Marine Cyanobacteria and Microalgae Metabolites—A Rich Source of Potential Anticancer Drugs

Cancer is at present one of the utmost deadly diseases worldwide. Past efforts in cancer research have focused on natural medicinal products. Over the past decades, a great deal of initiatives was invested towards isolating and identifying new marine metabolites via pharmaceutical companies, and research institutions in general. Secondary marine metabolites are looked at as a favorable source of potentially new pharmaceutically active compounds, having a vast structural diversity and diverse biological activities; therefore, this is an astonishing source of potentially new anticancer therapy. This review contains an extensive critical discussion on the potential of marine microbial compounds and marine microalgae metabolites as anticancer drugs, highlighting their chemical structure and exploring the underlying mechanisms of action. Current limitation, challenges, and future research pathways were also presented.

Microflora and, up to certain extent, microalgae could be regarded as chief drivers of nutrient transformations in a marine ecosystem. Shift in abiotic factors of marine ecosystem, such as temperature, salinity, nutrients, oxygen, solar energy, water clarity, tides, waves, aerial exposure and current has immensely influenced the production and secretion of marine metabolites/ bioactive chemical compounds from these organisms. To survive under these harsh abiotic factors, the marine microflora tends to form a symbiotic relationship with other marine microorganisms. These synergetic relationships enable them to endure and secrete a number of exotic secondary metabolites/bioactive compounds which they would not do under natural circumstances [25]. Over the last two decades, natural product research has observed a paradigm shift from terrestrial areas to oceans. Approximately 15,000 marine-origin metabolites, which have demonstrated cytotoxicity in cancer models, have been identified during the past three decades and from these, 28 agents are under clinical trials for their anticancer properties [26]. There is myriad of structural motifs undiscovered from the marine ecosystem and these metabolites can serve as new potent anticancer agents.
There are only a few previous reviews that present an in-depth overview of this important field of research. Many of the preceding publications focus exclusively on the compilation of marine secondary metabolites for study on natural products [27][28][29][30][31]. This review looks at the pharmacology of marine species with respect to anticancer drug molecules which have shown significant bioactivity to become drug or is in queue to enter clinical trials. The data procured covers published preclinical study, research papers and review of marine molecules isolated from a diverse group of marine algae, and cyanobacteria. This review evaluates the anticancer effects of numerous marine cyanobacterium and microalgae secondary metabolites, emphasizing on their chemical structures and highlighting the mechanisms of action that underlie their pharmacological activities.

Literature Search Methodology
In vitro, in vivo and clinical research investigating the anticancer potential of secondary secreted metabolites of marine species by modulating various pathways were screened utilizing credible repositories which include PubMed, ScienceDirect, Web of Science, SpringerLink, Scopus, and Google Scholar. Comprehensive papers released in peer-reviewed publications up to April 2020 have been included. There was no time restrainst on publication year. Only publications written in English have been listed and used in this article. The exclusion criteria for not setecting articles included non-English language publications, letters to editors, conference abstracts, and unpublisged reports. The keywords included within the literature quest are cancer, tumor, proliferation, cytotoxicity, apoptosis, marine, microbes, microalgae, cyanobacteria, prevention, therapy, In vitro, in vivo, and clinical trials. The bibliography of selected primary literature was also searched for additional relevant papers.

Alkaloids
Alkaloids are naturally occurring/synthetic organic compounds collectively used to describe the diverse groups of heterocyclic compounds having alkali-like properties and having at least one single nitrogen atom within its structure [32,33]. Presently, these nitrogen atoms containing heterocyclic compounds could be classified based on the basis of carbon skeleton resemblance contained in biochemical precursors such as ornithine, lysine, tyrosine, and tryptophan, having either indole and isoquinoline, or pyridine moieties [34]. Marine alkaloids could also be grouped into indoles, halogenated indoles, and phenylethylamines [35]. Marinoquinoline A is an anticancer alkaloid produced by Catalinimonas alkaloidigena, a marine bacterium [36] along with 13 other alkaloid metabolites. Pseudoalteromonas tunicata and P. citrea are two marine bacteria that secrete a yellow-pigmented alkaloid that belongs to a group called tambjamines, and this showed anti-tumor activity along with antimicrobial, antifungal, and antimalarial activity [37,38]. Calothrixins A and B are alkaloids containing a phenanthridine moiety that are isolated from Calothrix sp. Both the alkaloids showed significant cytotoxicity to human cervical carcinoma cells (HeLa) [39]. Demay et al. [40] have comprehensively reviewed a variety of bioactive metabolites of cyanobacteria, such as Hapalosiphon fontinalis, Fischerella musicola, F. ambigua, H. welwitschii, and Westiella intricate, as among which hapalindole-like alkaloid, and ambigols, possess cytotoxic activity.

Polyketides
Polyketides are a broad group of compounds which are biosynthesized as analogs generated by a sequence of modular enzymes act as biocatalysts from precursors which contain alternating carbonyl and methylene groups(-CO-CH 2 -) [41]. Then the compound undergoes decarboxylative condensation and modification of the acetate or the propionate chains primarily by reduction reaction, dehydration reaction, cyclization reaction, and aromatization reactions. Streptomyces koyangensis, a marine bacterium that produces two abyssomicins which have antitumor activity, but in-depth investigation is scanty [42]. Extracts of marine cyanobacteria like Nostoc spongiaeforme and N. linckia, contained a polyketide called borophycin that showed the strong anticancerous effect against human colon carcinoma cell lines (LoVo) [43].

Terpenes
Terpenes are the hydrocarbon compounds formed from 5-carbon isoprene units assembled to generate a vast range of skeletons, which are used by various enzymes to conjugate functionality and alter oxidation. These cyclic molecules can be categorized as monoterpenes, diterpenes, triterpenes (steroids), tetraterpenes (carotenoids), sesquiterpenes, and sesterterpenes based on the isoprene units it contains [44]. Presently it is acknowledged that marine microflora is an excellent source of these terpenes, and among which few terpenes exhibited their anticancer activity apart from other biological properties [22].

Peptides
Peptides are the definite protein fragments which provide optimistic impacts on human health [41][42][43][44][45]. Peptides are usually inert within the parent protein chain and may exhibit several physiological tasks upon proteolysis [46,47]. Enzymatic hydrolysis has played a significant role in the synthesis of peptide compounds in marine species [48,49]. Numerous prospective reports were documented about the utilization of marine peptides for pharmaceutical developments, including antitumor properties [50,51]. Cyclic and linear peptides have been established as potentially effective cytotoxic agents. These peptides possess cytotoxic, antimicrobial, specific ion channel-blocking, and other pharmacological activities with innovative chemical structures correlated with the actual mode of action [50].
A new peptide, polydiscamide A, and its analogs possess anti-tumor activity [50]. A certain number of marine peptides were successfully evaluated through clinical research and have now been available as formulated drugs in the market under various trade names.
A cyclic depsipeptide known as Apratoxin A exhibited effective cytotoxic potential against the human cervical carcinoma cells (HeLa) by triggering cell cycle arrest [52]. Peptides extracted from Lyngbya sp. and Nostoc sp., were reported to possess promising anticancer activity by the disruption of microfilaments, inhibition of secretory pathways, and influencing other intracellular pathways [26].

Nucleosides
Nucleosides belong to a class of organic compounds and are commonly known as the nitrogen glycosides of purines and pyrimidines; with their phosphate esters, they are called nucleotides [53,54]. These are the important constituents of all living cells and are associated with various fundamental physiological processes [53,54]. Marine microbes are skilled at manufacturing different nucleosides forms with unfamiliar structures and associated biological properties [53][54][55][56][57]. A few of these nucleosides with substantial pharmacological activities have been documented [56][57][58]. The potent biological properties of marine-derived nucleosides have stimulated the production of some analogs [59][60][61]. Marine nucleosides themselves have exhibited numerous bioactive potentials including the anticancer, antiviral, muscle relaxant, hypertensive and vasodilator activities [54].

Carbohydrates
Carbohydrates form the major component of aquatic organisms and are considered the significant food source of these organisms, particularly algae [62]. These compounds vary extremely in their molecular structure and resemble a pool of sulfated polysaccharides manufactured fixation of sulfur and carbon by the photosynthetic organisms [62]. Carbohydrates are classified by source in three groups: plant polysaccharides [63], animal polysaccharides [64], and microbial polysaccharides [65], which include both neutral and negatively charged saccharides with varying size [24]. Examples include either the nitrogen-linked or oxygen-linked oligosaccharides in glycoproteins, glycosaminoglycan in proteoglycans, glycolipids, sulfated fucans, and sulfated galactans [24,66]. However, their chemical compositions and arrangements are complex and heterogeneous in nature [67].
Marine carbohydrates are produced by a specific enzymatic hydrolysis process of polysaccharides [67]. The enzymatic breakdown of the sulfated polysaccharides involves a set of enzymes that can cleave the glycosidic bond and eliminate the sulfate groups from the carbohydrate backbone [62]. These marine-derived carbohydrates, including alginic acid, agar, carrageenan, chitin, cellulose, chitosan, fucan, glucan, glucosamine glycan, and laminaran; these possesses a wide number of substantial bioactive properties which includes the anticancer potentials [11,67,68]. These carbohydrate-based compounds exhibited anticancer effects against a number of carcinoma cells by modulating the innate immune system, which triggered the chemotactic response of macrophages and natural killer cells to the target location and have them produce their tumoricidal cytokines [12]. Fucoidan is a sulfated polysaccharide present as a metabolite in the brown algae cell wall and has been shown to inhibit atherosclerosis, angiogenesis, and metastasis [69] in the human lymphoma cell line (HS-Sultan) by subsequent activation of caspase-3, and downregulating kinase activity [12,70].

Secondary Metabolites of Marine Cyanobacteria and Microalgae at Various Phases of Clinical Research
Certain anticancer compounds from marine cyanobacteria and microalgae are currently undergoing clinical trials [12,22]. These natural bioactive compounds exhibited anticancer activity by regulating macromolecule expression induced in cancer cells via oncogenic signal transduction pathways [71].
Fewer than 10% of marine pharmacologically active compounds have been tested against diverse cancer types [60]. For example, the compounds dolastatin 10, ET-743, and bryostatin 1 are tested and analyzed in clinical research [12]. A variety of dolastatins and associated molecules were extracted from filamentous cyanobacteria of the genera Symploca and Lyngbya [72].These are small oligopeptides containing four unique non-protein amino acids-dolaphenine, dolaproline, dolaisoleucine, and dolavaline. Dolastatin 10 is a linear peptide while dolastatin 15 is a seven-unit depsipeptide agent, and both are potent cytostatic peptides that arrest cell division [59]. This was proven by studies that showed that dolastatins, especially dolastatin 10, attached to tubulin at the guanosine triphosphate position, leading to the disruption of its normal function and triggering metaphase cell cycle arrest [73]. Dolastatin 10 reached Phase I of clinical research in 1990s and has advanced to Phase II of clinical research [74]. However, because it developed peripheral neuropathy in over 40% of patients, it was discontinued [74]. Even so, it was the basis for more efficient derivatives to come into existence. One of its derivatives, the monoclonal antibody-drug conjugate Brentuximab vedotin (Adcetris) has been licensed for its anticancer activity against Hodgkin's lymphoma [75].  (TZT-1027) is a synthetic analog of dolastatin 10 that is more potent against cancer than its parent compound and other established anticancer medications such as podophyllotoxin and vincristine [76]. In addition to inhibiting tubulin function, TZT-1027, a vascular disrupting agent, causes a collapse in the vasculature of the tumor, causing cell death [77]. After completing Phase I and II trials, TZT-1027 is in the Phase III of clinical research under Aska Pharmaceuticals [78].
Synthadotin (ILX-651) is a synthetic pentapeptide derivative of dolastatin 15, is a potent antitumor agent in patients having metastatic melanoma in advanced stage [75]. It has been shown to inhibit microtubule nucleation [79]. Other dolastatins showed cardiovascular toxicity, but ILX-651 has not exhibited such toxicity [79]. It has successfully completed both Phase I and Phase II of clinical research and was found to be well tolerated and completely safe [80]. Another compound, bryostatin 1, has successfully completed Phase II of clinical research for the treatment of melanoma, non-Hodgkin's lymphoma, renal cancer, and colorectal cancer [12]. The marine bioactive compounds play a significant major role in the discovery of anticancer drugs, and these are classified as the antimigration, antimetastatic, anti-invasion, antitubulin agents and growth inhibitors, apoptosis inducers, autophagy, and antiangiogenic agents [22]. Also, because of their importance in signal transduction pathways, an additional family that includes proliferation inhibitors of mitogen-activated protein kinases have also been incorporated [22].
Salinosporamide Ais a cytotoxic bicyclic β-lactone-γ-lactam isolated from the Salinispora tropica, amarine actinobacterium [81]. It is an inhibitor of proteasome where the halogenation in its side chain containing ethyl functional group helps to irreversibly bind the 20S proteasome [81]. By binding to this enzyme, salinosporamide A triggers apoptosis of cancer cells [82]. Phase I and Phase II clinical research have been successfully completed for the treatment of solid tumors lymphoma and multiple myeloma [83]. The finding of the Phase 1 trial demonstrated good anticancer activity among multiple myeloma patients, with decent health and a non-cross-reactive toxicity profile [83]. Salinosporamide A did not induce peripheral neuropathy, thrombocytopenia, or myelosuppression, which were observed with other proteasome inhibitors [83]. Similarly, Phase II findings showed enhanced therapeutic function and increased duration of the inhibition of the proteasome [75]. Both the United States Food and Drug Administration (US-FDA) and European Medicines Agency (EMA) declared it to be an orphan medicine targeting multiple myeloma [75]. At present salinosporamide is undergoing Phase III studies for newly diagnosed cases of glioblastoma [75].

Marine Cyanobacteria Metabolites with Anticancer Property
Owing to their extraordinary abundance, marine cyanobacteria have drawn tremendous interest within the marine ecosystem. Some of the anticancer agents from marine cyanobacteria as shown in Table 1 are discussed below. Various symbols (↑, ↓ and ⊥ ) indicate increase, decrease and inhibition in the obtained variables, respectively.

Polyketides
Ankaraholide A (5) is a glycosylated swinholide compound, which is procured from Geitlerinema sp. [86]. It inhibited the proliferation of NCI-H460, Neuro-2a, and MDAMB-435 cells [96]. Swinholide A (6) was initially obtained from the marine sponge Theonella swinhoei. Later, it was also reported to be the metabolites of the marine cyanobacterium Symploca sp. It exhibited its antitumor activity by disruption of actin [86].
Curacin A (11) (Figure 2), a linear and complex hybrideketopeptide, was the first curacin that was isolated from extracts of the Caribbean cyanobacterium Lyngbya majuscule [90]. After curacin A, other curacin compounds, namely curacin B, C, and D, were also identified as the constituents of L. majuscule [91]. Amongst these, curacin A was the most active anticancer compound that inhibited the proliferation of non-small cell lung cancer cells (A549) by triggering apoptosis and causing cell cycle arrests in the G2-M Phase [92]. It acts by binding to tubulin at colchicine binding site and acting as a competitive antagonist, and tubulin polymerization inhibitor [93]. Its structural activity relationship (SAR) studies indicated that the existence of four double bonds, a conjugated diene, a readily oxidized thiazoline heterocycle, and high lipophilicity are the factors that govern the pharmacological function of curacin A [94]. Various analogues were prepared, but they exhibit activity, lower than that the parent compound [95]. Tasiamide B (12) (Figure 2), a linear peptide extracted from cyanobacteria Symploca sp., demonstrated potent toxicity against human nasopharyngeal carcinoma (ĸB) and LoVo cancer cells, respectively [96]. Apratoxins ( Figure 2) are cytotoxic cyclic depsipeptides that have a novel polyketide and peptide fragmented structure [98]. Apratoxin A (13) (Figure 2) was found in an aquatic cyanobacteria Lyngbya majuscula. It disrupted the secretory pathway of U2OS osteosarcoma cells and even induced arrest at G1 Phase of the cell cycle in HeLa cervical carcinoma. It also demonstrated significant cytotoxic behavior in human tumor cell lines, such as LoVo cells and epidermal ĸB carcinoma cancer cells, respectively [97]. Apratoxin A inhibited the translocation of proteins by specifically attacking a central subunit of the protein translocation receptor, Sec61α [99]. Apratoxin B (14) and C (15) (Figure 2), extracted from Lyngbya sp., has high cytotoxicity against ĸB oral epidermoid cancer cells and LoVo colon cancer cells [97]. Apratoxin D (16) (Figure 2), collected from Lyngbya majuscule and Lyngbya sordid, demonstrated significant cytotoxicity against the H-460 Tasiamide B (12) (Figure 2), a linear peptide extracted from cyanobacteria Symploca sp., demonstrated potent toxicity against human nasopharyngeal carcinoma (kB) and LoVo cancer cells, respectively [96]. Apratoxins ( Figure 2) are cytotoxic cyclic depsipeptides that have a novel polyketide and peptide fragmented structure [98]. Apratoxin A (13) (Figure 2) was found in an aquatic cyanobacteria Lyngbya majuscula. It disrupted the secretory pathway of U2OS osteosarcoma cells and even induced arrest at G1 Phase of the cell cycle in HeLa cervical carcinoma. It also demonstrated significant cytotoxic behavior in human tumor cell lines, such as LoVo cells and epidermal kB carcinoma cancer cells, respectively [97]. Apratoxin A inhibited the translocation of proteins by specifically attacking a central subunit of the protein translocation receptor, Sec61α [99]. Apratoxin B (14) and C (15) (Figure 2), extracted from Lyngbya sp., has high cytotoxicity against kB oral epidermoid cancer cells and LoVo colon cancer cells [97]. Apratoxin D (16) (Figure 2), collected from Lyngbya majuscule and Lyngbya sordid, demonstrated significant cytotoxicity against the H-460 lung cancer cell line [98]. Apratoxin E (17) (Figure 3) extracted from Lyngbya bouilloni demonstrated strong antiproliferative action against diverse cancer cell lines, such as U2OS osteosarcoma, HT29 colon adenocarcinoma, and HeLa epithelial carcinoma [99].
Apratoxin F (18) and G (19) (Figure 3), containing N-methyl alanine in their composition, has been reported to be isolated from Lyngbya bouilloni and have strong cytotoxicity against H-460 lung cancer and HCT-116 colorectal cell lines [100]. Cyclic depsipeptidesaurilide B (20) and C (21) (Figure 3), isolated from Lyngbya majusculus, demonstrated cytotoxicity in human lung tumor cell line NCIH460 and neuro-2a mouse neuroblastoma cells [101]. Aurilides binds prohibitin 1 (PHB1) in the mitochondria, stimulates optic atrophy 1 (OPA1) proteolytic synthesis and contributes to mitochondrial apoptosis [102].Coibamade A (22) (Figure 3) is a cyclic depsipeptide extracted from marine cyanobacterium Leptolyngbya sp. It showed strong cytotoxic activity against a triple-negative breast cancer cell line (MDA-MB-231) [103]. The antiproliferative activity of the active metabolite is associated with the cell cycle arrest at the G1 Phase. Similar observations were reported, where glioblastoma cell lines, such as U87-MG and SF-295, were treated with coibamide A. It also inhibited the proliferation of HUVECs by inducing cell morphology change and reducing the expression level of vascular endothelial growth factor receptor 2 (VEGFR2) [104]. Cobamide A induced autophagy associated cell death of human U87-MG glioblastoma cells and SF-295 glioblastoma cells and mouse embryonic fibroblasts [105].

Pigment
Scytonemin (106) (Figure 17) is a yellow-green pigment obtained from stigonema species of bluegreen algae (cyanobacteria). It showed antiproliferative activity against Jurkat T cells by inducing apoptosis. The formation of mitotic spindle and the protein serine/threonine kinase activity was inhibited by scytonemin [160,161].

Pigment
Scytonemin (106) (Figure 17) is a yellow-green pigment obtained from stigonema species of blue-green algae (cyanobacteria). It showed antiproliferative activity against Jurkat T cells by inducing apoptosis. The formation of mitotic spindle and the protein serine/threonine kinase activity was inhibited by scytonemin [160,161].

Phenanthridine Alkaloids
Calothrixins A (108) and B (109) (Figure 17) are phenanthridine alkaloids isolated from the marine cyanobacterium Calothrix sp. They possessed significant cytotoxicity and inhibited the proliferation of human carcinoma cell line (HeLa) [163,164]. It also inhibited the proliferation of CEM leukemia cells (human T-cell leukemia cells) by inducing cell cycle arrest at G1 and G2/M Phases [165].

Microalgae Metabolites as Anticancer Drugs with Their Mechanisms of Action
The following compounds have been reported from the microalgal species that have shown anticancer properties (Table 2).

Phenanthridine Alkaloids
Calothrixins A (108) and B (109) (Figure 17) are phenanthridine alkaloids isolated from the marine cyanobacterium Calothrix sp. They possessed significant cytotoxicity and inhibited the proliferation of human carcinoma cell line (HeLa) [163,164]. It also inhibited the proliferation of CEM leukemia cells (human T-cell leukemia cells) by inducing cell cycle arrest at G1 and G2/M Phases [165].

Avrainvillea nigricans
Human breast cancer MCF-7 cells and human colon cancer HCT-116 cells ⊥ Proliferation, antimitotic activity, ↑tubulin polymerization within the cell Not specified [218] Various symbols (↑, ↓ and ⊥ ) indicate increase, decrease and inhibition in the obtained variables, respectively.

Polysaccharide
Microalgae polysaccharides have shown bio-stimulant activity that has been proven to be effective for a number of industrial applications, although only a few studies have shown the potential to act as an anticancer agent.The potentiality of the polysaccharides varied with changes in molecular weight and sulfate content [169].

Chrysolaminaran Polysaccharide
Chrysolaminaran polysaccharide (115) (Figure 18) is a polysaccharide which is isolated from diatom Synedra acus. It belongs to chrysolaminaran family. It showed antitumor activity on human colon cancer cell lines HTC-116 and DLD-1 by inhibiting cancer cell proliferation [170].

Sulfated Polysaccharide
The sulfated polysaccharide was reported to be an important constituent of the brown seaweeds of Undaria pinnatifida and Saccharina japonica [171]. This has an antitumor function [171] and prevented the proliferation of human breast cancer (T-47D) and melanoma (SK-MEL-28) cell lines and prevents the colony development [172].
Fucoidans (116) (Figure 18), are sulfated polysaccharides isolated from the brown seaweeds of Sargassum hornery, Eclonia cava, and Costaria costata [173]. It showed antitumor activity against human skin melanoma cell line (SK-MEL-28) and human colon cancer cell line (DLD-1) [173]. It also has been documented that fucoidans are an active constituent of Fucus evanescens (a brown algae of the Okhotsk sea), exhibited in vivo antitumor and antimetastatic activity in C57Bl/6 mice, a preclinical animal model [174]. Low molecular weight fucoidan induced apoptosis through alteration of mitochondrial membrane potential by the release of cytochrome c, and inhibitions of Bcl-2, Bcl-xl, Mcl-1 antiapoptotic protein and also activated apoptosis-inducing factors, caspase-3, caspase-7, caspase-9, in MDA-MB-231 cells [175]. ERK1/2 pathway inhibition in human lung cancer cells (A549) led to antimetastatic effect imposed by fucoidan. It also inhibited the phosphoinositide 3-kinases/protein kinase B/ mechanistic target of rapamycin (PI3K/Akt/mTOR) pathway, associated with the downregulation of the expression levels of matrix metalloproteinase-2 (MMP-2) in the A549 human lung carcinoma cell line [176]. Fucoidan inhibited the phosphorylation of EGF receptor. Additionally, it also inhibited the phosphorylation of ERK, JNK, c-fos, and c-jun and activator protein-1 (AP-1) [177]. Over-sulfated fucoidan blocked the angiogenesis process by suppressing mitogenic and chemotactic response of the vascular endothelial growth factor (VEGF) [178]. In another reported study, fucoidan inhibited the proliferation of human hepatocellular carcinoma cells (Huh7) by downregulating the expression levels of chemotaxin CXCL12 and its receptor CXCR4 [179]. It also reduced the expression levels of transforming growth factor (TGF) receptor I and transforming growth factor receptor II proteins and controlled the associated signaling molecules of TGF and regulated the SMAD2, SMAD3, and SMAD4 protein phosphorylation. This could potentially be another type of novel revolutionary mechanism by which the fucoidans exhibited antitumor activity in the breast carcinoma cells by the influence of epithelial-mesenchymal transition [180].

Alginic Acid
Alginic acid (117) (Figure 18), commonly known as algin, is an anionic polysaccharide obtained from the cell wall of brown algae or seaweeds Sargassum wightii. Nanoparticles containing alginic acid provided stimulating antitumor effect on H22 tumor-bearing mice [41]. This polysaccharide also binds with toxic substances and heavy metals that cause cancer present in the intestine and it exerted its activity by converting these toxic substances into non-toxic ones [181].

Laminarin
Laminarin (118) (Figure 18) is a polysaccharide obtained from brown algae Eisenia bicyclis [182]. It exhibited anticancer activity by inhibiting the proliferation and inducing apoptosis, and cell cycle arrest at the subG1 Phase in ovarian clear cell carcinoma cells (ES2), and papillary serous adenocarcinoma (OV90) cell lines. PI3K/MAPK intracellular signaling mechanism is inhibited in ovarian cancer cells, as well as the increased release of cytochrome c associated with an increase in DNA fragmentation and expression level of apoptosis linked proteins. It also induced MMP loss in both the carcinoma cells, along with autophagy through the inactivation of ULK1 and P62 phosphorylation [183]. Similarly, laminarin and its sulfated analog displayed potential in vitro anticancer activity against JB6 Cl41 (normal mouse epidermal cells), and SK-MEL-28 (human malignant melanoma) cells. Inhibition of proliferation and migration of these cancer cells is associated with inhibition of MMP-2 and MMP-9 proteinases and down-regulation of ERK1/2 signaling mechanism [184]. Similarly, it also inhibited the colony formation of human colon cancer cell lines, such as HCT-116, HT-29, and DLD-1, and displayed cytotoxicity against various carcinoma cell lines [182,[185][186][187][188]. Ji and Ji [189] reported the anticancer activity of laminarin and its sulfated analog against LoVo cells. It is often associated with induction of apoptosis, upregulation of the expression levels of death receptor 4 (DR4) and DR5, TNF-related apoptosis-inducing ligand (TRAIL), Fas-associated protein with death domain(FADD), Bid, tBid and Bax, and downregulation of pro-caspase-8, pro-caspase-3, and Bcl-2 [190,191]. Additionally, activation of casapse-8, casapse-3, casapse-6, casapse-7, and casapse-9 and increased release of cytochromec were observed following the treatment with laminarin and its analogs [190,191]. The involvement of the laminarin on the ErbB signaling mechanism indicatesanother mechanism of action behind its apoptosis induction in human colon cancer cell line (HT-29) associated with cell cycle arrest at subG1 and G2-M Phases [192].

Carotenoids
Carotenoids are tetraterpenoids, which are also classified as the pigments formed by plants, algae, bacteria, and fungi [193]. There are more than 1100 carotenoids that have been identified so far. The carotenoid's general configuration is a polyene chain of 9-11 double bonds. This correlates with numerous pharmacological features, including anticancer behavior. Various xanthophylls carotenoids were identified, such as violaxanthin, siphonaxanthin, fucoxanthin, neoxanthin, zeaxanthin, lutein, and lactucaxanthin, to be the major constituents in microalgae [193][194][195].

Violaxanthin
Violaxanthin (119) (Figure 19) is the active metabolite reported to be present in the dichloromethane extract of the green algae Dunaliella tertiolecta [196]. It induced early apoptosis associated with biochemical and morphological changes in MCF-7 cancer cell line, but it did not contribute to the fragmentation of DNA. Additionally, it also reversed the multidrug resistance (MDR) by inhibiting the P-glycoprotein (P-gp) and MRP1 in L1210 (human MDR1 gene-transfected mouse lymphoma cells) and MDA-MB-231 (human breast cancer cells) [197]. Similar observations were recorded where violaxanthin reversed the MDR in human MDR1 (gene-transfected mouse lymphoma) and MCF-7 (human breast cancer cell) [198].

Fucoxanthin
Fucoxanthin (121) (Figure 19) is a pigment belonging to the xanthophylls family and found in brown algae Undaria pinnatifida as a major carotenoid. It shows antiproliferative activity against human leukemia cell line (HL-60) by inducing apoptosis [200]. Various studies revealed the anticancer nature in which it inhibited proliferation by inducing apoptosis and cell cycle arrest at the G0/G1 Phase or G2/M Phase through various molecules and pathways involving Bcl-2 protein, MAPK, NF-κB, caspase-3, caspase-8, caspase-9, and GADD45 in which their expression levels were regulated by fucoxanthin [201]. Among various carotenoids, fucoxanthin has been thoroughly researched as an anticancer agent and it has been established as having a significant anticancer activity [200,[202][203][204][205]].

Fucoxanthin
Fucoxanthin (121) (Figure 19) is a pigment belonging to the xanthophylls family and found in brown algae Undaria pinnatifida as a major carotenoid. It shows antiproliferative activity against human leukemia cell line (HL-60) by inducing apoptosis [200]. Various studies revealed the anticancer nature in which it inhibited proliferation by inducing apoptosis and cell cycle arrest at the G0/G1 Phase or G2/M Phase through various molecules and pathways involving Bcl-2 protein, MAPK, NF-κB, caspase-3, caspase-8, caspase-9, and GADD45 in which their expression levels were regulated by fucoxanthin [201]. Among various carotenoids, fucoxanthin has been thoroughly researched as an anticancer agent and it has been established as having a significant anticancer activity [200,[202][203][204][205]].

Siphonaxanthin
Siphonoxanthin (122) (Figure 19) is a keto-carotenoid present as an active metabolite in green algae Codium fragile, Caulerpa lentillifera, and Umbraulva japonica. Siphonaxanthin demonstrated the anticancer effect on the human leukemia cell line (HL-60) by inducing apoptosis and an increase in chromatin condensation, in association with the decreased expression level of Bcl-2 and increased caspase 3 activation. The expression level of GADD5α and DR5 were also upregulated [206]. The antiangiogenic effect was also displayed by siphonaxanthinin human umbilical vein endothelial (HUVEC) cells and aortic rings of rats [207]. It reduced the mRNA expression level of fibroblast growth factor 2 (FGF-2), fibroblast growth factor receptor (FGFR-1), and early growth response 1 (EGR-1) [208,209].
anticancer effect on the human leukemia cell line (HL-60) by inducing apoptosis and an increase in chromatin condensation, in association with the decreased expression level of Bcl-2 and increased caspase 3 activation. The expression level of GADD5α and DR5 were also upregulated [206]. The antiangiogenic effect was also displayed by siphonaxanthinin human umbilical vein endothelial (HUVEC) cells and aortic rings of rats [207]. It reduced the mRNA expression level of fibroblast growth factor 2 (FGF-2), fibroblast growth factor receptor (FGFR-1), and early growth response 1 (EGR-1) [208,209].

Stigmasterol
Stigmasterol (125) (Figure 20) is a sterol extracted from a microalga benthic diatom Navicula incerta. It showed significant anticancer activity by inhibiting the proliferation of the human liver cancer cell line (HepG2) by inducing apoptosis through mitochondrial membrane potential and cause morphological changes and damage of DNA [213,214]. The up-regulation of the expression of caspase-8, caspase-9, Bax, and p53 was induced by stigmasterol whereas antiapoptotic proteins, such as Bcl-2, and X-linked inhibitor of apoptosis protein (XIAP), was down-regulated. The result showed that the cell cycle arrest takes place at G 0 /G 1 and G 2 /M Phases due to cell component defects.

Nonyl 8-Acetoxy-6-Methyloctanoate
Nonyl 8-acetoxy-6-methyloctanoate (126) (Figure 20) is a fatty alcohol ester isolated from a marine diatom Phaeodactylum tricornutum. The anticancer activity of the secondary metabolite was established on the human promyelocytic leukemia cell line (HL-60), a human lung carcinoma cell line (A549), and a mouse melanoma cell line (B16F10). It induced damage of DNA and increased the apoptotic activity and triggered cell cycle arrest at the sub G1 Phase. It activated the pro-apoptotic protein Bax, and suppress the antiapoptotic protein Bcl-xL, and also increases the expression levels of both caspase-3 and p53 proteins [215].

Nigricanosides A (135) and B (136) and Methyl Esters of Nigricanosides A (137) and B (138)
These are glycolipids (Figure 21) extracted from a green algae Avrainvillea nigricans. These metabolites inhibited the proliferation of human breast cancer MCF-7 cells and human colon cancer HCT-116 cells and they also possess antimitotic activity which triggers tubulin polymerization within the cells [218].

Conclusions, Current Challenges and Future Perspectives
This review describes the most recently extracted or generated molecules from marine organisms, such as cyanobacteria and microalgae, with potential for cancer therapy. Marine resources certainly have valuable and undiscovered biochemical versatility and demonstrate a greater opportunity for the development of new anticancer agents. While a variety of compounds have been identified to suppress cell growth in a broad spectrum of cancer cell types, the mechanism of action still remains unclear. A handful of marine molecules have demonstrated possible cytotoxic actions

Nigricanosides A (135) and B (136) and Methyl Esters of Nigricanosides A (137) and B (138)
These are glycolipids (Figure 21) extracted from a green algae Avrainvillea nigricans. These metabolites inhibited the proliferation of human breast cancer MCF-7 cells and human colon cancer HCT-116 cells and they also possess antimitotic activity which triggers tubulin polymerization within the cells [218].

Conclusions, Current Challenges and Future Perspectives
This review describes the most recently extracted or generated molecules from marine organisms, such as cyanobacteria and microalgae, with potential for cancer therapy. Marine resources certainly have valuable and undiscovered biochemical versatility and demonstrate a greater opportunity for the development of new anticancer agents. While a variety of compounds have been identified to suppress cell growth in a broad spectrum of cancer cell types, the mechanism of action still remains unclear. A handful of marine molecules have demonstrated possible cytotoxic actions toward specific cancer by inhibition of cell proliferation, its antimitotic behavior (antitubulin impacts), induced apoptosis and inhibition of movement, invasion, or metastatic potential of cancer cells.
While these metabolites have shown potential for cancer treatment, there are several challenges associated with the development of these drugs that need consideration. Oceans certainly provide a large supply of valuable species, but the researchers still cannot access any of these regions. For years, the selection of entities in readily accessible places was preferred. Therefore, the problem is that marine research is not always easily accessible where researchers and the oceanographer need to have strong working ties. Genetic engineering is undergoing development in order to enhance drug production through the conversion of genetic data from the target compound into the host cells. It is an important field for regulation of the isolation and expression of aquatic genes, helping us produce lead compounds from the aquatic ecosystem in a more controlled way. Secondary metabolites are hard to produce independently from cultures, since their development is directly or indirectly dependent on host. Therefore, several of the main genes stay silent while these things are attempted to evolve In vitro. Another big concern is that the development of a specific molecule requires sufficient resources. There is still little research available on the toxicity studies of these marine metabolites in normal cells; this has to be addressed.
Marine cyanobacteria and microalgae tend to be an effective source of anticancer drugs. Nevertheless, more studies are required to understand the basic targets and pathways behind the cytotoxicity of these compounds in cancer cells.