Cytotoxic Alkaloids Derived from Marine Sponges: A Comprehensive Review

Marine sponges (porifera) have proved to be a prolific source of unique bioactive secondary metabolites, among which the alkaloids occupy a special place in terms of unprecedented structures and outstanding biological activities. Identification of active cytotoxic alkaloids extracted from marine animals, particularly sponges, is an important strive, due to lack of knowledge on traditional experiential and ethnopharmacology investigations. In this report, a comprehensive survey of demospongian bioactive alkaloids in the range 1987–2020 had been performed with a special emphasis on the potent cytotoxic activity. Different resources and databases had been investigated, including Scifinder (database for the chemical literature) CAS (Chemical Abstract Service) search, web of science, Marin Lit (marine natural products research) database. More than 230 representatives of different classes of alkaloids had been reviewed and classified, different genera belonging to the phylum porifera had been shown to be a prolific source of alkaloidal molecules, including Agelas sp., Suberea sp., Mycale sp., Haliclona sp., Epipolasis sp., Monanchora sp., Crambe sp., Reniera sp., and Xestospongia sp., among others. The sufficient production of alkaloids derived from sponges is a prosperous approach that requires more attention in future studies to consider the constraints regarding the supply of drugs, attained from marine organisms.


Introduction
Marine alkaloids present unique chemical structures that have been widely distributed among marine organisms. Some of them represent derivative molecules of the commonly encountered terrestrial alkaloids, whereas others show unprecedented novel structures confined to the marine systems. Their purification, structure elucidation, stereochemistry, chemical modification, synthesis, and pharmaceutical activity have acquired outstanding interdisciplinary attention from various fields of research aside from chemistry, including physiology, and pharmacology, ecology, biotechnology [1]. Alkaloids represent one of the most important classes of natural products that are widely distributed among different biological sources, including plants, animals, fungi, cyanobacteria, actinomycetes, dinoflagellates, red algae, cnidarians, and bryozoans; however, their presence in marine invertebrates as major constituents is limited to specific phylum, including some sponge genera, ascidians, mollusks, red algae, and bryozoans. Although the exact physiological function of alkaloids remains unclear, many of them had been developed as defense chemical weapons against predation-this is of great importance, especially for vulnerable sessile organisms, like sponges, and consequently, they are expected to be very potent molecules demonstrating toxicity at low doses [2][3][4].
In another study, ma'edamines C (16) and ma'edamines D (17) were isolated by Japanese researchers from the Okinawan marine sponge Suberea sp. compounds 16-17 revealed cyclization of the side-chain nitrogen of tyrosine to add a quaternary pyridinium nucleus to the structure. Both compounds have shown selective cytotoxicity against murine leukemia L1210 cell line though, and they did not express cytotoxicity against KB cell line [28]. Investigation of the marine sponge Psammoclemma sp. collected from Bommie Bay, Queensland, Australia, led to the isolation of psammaplysene C (18) and psammaplysene D (19). Compounds 18-19 showed C6C3N moiety in one of their bromotyrosine uints instead of the conventional C6C2N arrangement [29]; both compounds showed potent cytotoxic activities. Another bromotyrosyn alkaloids were isolated from the Fijian sponge Druinella sp. These 10 bromotyrosine alkaloids 20-29, were identified as purealidin S (20), purpuramine J (21). PurealidinQ (22), aplysamine 2 (23), purpureamine I (24), aerophobin2 (25), aerophobin1 (26), purealidin J (27), araplysillin1 (28), and araplysillin 2 (29). Compounds 25-27 revealed replacing the other bromobenzene moiety commonly encountered in bromotyrosine alkaloids with an imidazole ring. Compounds 20-29 were evaluated for their cytotoxicity against two cell lines showing potent to moderate activities. Among them, compound 21 was the most potent cytotoxic compound [30]. Suberedamine A (30) and suberedamine B (31) were isolated from Suberea sp. These two new cytotoxic bromotyrosine alkaloids exhibited potent cytotoxic activity against L1210 and KB cell lines [31]. Figure 3 illustrates the chemical structures of compounds 11-31; Table 3 summarizes the cytotoxic evaluation of compounds 11-31.    (33) with potent cytotoxicity against HeLa cells was identified from the sponge, Suberea sp., collected from the Red Sea in Yanbu, Saudi Arabia [33]. Two brominated indolosulfonic acid derivatives were reported from the hydroalcoholic extract of the Psammoclemma sp., collected from New Caledonia. The compounds were identified as echinosulfonic acid D (34) and echinosulfonic acid B (35) based on extensive LC/MS/MS analysis besides conventional 1 and 2 D NMR analysis. Both compounds showed potent cytotoxicity against KB cells with equal IC 50 of 2 µg/mL [34]. Figure 4 illustrates the chemical structures of compounds 32-35; Table 4 summarizes the cytotoxic evaluation of compounds 32-35.
Biomolecules 2021, 11, x FOR PEER REVIEW 7 of 40 analysis besides conventional 1 and 2 D NMR analysis. Both compounds showed potent cytotoxicity against KB cells with equal IC50 of 2 μg/mL [34]. Figure 4 illustrates the chemical structures of compounds 32-35; Table 4 summarizes the cytotoxic evaluation of compounds 32-35.

Aaptamine Alkaloids
Bioassay-guided fractionation of the Indonesian sponge Aaptos suberitoides resulted in the isolation of three benzonaphthryidine derivatives identified as aaptamine (36), isoaaptamine (37), and demethylaaptamine (38). Biological evaluation of compounds 36-38 revealed potent cytotoxicity against HeLa cell lines with IC50 values of 15, 3.1, and 1.4 μg/mL, respectively. Moreover, the activity had been attributed to proteasome inhibitory action [35]. Investigation of a different sample from the same sponge A. suberitoides, collected from Xisha islands in the South China Sea (NaiHai), has led to the isolation of additional aaptamine derivatives with dimerization in the benzonathyridine moiety, the compounds were identified as suberitine A (39), suberitine B (40), suberitine C (41) and suberitine D (42). Cytotoxic evaluation of 39-42 against P388, HeLa, and K562 cell lines revealed selective, potent activity of compounds 40 and 42 with IC50 values of 1.8 and 3.5 μM, respectively, against P388 cell line with the absence of activity on other lines. Interestingly, compounds 39 and 41 did not show significant effects against any of the cell lines, revealing the possible need of exocyclic double bond in one of the monomer units either as ketone as in 42 or exocyclic terminal methylene as in 40 [36]. Figure 5 illustrates the

Aaptamine Alkaloids
Bioassay-guided fractionation of the Indonesian sponge Aaptos suberitoides resulted in the isolation of three benzonaphthryidine derivatives identified as aaptamine (36), isoaaptamine (37), and demethylaaptamine (38). Biological evaluation of compounds 36-38 revealed potent cytotoxicity against HeLa cell lines with IC 50 values of 15, 3.1, and 1.4 µg/mL, respectively. Moreover, the activity had been attributed to proteasome inhibitory action [35]. Investigation of a different sample from the same sponge A. suberitoides, collected from Xisha islands in the South China Sea (NaiHai), has led to the isolation of additional aaptamine derivatives with dimerization in the benzonathyridine moiety, the compounds were identified as suberitine A (39), suberitine B (40), suberitine C (41) and suberitine D (42). Cytotoxic evaluation of 39-42 against P388, HeLa, and K562 cell lines revealed selective, potent activity of compounds 40 and 42 with IC 50 values of 1.8 and 3.5 µM, respectively, against P388 cell line with the absence of activity on other lines. Interestingly, compounds 39 and 41 did not show significant effects against any of the cell lines, revealing the possible need of exocyclic double bond in one of the monomer units either as ketone as in 42 or exocyclic terminal methylene as in 40 [36]. Figure 5 illustrates the chemical structures of compounds 36-42;

Guanidine Alkaloids
Monanchora pulchra collected from the Pacific Ocean near Urup islands, was the source of guanidine alkaloid monanchocidin A (43). It is composed of a five-membered spiro-ring in the pentacyclic guanidine core along with an uncommon branched long alkyl chain and a heavily oxygenated morpholinone fragment [37]. Evaluation of 43 against monocytic anemia cell lines THP-1, Hela, and JB6-C141 cell lines revealed potent cytotoxicity with IC50 values against 5.1 μM, 11.8 μM, and 12.3 μM, respectively [38]. As this sponge is a great source of guanidine alkaloids, additional pentacyclic guanidine alkaloids, monanchocidin B-E (44)(45)(46)(47) were reported by Makarieva at al.  (56). Cytotoxic evaluation of compounds 48-56 had been performed against a panel of cell lines revealed interesting cytotoxic activity [40]. Crambescidin-816 (57) has been isolated from Crambe crambe (oyster sponge) belongs to the pentacyclic guanidine alkaloid. Biological evaluation of 57 revealed its ability to reduce cell viability of the tumor-derived cell line HepG2 at concentrations higher than 150 nM. Compound 57 also affected the human tumor-derived cell

Peptide Alkaloid
Scleritodermin A (119), a potent sulfonated thiazol hexapeptide was isolated from the marine sponge Scleritoderma nodosum collected from the northwest side of Olango Island, Cebu, Philippines. The purified compound demonstrated low μM cytotoxicity to HCT116, A2780, and SKBR3 cell lines with IC50 of 1.9, 0.940, and 0.670 μM, respectively [67]. Figure 9 illustrates the chemical structures of compound 119; Table 9 summarizes the cytotoxic evaluation of compound 119.

Pyrimidine Alkaloids
Lanesoic acid (133), was isolated as zwitterionic alkaloid from Theonella swinhoei a sponge of Theonella family collected from Lanes in Indo-Pacific, Indonesia. Compound 133 displayed an unusual 1, 4, 5, 6-tetrahydropyrimidine cation rarely exhibits in natural sources. Compound 133 was evaluated for cytotoxicity against a panel of cell lines, such as PSN1, HT-29, breast MD-MB-231, and A549, that revealed selective cytotoxicity against PSN1cells with an IC 50 value of 28.2 µM and was inactive against NSCLC lung tumor, MD-MB-23, and HT-29 cells [75]. The alkaloid variolin B (134), heterocyclic constitute from the Antractic sponge Kirkpatrickia variolosa had been evaluated for cytotoxicity against a wide range of cancer cell lines, where it showed potent activity against most of the tested cell lines with the activity against androgen-sensitive prostate adenocarcinoma cell line with GI 50 0.05 µM [76]. Figure 11 illustrates the chemical structures of 133-134; Table 11 summarizes the cytotoxic evaluation of 133-134.
133 displayed an unusual 1, 4, 5, 6-tetrahydropyrimidine cation rarely exhibits in natural sources. Compound 133 was evaluated for cytotoxicity against a panel of cell lines, such as PSN1, HT-29, breast MD-MB-231, and A549, that revealed selective cytotoxicity against PSN1cells with an IC50 value of 28.2 μM and was inactive against NSCLC lung tumor, MD-MB-23, and HT-29 cells [75]. The alkaloid variolin B (134), heterocyclic constitute from the Antractic sponge Kirkpatrickia variolosa had been evaluated for cytotoxicity against a wide range of cancer cell lines, where it showed potent activity against most of the tested cell lines with the activity against androgen-sensitive prostate adenocarcinoma cell line with GI50 0.05 μM [76]. Figure 11 illustrates the chemical structures of 133-134; Table 11 summarizes the cytotoxic evaluation of 133-134. Figure 11. Pyrimidine alkaloid structure isolated from sponges.

Manzamine Alkaloids
Samoylenko et al. have reported two tertiary bases, (+)-8-hydroxymanzamine A (231) and (+)-manzamine A (232). In addition to their hydrochloride salts 231a and 232a, from Acanthostrongylophora ingens known for the biosynthesis of broad range of manzamine alkaloids as natural hydrochloride salts. Cytotoxic activity of 231, 232 and their salts versus a panel of cell lines were tested. The hydrochloride salt 231a was more toxic to HepG2, in comparison with 232 with IC 50 1.55 vs. 4.4 µg/mL, as well as being 3-fold more toxic than 232 towards kidney epithelial (non-cancer) cells (IC 50 0.69 vs. 2.15 µg/mL) [98]. Figure 18 illustrates the chemical structures of 231-232;

Conclusion
Isolation and identification of new cytotoxic alkaloids extracted from marine animals, particularly sponges, is a multidisciplinary and intricate endeavor. This is because of the lack of knowledge on traditional experiential and ethnopharmacology investigations that have endorsed this zoochemical scrutiny. Hence, not only is it fundamental to assemble a database to address the shortcomings in the scholarly publication, but it is also bound to be on natural products identification. The representative survey depicted the study on marine cytotoxic alkaloid originated from sponge accomplished between 1987 and 2020 in different resources and databases had been investigated, including Scifinder

Conclusions
Isolation and identification of new cytotoxic alkaloids extracted from marine animals, particularly sponges, is a multidisciplinary and intricate endeavor. This is because of the lack of knowledge on traditional experiential and ethnopharmacology investigations that have endorsed this zoochemical scrutiny. Hence, not only is it fundamental to assemble a database to address the shortcomings in the scholarly publication, but it is also bound to be on natural products identification. The representative survey depicted the study on marine cytotoxic alkaloid originated from sponge accomplished between 1987 and 2020 in different resources and databases had been investigated, including Scifinder (database for the chemical literature) CAS (Chemical Abstract Service) search, web of science, Marin Lit (marine natural products research) database. This investigation ended in 80 closelycorrelated articles, certifying, as an example, the considerable chemodiversity of sponges as original of secondary cytotoxic metabolite. During these 33 years database search, natural products originated, including novel, new, and known compounds, were reported as cytotoxic chemical structures. Several of these structures have demonstrated authenticated in vitro cytotoxic bioactivities versus a panel of cancer/tumor cell lines. In addition, several of them are attracting the interest for future in vivo assay. The main chemical classes of these alkaloid structures were confined, particularly to the Pyrroloiminoquinone alkaloids (16.4%), Guanidine alkaloids (13.9%), Indole alkaloids (12.6%), Pyridine alkaloids (10.6%), and Bromotyrosine alkaloids (8.9%)-demonstrating the five major chemical classes of alkaloid structures observed in sponges during this time, which along with other chemical classes as Quinolizidine and Brominated alkaloids, and others (47.6%), exhibited a various range of cytotoxicity activity. Verongida, Poecilosclerida, Agelasida, Haplosclerida, Halichondrida, Dictyonellidae, and Homosclerophorida were the sponge's main orders in this review, and were also discovered to be the most prosperous genera, due to their potential to produce promising bioactive alkaloid compounds. The examination of the contribution from an individual species revealed that regardless of the order, each species contributed, on average, 2-5 new compounds. In addition, alkaloids with cytotoxic activities have been isolated from 49 genus of sponges. Genus Agelas was discovered to be the most lucrative producer of potent alkaloid compounds, then Biemna, Suberea, and Pachychalina were established. The geniuses Mycale, kirkpatrickia, Scleritoderma, Zyzzya, and Dercitus only had one cytotoxic alkaloid. Among the reported alkaloid in this review, Dercitine (acridin alkaloid), Crambescidin 814 (guanidine alkaloid), Crambescidin 816 (guanidine alkaloid), Unguiculin A-C (guanidine alkaloid), Dihydrodiscorhabdin A and L (Pyrroloiminoquinone alkaloid), Discorhabdin A (Pyrroloiminoquinone alkaloid), Renierol (quinolone alkaloid), Renieramycin(quinolone alkaloid), Renieramycin J (quinolone alkaloid) and Jurunnamycin A (quinolone alkaloid) showed the best cytotoxic activity in micro molar to nano molar ranges.
Evaluation of cytotoxicity is requisite to approach the cytotoxic level of alkaloid structures. In this survey, the evaluation of cytotoxicity of the alkaloid compounds, extracted from marine sponges, was determined through the most usual panels of cytotoxic cell Nevertheless, in infrequent studies were wide-range and have consisted of the NCI-60 panel of cell lines for screening anticancer characteristics of the drug. These kinds of screening system are great implements for clarifying the relationship between cytotoxicity and anticancer drugs, for the exploration of molecular-targeted antineoplastic drugs, and improve the compounds, selection as candidates for antineoplastic drug. Even though it was not the object of this survey to consider preclinical and clinical perspective of the cytotoxic isolated alkaloids, it is obvious that the present gap with regard to the natural approachability of these cytotoxic alkaloids to carry out studies, since they need higher amounts of compounds. As far as the amounts of substances are concerned, preparing synthetic compounds influences the preclinical phase for drug candidates. Synthesis of natural products and their chemical derivatives display the potential to assert or modify isolated alkaloids, as well as to prepare effective analogs. Sponge aquatic environment and microorganisms engineering are attracted interesting to support these kinds of compounds. Sufficient production of alkaloids derived from sponges is a prosperous approach that requires more attention in future to consider the constraints regarding the supply of drugs, attained from marine organism. In conclusion, this review is able to point out the importance of cytotoxic alkaloids to explore new, effective anticancer drugs.