1. Introduction
Marine invertebrates are the most diverse group of marine life that exist in the oceans with highest biological and chemical diversity. A total of 38,925 marine-derived natural products, published in 38,645 articles, were identified from marine organisms
[1]. Such chemical biodiversity is attributed to the fact that these compounds are produced by mostly sessile organisms present in the marine environment. These sessile organisms are extremely susceptible to be attacked by predators. Marine invertebrates, such as sponges, tunicates, bryozoans, gorgonians, and soft corals evolve chemicals as a defense mechanism against highly mobile predators. Marine sponges are one of the most productive phyla of marine invertebrates and considered as an outstanding source of biologically active secondary metabolites
[2].
Members of the Phylum Porifera and their associated microbes represent the largest reservoir and supplier of secondary metabolites. Primitive and sessile animals, such as sponges, developed survival strategies depending on the production of defensive secondary metabolites for their own protection against different predators, fouling organisms, and invasion by different microbes and pathogens. Sponges are classified in four major classes including Calcarea, Demospongiae, Hexactinellida, and Homoscleromorpha. The chemical structural diversity of the secondary metabolites of the sponges includes several classes, such as alkaloids, macrolides, peptides, steroids, terpenoids, polyketides, and many others.
[3].
It was proven that the sponge-derived secondary metabolites produce an enormous array of antitumor, antiviral, anti-inflammatory, antibiotic, and other bioactive molecules that have the potential for therapeutic use. Studies have shown that different compounds affect the targeted disease through different modes of action. Chemical entities that can act as transcription factor inhibitors may be effective against both viral infections and malignant neoplasms. Most bioactive metabolites from sponges have proven to inhibit specific enzymes, which often mediate or produce mediators of intercellular or intracellular messengers involved in the development of diseases
[4].
Amongst different orders of marine sponges, the order Verongiida is considered as a reservoir of brominated tyrosine-derived secondary metabolites. This order belongs to the kingdom Animalia, phylum Porifera, class Demospongia, and subclass Verongimorpha. According to the World Porifera Database, order Verongiida includes five families viz. Aplysinellidae, Ernstillidae, Aplysinidae, Ianthellidae, and Pseudoceratinidae.
Members of the order Verongiida attracted researchers of the marine natural products community over the past 65 years due to the large number of bioactive bromotyrosine-derived alkaloids that they produce
[2]. Bromotyrosine-derived alkaloids display significant chemical diversity and offer effective chemical defense for these organisms against predators in the ocean
[5][6] and the fouling organisms
[7][8].
Bromotyrosine derivatives of the order Verongiida include several classes, such as spirooxepinisoxazolines, mono- and bis-configurated spiroisoxazolines, dibromocyclohexadienes, brominated phenolics, verongiaquinols, verongiabenzenoids, oxime disulfides, brominated oximes, bromotyramines, bromotyramine oximes, bastadins, and hemibastadins. Additional chemical classes that are not of bromotyrosine biosynthetic origin in this order are indole alkaloids, pyrroles, quinolines, hydroquinones guanidines, benzofurans isoprenoids, benzonaphthyridines, sesquiterpenoids, sesterterpenoids, merosesquiterpenoids, and macrolides
[9].
The bromotyrosine-derived compounds are considered as a class of interest due to their structural diversity and pharmacological and biological importance
[10][11][12][13]. Prominent members of the bromotyrosine derivatives include psammaplins, disulfide-linked compounds, that were first isolated from an unidentified specimen of
Psammaplysilla Keller, 1889 (=
Pseudoceratina Carter, 1885)
[14]. These compounds have stimulated further and deeper investigations on other Verongiid sponges as well as the synthesis of targeted anti-cancer drug analogs
[15][16][17]. The bromotyrosin-derived compounds of the Verongiid sponges display huge pharmaceutical and biomedical potential, with many viewed as being promising targets within the preclinical pipeline. Preclinical assays on bromotyrosines have highlighted many candidates for antiplasmodial
[18][19], antimicrobial
[19][20][21][22][23][24][25][26][27][28][29], antioxidant
[24][25], anti-invasion, and antimigratory
[30][31][32], parasympatholytic
[33], as well as compounds that affect the central nervous system
[20][26][34][35]. These significant and broad-spectrum activities have provided much motivation for further investigations of the members of this order for the exploration of its secondary metabolites and biomedical importance.
To date, more than 633 natural products, mostly bromotyrosine-derived, are reported from over 43 different species of the order Verongiida in the literature
[9]. Among these, forty-one bromotyrosine alkaloids possessing the 1,6-dioxa-2-azaspiro [4.6] undecane skeleton have been reported from marine sponges of the Verongiida, including members of genera
Aplysinella, Psammaplysilla, Pseudoceratina, and
Subarea, and three compounds from the order Dictyoceratida (including the genera
Dysidea and
Hyattella).
2. Compounds with Antimicrobial Properties
Psammaplysins A (
1) and B (
2) have been reported to show in vitro activity towards gram positive bacteria as well as
E. coli [36]. In addition, psammaplysin A (
1) has found to possess antibacterial activity against
Flavobacterium marinotypicum with an inhibition zone of 10 mm at a concentration of 10 µg/disc
[37]. Furthermore, Psammaplysins A (
1) and B (
2) have been described to inhibit the
Mycobacterium tuberculosis detoxification enzyme mycothiol-
S-conjugate amidase in a fluorescence-detected assay
[38]. Furthermore, psammaplysins F (
10) and H (
12) have been found to inhibit the growth of six Gram-positive strains,
S. aureus NCTC 6571,
S. aureus 1H,
E. facecalis NCTC-775, B. cereus NCTC-7464, MRSA MW2 and MRSA USA-300
[38] (
Table 1).
Table 1. Compounds with reported antimicrobial activities.
3. Compounds with Growth Inhibition and Cytotoxic Activities
Psammaplysin A (
1) has been reported as a growth inhibitor of many cancer cell lines including, HCT-116, HCT-15 (colon cancer), PC-3 (prostate cancer), ACHN (renal cancer), MDA-MB-231 (breast cancer), NUGC-3 (stomach cancer), NCI-H23 (lung cancer), and Hela (Cervical)
[40][41][42][43] (
Table 2).
Table 3. Compounds with reported cancer cells’ growth inhibition activities.
Similarly, psammaplysin B (
2) was found to inhibit the growth of several cell lines, including HCT-116, HCT-15 (colon cancer), PC-3 (prostate cancer), ACHN (renal cancer), MDA-MB-231 (breast cancer), NUGC-3 (stomach cancer), and NCI-H23 (lung cancer)
[41][42] (
Table 2).
Psammaplysin C (
3) has been reported to inhibit the growth of HCT-116 cell line
[41] (
Table 1). Likewise, psammaplysin D (
4) has been reported to inhibit the growth of HCT-15 (colon cancer), PC-3 (prostate cancer), ACHN (renal cancer), MDA-MB-231 (breast cancer), NUGC-3 (stomach cancer), and NCI-H23 (lung cancer) cell lines
[42] (
Table 2).
Psammaplysin E (
5) has been reported as a potent growth inhibitor of several cancer cell lines, including KB (human oral, epidermoid carcinoma), LoVo (human colon, adenocarcinoma), HCT-15, PC-3, ACHN, MDA-MB-231, NUGC-3, and NCI-H23
[37][40][42][44]. In addition, it has been found that psammaplysin E possesses a potent antimigratory effect against MDA-MB-231 and Hela cells
[40] and has moderate immunosuppressive activity as well
[44] (
Table 2).
Psammaplysin F (
10) has been reported as a moderate inhibitor of HEK293 mammalian cell line
[45] (
Table 2).
Psammaplysin X (
35) and 19-hydroxypsammaplysin X (
36) were reported to inhibit the growth of HCT-15, PC-3, ACHN, MDA-MB-231, NUGC-3, and NCI-H23 cancer cell lines
[42] (
Table 2).
Psammaplysin Z (
38) and 19-hydroxypsammaplysin Z (
39) were found to inhibit the growth of MDA-MB-231 and HeLa cancerous cell lines
[40] (
Table 2).
Finally, psammaceratin A (
44) has been reported to inhibit the growth of MDA-MB-231, Hela, and HCT116 cell lines
[43] (
Table 2).
From the above results, it could be concluded that the growth inhibition of the psammaplysins towards cancerous cell lines suggests that the spirooxepinisoxazoline ring system is an essential element for the activity. The N-terminal substitution with a cyclopentene-dione moiety, as in psammaplsyins E (5) and 19-hydroxypsammaplysin E (6), or 4-chloro-2-methylenecyclopentane-1,3-dione moiety, as in psammaplysin X (35) and 19-hydroxypsammaplysin X (36), increases the activity. Further, introduction of 19-OH group, as in psammaplysin B (2) versus psammaplysin A (1), diminishes the activity (Table 2).
On the contrary, psammaplysin D (4), however, lacked activity (GI50 > 10 μM), which might be explained by its high lipophilicity. Furthermore, the existence of a terminal N-methyl group, as in psammaplysin F (10), or an urea moiety, as in psammaplysin Z (38), diminished the growth inhibition effect (Table 2).
4. Compounds with Antimalarial Activities
19-Hydroxypsammaplysin E (
6), psammaplysin K (
15), psammaplysin L (
17), psammaplysin M (
18), psammaplysin N (
19), 19-hydroxypsammaplysin P (
22), psammaplysin T (
28), and psammaplysin V (
32) have been evaluated for their antimalarial activity, at 10 μM, against the chloroquine-sensitive
Plasmodium falciparum 3D7 malaria parasite line. Only 19-hyroxypsammaplysin E (
6) was found to have antimalarial effect against this strain
[46] (
Table 3).
Table 3. Compounds with reported antimalarial activities.
Similarly, psammaplysin F (
10) has been reported to inhibit chloroquine-sensitive (3D7), Dd2
[45], the drug-resistant (K1) and drug-sensitive (FCR3) strains of
P. falciparum [47] (
Table 3). Furthermore, psammaplysin G (
11) has been found to inhibit the chloroquine-resistant (Dd2)
P. falciparum strain without any toxicity towards the HEK293 cell line
[45]. Likewise, psammaplysin H (
12) was described to have a potent antiplasmodial activity against 3D7 strain with an excellent selectivity index
[48] (
Table 3).
Finally, ceratinadins E (
40) and F (
41) were reported to show antiplasmodial activities against the drug-resistant (K1) and drug-sensitive (FCR3) strains of
P. falciparum. Moreover, ceratinadin E (
40) was found to display higher selectivity indices (SI) than ceratinadin F (
41)
[47] (
Table 3).
The antimalarial evaluation of 13 psammaplysins analogs clearly shows that psammaplysin F (
10) is the most potent active compound against Dd2 strain, while ceratinadin E (
40) possesses a greater antimalarial activity towards K1 strain and a better selectivity index than psammaplysins F (
10). The addition of a terminal
N-methyl, as in psammaplysin F (
10), enhances the activity. However, ceratinadin F (
41), which possesses several
N-methyls, did not show significant antimalarial activity, which could be attributed to the high lipophilicity of the compound. Though the antimalarial activity against drug-resistant strains of
P. falciparum is unknown, psammaplysin H (
12), a quaternary analog with a trimethylamino group instead of a methylamino group, possesses a potent antimalarial activity and better selectivity against a drug-sensitive strain of
P. falciparum than psammaplysins F (
10) without any significant cytotoxicity against the HEK293 384 cell line
[45][46][48] (
Table 3).
Comparing the activities of psammaplysins F (
10), G (
11), and H (
12) towards two mammalian cell lines (HEK293 and HepG2), psammaplysin H was found to display a minimal toxicity at the highest concentration tested (40 μM), giving this compound a parasite-specific selectivity index (SI) of >97. In contrast, psammaplysins G and F display higher toxicity to these cell lines with IC
50 values between 3.71 and 18.96 μM, respectively. These preliminary structure–activity data suggest that full methyl-substitution of the terminal amine (
N-quaternization) is essential for optimal antimalarial activity and better selectivity
[48].
Likewise, the replacement of an urea, amine, or enamine derivative with a secondary amide group adversely affects the antimalarial activity. However, the higher lipophilicity (i.e., log P) and larger molecular weights associated with the amide analogs including psammaplysin M (
18), psammaplysin N (
19), 19-hyroxypsammaplysin P (
22), psammaplysin T (
29, and psammaplysin V (
32) would also minimize the bioavailability
[49], thus reducing the antimalarial effect.
5. Compounds with Antifouling Activities
When evaluated for their antifouling activity, psammaplsyins A (
1), E (
5), ceratinamides A (
7) and B (
8) have reported to inhibit the metamorphosis and settlement of the barnacle
B. Amphitrite [37] (
Table 4). The highest activities of psammaplysin A (
1) and ceratinamide A (
7) suggests the importance of a terminal amine or an
N-formyl moiety for a maximum antifouling activity. Furthermore, ceratinamide A (
7) was found to induce a larval metamorphosis of the ascidian
Halocynthia roretzi [37] (
Table 4).
Table 4. Compounds with reported antifouling activities.
6. Compounds with Other Reported Activities
Psammaplysin D (
4) was reported to display anti-HIV towards the Haitian RF strain of HIV-I
[44] (
Table 4). Recently, psammaplysin F (
10) was reported to increase the efficacy of the antitumor drugs bortezomib and sorafenib through regulation of the synthesis of stress granules
[50] (
Table 5).
Table 5. Compounds with other reported activities.
Frondoplysins A (
42) and B (
43) were described to inhibit protein-tyrosine phosphatase IB (PTP1B). The compounds were found to have a higher activity than the positive control oleanolic acid
[51] and thiazolidinediones
[53] and were similar to benzofuran and benzothiophene biphenyls
[54] (
Table 5). Further, frondoplysin A was found to possess in vivo antioxidant activity in transgenic fluorescent zebrafish over five times stronger than that of vitamin C
[51] without any cytotoxicity
[51] (
Table 5).
It has been described that psamaplysin F (
10) andits urea semisynthetic analogs (
45,
51,
53 and
54) strongly reduce the mitochondrial membrane potential (MMP). Further, it was found that psammaplysin F strongly affects the mitochondrial morphology and reduced the number of end points and branch points within the tubular structure of individual mitochondria, leading to visible fragmentation of the mitochondrial tubular network. These findings provide a strong rationale for more detailed mechanistic studies of psammaplysin F and derivatives as novel mitochondrial poisons
[52].