Bioactive Natural Products of Marine Sponges from the Genus Hyrtios

Marine sponges are known as a rich source for novel bioactive compounds with valuable pharmacological potential. One of the most predominant sponge genera is Hyrtios, reported to have various species such as Hyrtios erectus, Hyrtios reticulatus, Hyrtios gumminae, Hyrtios communis, and Hyrtios tubulatus and a number of undescribed species. Members of the genus Hyrtios are a rich source of natural products with diverse and valuable biological activities, represented by different chemical classes including alkaloids, sesterterpenes and sesquiterpenes. This review covers the literature until June 2016, providing a complete survey of all compounds isolated from the genus Hyrtios with their corresponding biological activities whenever applicable.


Introduction
Marine ecosystems contain enormous, still unexplored and taxonomically diverse macroand microorganisms. These marine organisms are able to produce novel molecules with large structural diversity and various interesting pharmacological activities [1][2][3][4]. Since 1985, more than 4000 marine natural products possessing a wide range of biological activities have been isolated and characterized [5]. About twenty-four marine natural products are currently in phase I-III clinical trials [6,7]. Moreover, there are currently eight marine natural products on the market, possessing different pharmacological activities [8]. Sponges (phylum Porifera) are among the oldest multicellular animals with a fossil record dating back to Precambrian times [9]. Sponges are widespread in tropical reefs in a great abundance, but can also be found in polar latitudes and the deep, sea as well as in fresh water lakes and rivers [10]. Marine sponges continue to attract attention as rich sources of structurally novel secondary metabolites that are potential lead compounds for the development of new drugs [11]. With more than 280 new isolated compounds from sponges reported during 2014, sponges have returned again to be a superior source of new biologically active marine natural products [11].
Sponges belonging to the genus Hyrtios (Kingdom: Animalia, phylum: Porifera, class: Demospongiae, order: Dictyoceratida, family: Thorectidae) are reported to be rich sources of bioactive secondary metabolites. Among marine sponges of the genus Hyrtios, the sponge H. erectus is the most frequently investigated source of bioactive natural products. For example, several indole alkaloids [12,13], β-carboline alkaloids [14,15] and sesterterpenes [16,17] were isolated from this Hyrtios species. Biological investigations of alkaloids and sesterterpenes isolated from H. erectus revealed that some of these compounds possess noteworthy anticancer [18,19] and antimicrobial [20,21] activities. The marine sponge H. erectus has been collected from different marine environments, including the Red Sea in Egypt [13,16] and Okinawa in Japan [18,22]. The Indonesian marine sponge H. reticulatus is another frequently studied marine sponge of this genus reported as a good source of novel β-carboline alkaloids [23][24][25].
In addition, natural product discovery projects from the three sponges H. gumminae, H. communis and H. tubulatus (which has been allocated to the new genus Dysidea tubulata) have been reported in the literature. H. gumminae was collected from the Andaman Sea in Thailand and was found to be a source of novel sesterterpenoids [26]. Moreover, biological and chemical investigations of the extracts of several undescribed species of the genus Hyrtios can be found in the literature. Novel derivatives of puupehenone, a sesquiterpene-methylene quinone, [27][28][29] and alkaloids [30][31][32][33][34][35][36] were the most predominant isolated compounds from the undescribed species of the genus Hyrtios. Here, we are reporting an overview on the chemical structures of marine natural products isolated from diverse marine sponges of the genus Hyrtios, together with their isolation sources as well as their biological activities whenever applicable.

H. erectus
5-Hydroxyindole-3-aldehyde (1) together with the related compounds hyrtiosins A (2) and B (3) were isolated from the Okinawan marine sponge H. erectus collected near Ishigaki Island ( Figure 1). Compound 1 showed in vitro cytotoxic activity against human epidermoid carcinoma KB cells with IC50 (half maximal inhibitory concentration, concentration causing 50% of the desired activity) value of 4.3 µg/mL, while hyrtiosins A (2) and B (3) were less cytotoxic than 1 [18]. Hyrtiomanzamine 4, a β-carboline alkaloid, was isolated from the marine sponge H. erectus collected in the Red Sea ( Figure 2). Compound 4 exhibited immunosuppressive activity with an EC50 (half maximal effective concentration, concentration causing 50% of the desired activity) of 2 µg/mL in the B lymphocytes reaction assay and no cytotoxic activity on KB cells was observed [37].  Hyrtiomanzamine 4, a β-carboline alkaloid, was isolated from the marine sponge H. erectus collected in the Red Sea ( Figure 2). Compound 4 exhibited immunosuppressive activity with an EC 50 (half maximal effective concentration, concentration causing 50% of the desired activity) of 2 µg/mL in the B lymphocytes reaction assay and no cytotoxic activity on KB cells was observed [37]. secondary metabolites. Among marine sponges of the genus Hyrtios, the sponge H. erectus is the most frequently investigated source of bioactive natural products. For example, several indole alkaloids [12,13], β-carboline alkaloids [14,15] and sesterterpenes [16,17] were isolated from this Hyrtios species. Biological investigations of alkaloids and sesterterpenes isolated from H. erectus revealed that some of these compounds possess noteworthy anticancer [18,19] and antimicrobial [20,21] activities. The marine sponge H. erectus has been collected from different marine environments, including the Red Sea in Egypt [13,16] and Okinawa in Japan [18,22]. The Indonesian marine sponge H. reticulatus is another frequently studied marine sponge of this genus reported as a good source of novel β-carboline alkaloids [23][24][25].
In addition, natural product discovery projects from the three sponges H. gumminae, H. communis and H. tubulatus (which has been allocated to the new genus Dysidea tubulata) have been reported in the literature. H. gumminae was collected from the Andaman Sea in Thailand and was found to be a source of novel sesterterpenoids [26]. Moreover, biological and chemical investigations of the extracts of several undescribed species of the genus Hyrtios can be found in the literature. Novel derivatives of puupehenone, a sesquiterpene-methylene quinone, [27][28][29] and alkaloids [30][31][32][33][34][35][36] were the most predominant isolated compounds from the undescribed species of the genus Hyrtios. Here, we are reporting an overview on the chemical structures of marine natural products isolated from diverse marine sponges of the genus Hyrtios, together with their isolation sources as well as their biological activities whenever applicable.

H. communis
The extract of marine sponge H. communis, collected from a depth of 18−21 m from the northern reefs region off the coast of Palau, was found to inhibit transcription factor hypoxia-inducible factor-1 (HIF1) activation in T47D human breast tumor cells. Bioassay-guided fractionation of the H. communis extract led to the isolation and identification of the sesterterpenes 90-102 ( Figure 22). Thorectidaeolide A (90), 4-acetoxythorectidaeolide A (91) and luffariellolide (100) showed potent inhibition activities of HIF-1 activation, with IC50 values of 3.2, 3.5, and 3.6 µM, respectively. Compound 100 exhibited a significant cytotoxic activity, which can be explained by its HIF-1 inhibitory activity [43].

H. communis
The extract of marine sponge H. communis, collected from a depth of 18-21 m from the northern reefs region off the coast of Palau, was found to inhibit transcription factor hypoxia-inducible factor-1 (HIF1) activation in T47D human breast tumor cells. Bioassay-guided fractionation of the H. communis extract led to the isolation and identification of the sesterterpenes 90-102 ( Figure 22). Thorectidaeolide A (90), 4-acetoxythorectidaeolide A (91) and luffariellolide (100) showed potent inhibition activities of HIF-1 activation, with IC 50 values of 3.2, 3.5, and 3.6 µM, respectively. Compound 100 exhibited a significant cytotoxic activity, which can be explained by its HIF-1 inhibitory activity [43].

H. communis
The extract of marine sponge H. communis, collected from a depth of 18−21 m from the northern reefs region off the coast of Palau, was found to inhibit transcription factor hypoxia-inducible factor-1 (HIF1) activation in T47D human breast tumor cells. Bioassay-guided fractionation of the H. communis extract led to the isolation and identification of the sesterterpenes 90-102 ( Figure 22). Thorectidaeolide A (90), 4-acetoxythorectidaeolide A (91) and luffariellolide (100) showed potent inhibition activities of HIF-1 activation, with IC50 values of 3.2, 3.5, and 3.6 µM, respectively. Compound 100 exhibited a significant cytotoxic activity, which can be explained by its HIF-1 inhibitory activity [43].

Undescribed Marine Sponges of the Genus Hyrtios
Dipuupehedione (106) has been isolated from the dichloromethane extract of a Caledonian marine sponge Hyrtios sp. (Figure 24), collected by SCUBA diving in New Caledonia (East Coast). Compound 106 showed significant cytotoxic activity on KB cells with IC50 value of 3 µg/mL [27]. Fractionation and chemical investigation of the dichloromethane extract of the marine sponge Hyrtios sp. collected from the East Coast of New Caledonia afforded dipuupehedione (106), puupehenone (107) and 15α-methoxypuupehenol (108) (Figure 25). Compound 108 exhibited similar antimicrobial and antifungal activity as puupehenone (107) and a lower cytotoxity towards KB cells with ED50 values of 6 and 0.5 µg/mL, respectively. Compound 108 exhibited slightly higher in vitro antimalarial activity than puupehenone 107, against three strains of Plasmodium falciparum [28].

Undescribed Marine Sponges of the Genus Hyrtios
Dipuupehedione (106) has been isolated from the dichloromethane extract of a Caledonian marine sponge Hyrtios sp. (Figure 24), collected by SCUBA diving in New Caledonia (East Coast). Compound 106 showed significant cytotoxic activity on KB cells with IC 50 value of 3 µg/mL [27].

Undescribed Marine Sponges of the Genus Hyrtios
Dipuupehedione (106) has been isolated from the dichloromethane extract of a Caledonian marine sponge Hyrtios sp. (Figure 24), collected by SCUBA diving in New Caledonia (East Coast). Compound 106 showed significant cytotoxic activity on KB cells with IC50 value of 3 µg/mL [27]. Fractionation and chemical investigation of the dichloromethane extract of the marine sponge Hyrtios sp. collected from the East Coast of New Caledonia afforded dipuupehedione (106), puupehenone (107) and 15α-methoxypuupehenol (108) (Figure 25). Compound 108 exhibited similar antimicrobial and antifungal activity as puupehenone (107) and a lower cytotoxity towards KB cells with ED50 values of 6 and 0.5 µg/mL, respectively. Compound 108 exhibited slightly higher in vitro antimalarial activity than puupehenone 107, against three strains of Plasmodium falciparum [28].  Fractionation and chemical investigation of the dichloromethane extract of the marine sponge Hyrtios sp. collected from the East Coast of New Caledonia afforded dipuupehedione (106), puupehenone (107) and 15α-methoxypuupehenol (108) (Figure 25). Compound 108 exhibited similar antimicrobial and antifungal activity as puupehenone (107) and a lower cytotoxity towards KB cells with ED 50 values of 6 and 0.5 µg/mL, respectively. Compound 108 exhibited slightly higher in vitro antimalarial activity than puupehenone 107, against three strains of Plasmodium falciparum [28].

Undescribed Marine Sponges of the Genus Hyrtios
Dipuupehedione (106) has been isolated from the dichloromethane extract of a Caledonian marine sponge Hyrtios sp. (Figure 24), collected by SCUBA diving in New Caledonia (East Coast). Compound 106 showed significant cytotoxic activity on KB cells with IC50 value of 3 µg/mL [27]. Fractionation and chemical investigation of the dichloromethane extract of the marine sponge Hyrtios sp. collected from the East Coast of New Caledonia afforded dipuupehedione (106), puupehenone (107) and 15α-methoxypuupehenol (108) (Figure 25). Compound 108 exhibited similar antimicrobial and antifungal activity as puupehenone (107) and a lower cytotoxity towards KB cells with ED50 values of 6 and 0.5 µg/mL, respectively. Compound 108 exhibited slightly higher in vitro antimalarial activity than puupehenone 107, against three strains of Plasmodium falciparum [28].   Two sesquiterpene γ-methoxybutenolides, hyrtiosenolide A (117) and hyrtiosenolide B (118), along with a 4α-methyl polyoxygenated steroid, hyrtiosterol (119), were obtained from a marine sponge of the genus Hyrtios collected from the Red Sea, Hurghada, Egypt ( Figure 28). Hyrtiosenolides A (117) and B (118) displayed weak in vitro antibacterial activity against Escherichia coli. An inhibition zone of 7 mm was observed when 100 µg of 117 or 118 was applied to a 6 mm diameter paper disk on an agar plate inoculated with E. coli [45].   Two sesquiterpene γ-methoxybutenolides, hyrtiosenolide A (117) and hyrtiosenolide B (118), along with a 4α-methyl polyoxygenated steroid, hyrtiosterol (119), were obtained from a marine sponge of the genus Hyrtios collected from the Red Sea, Hurghada, Egypt ( Figure 28). Hyrtiosenolides A (117) and B (118) displayed weak in vitro antibacterial activity against Escherichia coli. An inhibition zone of 7 mm was observed when 100 µg of 117 or 118 was applied to a 6 mm diameter paper disk on an agar plate inoculated with E. coli [45].  Two sesquiterpene γ-methoxybutenolides, hyrtiosenolide A (117) and hyrtiosenolide B (118), along with a 4α-methyl polyoxygenated steroid, hyrtiosterol (119), were obtained from a marine sponge of the genus Hyrtios collected from the Red Sea, Hurghada, Egypt ( Figure 28). Hyrtiosenolides A (117) and B (118) displayed weak in vitro antibacterial activity against Escherichia coli. An inhibition zone of 7 mm was observed when 100 µg of 117 or 118 was applied to a 6 mm diameter paper disk on an agar plate inoculated with E. coli [45].  Two sesquiterpene γ-methoxybutenolides, hyrtiosenolide A (117) and hyrtiosenolide B (118), along with a 4α-methyl polyoxygenated steroid, hyrtiosterol (119), were obtained from a marine sponge of the genus Hyrtios collected from the Red Sea, Hurghada, Egypt ( Figure 28). Hyrtiosenolides A (117) and B (118) displayed weak in vitro antibacterial activity against Escherichia coli. An inhibition zone of 7 mm was observed when 100 µg of 117 or 118 was applied to a 6 mm diameter paper disk on an agar plate inoculated with E. coli [45].  Hyrtinadine A (121), a cytotoxic bis-indole alkaloid with a pyrimidine moiety, was isolated from a marine sponge Hyrtios sp. collected off Unten-Port, Okinawa ( Figure 30). Compound 121 was the first example of a bis-indole alkaloid with a 2,5-disubstituted pyrimidine ring between two indole rings. Hyrtinadine A (121) showed in vitro cytotoxic activity against murine leukemia L1210 cells with IC50 value of 1 µg/mL and against human epidermoid carcinoma KB cells with IC50 value of 3 µg/mL [47].  The sesquiterpene-dihydroquinone derivative puupehanol (123) and chloropuupehenone (124) were isolated from a marine sponge of the genus Hyrtios collected in Papua New Guinea, together with the known compound puupehenone (107) (Figure 32). Compound 107 showed potent antifungal activity against Cryptococcus neoformans and Candida krusei with minimum fungicidal concentration (MFC) values of 1.25 and 2.50 µg/mL, respectively [49].

Conclusions
Marine sponges harbor a huge repertoire of yet undiscovered natural products possessing a broad-spectrum of pharmacological applications. Among the several Hyrtios species discovered, H. erectus, H. reticulatus, H. gumminae, H. communis, and H. tubulatus were the most prolific producers of secondary metabolites with various pharmaceutically and medically relevant bioactivities (Table 1). A total of 146 natural products from various marine sponges belonging to the genus Hyrtios were reported in MarinLit database until 2016 as well as in the literature. H. erectus represents the most frequently investigated source of bioactive natural products from Hyrtios sp. in terms of number of natural products isolated. The discovery of new species from the genus Hyrtios indicates that there is room for new natural products discovery. Mass-guided fractionation of the methanolic extract from a specimen of the Australian marine sponge Hyrtios sp. led to the isolation of two tryptophan alkaloids, 6-oxofascaplysin (142), and secofascaplysic acid (143), in addition to the two metabolites fascaplysin (11) and reticulatate (144) (Figure 38). Compounds 11 and 142-144 displayed in vitro cytotoxic activity against a prostate cancer cell line (LNCaP) with IC50 values ranging from 0.54 to 44.9 µM [35].  Genus Hyrtios attracted scientists' attention and sparked high synthetic efforts for the synthesis of the isolated compounds from its members [37,38]. Several compounds have been synthesized such as Salmahyrtisol A from Hyrtios erecta [39,40], Similan A from the Thai sponge Hyrtios gumininae [41,42], sesterstatin 1 from Hyrtios erecta [43,44], Spongistatins 1 (Altohyrtin A ) from Hyrtios erecta [45,46], (−)-Hyrtiosal and its C-16 epimer have been synthetized from sclareol [47] which was previously isolated from the sponge Hyrtios erectus [48].

Conclusions
Marine sponges harbor a huge repertoire of yet undiscovered natural products possessing a broad-spectrum of pharmacological applications. Among the several Hyrtios species discovered, H. erectus, H. reticulatus, H. gumminae, H. communis, and H. tubulatus were the most prolific producers of secondary metabolites with various pharmaceutically and medically relevant bioactivities (Table 1). A total of 146 natural products from various marine sponges belonging to the genus Hyrtios were reported in MarinLit database until 2016 as well as in the literature. H. erectus represents the most frequently investigated source of bioactive natural products from Hyrtios sp. in terms of number of natural products isolated. The discovery of new species from the genus Hyrtios indicates that there is room for new natural products discovery. Genus Hyrtios attracted scientists' attention and sparked high synthetic efforts for the synthesis of the isolated compounds from its members [37,38]. Several compounds have been synthesized such as Salmahyrtisol A from Hyrtios erecta [39,40], Similan A from the Thai sponge Hyrtios gumininae [41,42], sesterstatin 1 from Hyrtios erecta [43,44], Spongistatins 1 (Altohyrtin A ) from Hyrtios erecta [45,46], (−)-Hyrtiosal and its C-16 epimer have been synthetized from sclareol [47] which was previously isolated from the sponge Hyrtios erectus [48].

Conclusions
Marine sponges harbor a huge repertoire of yet undiscovered natural products possessing a broad-spectrum of pharmacological applications. Among the several Hyrtios species discovered, H. erectus, H. reticulatus, H. gumminae, H. communis, and H. tubulatus were the most prolific producers of secondary metabolites with various pharmaceutically and medically relevant bioactivities (Table 1). A total of 146 natural products from various marine sponges belonging to the genus Hyrtios were reported in MarinLit database until 2016 as well as in the literature. H. erectus represents the most frequently investigated source of bioactive natural products from Hyrtios sp. in terms of number of natural products isolated. The discovery of new species from the genus Hyrtios indicates that there is room for new natural products discovery.
With the currently available data a correlation between geographical area where the sponges were collected and the type of metabolites found for this particular species can be concluded. Sponges collected off Okinawa (Japan) were richer in alkaloids, especially indole alkaloids (indole alkaloids possessing β-carboline skeleton with an imidazolium unit, azepino-hydroxyindole moieties) and bisindole alkaloids. In addition, sponges collected off Fiji were rich with brominated alkaloids and sesterterpenes. Sponges collected from the Republic of Maldives were very rich in scalarane-type pentacyclic sesterterpene and sesterstatins. Furthermore, sponges collected off Indonesia are rich in β-carboline alkaloids. On the other hand, sponges collected off the Red sea were rich in terpenoids, especially sesterterpenes, sesterstatins as well as indole alkaloids and azepino alkaloids, with the majority of the isolated compounds being terpenoids. The different geographical chemotypes might be explained by variations in the microbial community associated with the respective sponges. Sponges have developed intimate association with a huge diversity of microorganisms, such as viruses, bacteria, archaea, fungi, protozoa and single-celled algae. It is often unclear whether the compounds of interest are biosynthesized by the sponges or their associated microbes [50,51]. Many bioactive natural products from marine invertebrates have striking similarities to metabolites of their associated microorganisms, especially bacteria [52][53][54][55][56][57]. In most cases, the development of sponge-derived drugs is challenged by environmental concerns and technical problems associated with harvesting large amounts of sponges. Sponge-associated microorganisms may represent a sustainable source of sponge-derived natural products that could be established through a symbiont culture or by transferring its biosynthetic genes into culturable microorganisms [58]. Based on available scientific literature, it is evident that marine sponges within genus Hyrtios represent a rich source of natural products with various biological activities.

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Antimicrobial and antifungal activity.
In vitro antimalarial activity. Antibacterial activity against Escherichia coli.
In vitro cytotoxic activity against murine leukemia L1210 cells and against human epidermoid carcinoma KB cells.
A potent inhibitory activity against isocitrate lyase (ICL) of Candida albicans. Potent antifungal activity against Cryptococcus neoformans and Candida krusei with minimum fungicidal concentration (MFC).
Antifungal activity against C. albicans and C. neoformans and against Trichophyton mentagrophytes. Potent antioxidant activity. Antifungal activities against Aspergillus niger and against Cryptococcus neoformans.
In vitro cytotoxic activity against human epidermoid carcinoma KB cells and murine leukemia L1210 cells. Inhibitory effects against A. niger and inhibitory effect against C. neoformans. In vitro cytotoxic activity against a prostate cancer cell line. Antifungal activity against A. niger.