Cytotoxic Compounds from Alcyoniidae: An Overview of the Last 30 Years

The octocoral family Alcyoniidae represents a rich source of bioactive substances with intriguing and unique structural features. This review aims to provide an updated overview of the compounds isolated from Alcyoniidae and displaying potential cytotoxic activity. In order to allow a better comparison among the bioactive compounds, we focused on molecules evaluated in vitro by using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay, by far the most widely used method to analyze cell proliferation and viability. Specifically, we surveyed the last thirty years of research, finding 153 papers reporting on 344 compounds with proven cytotoxicity. The data were organized in tables to provide a ranking of the most active compounds, to be exploited for the selection of the most promising candidates for further screening and pre-clinical evaluation as anti-cancer agents. Specifically, we found that (22S,24S)-24-methyl-22,25-epoxyfurost-5-ene-3β,20β-diol (16), 3β,11-dihydroxy-24-methylene-9,11-secocholestan-5-en-9-one (23), (24S)-ergostane-3β,5α,6β,25 tetraol (146), sinulerectadione (227), sinulerectol C (229), and cladieunicellin I (277) exhibited stronger cytotoxicity than their respective positive control and that their mechanism of action has not yet been further investigated.


Introduction
Marine environments, and especially coral reefs, are among the richest and most complex Earth ecosystems in terms of biodiversity [1]. Although their surface may appear small when compared to the total ocean floor, coral reefs provide a plethora of ecosystem goods and services [2,3] and represent the most important marine biodiversity hotspots [4]. Generally, biodiversity is the result of complex interactions between biotic and abiotic factors, resulting in a wide array of forms, behaviors, and consequently molecules. Over the years, the vast marine biodiversity has been largely investigated as a resource of therapeutic drugs with alternate success. Roughly 30,000 marine natural products are known so far and more than 1000 new compounds are isolated every year [5]. The oldest marine-derived drug of this type is cytarabine, which is the synthetic analog of spongeisolated cytostatics spongothymidine and spongouridine. Cytarabine was approved as an antileukemic agent in 1969 and has been in clinical use for decades. At the present time, 17 marine drugs are approved, four are in phase III, nine in phase II, and 16 in phase I clinical trials [6]. Among these 46 molecules, 35 are anti-cancer agents. Corals have been the target of study for drug discovery since the nineteenth century and since the 70s, many natural products with diverse and important biological activities have been isolated and characterized. The phylum Cnidaria, and especially corals, accounts, along with Porifera, for 57% of the total bioactive marine compounds discovered [7]. However, despite the extreme coral biodiversity, none of these products have been placed under clinical evaluation yet.
Corals mostly belong to the class Anthozoa, which is currently divided into two subclasses: Hexacorallia, including anemones, black corals, hard corals, and Octocorallia, including blue corals, sea pens, gorgonians, and soft corals [8]. Soft corals ( Figure 1) have so far received more attention as potential sources of drug candidates, as they produce a greater number of secondary metabolites compared to hard corals. The most typical metabolites produced by soft corals are steroids, polyhydroxysterols, sesquiterpenes, diterpenoids, and biscembranoids [9]. These molecules may act as chemical defense compounds toward predators, competing reef organisms, and also against bacteria and parasites, to secure their protection and survival, and to control epibiont overgrowth [10]. For example, sarcophytoxide (132) is an allelopathic molecule usually found in Sarcophyton and used by this soft coral genus in the competition for space with scleractinian corals [11]; sarcophine (121) is another metabolite used for ecological defense, displaying toxic effects in fish [12]. In addition, several compounds may not be directly produced by the coral itself, but rather originate from or be made in concert with single-celled organisms (e.g., algae and bacteria) that act as symbionts and contribute significantly to the nutrition of their host. Production of these compounds may also be specific to the host-symbiont interaction, as the case of gogosterol, synthesized by zooxanthellae [13]. Recent studies also highlighted strong correlation between diversity of microbial communities and coral metabolome [14]. These compounds have been largely investigated given their potential to possess pharmacological properties of interest for human health [15]. Compounds extracted from soft corals have been frequently investigated in relation to their possible anticancer activity by testing their cytotoxic and antiproliferative activity against cancer cell lines, due the postulated natural function of these compounds as part of the chemical defense machinery of the producing organism or the host organism harboring the producing species [5]. In this framework, since the 2000s, the number of studies and scientific publications has grown exponentially, and consequently also the number of compounds isolated from soft corals with cytotoxic activity has reached considerable dimension. This is not surprising since cancer is a disease with a very high incidence in the developed countries, rising year by year and occupying the second ranking as cause for death after strokes. About 19.3 million new cancer cases and almost 10.0 million cancer deaths are estimated to have occurred in 2020 worldwide [16]. Currently, anticancer treatments involve the use of cytotoxic agents supplemented by targeted therapies to improve treatment efficacy and reduce side effects. The idea of using natural compounds to fight cancer is a long-and well-established tradition mainly originating from the availability of natural products and their traditional use that render public opinion open to their use as drugs. Indeed, almost 60% of drugs approved for cancer therapy are of natural origin, mostly from plants, such as vincristine (VCR), irinotecan etoposide, taxanes, and campthotecins, and from microbes, such as actinomycin D, mitomycin C, bleomycin, doxorubicin, and l-asparaginase [17]. In this context, the marine environment can be considered still underexplored, even if many papers that describe the application of marine metabolites are available, highlighting how the variety that populates the oceans overwhelms our discovery possibility. For instance, several halogenated compounds that are not present in the terrestrial environment were discovered in the sea [18]. Therefore, it is expected that future research, supported by the current knowledge, will bring to light new therapeutics.
Starting from this basis, in this review work, we focused on Alcyoniidae, one of the largest octocoral family belonging to the order Alcyonacea, with a circumglobal distribution, especially in the tropics and subtropics, and including several genera associated with intracellular symbiotic dinoflagellates [19]. Multiple genera in this family have been investigated for the presence of cytotoxic metabolites against cancer cell lines, namely Lobophytum, Sarcophyton, Sinularia, Cladiella, Klyxum, Alcyonium, Bellonella, Anthomastus, Paraminabea and Protodendron.
This literature review has been carried out using three different online databases (Scopus, Web of Science, and Google Scholar) and applying the following keywords: cytotoxic; anticancer, cytotoxicity; Alcyoniidae; Lobophytum; Sarcophyton; Sinularia; Cladiella; Klyxum. Among the retrieved papers, we selected only those reporting the full structural characterization of metabolites (i.e., with multi-dimensional NMR and HRMS data available) and the data regarding their cytotoxic activity against any type of cancer cell line evaluated with 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay or a similar test related to the NAD(P)H-dependent oxidoreductase enzymes activity. As a result, a total of 164 articles were surveyed containing information related to 344 compounds. Most of the papers dated from 1992 to 2021. Data were then organized into three tables on the basis of the in vitro results of the MTT method [20,21] as reported in the related paper.
Overall, the aim of the presented review was to provide researchers with a dataset handily exploitable for the selection of the most promising leads for further experiments and pre-clinical development studies, obtained from the actual body of knowledge regarding the last discovered cytotoxic compounds from soft corals under the lens of the same in vitro assay. Specifically, this work is the first providing a ranking of the cytotoxic coral metabolites based on the results of the same assay from the beginning of reporting of active compounds from Alcyoniidae. leukemia), and HepG2 (hepatocellular carcinoma) cells with (IC 50 values ranging from 0.34 to 3.44 µM.
Roy et al. [38] isolated three compounds (35)(36)(37)  Three new polyoxygenated steroids, michosterols A-C, and four known compounds were isolated from Lobophytum michaelae by Huang et al. [39]. The cytotoxicity of the new metabolites against A-549 (human lung epithelial carcinoma), DLD-1 (human colon adenocarcinoma), and LNCap (human prostate adenocarcinoma) cells was evaluated, and the results showed that only michosterol A (38) exhibited a cytotoxicity effect against the A549 cell line with an (IC 50 value of 14.9 ± 5.7 µg/mL.

Cytotoxic Compounds from the Genus Sarcophyton
The genus Sarcophyton is by far the most studied source of bioactive compounds among soft corals, and several scientific papers dealt with cytotoxic compounds extracted from this taxon. The very early and iconic example dates back to 1989 with the discovery by Fujiki et al. [44] of sarcophytol A (55) from Sarcophyton sp. in Ishigaki Island (Okinawa, Southern Japan).
Su et al. [58] investigated the cytotoxic effects of 13-acetoxysarcocrassolide (57), isolated from S. crassocaule, on bladder female transitional cancer (BFTC) cells. The concentration of 13-acetoxysarcocrassolide, ranging from 0.5 µg/mL to 5 µg/mL dose-dependently, inhibited the growth of BFTC cells: the population of cancer cells was clearly reduced after treatment with 1.5 and 3 µg/mL.

Cytotoxic Compounds from the Genus Sinularia
The genus Sinularia includes an estimated number of~190 nominal species, 47 of which have been described only in the last 25 years [76]. Many of these species have been chemically evaluated, including hybrid species. It is by far the most studied genus of corals as a resource of metabolites with potential use as anticancer drugs.
A chemical investigation on the dichloromethane extract of Sinularia parva led to the isolation of three new bicyclic norcembranoids, leptocladolide A, 1-epi-leptocladolide A, and 7E-leptocladolide A [86]. Leptocladolide A (177) exhibited cytotoxicity against KB and Hepa 59T/VGH cancer cells lines with ED 50 values ranging from 2.6 to 15.1 µg/mL.
A novel steroid, gibberoketosterol, along with four known steroids, were extracted from Sinularia gibberosa by Ahmed et al. [87]. Gibberoketosterol (178) and two of the known steroids showed a cytotoxicity against the growth of Hepa 59T/VGH cancer cells with ED 50 values of 10.0, 9.3, and 6.8 µg/mL, respectively.
Ahmed et al. [88] also isolated three new norditerpenoids, scabrolides E-G, along with dissectolide A, from Sinularia scabra. The tricyclic norditerpenoid scabrolide E (179) was found to be cytotoxic against the proliferation of Hepa 59T/VGH and KB cells with ED 50 values of 0.5 and 0.7 µg/mL, respectively.
Nanolobatin A (180) and nanolobatin B (181), two new norsesquiterpenoids isolated by Ahmed et al. [89] from Sinularia nanolobata, were found to be cytotoxic toward KB and Hepa 59T/VGH cancer cell lines with ED 50 values ranging from 4.5 to 8.3 µg/mL.
Liu et al. [100] investigated the cytotoxic effects of 11-dehydrosinulariolode, isolated from S. leptoclados, on CAL-27 (oral adenosquamous carcinoma) cells. When a concentration of 1.5 µg/mL of 11-dehydrosinulariolide (197) was applied, the results showed that CAL-27 cells viability was reduced to a level of 70% of the control sample.

Cytotoxic Compounds from the Genus Cladiella
The genus Cladiella has proven to be a rich source of cytotoxic eunicellin-based diterpenoids. In 2005, Ahmed et al. [142]   Hirsutalin R (283), isolated from C. hirsuta by Huang et al. [154], exhibited cytotoxicity toward the proliferation of P-388 (murine leukemia) and K-562 (human erythro myeloblastoid leukemia) cell lines with (IC 50 values of 13.8 and 36.3 µg/mL, respectively, which compared to those of the positive control 5-fluorouracil (8.5 and 24.6 µg/mL, respectively) showed cytotoxicity. Unlike the drug, however, hirsutalin R (283) was inactive on HT-29 (human colon adenocarcinoma) and A-549 (human lung adenocarcinoma) cell lines.
Two new xenicanes, named protoxenicins A and B, were isolated by Urda et al.

Thirty Years of Compounds Discovery
For a better reading of the data, data reported in the previous section was organized in three tables (Supplementary Materials, Tables S1-S3) compiled by dividing compounds according to their structures: cembranes, cembrane lactones, other terpenes, steroids. The tables report the common chemical names of the selected compounds, the species from which these compounds were isolated, the extraction yields claimed by the authors of the research, the cell lines used for the cytotoxicity test and the results of the assay.
Considering doxorubicin as positive control, the most active compounds discovered in the last thirty years are 5-episinuleptolide acetate (207) (16), and 3β,11-dihydroxy-24-methylene-9,11-secocholestan-5-en-9-one (23). Noteworthily, the experiments carried out using the anticancer drugs paclitaxel, cisplatin, and vinblastine as positive controls highlighted that no molecules from corals are so far capable to display a comparable cytotoxic activity. This suggests that further tests employing these controls are needed to better verify the cytotoxicity of coral-derived natural products and to identify possible differences in the mechanism of action. The structures of the anticancer drugs used as positive control for the MTT assay in the considered studies are shown in Figure 15. Considering the isolation yields, the best results were obtained in the extraction of emblide (70) with 2790.7 mg per kg of coral (dry weight), 13-acetoxysarcocrassocolide (42) with 2033 mg per kg of coral (wet weight), 11-episinulariolide acetate (202) with 1757.1 mg per kg of coral (dry weight), and lobohedleolide (4) with 1090.9 mg per kg of coral. For the other compounds, the very low yields (more than 1 ton to isolate 1 g of active compound) represent a significant obstacle for the preclinical development, and a synthetic replacement would be required.
Overall, this review clearly shows that considering the chemical structure, the most effective compounds isolated from the Alcyoniidae showed to belong to the group of diterpenoids and steroids. Significant, but scattered, activity was found also from compounds belonging to monoterpenoids, sesquiterpenoids, diterpenoids, triterpenoids, and tetraterpenoids. Most of the described structures are rare and unusual in the natural compounds' literature and may be specific to the cnidarian world.
The fact that the strongest cytotoxicity was found in compounds belonging to the group of terpenoids is not surprising, since terpenes (and derivatives) represent the largest class of natural products in the biosphere, and their variety of functions in mediating antagonistic and beneficial interactions among organisms is still largely unexplored [180]. Terpenes already include very important chemotherapeutics, such as the taxanes (diterpenes), discovered from the plant world and commonly used in clinical studies, since they are mitotic inhibitors binding tubulin [181].
Among the diterpenoids, cembranoids were the most represented. This name indicates the structure of an isopropyl-and three methyl-substituted 14-membered rings originated by the cyclization of a geranylgeraniol derived precursor between carbons 1 and 14 [182]. Structural changes in the position of double bonds, epoxidation, allylic and isopropyl oxidation, and carbon cyclization are widely observed in these compounds [183] and often correlated with geographic variation and environmental conditions [184], since cembranoids are used by corals as chemical defense to ensure protection against predators, parasites and competing reef organisms [185]. According to a recent review by Nurrachma et al. [186], more than 360 cembranoids are known in the literature. Following Yang et al. [187], cembranoids may be divided in 5 subclasses: (1) simple cembranes, including isopropyl cembranes, isopropenyl cembranes, and isopropyl/isopropenyl acid cembranes subtypes; (2) cembranolides that possess a 14-membered carbocyclic nucleus generally fused to a 5-, 6-, 7-, or 8-membered lactone ring, as observed in Sarcophyton with durumolide C (211) (5-membered), and in Sinularia with manaarelonide A (6-membered) and sinuladiterpene (7-membered); (3) furanocembranoids characterized by a 14-membered carbocyclic nucleus, as well as a furan heterocycle, as observed in Sinularia with pukalide; (4) biscembranoids showing a 14-6-14 membered tricyclic backbone of tetraterpenoids, as in lobophytone A found in Lobophytum pauciflorum. (5) special cembranes that in turn include several subtypes. Our review clearly shows that among the cembranoids, the best activity at the MTT assay is obtained by molecules owing an odd membered lactone ring (five and seven) such as in the case of deoxycrassin (44), flexilarin D (196), 11-dehydrosynulariolide (197), but also simple cembranes bearing various oxidation display a potent cytotoxic action, such as in sinulerectol C (229).
Steroids resulted in the second most represented group in terms of compounds active against cancer cell lines, with a total of 72 compounds. The use of steroids for cancer treatment is clinically well established, and the pharmacology of several active compounds has been described. Steroids mostly work by interacting with specific nuclear hormone receptors that are known to drive cell growth, proliferation, and metastasis for specific tumors (i.e., the estrogen receptor for breast cancer and the androgen receptor for prostate). In contrast, glucocorticoids work through glucocorticoid receptors, which are not considered oncogenes, to perform a variety of functions, including arresting growth or inducing apoptosis in lymphocytes; for these reasons they are widely used in the treatment of all lymphatic cancers [188]. On the other hand, the contribution of the discovery of new steroid structures from the natural word is fundamental to open new clinical development and inspire the work of synthetical chemists.
According to a recent review of Ermolenko et al. [189], about 200 different steroids were discovered from corals, displaying structures belonging to several different types (i.e., secosteroids, spirosteroids, epoxy-and peroxy-steroids, steroid glycosides, halogenated steroids, polyoxygenated steroids, and steroids containing sulfur or nitrogen heteroatoms). In this review, more than 40 steroids were considered potentially capable of displaying cytotoxic activity based on structure-activity relationship predictions (but only a few were tested experimentally). Our review can be considered a completion of this work, since the results of MTT were retrieved and organized for further evaluation: we, in fact, highlighted the presence in nature of at least 33 compounds (from 13 papers) with a very potent cytotoxic action, still not considered in a structure-activity relationship (SAR) study.
A variety of structural variations from the classical steroidal architecture was found in both the tetracyclic ring and the alkyl chain, such as in the case of klyffacisteroid A (304) and (22S,24S)-24-methyl-22,25-epoxyfurost-5-ene-3 (16). Interestingly, several active molecules showed the methyl/ethyl substitution typical of phytosterol, as in sarcoaldosterol B (148) and acutumosterol A (147). Another interesting modification displayed in the coral cytotoxic steroids was the change of the position of the unsaturation in the steroid ring, something commonly observed in algae. On the other hand, the loss of the methyl substituents, which is equally common in the algae, was not observed. Since corals are not capable of providing de novo synthesis of cholesterol and its derivatives, these structures are obtained from phytosterols (provided by intracellular algal symbionts) through dealkylation and successive oxidation reactions. Signs of the oxidative pathway are clearly observable in various cytotoxic structures, such as the addition of an 11α-hydroxyl group in acutumosterol A (147) and sarcoaldosterol B (148), a 17α hydroxylation in klyfaccisteroid A (304), the oxidation in the lateral chain, even if this does not come with extensive dealkylation, the presence of a conjugated ∆4-3-keto and of an additional unsaturation at ∆1 as in sinubrasolide A (236), K (235) and in sinubrasones (247, 248).
For some of the more potent cytotoxic compounds listed before, the literature is not limited to in vitro cytotoxicity tests, but studies have gone further by investigating the cellular mechanism of induction of apoptosis and the ability to alter some fundamental properties of cancer cells, such as the capacity to migrate and the potential colony formation were investigated in detail (Suppplementary Materials, Table S4).
Lobophytosterol (15) was found to induce apoptosis, in fact chromatin condensation in apoptotic bodies was observed [28].
Sinularin (205) induced apoptosis and suppressed the migration capacity of A2058 cancer cells [105]. Wu et al. [123] found that sinularin (205) inhibited cell migration capacity and induced apoptosis through mitochondrial dysfunction and inactivation of the pI3K/AKT/mTOR pathway in human gastric carcinoma cell lines. Moreover, sinularin (205) activated ATM/Chk2 DNA damage pathway, induced G2/M phase arrest, decreased mitochondial memebrane potential and caused apoptosis in HepG2 (human hepatocellular carcinoma) cell line [131]. Moreover, Sinularin (205) induced oxidative stress mediated G2/M arrest and apoptosis in Ca9-22 cancer cells [190] as well as caused G2/M arrest, apoptosis, and oxidative DNA damage in human breast carcinoma (SKBR3) cells [191]. The studies conducted by Ma et al. [192] revealed that sinularin (205) induced G2/M arrest and induced apoptosis as well as activation of MAPKs and repression of PI3K/AKT pathways, which are dependent on ROS generation, in human renal cancer (786-O) cells. Recently, Ko et al. [193] have thoroughly investigated the effect of sinularin (205) on human hepatocellular cancer (SK-HEP-1) cells and found that wound healing, cell migration, and potential colony formation were attenuated as well as DNA fragmentation and apoptosis were induced and mitochondrial and intracellular reactive oxygen species (ROS) levels were significantly increased following sinularin treatment, which also decreased the mitochondrial membrane potential and caused cytoskeleton disruption [194].
5-Episinuleptolide acetate (207) was investigated by Huang et al. [115] and it was found that the compound increased the generation of ROS, the accumulation of intracellular Ca 2+ as well as decreased the mitochondrial membrane potential and induced apoptosis through the Hsp90 chaperone machinery inhibition in HL-60 (human promyelocytic leukemia) cells.
Sinulariaoid A (215) induced apoptosis but its selective toxicity toward HepG2/ADM cells was not related to P-glycoproteins [116].
In addition to these studies, Peng et al. [195] researched the potential synergistic antiproliferation of the combined treatment of ultraviolet-C and sinularin (UVC/sinularin) in oral cancer cells. The results showed that UVC/sinularin synergistically and selectively inhibits cancer cells proliferation with low cytotoxicity on normal oral cells. The cytotoxic mechanism was found to involve apoptosis by increasing cellular and mitochondrial oxidative stress, DNA damage, and mitochondrial dysfunction.

Challenges and Future Perspective
After several decades of research, marine organisms still deserve to be considered one of the largest untapped resources in the study of natural and chemical diversity. Estimates suggest that from one third to two thirds of eukaryotic marine species still have to be described [196] and the diversity of non-eukaryotic marine microorganisms is also likely far from being characterized [197], suggesting that not only the biodiversity but also the chemical diversity of the marine environments has been only superficially described. A convincing example is the fact that at the present time, less than 5% of the deep-sea has been investigated and an even lower percentage (0.01%) of the deep-sea floor has been sampled and studied [198]. On the other hand, the fast-paced improvement of the explorative technologies is allowing scientists to explore remote and extreme marine environments, such as mesophotic ecosystems and even the deep sea. Indeed, technological advances in remotely operated vehicles, autonomous underwater vehicles, human occupied vehicles, technical SCUBA diving, or even the development of portable hyperbaric chambers, are allowing the discovery and characterization of several previously poorly explored and unknown ecosystems with great potential for biological diversity [199][200][201]. Thus, there is a strong possibility of finding new bioactive natural products in these environments, given the fact that chemical diversity is directly proportional to biological diversity. Soft corals, the taxon considered in this review, with 5800 secondary metabolites already described [202], provide the most astonishing example that support these considerations.
Nowadays, several high throughput technologies are available to study diversity, which can therefore rely on the constantly improved 'omics' sciences, such as genomics, transcriptomics, metabolomics, and proteomics [203]. In this sense, the term 'taxonomics' has been also proposed to accommodate this highly integrative approach to characterize the Earth's biodiversity [204]. Specifically, these technologies permit a relatively fast and cost-effective production of huge amounts of data, that can facilitate the description or characterization of species, the deep understanding of their evolution, and the clarification of their relationships with the surrounding environment and organisms. Moreover, these data are useful not only to characterize the diversity of life, but also assist in the discovery of compounds produced by the species and potentially useful for humans [205].
Corals are no exception to this trend, with the knowledge on their diversity, evolution, and ecology being continuously updated, especially in recent years, thanks to the aforementioned 'omics' technologies. Indeed, new species are being discovered [206], the diversity of taxonomically problematic taxa is being elucidated [207,208], the evolutionary history of the Cnidaria is being clarified [209][210][211], and highly integrative studies are also revealing that morphologically similar corals ascribable to the same species can be characterized by deep genetic divergence and ecological and physiological differences [212]. As reviewed herein, cnidarians, and in particular octocorals, are promising organisms for the discovery of bioactive compounds [213] and the implementation of the 'omics' approach to this group will likely largely enhance the known diversity of the chemical compounds associated with octocorals.
The enormous biological diversity found in the marine environment, as well as the diversity of conditions of sea life, underlie the huge chemical diversity found in marine organisms, exceeding that of terrestrial organisms and representing subspaces in the universe of the chemical diversity different from those available to laboratory synthetic chemistry and terrestrial natural synthesis. Marine compounds are in fact characterized by the high presence of chiral centers and heteroatoms, not only oxygen and nitrogen, but even halogens, as result of their high concentration in seawater and the presence of specialized enzymes [214]. Currently, more than 36,000 compounds of marine origin are already described in the scientific literature [215] and about 1000 more compounds are discovered every year [5]. This is obtained thanks to the fast-pacing improvements in the analytical technology, which provide the access to structure elucidation of unknown compounds even when present at trace concentration levels. Notably, aquaculture techniques applied to corals have experienced significant improvements in the last years, due to aquarium trade, reef restoration, and bioprospecting demand [216]. Accordingly, it has been suggested that coral aquaculture has great potential for metabolite discovery and large-scale production of bioactive compounds [217]. Some examples of bioactive substances extracted from cultivated soft corals, such as Sinularia, Lobophytum, and Sarcophyton species, are already available in literature [218][219][220][221], demonstrating the efficacy of this approach. Therefore, coral aquaculture seems extremely promising for both metabolite discovery, in vitro and in vivo studies, and subsequent production for pharmaceutical use.
In this regard, it is advisable that pharmaceutical industries re-establish natural product-based discovery programs, which may provide new compounds in strategic fields, such as cancer treatments. Drivers for this innovation may be the small biotech companies, which may play important roles in the discovery process if positively supported by academia and government through financed projects. The support of these institutions has a paramount importance since the discovery process is risky, time consuming and needs strong capital investment. An additional challenge is represented by intellectual property protection issues. Also in this case, the involvement of universities and government entities may establish mechanisms that encourage partnerships. Access to the scientific data regarding previous research is equally important.
Moreover, this review clearly showed how corals may be still considered unexploited since the number of published works (164 were retrieved in this review) looks still small when compared to the vastness of biodiversity available and the possibilities offered by coral farming.

Conflicts of Interest:
The authors declare no conflict of interest.