Alcyonacea: A Potential Source for Production of Nitrogen-Containing Metabolites

Alcyonacea (soft corals and gorgonia) are well known for their production of a wide array of unprecedented architecture of bioactive metabolites. This diversity of compounds reported from Alcyonacea confirms its productivity as a source of drug leads and, consequently, indicates requirement of further chemo-biological investigation. This review can be considered a roadmap to investigate the Alcyonacea, particularly those produce nitrogen-containing metabolites. It covers the era from the beginning of marine nitrogen-containing terpenoids isolation from Alcyonacea up to December 2018. One hundred twenty-one compounds with nitrogenous moiety are published from fifteen genera. Their prominent biological activity is evident in their antiproliferative effect, which makes them interesting as potential leads for antitumor agents. For instance, eleutherobin and sarcodictyins are in preclinical or clinical stages.


Introduction
The marine biota is characterized by living under harsh environmental conditions (e.g., high salinity, variable pressures, hydrothermal vents and variable nutrient accessibility) [1,2]. The marine invertebrates, particularly soft corals, suffer from absence of mechanical defenses [3][4][5]. They produce chemical agents to maintain their life. These defenses can be represented by production of secondary metabolites that possess ecological functions including anti-predatory protection, thus several natural metabolites were discovered from marine sources [6]. Most of these substances have unprecedented structures with diversity of pharmacological application [7][8][9][10][11][12][13]. Alcyonacea comprises marine invertebrates that live ubiquitously in tropical sea waters, particularly intertidal zones or inner reefs below the stony corals, and are less prone to damage from collecting or shipping than the stony corals [14]. They are animals, provide stinging cells in the form of toxic stinging nematocysts with absence of the rigid protective skeleton of scleractinians, and possess allelopathic capabilities of chemical production [15,16]. This enables the sessile corals to strive and reduce their palatability, by reducing fouling effect through producing chemical substances, mucus or terpenoids, aiming at protecting them against predators [17][18][19]. Alcyonacea is well known for the production of terpenoids, while the occurrence of nitrogenous terpenoidal derivatives is rare. The bio-synthetic pathway of the nitrogenous moiety is unclear [6,20,21]. Conclusively, Alcyonacea has conceivable therapeutics, which includes immunomodulator, anticancer and useful antifouling agents.
Amazingly, 40% of the chemical frameworks that appear in different databases are natural compounds. Approximately, half of new drugs recently reported are of natural origin or constructed based on natural architectures [22]. A comparative analysis study indicates that marine products are superior to earthly metabolites in terms of chemical uniqueness [23]. An investigation has led to establishing that 70% of metabolites that appear in Dictionary of Marine Natural Products (DMNP) are completely exploited by marine invertebrates. Additionally, marine drugs have successfully launched in the market, and others are still in different phases of clinical trials. A recent review reports the marine pharmaceutical products [24][25][26].
In 1969, FDA approved cytarabine as an anticancer agent, while vidarabine was permitted in 1976 as an antiviral agent. Around fourteen years later, ziconotide was approval by FDA for treatment of severe chronic pain. Trabectedin was approved from FDA in 2010 for metastatic breast cancer [25]. On their way to the market, another twelve marine-derived drugs are being clinically investigated [26]; for instance, the bryostatin and dolastatin derivatives, soblidotin and synthadotin, respectively. A tricyclic diterpenoid, eleutherobin, has been reported from Eleutherobia sp., with potent inducer of tubulin polymerization in vitro, which mimics taxol-like effect [25][26][27][28].
In the current review, marine nitrogen-containing metabolites isolated from Alcyonacea are presented. These compounds show significant effects toward certain diseases and/or have a role in drug discovery. It is interesting to discuss the future perspectives of the structure-activity relationship of these metabolites. An extensive bibliographic literature survey was conducted employing different scientific databases including Scopus, Pubmed, SciFinder, Google scholar and Web of Science.
Eleuthosides A (34) and B (35), along with sarcodictyin A (23) were reported from Eleutherobia aurea, gathered around Kwazulu-Natal Coast, South Africa [44]. Both 34 and 35 are novel diterpenoidal glycosides, and were proven to have potent microtubule stabilizing effect. The detailed information is registered in a patent with publication Number WO1999021862 A1.
Eleutherobin has potent inducing in vitro tubulin polymerization effect, although its cytotoxicity is less than those obtained from paclitaxel [35][36][37][38][39]44]. P-glycoprotein is a target substance of Eleutherobin, similar to paclitaxel. Both showed cross-resistance in MDR1-expressing lines. Sarcodictyins are reported from Sarcodictyon roseum and seemed to be more promising than eleutherobin, despite their lower effects. Eleutherobins and sarcodictyins have been extensively modified by employing conventional and combinatorial chemistry techniques, which have also allowed the formation of hybrid molecules of the two base structures. These investigations indicated their SARS ( Figure 4) [36,44].
The side chain and the imidazole ring are important. Both OH and OCH 3 groups are tolerated, with little difference in effects. Elimination or alteration of the aglycone moiety of eleutherobin changes the cytotoxicity and resistance pattern.
Caucanolide B (61), a possible pesudopterane-type diterpenoid successor through oxidation cleavage at C-2/C-3 together with five rare diterpenes, was isolated from Pseudoptergorgia bipinnata collected near the Colombian Southwestern Caribbean Sea. These caucanolides were evaluated against the malaria parasite, Plasmodium falciparum. Unfortunately, Caucanolide B showed no significant activity, although it is the only example from nature of a secondary metabolite possessing the N1,N1-dimethyl-N2acylformamidine functionality [64].
Fenical and Clardy (1982) examined the constituents of Floridian specimens of Pseudopterogorgia acerosa, leading to the isolation of pseudopterolide, a remarkable metabolite based on the 12-membered carbocyclic pseudopterane skeleton [64]. Further examination of extracts of several Pseudopterogorgia spp. has since resulted in the isolation of other pseudopterane metabolites, many of which possess chemically unique structural features ( Figure 6). Tobagolide (62), one such metabolite, is a rare nitrogen-containing diterpenoid isolated from a Trinidadian specimen of Pseudoptergorgia acerosa [65]. Another examination of Puerto Rican specimens of Pseudoptergorgia acerosa led to isolation and structural determination of alanolide (63), a novel tetracyclic norditerpene, which appears to be biogenetically related to tobagolide [66]. The structure of aceropterine (64), the first pseudopterane with a transposed lactone moiety, is closely related to that of tobagolide, which was isolated from a Trinidadian specimen of Pseudoptergorgia acerosa [67,68].

Terpenoidal Alkaloids
Zoanthamine-type alkaloid (Lobozoantamine, 65) was identified from an Indonesian coral, Lobophytum sp. It belongs to a unique class of alkaloids, the precursor type of which is still ambiguous, whether it is a triterpenoid or a polyketide [68]. The first member of the zoanthamine-type alkaloid was isolated from Zoanthus sp. [69,70], followed by a series of analogs isolated from the genus Zoanthus, with the single exception of zooxanthellamine, which was isolated from the unicellular dinoflagellate Symbiodinium sp. [71]. Lobozoanthamine (65) was evaluated against AGS and C6, and showed cytotoxic effect with IC 50 value > 50 µM on both cell lines.
A rare pyrroloindoline alkaloid verrupyrroloindoline (89) was isolated from Sinularia verruca. It showed no protection towards the cytopathic effects of HIV-1 infection and no effect was observed against LPS-induced NO production [79].

Miscellaneous Nitrogen-Containing Metabolites
Spermidine is a polyamine found in ribosomes and living tissues. It has various metabolic functions within organisms and was isolated originally from semen. Spermidine is commonly used for in vitro molecular biology reactions, particularly in vitro transcription by Phage RNA polymerases, in vitro transcription by human RNA polymerase II, and in vitro translation. Spermidine increases specificity and reproducibility of Taq-mediated PCR by neutralizing and stabilizing the negative charge on DNA phosphate backbone. Spermidine is, at physiological pH, a polycationic reagent that aids in enzyme digestion by forcing apart DNA molecules. Two cytotoxic spermidine derivatives (115 and 116) (Figure 9) were reported from Pacific Sinularia brongersmai [95]. Sinulamide (117), a tetraprenylatedspermine derivative, was identified from Japanese Sinularia sp. Sinulamide (117) not only inhibits H, K-ATPase with an IC 50 value of 5.5 µM, but also is cytotoxic against L1210 and P388 with IC 50 values of 3.1 and 4.5 µg/mL, respectively [96]. Two N-methylated spermidine amides, 118 and 119, were isolated from Sinularia sp. [97]. An acylated spermidine (120) was identified from Sinularia sp. and was found to be cytotoxic against P-388 cells (ED50 0.04 µg/mL) [98].

Summary and Conclusions
Natural products possess a characteristic chemical spatial orientation. This enables them to interact with their biological targets, which validates initial points for drug discovery. Recently, half of new drugs reported are naturally occurring or constructed on the basis of natural chemical frame. Forty percent of the bioactive compounds are natural metabolites and appear in the Dictionary of Natural Products. Chemical novelty of marine products is superior to terrestrial metabolites. Approximately 70% of the molecular skeletons that appear in databases are produced by marine organisms. Additionally, marine drugs have successfully been purchased and others are in different clinical phases.
Alcyonacea will be considered as potential source of bioactive nitrogen containing metabolites. The engagement of different approaches played a significant role in the facilitation of the forthcoming drug discovery process. Noteworthy, many marine metabolites displaying fascinating molecular structures with diverse pharmacological effects have been reported from Alcyonacea during the last four decades . Of the 121 distinctive structures accounted for in this review, 61 (50.4%) are nitrogen-containing terpenoidal metabolites. Figure 11 illustrates the number of nitrogenous metabolites, as reported from 44 species. These species belong to six genera, as illustrated in Figure 12.    A remarkable 121 metabolites reported from Alcyonacea are discussed in the current review. Of these distinctive structures, 121 (100%) are nitrogen-containing terpenes. The major classes of nitrogen-containing metabolites produced by Alcyonacea are sesquiterpenes, diterpenes and ceramides. The estimated analysis per class is as follows: 46 (38%) are diterpenes, 15 (12.4%) are sesquiterpenes, 24 (19.8%) are alkaloids and the remaining 36 (28.8%) are spermidins, cerebrosides, and ceramides, as appeared in Table 2.

93-94,114
Ceramides and cerebrosides Diterpenoids, the major division, are in turn further analyzed for each Alcyonacean species, and the distribution by skeleton classes of compounds in this group is shown in Table 3, which summarizes the impressive structural variety of terpenoid carbon skeletons found in these animals. The diterpene skeleton is most frequently elaborated by the genera Pseudopterogorgia, Erythropodium, Sarcodictyon, Lobophytom and Sinularia. These diterpenes have highly functionalized moieties with potent cytotoxicity that could be mimetic the mode of action of taxol. Some of them are currently in clinical trials.