Dactylospongia elegans—A Promising Drug Source: Metabolites, Bioactivities, Biosynthesis, Synthesis, and Structural-Activity Relationship

Marine environment has been identified as a huge reservoir of novel biometabolites that are beneficial for medical treatments, as well as improving human health and well-being. Sponges have been highlighted as one of the most interesting phyla as new metabolites producers. Dactylospongia elegans Thiele (Thorectidae) is a wealth pool of various classes of sesquiterpenes, including hydroquinones, quinones, and tetronic acid derivatives. These metabolites possessed a wide array of potent bioactivities such as antitumor, cytotoxicity, antibacterial, and anti-inflammatory. In the current work, the reported metabolites from D. elegans have been reviewed, including their bioactivities, biosynthesis, and synthesis, as well as the structural-activity relationship studies. Reviewing the reported studies revealed that these metabolites could contribute to new drug discovery, however, further mechanistic and in vivo studies of these metabolites are needed.


Introduction
The marine environment is extraordinarily rich in diverse species of organisms that represent an enormous source of biometabolites, many of which have unique chemical entities not present in terrestrial sources [1][2][3]. The molecular diversity and exceptional complexity of marine metabolites have been highlighted in many studies [4,5]. The marine environment investigations have greatly been limited to subtropical and tropical regions, in addition, the exploration has been recently extended to colder regions. However, it is fact that many of these unique marine resources have barely been investigated.
Sponges (filter feeders, phylum Porifera) are evolutionarily ancient metazoans that occur in marine benthic, quasi-terrestrial, deep-sea, and fresh-water ecosystems [6,7]. They represent one of the important members of marine communities that have potential biotechnological and ecological roles [8,9]. They are sessile multicellular invertebrates, having an enormous amount of tiny pores on their surfaces that allow entrance and circulation of water through canals where organic particles and microorganisms are filtered out and eaten [10]. Calcarea, Demospongiae, Homoscleromorpha, and Hexactinellida are the four main classes of sponges [11]. It is noteworthy that the secondary metabolites distribution among these four different classes of sponges is greatly varied as shown in Table 1. Table 1. Reported secondary metabolites from the four main class of sponges [12].

Sponge Class Compounds Classes
Calcarea C 27 to C 29 ∆ 5,7,9(11), 22 and C 27  Sponges are devoid of any physical capacity for defense; therefore, they need to develop specific means and adaptive responses for self-protection [13]. They produce diverse secondary metabolites as defense ways against pathogenic fungi, algae, bacteria, and other predators, also to modulate and/or enable cellular communication [14]. Sponges have been known as a fertile field for the discovery of bioactive metabolites with diverse structural features that have been proven to have a beneficial potential for humans as agricultural medicines, drugs, health foods, and cosmetics [14,15].
Sponges belonging to Dactylospongia, particularly D. elegans Thiele (Thorectidae), have been vastly recognized as a wealth pool of variable metabolites with a wide array of potent bioactivities. Most of these reported metabolites are sesquiterpenes, including sesquiterpene hydroquinones, sesquiterpene quinones, and sesquiterpene tetronic acids, in addition to few sesterterpenes, sterols, and pregnanes. Further, these metabolites displayed relevant bioactivities, such as antitumor, cytotoxicity, antibacterial, and anti-inflammatory. Diverse studies focusing on the separation, characterization, and bioactivities of D. elegans metabolites are reported. Therefore, this works aims to summarize all reported molecules, including their activities, biosynthesis, and synthesis, as well as highlighting the structureactivity relationship studies, which could be used as an extensive reference for further studies on this sponge and its metabolites. Additionally, this work magnifies the relevance of D. elegans in the field of marine metabolites production and its significance in the discovery of naturally derived biometabolites. The literature search on D. elegans was done by collecting the information on the conducted studies, using scientific databases and websites of various journals, such as Google Scholar, ACS (American Chemical Society),  (−)-Ilimaquinone (1)       Mamanuthaquinone (                   Sesquiterpenic quinones/hydroquinones are a class of natural marine metabolites that are mainly reported from order Dictyoceratida sponges, including various genera such as Dysidea, Fenestraspongia, Hyrtios, Dactylospongia, Petrosaspongia, Spongia, and Hippospongia [27,32]. They can be either with the drimane or rearranged drimane (clerodane-decalin or 4,9-friedodrimane) skeleton, having four stereo-genic centers. The quinone/hydroquinone moiety could be mono, di, tri, tetra, or penta-substituted. These substituents can be hydroxy, methoxy, methyl ester, or amino groups. These metabolites possess variable structures based on the configuration at C-5 (cis-or trans-clerodane skeleton), C-9, C-8 and/or the position of the double bond. These metabolites have drawn remarkable interest due to their diverse structures and bioactivities.
Cancer represents one of the main reasons for death that has the second-highest incidence of mortality after cardiovascular diseases [38]. Chemotherapeutic treatment is the most common strategy for cancer treatment. However, the chemotherapeutic agents influence not only tumor cells but also normal cells resulting in hazardous side effects [39]. D. elegans reported sesquiterpenes have been evaluated for their anticancer potential towards various cancer cell lines. Further, some studies reported the structure-activity relationship and synthesis of some analogs have been discussed as shown here.
Differentiation induction therapy is one of the alternative therapeutic methods that is based on the differentiation of tumor cells to normal cells using a differentiation inducer, ATRA (all-trans-retinoic acid) [41]. However, different types of leukemia were found to be unresponsive to ATRA, such as CML (human chronic myelogenous leukemia). So, several exploratory research was carried out to discover new differentiation inducers from marine organisms. CML is a hematopoietic stem cell cancer produced by the Bcr-Abl tyrosine kinase that is resulted from Philadelphia chromosome (Ph) translocation [42]. Allogeneic BMT (bone marrow transplant) is the only common curative therapy for CML. However, the treatment-associated toxicity is dangerous with about 30% reported mortality [43]. Aoki et al. stated that smenospongine (25) (Conc. 3-15 µM) induced K562 CML cells differentiation into erythroblasts alongside with cell cycle arrest at the G1 phase and increased expression of p21 protein, which had an important role in differentiation. Further, it prohibited the phosphorylation of Crkl, which is a substrate of Bcr-Abl tyrosine kinase [31]. Smenospongine 25 an aminoquinone sesquiterpene was firstly reported in 1987 as an antimicrobial and cytotoxic metabolite from Smenospongia sp. [44].
Further, in 2008, Kong et al. investigated the influence of 25 on the cell cycles of various cells, including HL60 (human acute promyelocytic leukemia) and U937 (human histiocytic lymphoma) cells, as well as the mechanism of K562 cells G1-phase arrest. It was found to induce dose-dependent apoptosis in U937 and HL60 cells and G1 arrest in K562 cells. In K562 cells, it boosted p21 expression and suppressed Rb phosphorylation, revealing the remarkable function of the p21-Rb pathway in G1 arrest. In addition, it could enhance p21 expression through another mechanism than p21 promoter transactivation [43]. Further, its effect versus a panel of 39 solid cancer cell lines was assessed in the SRB and WST-8 (watersoluble tetrazolium salt-8) assays. Smenospongine (25) suppressed the growth of these cells in vitro (mean Log GI 50 -5.55). Additionally, it prohibited migration, proliferation, and HUVEC (human umbilical vein endothelial cells) tube formation. Hence, it demonstrated antitumor potential versus solid tumors through direct growth inhibition of the tumor cells and anti-angiogenic effectiveness on endothelial cells, indicating its potential as a lead compound for discovering a prominent anticancer [45].
In another study by Aoki et al., a new aminoquinone sesquiterpene, 5-(+)-Epi-Smenospongorine (31) and known quinone/hydroquinone sesquiterpenes; 1, 2, 25, 26, 30, 35, 36, 48, and 52 were separated. Compound 31 was presumed to be a hybrid of 30 and 36 and identified a C-5 epimer 30 as confirmed by NOESY (nuclear Overhauser effect spectroscopy). These metabolites were assessed for differentiation-producing potential by induction of hemoglobin production in K562 cells, where the hemoglobin pseudo-peroxidase effect was estimated colorimetrically using diaminofluorene. It was found that 25, 26, 30, 31, 35, and 36 possessed similar K562 cells differentiation-inducing capacity into erythroblasts and were more powerful than aphidicolin; while 48 and 52 had no activity and 2 and 1 had only activity at a higher concentration than those of 25, 26, 30, 31, 35, and 36 [19]. Structure-activity relationship studies revealed that quinone moiety and amino group were crucial for the activity, whereas the substituents at the amino group and C-5 configuration were not essential [20].
A tumor environment's hypoxic condition is now known as an essential factor for angiogenesis, tumor growth, and metastasis; additionally, at this condition the tumor cells become resistant to irradiation and chemotherapy [23,24]. Thus, the metabolites that selectively prohibit tumor cells growth in the hypoxic environment are expected to be a promising new lead for anticancer agents.
Hypoxia-inducible factor-1 (HIF-1) is a hetero-dimeric transcription factor that comprises an O 2 -regulated α-subunit and a constitutively expressed β-subunit. Hypoxia prohibited the HIF-1α subunit O 2 -dependent hydroxylation, leading to degradation by the proteasome, dimerization of accumulated HIF-1α with HIF-1β, and activation of target genes transcription. HIF-1 activation enhances cancer progression and/or oncogenesis. Furthermore, HIF-1 inhibition causes a decrease in VEGF (vascular endothelial growth factor) expression [24]. As such, HIF-1 has been drawn much interest as a target for chemotherapeutic drugs.
Additionally, in 2015 Sumii et al. reported the selective prohibition of DU145 proliferation by 49 and 52 under hypoxic conditions and their in vivo antitumor effects in subcutaneously inoculated mice with sarcoma S180 cells with no observed acute toxicities during the study period for these compounds [46]. It was implied that methyl ester, exo-olefinic bond, 8-methyl, and OH group were crucial for the hypoxia selective growth inhibition activities of 49 and 52 [45]. Collectively, not only the para-hydroxy-benzoyl moiety but also the decalin skeleton with 8-methyl and 4-exo-cyclic olefinic bond were significant for hypoxia-targeted growth inhibition of 49 and 52 [46,47].

Diterpenes
Dolabellane diterpenes are diterpenoids with a dolabellane skeleton, consisting of an unusual trans-bicyclo

Other Metabolites
Neupane et al. separated kauamide (101), a new chlorinated metabolite with a rare 11-membered heterocyclic skeleton. The structure of 101 was verified by spectroscopic analyses and its 3S, 6S, 11S, and L-leucine stereoconfigurations were established from GIAO (gauge-independent atomic orbital) NMR shielding tensors DFT (density functional theory) calculations, and Marfey's analysis. It had no BACE1 inhibitory potential and cytotoxic activity against U251 and Panc-1 cell lines in the MTT assay [26].

Mar. Drugs 2022, 20, x FOR PEER REVIEW 12 of 43
Cancer represents one of the main reasons for death that has the second-highest incidence of mortality after cardiovascular diseases [38]. Chemotherapeutic treatment is the most common strategy for cancer treatment. However, the chemotherapeutic agents influence not only tumor cells but also normal cells resulting in hazardous side effects [39]. D. elegans reported sesquiterpenes have been evaluated for their anticancer potential towards various cancer cell lines. Further, some studies reported the structure-activity relationship and synthesis of some analogs have been discussed as shown here.

Biosynthetic Pathways of D. elegans Metabolites
Several studies reported the biosynthetic pathways of the reported sesquiterpenes from this sponge. In this work, the reported postulated pathways were summarized. It was reported that the observed differences in stereochemistry among the marine sesquiterpene metabolites could be inferred from the precursor binding preferences within a sin- Figure 9. Structures of compounds 92-101.

Biosynthetic Pathways of D. elegans Metabolites
Several studies reported the biosynthetic pathways of the reported sesquiterpenes from this sponge. In this work, the reported postulated pathways were summarized. It was reported that the observed differences in stereochemistry among the marine sesquiterpene metabolites could be inferred from the precursor binding preferences within a single cyclase enzyme active site [54]. Additionally, this may be due to the existence of various synthase enzymes, whereas each individual enzyme can create a range of diverse metabolites, as well as presumably the potential to change the stereochemical outcomes, relying on the provided substrate nature. Thus, the possibility of enantiomeric metabolites should be considered that emphasize the significance of reporting [α] D values for these terpenes in cases where they are utilized as a reference in the stereochemical determination [21,55].
Boufridi et al. hypothesized that the biosynthetic process of 1 and 2 started with the farnesylation of the aromatic ring, which is the quinone moiety's precursor to obtain I. This involves the initial folding of I within the active site of a specific terpene cyclase (Scheme 1). Successively, two carbocationic intermediates II and III are resulted from peri-planar Wagner-Meerwein hydrogen and methyl shifts. Finally, III may undergo two pathways (A or B) for the formation of 1 and 2 through the loss of a proton from carbocations IV and V, respectively [40].  Scheme 3 shows that the farnesyl precursor (I) initial cyclization gives II ( meric cation) that undergoes a loss of a proton to give 16 and 21. On the other sid II's hydride and methyl migrations yield compounds 1, 14, and 18 [21]. Scheme 3. Postulated biosynthetic pathways for 1, 14, 16, 18, and 21 [21].

Synthesis of D. elegans Metabolites
Some of the reported metabolites from this sponge possessed fascina ties, such as anticancer. Nevertheless, further biological investigation is lim not enough isolated metabolites. Therefore, research interests have been di synthesis and structural modification of these metabolites to improve the bi study structural/activity relations, which could help in drug development Some of these studies have been highlighted here.
It was found that only the analog (FA) with desmethyl near to the fur excellent hypoxia-specific growth inhibitory potential such as furospinos displayed greater in vivo antitumor potential in oral administration (doses sus DU145 cells, as well as lower toxicity in normal conditions (dose 300 µ fore, this analog might be better than furospinosulin-1 for drug candidates Scheme 4. Proposed biosynthetic pathway for 88 [36].

Synthesis of D. elegans Metabolites
Some of the reported metabolites from this sponge possessed fascinating bioactivities, such as anticancer. Nevertheless, further biological investigation is limited due to the not enough isolated metabolites. Therefore, research interests have been directed towards synthesis and structural modification of these metabolites to improve the bioactivities and study structural/activity relations, which could help in drug development and discovery. Some of these studies have been highlighted here.

Synthesis of D. elegans Metabolites
Some of the reported metabolites from this sponge possessed fascina ties, such as anticancer. Nevertheless, further biological investigation is lim not enough isolated metabolites. Therefore, research interests have been di synthesis and structural modification of these metabolites to improve the bi study structural/activity relations, which could help in drug development Some of these studies have been highlighted here.
It was found that only the analog (FA) with desmethyl near to the fur excellent hypoxia-specific growth inhibitory potential such as furospinos displayed greater in vivo antitumor potential in oral administration (doses sus DU145 cells, as well as lower toxicity in normal conditions (dose 300 µ fore, this analog might be better than furospinosulin-1 for drug candidates Kotoku et al. reported that the modifications of 92 structure such as methyl group close to the furan ring, changing the aromatic ring, and side tion led to a remarkable loss of selective hypoxia growth inhibition capa Scheme 5. Synthesis of furospinosulin-1 (92) analogues [57].
It was found that only the analog (FA) with desmethyl near to the furan ring had an excellent hypoxia-specific growth inhibitory potential such as furospinosulin-1 (92) and displayed greater in vivo antitumor potential in oral administration (doses 1-10 µM) versus DU145 cells, as well as lower toxicity in normal conditions (dose 300 µM) [57]. Therefore, this analog might be better than furospinosulin-1 for drug candidates. Kotoku et al. reported that the modifications of 92 structure such as elongating the methyl group close to the furan ring, changing the aromatic ring, and side-chain truncation led to a remarkable loss of selective hypoxia growth inhibition capacity, obviously revealing that the entire structure was substantial for the binding with the target molecule [58]; whilst the analog had a longer side chain partially retained hypoxia-selective inhibitory potential. Therefore, Kotoku  The analog X was found to be much more potent than furospinosulin-1 as it possessed hypoxia-selective growth-inhibitory potential (Conc. 1-300 µM) and had powerful in vivo antitumor effectiveness after oral administration (doses 5-25 mg/kg) without side effects [58].
The analog X was found to be much more potent than furospinosulin-1 as it possessed hypoxia-selective growth-inhibitory potential (Conc. 1-300 µM) and had powerful in vivo antitumor effectiveness after oral administration (doses 5-25 mg/kg) without side effects [58].
Pelorol (64), sesquiterpene hydroquinone having C8-C21 cyclization had a potent and selective SHIP1 (Src homology 2-containing inositol 5-phosphatase 1) activating potential [59]. However, this compound had catechol moiety that can be either enzymatic or chemically oxidized to orthoquinones, forming covalent linkages with proteins resulting in losing the activity. Additionally, it is highly water-insoluble, which limits its in vivo bioactivity as a SHIP1 activator. Meimetis et al. synthesized more water-soluble amino analogs ent-PA or (±)-PA (Scheme 8). Synthesis of (±)-PA characterized by generating a tetracyclic ring system through a cation-initiated polyene cyclization [60]. Then, 3,5-Dimethoxybromobenzene lithiation subsequent alkylation with farnesyl bromide produced the 3,5-dimethoxybenzene prenylated intermediate. A racemic epoxide was produced utilizing m-chloroperbenzoic acid with subsequent steps giving (±)-PA. Repeating the same procedure using ent-Shi catalyst produced ent-PA analog. These analogs in vitro activated SHIP1, prohibited Akt phosphorylation and had potent in vivo anti-inflammation potential (ED50 0.1 mg/kg, oral gavage) using passive cutaneous anaphylaxis mouse model [60]. The results suggested that ent-PA or (±)-PA pelorol analogs are promising candidates for further in vivo preclinical investigation as SHIP1-activating therapeutics for treating hematopoietic illnesses, involving aberrant PI3K cell signaling activation. Pelorol (64), sesquiterpene hydroquinone having C8-C21 cyclization had a potent and selective SHIP1 (Src homology 2-containing inositol 5-phosphatase 1) activating potential [59]. However, this compound had catechol moiety that can be either enzymatic or chemically oxidized to orthoquinones, forming covalent linkages with proteins resulting in losing the activity. Additionally, it is highly water-insoluble, which limits its in vivo bioactivity as a SHIP1 activator. Meimetis et al. synthesized more water-soluble amino analogs ent-PA or (±)-PA (Scheme 8). Synthesis of (±)-PA characterized by generating a tetracyclic ring system through a cation-initiated polyene cyclization [60]. Then, 3,5-Dimethoxybromobenzene lithiation subsequent alkylation with farnesyl bromide produced the 3,5-dimethoxybenzene prenylated intermediate. A racemic epoxide was produced utilizing m-chloroperbenzoic acid with subsequent steps giving (±)-PA. Repeating the same procedure using ent-Shi catalyst produced ent-PA analog. These analogs in vitro activated SHIP1, prohibited Akt phosphorylation and had potent in vivo anti-inflammation potential (ED50 0.1 mg/kg, oral gavage) using passive cutaneous anaphylaxis mouse model [60]. The results suggested that ent-PA or (±)-PA pelorol analogs are promising candidates for further in vivo preclinical investigation as SHIP1-activating therapeutics for treating hematopoietic illnesses, involving aberrant PI3K cell signaling activation.

Activities of D. elegans Extracts and Fractions
BACE1 (β-site of amyloid precursor protein cleaving enzyme) is an enzyme involved in Alzheimer's disease pathogenesis. The 75% and 100% MeOH C8 fractions of D. elegans obtained from the coast of Kauai had significant in vitro BACE1 inhibition (%inhibition 66% and 73%, respectively, at Conc. 30 µg/mL) [61]. Li et al. revealed that the CH 2 Cl 2 /MeOH extract possessed marked IL-6 and TNF-α inhibitory potential [34]. Additionally, the antioxidant potential testing revealed that D. elegans hexane extract exhibited significant antioxidant potential in comparison to the ascorbic acid [62].

Conclusions
The marine environment is a wealth of biological and chemical diversity. Biometabolites from marine organisms have been proven to be beneficial sources for the discovery of novel drug targets. Among these organisms, sponges are a fascinating marine invertebrate' phylum, which have been recognized as a big reservoir of biometabolites. The current work highlights one of the most interesting sponge species, D. elegans. Over the last 30 years, 101 metabolites have been separated and characterized from D. elegans. The results displayed that sesquiterpenes of various classes represented the major metabolites of this sponge. Additionally, few studies reported the isolation of sesterterpenes, sterols, pregnanes, and diterpenes ( Figure 10).

Activities of D. elegans Extracts and Fractions
BACE1 (β-site of amyloid precursor protein cleaving enzyme) is an enzyme involved in Alzheimer's disease pathogenesis. The 75% and 100% MeOH C8 fractions of D. elegans obtained from the coast of Kauai had significant in vitro BACE1 inhibition (%inhibition 66% and 73%, respectively, at Conc. 30 µg/mL) [61]. Li et al. revealed that the CH2Cl2/MeOH extract possessed marked IL-6 and TNF-α inhibitory potential [34]. Additionally, the antioxidant potential testing revealed that D. elegans hexane extract exhibited significant antioxidant potential in comparison to the ascorbic acid [62].

Conclusions
The marine environment is a wealth of biological and chemical diversity. Biometabolites from marine organisms have been proven to be beneficial sources for the discovery of novel drug targets. Among these organisms, sponges are a fascinating marine invertebrate' phylum, which have been recognized as a big reservoir of biometabolites. The current work highlights one of the most interesting sponge species, D. elegans. Over the last 30 years, 101 metabolites have been separated and characterized from D. elegans. The results displayed that sesquiterpenes of various classes represented the major metabolites of this sponge. Additionally, few studies reported the isolation of sesterterpenes, sterols, pregnanes, and diterpenes ( Figure 10). These metabolites have been assessed mainly for their antitumor and antiproliferative potential. Limited studies investigated the antibacterial, anti-inflammatory, β-secretase 1 inhibitory, and antiprotozoal activities ( Figure 11) These metabolites have been assessed mainly for their antitumor and antiproliferative potential. Limited studies investigated the antibacterial, anti-inflammatory, β-secretase 1 inhibitory, and antiprotozoal activities ( Figure 11) Some D. elegans sesquiterpenes demonstrated remarkable cytotoxic and antiproliferative activities either in vivo or in vitro towards various cancer cell lines. Further, some sesquiterpenes had promising HIF-1 activating capacity, therefore they could have therapeutic potential as future drug targets for chemotherapeutic drugs, as well as for treating ischemic diseases.
It is noteworthy that limited studies exploring the mechanism of anticancer potential of these metabolites were reported. It was found that these metabolites induced their effectiveness through any of the following mechanisms: hypoxia-targeted growth inhibition (e. g., 48, 49, and 52), inhibition of IGF-IR signaling via IGF-2 transcription suppression (e.g., 92), inhibiting accumulation of HIF-1 and decreasing production of VEGF (e.g., 52), induction of apoptosis through increasing proteolytic activity of caspases (e.g., 1 and 2), and cell cycle arrest through boosting p21 expression and suppressing Rb phosphorylation (e.g., 25). Further, one report revealed the anti-inflammatory potential of 69 through inhibition of IL-6 production.
Mar. Drugs 2022, 20, x FOR PEER REVIEW Figure 11. Bioactivities and the number of tested metabolites.
Some D. elegans sesquiterpenes demonstrated remarkable cytotoxic and a ative activities either in vivo or in vitro towards various cancer cell lines. Fu sesquiterpenes had promising HIF-1 activating capacity, therefore they could peutic potential as future drug targets for chemotherapeutic drugs, as well as ischemic diseases.
It is noteworthy that limited studies exploring the mechanism of anticanc of these metabolites were reported. It was found that these metabolites induced tiveness through any of the following mechanisms: hypoxia-targeted growth (e.g., 48, 49, and 52), inhibition of IGF-IR signaling via IGF-2 transcription s (e.g., 92), inhibiting accumulation of HIF-1 and decreasing production of VEG induction of apoptosis through increasing proteolytic activity of caspases (e.g., 1 cell cycle arrest through boosting p21 expression and suppressing Rb phosp (e.g., 25). Further, one report revealed the anti-inflammatory potential of 69 th bition of IL-6 production.
The structure-activity studies demonstrated that the chemical nature of t lites' skeletons, as well as the substituents, were greatly influenced the activit marized in Figures 12 and 13. The structure-activity studies demonstrated that the chemical nature of the metabolites' skeletons, as well as the substituents, were greatly influenced the activities as summarized in Figures 12 and 13.   Figure 11. Bioactivities and the number of tested metabolites.
Some D. elegans sesquiterpenes demonstrated remarkable cytotoxic and antiproliferative activities either in vivo or in vitro towards various cancer cell lines. Further, some sesquiterpenes had promising HIF-1 activating capacity, therefore they could have therapeutic potential as future drug targets for chemotherapeutic drugs, as well as for treating ischemic diseases.
It is noteworthy that limited studies exploring the mechanism of anticancer potential of these metabolites were reported. It was found that these metabolites induced their effectiveness through any of the following mechanisms: hypoxia-targeted growth inhibition (e.g., 48, 49, and 52), inhibition of IGF-IR signaling via IGF-2 transcription suppression (e.g., 92), inhibiting accumulation of HIF-1 and decreasing production of VEGF (e.g., 52) induction of apoptosis through increasing proteolytic activity of caspases (e.g., 1 and 2), and cell cycle arrest through boosting p21 expression and suppressing Rb phosphorylation (e.g., 25). Further, one report revealed the anti-inflammatory potential of 69 through inhibition of IL-6 production.
The structure-activity studies demonstrated that the chemical nature of the metabolites' skeletons, as well as the substituents, were greatly influenced the activities as summarized in Figures 12 and 13.  lites through click chemistry which is a new field for synthesizing drug-like molecules that can accelerate the process of drug discovery. Interestingly, kauamide was proposed to be biosynthesized by cyanobacteria harboring D. elegans due to its structure similarity to cyanobacteria reported metabolites, therefore the chemical investigation of symbiotic microorganisms of D. elegans could be potential producers of biometabolites originally derived from this sponge. Moreover, some bioassays of the reported metabolites revealed no potential effectiveness, providing more potentiality for carrying out other pharmacologic evaluations. Advanced techniques, such as metabolomics, LC/MS/NMR (liquid chromatography/mass spectrometry/nuclear magnetic resonance), and UPLC/MS (Ultra performance liquid chromatography-tandem mass spectrometer) should be employed to explore more biometabolites from this sponge. A combination of pathway reconstructing, genetic and enzyme engineering, and metabolic networks could modify this sponge to biosynthesize more novel metabolites having promised structural features or a large quantity of the valuable known ones for pharmaceutical application. Structure-activity relationship studies and chemical syntheses of the promising metabolites give further chances to generate more significant drug leads with optimized chemical stability, activity, and accessibility. Finally, we believed that the metabolites of this sponge deserve much more research attention.   On the other side, synthesis studies revealed that the structural modifications and replacement of some functional groups with the more physiologically stable functionalities resulted in more active and useful tags for further functionalization of these metabolites through click chemistry which is a new field for synthesizing drug-like molecules that can accelerate the process of drug discovery. Interestingly, kauamide was proposed to be biosynthesized by cyanobacteria harboring D. elegans due to its structure similarity to cyanobacteria reported metabolites, therefore the chemical investigation of symbiotic microorganisms of D. elegans could be potential producers of biometabolites originally derived from this sponge.
Moreover, some bioassays of the reported metabolites revealed no potential effectiveness, providing more potentiality for carrying out other pharmacologic evaluations. Advanced techniques, such as metabolomics, LC/MS/NMR (liquid chromatography/mass spectrometry/nuclear magnetic resonance), and UPLC/MS (Ultra performance liquid chromatography-tandem mass spectrometer) should be employed to explore more biometabolites from this sponge. A combination of pathway reconstructing, genetic and enzyme engineering, and metabolic networks could modify this sponge to biosynthesize more novel metabolites having promised structural features or a large quantity of the valuable known ones for pharmaceutical application. Structure-activity relationship studies and chemical syntheses of the promising metabolites give further chances to generate more significant drug leads with optimized chemical stability, activity, and accessibility. Finally, we believed that the metabolites of this sponge deserve much more research attention.