity assays against a monkey kidney cell line (BSC-1) and a human colorectal tumor cell line (HCT-116) [155]. A new brominated indole 6-Br-8keto-conicamin A (373) was identified from Haplosclerida sponge, and it showed moderate cytotoxic activity against the PANC-1 tumor cell line with the IC 50 value of 1.5 µM [156]. Two new brominated bisindole alkaloids, dragmacidins I (374) and J (375), were isolated from the Tanzanian sponge Dragmacidon sp. They showed low micromolar cytostatic activity against A549, HT-29 and MDA-MB-231. The mechanism of the action was investigated through different molecular biology experiments, which indicated that these two dragmacidins act via the inhibition of Ser-Thr PPs [157]. Six new cyclopenta[g]indole natural products, trans-herbindole A (376) and trikentramides E-I (377-381), were isolated from the sponge Trikentrion flabelliforme, and there is no bioactivity reported [158]. Antibacterial-guided fractionation of an extract from a deep-water Topsentia sp. marine sponge led to the isolation of two new indole alkaloids, tulongicin A (371) and dihydrospongotine C (372) (Figure 38). Antibacterial, anti-HIV activity and cytotoxicity were evaluated for compounds 371 and 372. They showed strong antibacterial effects toward S. aureus with 1.2 and 3.7 μg/mL MICs. However, only weak to no inhibition toward E. coli  [157]. Two new brominated bisindole alkaloids, dragmacidins I (374) and J (375), were isolated from the Tanzanian sponge Dragmacidon sp. They showed low micromolar cytostatic activity against A549, HT-29 and MDA-MB-231. The mechanism of the action was investigated through different molecular biology experiments, which indicated that these two dragmacidins act via the inhibition of Ser-Thr PPs [158]. Six new cyclopenta[g]indole natural products, trans-herbindole A (376) and trikentramides E−I (377−381), were isolated from the sponge Trikentrion flabelliforme, and there is no bioactivity reported [159].  Guitarrins A−E (390−394), the first natural 5-azaindoles, and aluminumguitarrin A (395), the first aluminum-containing compound from marine invertebrates, were isolated from the sponge Guitarra fimbriata (Figure 40). Guitarrin C (392) inhibited alkaline phosphatase from the marine bacterium Cobetia marina with an IC50 value of 2.0 μM, being a natural inhibitor of alkaline phosphatase [162]. Two brominated oxindole alkaloids (396 and 397) were isolated from sponge Callyspongia siphonella with LC-HRESIMS-assisted Guitarrins A-E (390-394), the first natural 5-azaindoles, and aluminumguitarrin A (395), the first aluminum-containing compound from marine invertebrates, were isolated from the sponge Guitarra fimbriata (Figure 40). Guitarrin C (392) inhibited alkaline phosphatase from the marine bacterium Cobetia marina with an IC 50 value of 2.0 µM, being a natural inhibitor of alkaline phosphatase [161]. Two brominated oxindole alkaloids (396 and 397) were isolated from sponge Callyspongia siphonella with LC-HRESIMS-assisted dereplication and bioactivity-guided isolation. The sponge was collected from Hurghada along the Red Sea Coast. Oxindoles 396 and 397 exhibited diverse pharmacological activities, including antibacterial activity, biofilm inhibitory activity, antitrypanosomal activity and antitumor activity. They inhibited the growth of Staphylococcus aureus (MIC = 8 and 4 µg/mL), Bacillus subtilis (MIC = 16 and 4 µg/mL), Pseudomonas aeruginosa (49.32% and 41.76% inhibition at the concentration of 128 µg/mL), and T. brucei (13.47 and 10.27 µM for 72 h). In addition, they showed good cytotoxic effect toward HT-29, OVCAR-3 and MM.1S with IC 50 values ranging from 7 to 12 µM through non-programmed cell necrosis [162]. A naturally new alkaloid (398) was isolated from Gelliodes sp. collected in Vietnam, and showed no cytotoxicity against Hela, MCF-7 and A549 cell lines [163]. Myrindole A (399), a bis-indole alkaloid, was isolated from the deep-sea sponge Myrmekioderma sp. Myrindole A inhibits the growth of E. coli and Bacillus subtilis with MIC values of 37.5 and 18.5 µM, respectively [164]. The structures of a series of incorrectly reported sponge-derived dibrominated indole alkaloids, echinosulfone A (400) and the echinosulfonic acids A-D (401-404) were corrected [165][166][167]. Another two papers have also disclosed identical structure revisions for these dibrominated indole alkaloids (400-404) [168,169]. A bis-indole (405) and an alkynyl indole alkaloid (406) were isolated from the sponge Plakortis sp. collected from Zampa in Okinawa (Figure 41). The bis-indole was inactive against both P388 and B16 cells even at 100 μg/mL, while 406 showed cytotoxicity against P388 at 1 μg/mL (IC50 = 0.6 μg/mL) and B16 cells at 100 μg/mL [171]. Zamamidine D (407) was isolated from an Okinawan Amphimedon sp. marine sponge, and it exhibited obvious antibacterial activity against the eight tested strains (Escherichia coli, Stapylococcus aureus, A bis-indole (405) and an alkynyl indole alkaloid (406) were isolated from the sponge Plakortis sp. collected from Zampa in Okinawa (Figure 41). The bis-indole was inactive against both P388 and B16 cells even at 100 µg/mL, while 406 showed cytotoxicity against P388 at 1 µg/mL (IC 50 = 0.6 µg/mL) and B16 cells at 100 µg/mL [170]. Zamamidine D (407) was isolated from an Okinawan Amphimedon sp. marine sponge, and it exhibited obvious antibacterial activity against the eight tested strains (Escherichia coli, Stapylococcus aureus, Micrococcus luteus, Aspergillus niger, Trichophyton mentagrophytes, Candida albicans and Cryptococcus neoformans) with IC 50 values ranging from 2 to 32 µg/mL [171]. An extract of the marine sponge Damiria sp., which represents an understudied genus, provided two novel alkaloids named damirines A (408) and B (409). Compound 408 showed selective cytotoxic properties toward six different cell lines in the NCI-60 cancer screen [172]. Makaluvamine W (410) was isolated from the Tongan sponge Strongylodesma tongaensis. Compound 410 was inactive to the tested HL-60 cell line and confirmed the requirement of an intact iminoquinone functionality required by these metabolites to be bioactive [173]. In the study of developing a metric-based prioritization approach by exact LC-HRMS, 411 and 412 were isolated in a case study from a sponge collected from a reef on the island of Tavarua, Fiji Islands. No activity was evaluated for 411 and 412 [174]. Five dibromoindole alkaloids (413-417) were isolated from sponge Narrabeena nigra collected around the Futuna Islands ( Figure 42). They reduced the TBHP-induced cell death, which demonstrated their potential in neuroprotection, and showed almost no cytotoxic effect up to 10 μM on human neuroblastoma SH-SY5Y and microglia BV2 cells [176]. Five dibromoindole alkaloids (413-417) were isolated from sponge Narrabeena nigra collected around the Futuna Islands ( Figure 42). They reduced the TBHP-induced cell death, which demonstrated their potential in neuroprotection, and showed almost no cytotoxic effect up to 10 µM on human neuroblastoma SH-SY5Y and microglia BV2 cells [175].
A highly modified hexapeptide friomaramide (424) was isolated from the Antarctic sponge Inflatella coelosphaeroides, and it blocks more than 90% of Plasmodium falciparum liverstage parasite development at 6.1 µM (Figure 44) [177]. Halicylindramides F-H (425-427) were isolated from a Petrosia sp. marine sponge collected off the shore of Youngdeok-Gun, East Sea, Republic of Korea. Halicylindramides F (425) showed human farnesoid X receptor (hFXR) antagonistic activity, but it did not bind directly to hFXR [178]. Five dibromoindole alkaloids (413-417) were isolated from sponge Narrabeena nigra collected around the Futuna Islands ( Figure 42). They reduced the TBHP-induced cell death, which demonstrated their potential in neuroprotection, and showed almost no cytotoxic effect up to 10 μM on human neuroblastoma SH-SY5Y and microglia BV2 cells [176].   A highly modified hexapeptide friomaramide (424) was isolated from the A sponge Inflatella coelosphaeroides, and it blocks more than 90% of Plasmodium fal liver-stage parasite development at 6.1 μM (Figure 44) [178]. Halicylindramid (425−427) were isolated from a Petrosia sp. marine sponge collected off the shore of deok-Gun, East Sea, Republic of Korea. Halicylindramides F (425) showed hum nesoid X receptor (hFXR) antagonistic activity, but it did not bind directly to hFX Microsclerodermins B and J (428 and 429) were reported by Faulkner and co-workers and Li and co-workers from marine sponge Microscleroderma ( Figure 45) [179,180]. The configuration of the C44 stereocenter of microsclerodermin B was confirmed by total synthesis of dehydromicosclerodermin B, which was revised from 44S to 44R. The same method was applied with microsclerodermins J, and the configuration was revised to be 44R [181]. The structure of topsentin C (430) was revised by an efficient total synthesis [182].
A highly modified hexapeptide friomaramide (424) was isolated from the Antarctic sponge Inflatella coelosphaeroides, and it blocks more than 90% of Plasmodium falciparum liver-stage parasite development at 6.1 μM (Figure 44) [178]. Halicylindramides F−H (425−427) were isolated from a Petrosia sp. marine sponge collected off the shore of Youngdeok-Gun, East Sea, Republic of Korea. Halicylindramides F (425) showed human farnesoid X receptor (hFXR) antagonistic activity, but it did not bind directly to hFXR [179].  Microsclerodermins B and J (428 and 429) were reported by Faulkner and co-work and Li and co-workers from marine sponge Microscleroderma ( Figure 45) [180,181]. T configuration of the C44 stereocenter of microsclerodermin B was confirmed by total sy thesis of dehydromicosclerodermin B, which was revised from 44S to 44R. The sa method was applied with microsclerodermins J, and the configuration was revised to 44R [182]. The structure of topsentin C (430) was revised by an efficient total synthe [183].

Cyanobacteria
Two proline-rich cyclic peptides (455 and 456) were isolated from marine cyanobacterium Symploca sp., collected from Minna Island, Japan and Bintan Island, Indonesia ( Figure 48). Compound 456 possessed cytotoxicity against the MOLT4 and AML2 cancer cell lines with IC 50 values of 4.8 and 8.2 µM, respectively [194,195]

Red Algae
Eleven new tetrahalogenated indoles (457-467) were isolated from the red alga Rhodophyllis membranacea, collected from Moa Point, New Zealand ( Figure 49). Compounds 457 and 459−461 showed no antifungal activity against wild-type Saccharomyces cerevisiae (baker's yeast) and cytotoxicity against HL-60 promyelocytic leukemia cell line with IC50 values higher than 10 μM [197]. Compounds 468-470 were isolated and identified from the red algae Laurencia similis, and 468 showed potent antibacterial activity against seven bacterial strains with MIC values ranging from 2 to 8 μg/mL [198].

Red Algae
Eleven new tetrahalogenated indoles (457-467) were isolated from the red alga Rhodophyllis membranacea, collected from Moa Point, New Zealand ( Figure 49). Compounds 457 and 459-461 showed no antifungal activity against wild-type Saccharomyces cerevisiae (baker's yeast) and cytotoxicity against HL-60 promyelocytic leukemia cell line with IC 50 values higher than 10 µM [196]. Compounds 468-470 were isolated and identified from the red algae Laurencia similis, and 468 showed potent antibacterial activity against seven bacterial strains with MIC values ranging from 2 to 8 µg/mL [197].

Red Algae
Eleven new tetrahalogenated indoles (457-467) were isolated from the red alga Rhodophyllis membranacea, collected from Moa Point, New Zealand ( Figure 49). Compounds 457 and 459−461 showed no antifungal activity against wild-type Saccharomyces cerevisiae (baker's yeast) and cytotoxicity against HL-60 promyelocytic leukemia cell line with IC50 values higher than 10 μM [197]. Compounds 468-470 were isolated and identified from the red algae Laurencia similis, and 468 showed potent antibacterial activity against seven bacterial strains with MIC values ranging from 2 to 8 μg/mL [198].

Conclusions
In this article, we reviewed 472 indole alkaloids discovered from marine organisms with a vast of bioactivities during the year from 2015 to 2021. The alkaloids were grouped according to the sources, divided into marine microbes, invertebrates, and plants. Marine microbes are the main source of natural marine products, which is the case for indole alkaloids. A total of 321 new indole metabolites were isolated from marine microorganisms, including 64 from marine-sourced bacteria and 257 from marine-sourced fungi. Sponges have been abundant and stable sources of marine natural products over the years. Among the indole alkaloids discovered from marine invertebrates, sponge-derived make up the vast majority, with 109 out of 133 in total. Marine plants only contributed 18 indole compounds isolated from cyanobacteria, red algae and mangrove.
Due to the insufficient amount of compounds isolated, the bioactivity determination of natural products has always been a significant challenge. Although a number of top chemists are devoted to moving natural products synthesis from the laboratory to the factory, the total synthesis of some complex marine natural products remains a challenge, and not to mention industrialization. However, marine microbial fermentation has the characteristics of non-destruction of ecological resources, relatively low cost, and good reproducibility. Mass fermentation assisted by genetic engineering transformation can better realize industrial production. In addition, there are many kinds of marine microorganisms, which are inexhaustible sources for marine drugs development. At present, most marine indole alkaloids were evaluated for their antitumor and antimicrobial activities. This is not only because this class of compounds is more likely to exhibit such activity, but also because antitumor and antimicrobial activity measurements are generally easier to perform. It can be summarized from this review that marine indole alkaloids have rich skeleton structures and various pharmacological activities. How to transfer the chemical diversity to pharmacological activity diversity is another challenge. Therefore, it is expected that a more general and practical pharmacological activity assay should be developed and applied to the early drug development process.
Compounds with indole moiety typically have significant pharmacological activity. We hope that this review will promote the development of marine indole alkaloids in medicinal chemistry and pharmacology in terms of the extent of drug discovery.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Table S1: General information of the cell lines; Table S2: Summary of the marine indole alkaloids isolated from marine microorganisms; Table S3: Summary of the marine indole alkaloids isolated from marine invertebrates and plants.

Conclusions
In this article, we reviewed 472 indole alkaloids discovered from marine organisms with a vast of bioactivities during the year from 2015 to 2021. The alkaloids were grouped according to the sources, divided into marine microbes, invertebrates, and plants. Marine microbes are the main source of natural marine products, which is the case for indole alkaloids. A total of 321 new indole metabolites were isolated from marine microorganisms, including 64 from marine-sourced bacteria and 257 from marine-sourced fungi. Sponges have been abundant and stable sources of marine natural products over the years. Among the indole alkaloids discovered from marine invertebrates, sponge-derived make up the vast majority, with 109 out of 133 in total. Marine plants only contributed 18 indole compounds isolated from cyanobacteria, red algae and mangrove.
Due to the insufficient amount of compounds isolated, the bioactivity determination of natural products has always been a significant challenge. Although a number of top chemists are devoted to moving natural products synthesis from the laboratory to the factory, the total synthesis of some complex marine natural products remains a challenge, and not to mention industrialization. However, marine microbial fermentation has the characteristics of non-destruction of ecological resources, relatively low cost, and good reproducibility. Mass fermentation assisted by genetic engineering transformation can better realize industrial production. In addition, there are many kinds of marine microorganisms, which are inexhaustible sources for marine drugs development. At present, most marine indole alkaloids were evaluated for their antitumor and antimicrobial activities. This is not only because this class of compounds is more likely to exhibit such activity, but also because antitumor and antimicrobial activity measurements are generally easier to perform. It can be summarized from this review that marine indole alkaloids have rich skeleton structures and various pharmacological activities. How to transfer the chemical diversity to pharmacological activity diversity is another challenge. Therefore, it is expected that a more general and practical pharmacological activity assay should be developed and applied to the early drug development process.
Compounds with indole moiety typically have significant pharmacological activity. We hope that this review will promote the development of marine indole alkaloids in medicinal chemistry and pharmacology in terms of the extent of drug discovery.