Molecular architecture and biomedical leads of terpenes from red sea marine invertebrates.

Marine invertebrates including sponges, soft coral, tunicates, mollusks and bryozoan have proved to be a prolific source of bioactive natural products. Among marine-derived metabolites, terpenoids have provided a vast array of molecular architectures. These isoprenoid-derived metabolites also exhibit highly specialized biological activities ranging from nerve regeneration to blood-sugar regulation. As a result, intense research activity has been devoted to characterizing invertebrate terpenes from both a chemical and biological standpoint. This review focuses on the chemistry and biology of terpene metabolites isolated from the Red Sea ecosystem, a unique marine biome with one of the highest levels of biodiversity and specifically rich in invertebrate species.

1. Red Sea Ecosystem

Marine ecosystems cover nearly 70% of the earth’s surface, averaging almost 4 km in depth and are proposed to contain over 80% of the world’s plant and animal species [1]. Exact marine biodiversity is less certain since between one-third and two-thirds of marine organisms have yet to be described [2]. Worldwide there are approximately 226,000 marine eukaryotes currently reported, while close to a million total species are estimated, based on calculations by marine biologists using statistical predictions [2]. Considering that constituents from higher plants along with metabolites from terrestrial microorganisms have provided a substantial fraction of the natural-product-derived drugs currently used in Western medicine [3], the potential to vastly expand the number and diversity of natural products by mining marine eukaryotes as well as associated prokaryotes from the richly diverse Red Sea ecosystem seems attainable. In fact, just within the past quarter century, the search for new marine metabolites has resulted in the isolation of upward of 10,000 compounds [4], many of which exhibit biological activity. Despite the fact that marine biodiversity far exceeds that of terrestrial ecosystems, research of marine natural products as pharmaceutical agents, is still in its infancy. Factors that contribute to the gap between terrestrial and marine derived natural products include a paucity of ethno-medical history from marine sources as well as impediments associated with collecting, identifying and chemical analysis of marine materials [5].

Notwithstanding, a combination of new diving techniques and implementation of remotely operated pods over the last decade has facilitated the characterization of marine-derived metabolites. This review encompasses secondary metabolites derived from marine invertebrates, a largely diverse group of fixed or sessile organisms, many in a stationary form although some are capable of slow primitive movement. While invertebrates lack physical defences such as a protective shell or spines, they are often rich in defence metabolites that can be utilized to attack prey or defend a habitat.

This review focused on a class of secondary defence metabolites abundant in marine invertebrates, five-carbon isoprenoid-derived terpenes. Extensive speciation from microorganisms to mammals can be attributed, at least in part to a wide range of temperatures (0 to 35 °C in arctic waters versus hydrothermal vents, respectively), pressures (1–1000 atm.), nutrient availabilities (oligotrophic to eutrophic) and lighting conditions that exist in this marine biome [6]. The analysis will be limited to the Red Sea which is considered an epicenter for marine biodiversity with an extremely high endemic biota including over 50 genera of hermatypic coral. Indeed soft coral (Cnidaria: Anthozoa: Octocorallia), which are an important structural component of coral reef communities [7,8], are approximately 40% native to the Red Sea [6]. The Red Sea, in which extensive reef formation occurs, is arguably the world’s warmest (up to 35 °C in summer) and most saline habitat (ca. 40 psu in the northern Red Sea) [6]. Despite the Red Sea’s size and diversity of reef-associated inhabitants (for examples, see Figure 1), marine invertebrates in this ecosystem remain poorly studied compared to other large coral reef systems around the world such as the Great Barrier Reef or the Caribbean. This review will cover terpenes isolated from marine invertebrates of the Red Sea (Figure 2) as well as identified biological activities for compounds reported during the time period from 1980 to 2014.

Figure 1

Samples of marine invertebrate diversity from the Red Sea including (from left to right starting at the top left corner) Sarcophyton glaucum, S. regulare, S. ehrenbergi, Nephthea molle, Acropora humilis, Porites solida, Pocillopora verrucosa, Clothraria rubrinoidis and Cystoseira trinode. Marine species exhibit greater phyta diversity than land species.

Figure 2

Terpene skeletal types including ylangene (A), aromadendrane (B), tricycle-[6,7,5]-sesquiterpene (C), cembrane (D), xeniolide and xeniaphyllane (E), eunicellin diterpene (F), sesterterpene (G), norsesterterpene (H), triterpene (I) and steroid (J) types.

2. Sesquiterpenes

Sesquiterpenes are secondary metabolites present in many marine organisms including soft coral (e.g., Dendronephthya sp., Sinularia gardineri, Litophyton arboreum, Sarcophyton trocheliophorum, S. glaucum and Parerythropodium fulvum fulvum) [9,10,11,12,13,14], and sponges (e.g., Hyrtios sp. and Diacarnus erythraenus) [15,16].

2.1. Ylangene-Type Sesquiterpenes

Tricyclo-[4,6,6]-sesquiterpenes, Dendronephthol A–C (1–3) have been isolated from the soft coral Dendronephthya, family Nephtheidae (Figure 3). Cytotoxic activity was observed for 1 and 3 against the murine lymphoma L5187Y cancer cell line with ED50 values of 8.4 and 6.8 μg/mL, respectively [9].

Figure 3

Representative structures of ylangene-type sesquiterpenes (1–3).

2.2. Aromadendrane-Type Sesquiterpenes

Bicyclico [5,7] sesquiterpenes have been isolated from several different coral with examples shown in Table 1 and Figure 4. Cytotoxicity to murine leukemia (P-388), human lung carcinoma (A-549), human colon carcinoma (HT-29), and human melanoma cells (MEL-28) [11] was observed with exposure to 4. Inhibitory activity against HIV-1 protease (PR) at an IC50 of 7 μM was observed for 5. Compounds 5 and 8 demonstrated cytostatic action with assaying HeLa cells, revealing potential use in virostatic cocktails [11]. Antitumor activity against lymphoma and Ehrlich cell lines was observed for 9 with LD50 in the range of 2.5–3.8 μM; antibacterial and antifungal activities were also observed [12]. Compound 10 showed potent activity against the prostate cancer line PC-3 with an IC50 of 9.3 ± 0.2 μM. Anti-proliferative activity of 9 can be attributed, at least in part, to its ability to induce cellular apoptosis [13]. Compound 12 exhibited a promising IC50 > 1 μg/mL against three cancer cell lines including murine leukemia (P-388; ATCC: CCL-46), human lung carcinoma (A-549; ATCC: CCL-8) and human colon carcinoma (HT-29; ATCC: HTB-38) [15].

Table 1

Aromadendrane sesquiterpenes, sources and activities.

No.	Name	Sources	Activities
4	Guaianediol [10]	Sinularia gardineri	anti-tumor
5	Alismol [11]	Litophyton arboreum	cytostatic
6	Lactiflorenol [17]	Sinularia polydactyla
7	10-O-Methyl alismoxide [11]	L. arboreum
8	Alismoxide [11]	L. arboreum	cytostatic
9	Palustrol [12]	Sarcophyton trocheliophorum	anti-tumor, antibacterial and antifungal
10	10(14)-Aromadendrene [13]	Sarcophyton glaucum	anti-tumor, antiproliferative
11	Fulfulvene [14]	Parerythropodium fulvum fulvum
12	O-Methyl guaianediol [15]	Diacarnus erythraenus	cytotoxic

Figure 4

Representative structures of aromadendrane-type sesquiterpenes (4–12).

2.3. γ-Methoxybutenolide-Type Sesquiterpenes

Tricyclo-[6,7,5]-sesquiterpenes, Hyrtiosenolide A and B have been isolated from the sponge Hyrtios sp., and compounds 13 and 14 exhibit weak antibacterial activity against Escherichia coli [16] (Figure 5).

Figure 5

Structures of sesquiterpenes-γ-methoxybutenolides and sesquiterpene derivatives (13–28).

2.4. Miscellaneous Sesquiterpenes

Additional sesquiterpenes have been isolated from several coral genera with examples reported in Table 2 and Figure 5. Compound 28 exhibits cytotoxic activity against human hepatocarcinoma (HepG2) and breast adenocarcinoma (MCF-7) [17].

Table 2

Other sesquiterpenes, sources and activities.

No.	Name	Sources	Activities
15	5-Hydroxy-8-methoxy-calamenene [14]	Parerythropodium fulvum fulvum
16	5-Hydroxy-8-methoxy-calamenene-6-al [14]	Parerythropodium fulvum fulvum
17	Peyssonol A [17]	Peyssonnelia sp.
18	Ilimaquinone [18]	Smenospongia sp.
19	Avarol [18,19]	Dysidea cinerea	HIV
20	3′-Hydroxyavarone [20]	D.cinerea
21	3′,6′-Dihydroxyavarone [20]	D.cinerea
22	6′-Acetoxyavarone [20]	D.cinerea
23	6′- Hydroxy4′-methoxyavarone [20]	D.cinerea
24	6′-Hydroxyavarol [20]	D.cinerea
25	6′-Acetoxyavarol [20]	D.cinerea
26	Smenotronic acid [18]	Smenospongia sp.
27	Dactyltronic acids [18]	Smenospongia sp.
28	(E)-Methyl-3-(5-butyl-1-hydroxy-2,3-dimethyl-4-oxocyclopent-2-enyl)acrylate [21]	Sarcophyton ehrenbergi	cytotoxic (HepG2) (anti-tumor)

3. Diterpenes

Diterpenoids are widespread in various marine organisms including soft coral (Sarcophyton glaucum, S. trocheliophorum, Sinularia polydactyla, S. gardineri, Litophyton arboreum, Lobophyton sp., Xenia sp. and Cladiella pachyclados) [10,11,12,13,22,23,24,25,26,27,28,29], and sponges (Leucetta chagosensis) [23].

3.1. Cembrane-Based Diterpenes

Fourteen-membered cyclic and bicycle-[5,14]-diterpenes have been isolated from numerous coral genera with examples shown in Table 3 and Figure 6. Compounds 29, 41 and 42 exhibited antibacterial and antifungal activity against Aspergillus flavus and Candida albicans with low μM MIC values [12]. Lack of cytotoxicity against monkey kidney CV-1 cells suggests that 30, 32, and 33 may prove to be good candidate drugs against melanoma and warrant further studies in the development as antitumor agents [19]. Compound 30 exhibits moderate antifungal activity against Cryptococcus neoformans with an IC50 of 20 μg/mL [22]. Compound 43 showed selective cytotoxicity against HepG2 (IC50 1.0 μg/mL) [24]. Compounds 44 and 45 were found to be inhibitors of cytochrome P450 1A activity [25]. Compound 47 exhibits cytotoxic activity against HepG2, HCT-116, and HeLa cells with low IC50 μg/mL values [26]. Cytotoxic activity against human hepatocarcinoma (HepG2) and breast adenocarcinoma (MCF-7) cell lines was observed for 48 and 49 [21].

Table 3

Cembrane diterpenes, sources and activities.

No.	Name	Source	Activity
29	Cembrene-C [12]	Sarcophyton trocheliophorum	anti-fungal, anti-bacterial
30	Sarcophine [19,22]	S. glaucum	anti-tumor, antifungal
31	(+)-7α,8β-Dihydroxydeepoxy-sarcophine [22]	S. glaucum
32	Sarcophytolide 1 [19,30]	S. glaucum	anti-tumor
33	(1S,2E,4R,7E,11E,13S)-Cembratrien-4,13-diol [22]	S. glaucum	anti-tumor
34	(1S,2E,4R,6E,8R,11S,12R)-8,12-Epoxy-2,6-cembradiene-4,11-diol [22]	S. glaucum	anti-tumor
35	(1S,2E,4R,6E,8S,11R,12S)-8,11-Epoxy-4,12-epoxy-2,6-cembradiene [22]	S. glaucum	anti-tumor
36	Trochelioid A [23]	S. trocheliophorum
37	Trochelioid B [23]	S. trocheliophorum
38	16-Oxosarcophytonin E [23]	S. trocheliophorum
39	ent-Sarcophine [23]	S. trocheliophorum
40	8-epi-Sarcophinone [23]	S. trocheliophorum
41	Sarcotrocheliol acetate [12]	S. trocheliophorum	anti-tumor
42	Sarcotrocheliol [12]	S. trocheliophorum	anti-tumor
43	Durumolide C [24]	Sinularia polydactyla	anti-fungal, anti-bacterial
44	11(S)-Hydroperoxylsarcoph-12(20)-ene [22]	S. glaucum	anti-fungal, anti-bacterial
45	12(S)-Hydroperoxylsarcoph-10-ene [25]	S. glaucum	cytotoxic HepG2 (anti-tumor)
46	(2R,7R,8R)-Dihydroxy-deepoxysarcophine [26]	S. glaucum	anti-tumor
47	7β-Acetoxy-8α-hydroxy-deepoxysarcophine [26]	S. glaucum	cytotoxic (HepG2)( anti-tumor)
48	7-Keto-8α-hydroxy-deepoxysarcophine [21]	S. ehrenbergi	cytotoxic (HepG2) (anti-tumor)
49	7β-Chloro-8α-hydroxy-12-acetoxy-deepoxysarcophine [21]	S. ehrenbergi	cytotoxic (HepG2) (anti-tumor)
50	Nephthenol [27]	Lobophytum pauciflorum
51	Cembrene-A [27]	Alcyonium utinomii
52	Alcyonol A [27]	A. utinomii
53	Alcyonol B [27]	A. utinomii
54	Alcyonol C [27]	A. utinomii
55	Pauciflorol A [27]	L. pauciflorum
56	Pauciflorol B [27]	L. pauciflorum
57	Thunbergol [27]	L. pauciflorum
58	Labolide [27]	L. crassum
59	20-Acetylsinularolide B [27]	L. crassum
60	20-Acetylsinularolide C [27]	L. crassum
61	Sinularolide C [27]	L. crassum
62	Sinularolide C diacetate [27]	L. crassum
63	3-Deoxypresinularolide B [27]	L. crassum
64	3-Deoxy-20-acetylpresinularolide B [27]	L. crassum
65	Sarcophytol M [11]	Litophyton arboreum
66	Sarcophytolol [13]	Sarcophyton glaucum	cytotoxic HepG2 (anti-tumor) antiproliferative
67	Sarcophytolide B [13]	S. glaucum
68	Sarcophytolide C [13]	S. glaucum
69	Deoxosarcophine [13]	S. glaucum	cytotoxic against MCF-7 (anti-tumor)
70	2-epi-Sarcophine [31]	S. auritum	cytotoxic
71	(1R,2E,4S,6E,8R,11R,12R)-2,6-cembradiene-4,8,11,12-tetrol [31]	S. auritum	cytotoxic
72	Singardin [31]	Sinularia gardineri	anti-tumor

Figure 6

Structures of cembrane-based diterpenes (29–72).

Compounds 66 and 68 have significant cytotoxic activity against the human hepatocellular liver carcinoma cell line HepG2 with an IC50 of 20 μM while 67 and 68 show activity against the human breast adenocarcinoma cell line MCF-7, also with an IC50 in the low μM range. The anti-proliferative activity of 66 and 68 can be attributed, at least in part, to observed cellular apoptosis activity [13,30]. Compound 70 exhibits cytotoxicity to a variety of cell lines including murine leukemia (P-388), human lung carcinoma (A-549), human colon carcinoma (HT-29) and human melanoma (MEL-28) [31].

3.2. Xenicane Diterpenes

Bicyclo-[6,9]/[4,9]-diterpenes have been isolated from the coral genus Xenia with examples shown in Table 4 and Figure 7.

Table 4

Xenia diterpenes, sources and activities.

No.	Name	Source
73	Xenicin [28]	Xenia macrosoiculata
74	Xenialactol-D [28]	X. obscuronata
75	Xenialactol-C [28]	X. obscuronata
76	Xeniolide-E [28]	X. obscuronata
77	14(15)-Epoxyxeniaphyllene [28]	X. lilielae
78	Xeniaphyllene-dioxide [28]	X. lilielae
79	Xeniaphyllenol-C [28]	X. macrosoiculata
80	Epoxyxeniaphyllenol-A [28]	X. lilielae, X. macrosoiculata
81	l4,15-Xeniaphyllandiol-4,5-epoxide [28]	X. macrosoiculata
82	Xeniaphyllenol-B [28]	X. macrosoiculata

Figure 7

The structures of Xenicane diterpenes (73–82).

3.3. Eunicellin-Based Diterpenes

Tricyclo-[6,5,10]-diterpenes have been isolated from the soft coral genus Cladiella with examples shown in Table 5 and Figure 8. Eunicellin-based diterpenes display a wide range of bioactivities including anti-inflammatory and antitumor activities [26]. Compounds 83–104 have been evaluated for activity to inhibit growth, proliferation, invasion and migration of a prostate cancer cell line with potent anti-migratory and anti-invasive activities observed. Compounds with exomethylene functionalities at C-7 and C-11 demonstrate low anti-migratory activity, however replacement of the exomethylene moiety at C-7 with a quaternary oxygenated carbon, appreciatively increases the activity, as observed for compounds 93–94 and 96 [29].

Table 5

Eunicellin diterpenoids, sources and activities.

No.	Name	Source	Activity
83	Pachycladin A [29]	Cladiella pachyclados	anti-tumor, anti-invasive
84	Klysimplexin G [29]	C. pachyclados	anti-tumor, anti-invasive
85	Pachycladin B [29]	C. pachyclados	anti-tumor, anti-invasive
86	Klysimplexin E [29]	C. pachyclados	anti-tumor, anti-invasive
87	Pachycladin C [29]	C. pachyclados	anti-tumor, anti-invasive
88	Cladiellisin [29]	C. pachyclados	anti-tumor, anti-invasive
89	3-Acetyl cladiellisin [29]	C. pachyclados	anti-tumor, anti-invasive
90	3,6-Diacetyl cladiellisin [29]	C. pachyclados	anti-tumor, anti-invasive
91	Pachycladin D [29]	C. pachyclados	anti-tumor, anti-invasive
92	Pachycladin E [26]	C. pachyclados	anti-tumor, anti-invasive
93	Sclerophytin A [29]	C. pachyclados	anti-tumor, anti-invasive
94	Sclerophytin F methyl ether [29]	C. pachyclados	anti-tumor, anti-invasive
95	Sclerophytin B [29]	C. pachyclados	anti-tumor, anti-invasive
96	(+)-Polyanthelin A [29]	C. pachyclados	anti-tumor, anti-invasive
97	Cladiella-6Z,11(17)-dien-3-ol [29]	C. pachyclados	anti-tumor, anti-invasive
98	Briarein A [32]	Junceella juncea
99	Juncins A [32]	J. juncea
100	Juncins B [32]	J. juncea
101	Juncins C [32]	J. juncea
102	Juncins D [32]	J. juncea
103	Juncins E [32]	J. juncea
104	Juncins [32]	J. juncea

Figure 8

The structure of eunicellin-type diterpenes (83–104).

3.4. Miscellaneous Diterpenes

Miscellaneous diterpenes were isolated from three different genus Xenia, Chelonaplysilla and Dysidea. These compounds were classified as: prenylated germacrenes (105), bicyclic diterpenes (108, 109), clerodane diterpenes (107), carbo-tricyclic diterpenes (108) and re-arranged spongian diterpenes (110–113) as shown in Table 6 and Figure 9.

Table 6

Macrocyclic diterpenes, sources and activities.

No.	Name	Source
105	Obscuronatin [28]	Xenia obscuronata
106	Biflora-4,10(19),15-triene [28,33]	X. obscuronata
107	Chelodane [34]	Chelonaplysilla erecta
108	Barekoxide [34]	C. erecta
109	Zaatirin [34]	C. erecta
110	Norrisolide [35]	Dysidea sp.
111	Norrlandin [35]	Dysidea sp.
112	Seco-norrlandin B [35]	Dysidea sp.
113	Seco-norrlandin C [35]	Dysidea sp.

Figure 9

The structure of the macrocyclic type diterpenes (105–113).

4. Sesterterpenes and Norsesterterpenes

4.1. Sesterterpenes

Pentacyclo-[6,6,6,6,5]-sesterterpenes have been isolated from two different sponges with examples shown in Table 7 and Figure 10. Compound 116 exhibits antimycobacterial inhibition against Mycobacterium tuberculosis (H37Rv) at a concentration of 6 μg/mL while 117–119 displayed moderate to weak inhibitory activity [36]. Compounds 122–123 showed significant cytotoxicity against murine leukemia (P-388), human lung carcinoma (A-549) and a human colon carcinoma (HT-29) [37].

Table 7

Sesterterpenes, sources and activities.

No.	Name	Source	Activity
114	Scalardysin [18]	Dysidea herbacea
115	25-Dehydroxy-12- epi-deacetylscalarin [36]	Hyrtios erecta	antimycobacterial
116	Sesterstatin [36]	H. erecta	antimycobacterial
117	16-epi-Scalarolbutenolide [36]	H. erecta	antimycobacterial
118	3-Acetylsesterstatin [36]	H. erecta	antimycobacterial
119	Salmahyrtisol A [37]	H. erecta
120	Hyrtiosal [37]	H. erecta
121	Salmahyrtisol B [37]	H. erecta	cytotoxic (anti-tumor)
122	19-Acetyl sesterstatin [37]	H. erecta	cytotoxic (anti-tumor)
123	Scalarolide [37]	H. erecta
124	Salmahyrtisol C [37]	H. erecta
125	16-Hydroxyscalarolide [38]	H. erecta	Cytotoxic, antimycobacterial
126	12-O-Deacetyl-12-epi-scalarine [38]	H. erecta	Cytotoxic, antimycobacterial
127	(−)-Wistarin [39]	Ircinia wistarii
128	(+)-Wistarin [39]	I. wistarii
129	(−)-Ircinianin [39]	I. wistarii
130	Bilosespens A [40]	Dysidea cinerea	Cytotoxic
131	Bilosespens A [40]	D. cinerea	Cytotoxic

Figure 10

Structures of sesterterpenes (114–131).

4.2. Norsesterterpenes

Norsesterterpenes have been isolated from the sponge species Diacarnus erythraeanus with examples shown in Table 8 and Figure 11. Antitumor natural peroxide products are known to induce cytotoxicity in cancer cells through the generation of particular reactive oxygen species (ROSs). Compounds 134–135 displayed mean IC50 growth inhibitions less than 10 μM with several tumor cell lines [41]. However, additional studies with 135 established no in vitro selective growth inhibition between normal and tumor cells. In assaying three cancer cells including murine leukemia (P-388), human lung carcinoma (A-549) and human colon carcinoma (HT-29), 140–143 exhibited an IC50 greater than 1 μg/mL [15] while 145 showed lower cytotoxicity against the same lines [42].

Table 8

Norsesterterpenes, sources and activities.

No.	Name	Source	Activity
132	Nuapapuin A methyl ester [41]	Diacarnus erythraeanus
133	Methyl-2-epinuapapuanoate [41]	D. erythraeanus
134	(−)-13,14-Epoxymuqubilin A [41]	D. erythraeanus	anti-tumor
135	(−)-9,10-Epoxymuqubilin A [41]	D. erythraeanus	anti-tumor
136	(−)-Muqubilin A [41,43]	D. erythraeanus	anti-tumor
137	Hurghaperoxide [41]	D. erythraeanus
138	Sigmosceptrellin B [41]	D. erythraeanus
139	Sigmosceptrellin B methyl ester [41]	D. erythraeanus
140	Aikupikoxide A [15]	D. erythraeanus	cytotoxic
141	Aikupikoxide D [15]	D. erythraeanus	cytotoxic
142	Aikupikoxide C [15]	D. erythraeanus	cytotoxic
143	Aikupikoxide B [15]	D. erythraeanus	cytotoxic
144	Tasnemoxide A [42]	D. erythraeanus	cytotoxic (anti-tumor)
145	Tasnemoxide B [42]	D. erythraeanus	cytotoxic (anti-tumor)
146	Tasnemoxide C [42]	D. erythraeanus	cytotoxic (anti-tumor)
147	epi-Sigmosceptrellin B [44]	D. erythraeanus
148	Muqubilone [45]	D. erythraeanus	antimalarial

Figure 11

Structures of norterpenes (132–148).

5. Triterpenes

Structurally diverse triterpenes are widespread in Red Sea sponges with examples shown in Table 9 and Figure 12. Compound 149 inhibits growth of human breast cancer cells, MDA-MB-231, MCF-7, BT-474 and T-47D, in a dose-dependent manner [46,47]. Triterpenes have also been studied for their efficacy in reducing the appearance of drug resistance. In the presence of many cytotoxic drugs, resistant cell variants appear, a process referred to as multidrug resistance (MDR). Overexpression of the ATP-binding cassette (ABC) transporter ABCB1/P-glycoprotein (P-gp) is one of the most common causes of MDR in cancer cells. P-gp a 170-kD transmembrane glycoprotein functions as a drug efflux pump that extrudes a wide spectrum of compounds including amphipathic and hydrophobic drugs. Sipholane triterpenoids can serve as P-gp inhibitors and are being developed to enhance the effect of chemotherapeutic drugs with MDR cancer cells in vitro and in vivo [33,36]. Compounds 162–163 enhanced cytotoxicity of several P-gp substrate-anticancer drugs, including colchicine, vinblastine and paclitaxel. These sipholane triterpenes significantly reversed the MDR-phenotype in P-gp-over expressing MDR cancer cells, KB-C2, in a dose-dependent manner. Moreover, these sipholanes have no effect on the response to cytotoxic agents in cells lacking P-gp expression or expressing MRP1 (ABCC1) or MRP7 (ABCC10) or with the breast cancer resistance protein (BCRP/ABCG2). Perhaps most importantly, these sipholanes with a low IC50 of ca. 50 μM are not toxic to the assayed cell lines [48].

Table 9

Triterpenes, sources and activities.

No.	Name	Source	Activity
149	Neviotine-A [46,47]	Siphonochalina siphonella
150	Sipholenol A [47,49,50,51,52,53]	S. siphonella	anti-tumor
151	SipholenolA-4-O-3′,4′-dichlorobenzoate [49]	S. siphonella
152	Shaagrockol B [54]	Toxiclona toxius
153	Shaagrockol C [54]	T. toxius
154	Sipholenol G [55]	S. siphonella
155	Sipholenone D [55]	S. siphonella
156	Sipholenol F [55]	S. siphonella
157	Sipholenol H [55]	S. siphonella
158	Neviotine B [55]	S. siphonella
159	Sipholenoside A [55]	S. siphonella
160	Sipholenoside B [55]	S. siphonella
161	Siphonellinol B [55]	S. siphonella
162	Dahabinone A [55]	S. siphonella
163	Sipholenone E [51]	S. siphonella	anti-tumor
164	Sipholenol L [47,51]	S. siphonella	anti-tumor
165	Sipholenol J [51]	S. siphonella
166	(2S,4aS,5S,6R,8aS)-5-(2-((1S,3aS,5R,8aS,Z)-1-hydroxy-1,4,4,6-tetramethyl-1,2,3,3a,4,5,8,8a-octahydroazulen-5-yl)-ethyl)-4a,6-dimethyloctahydro-2H-chromene-2,6-diol [51]	S. siphonella
167	Sipholenol K [51]	S. siphonella
168	Sipholenol M [51]	S. siphonella
169	Siphonellinol D [51]	S. siphonella
170	Siphonellinol E [51]	S. siphonella
171	Siphonellinol-C-23-hydroperoxide [51]	S. siphonella
172	Siphonellinol C [56]	S. siphonella
173	epi-Sipholenol I [56]	S. siphonella
174	Sipholenol I [51]	S. siphonella
175	Sipholenone A [56,47]	S. siphonella
176	Sipholenol D [52]	S. siphonella
177	Neviotine-C [47]	Siphonochalina siphonella	cytotoxic

Figure 12

Structures of triterpenes (149–177).

6. Steroids

Steriods are widespread throughout the marine biome with recent chemical reports including soft coral (Sinularia candidula, S. polydactyla, Heteroxenia ghardaqensis, Dendronephthya sp., Lobophytom depressum and Litophyton arboreum) [9,11,24,57,58,59,60], black coral (Antipathes dichotoma) [38,40], and sponges (Echinoclathria gibbosa, Hyrtios sp., Erylus sp., and Petrosia sp.) [18,64,65,66]. Steroid examples are shown in Table 10 and Figure 13.

Table 10

Steroids, sources and activities.

No.	Name	Source	Activity
178	3β-25-Dihydroxy-4-methyl-5α,8α-epidioxy-2-ketoergost-9-ene [57]	Sinularia candidula	anti-viral
179	Gorgosten-5(E)-3β-ol [58]	Heteroxenia ghardaqensis	anti-tumor
180	Gorgostan-3β,5α,6β,11α-tetraol (sarcoaldosterol A) [58]	H. ghardaqensis
181	Gorgostan-3β,5α,6β-triol-11α-acetate [58]	H. ghardaqensis
182	5α-Pregna-3β-acetoxy-12β,16β-diol-20-one [59]	Echinoclathria gibbosa	anti-tumor
183	β-Sitosterol-3-O-(3Z)-pentacosenoate [59]	E. gibbosa	anti-tumor
184	Cholesterol [9]	Dendronephthya
185	Dendronesterone A [9]	Dendronephthya
186	24-Methylcholestane-3β,5α,6β,25-tetrol-25-monoacetate [24]	Sinularia polydactyla	anti-tumor
187	24-Methylcholestane-5-en-3β,25-diol [24]	S. polydactyla	antimicrobial
188	Lobophytosterol [60]	L. depressum
189	5β,6β-Epoxy-24E-methylchloestan-3β,22(R),25-triol [60]	L. depressum
190	Depresosterol [60]	L. depressum
191	(22R,24E,28E)-5β,6β-Epoxy-22,28-oxido-24-methyl-5α-cholestan-3β,25,28-triol [60]	L. depressum
192	(22R,24E)-24-Methylcholest-5-en-3β,22,25,28-tetraol [60]	L. depressum
193	24-Methylcholesta-5,24(28)-diene-3β-ol [11]	Litophyton arboreum
194	7β-Acetoxy-24-methylcholesta-5-24(28)-diene-3,19-diol [11]	L. arboreum	cytotoxic
195	24-Methylcholesta-5,24(28)-diene-3β,7β,19-triol [11]	L. arboreum
196	Hyrtiosterol [16]	Hyrtios Species
197	Eryloside A [61,62,63]	Genus Erylus	cytotoxic
198	(22E)-Methylcholesta-5,22-diene-1α,3β,7α-triol [64]	Antipathes dichotoma	anti-bacterial
199	3β,7α-Dihydroxy-cholest-5-ene [64]	A. dichotoma	anti-bacterial
200	(22E,24S)-5α,8α-Epidioxy-24 methylcholesta -6,22-dien-3β-ol [64]	A. dichotoma	anti-bacterial
201	(22E,24S)-5α,8α-Epidioxy-24-methylcholesta-6,9(11),22-trien-3β-ol [64]	A. dichotoma	anti-bacterial
202	3β-Hexadecanoylcholest-5-en-7-one [65]	A. dichotoma	anti-tumor
203	3β-Hexadecanoylcholest-5-en-7β-ol [65]	A. dichotoma	anti-tumor
204	Cholest-5-en-3β-yl-formate [65]	A. dichotoma	anti-tumor
205	3β-Hydroxycholest-5-en-7-one [65]	A. dichotoma
206	Cholest-5-en-3β,7β-diol [65]	A. dichotoma
207	22-Dehydrocholestrol [65]	A. dichotoma
208	3β,7β,9α-Trihydroxycholest-5-en [66]	Petrosia	cytotoxic (anti-tumor)
209	Cholest-5-en-7β-methyl-3β-yl formate [66]	Petrosia sp.	cytotoxic (anti-tumor)
210	Dehydroepiandrosterone [66]	Petrosia sp.	cytotoxic (anti-tumor)
211	7-Dehydrocholesterol [66]	Petrosia sp.	cytotoxic (anti-tumor)
212	5α,6α-Epoxycholest-8(14)-ene-3β,7α-diol [66]	Petrosia sp.	cytotoxic (anti-tumor)
213	5α,8α-Epidioxycholesta-6-en-3β-ol [66]	Petrosia sp.	cytotoxic (anti-tumor)
214	Cholesta-8-en-3β,5α,6α,25-tetrol [67]	Lamellodysidea herbacea
215	Cholesta-8(14)-en-3β,5α,6α,25-tetrol [67]	L. herbacea
216	Cholesta-8,24-dien-3β,5α,6α-triol [67]	L. herbacea	anti-fungal
217	Cholesta-8(14),24-dien-3β,5α,6α-triol [67]	L. herbacea	anti-fungal
218	Clathsterol [68]	Clathria sp.
219	Clionasterol [69]	Dragmacidon coccinea
220	Stigmasterol [69]	D. coccinea
221	Campesterol [69]	D. coccinea
222	Brassicasterol [69]	D. coccinea
223	Dendrotriol [70]	Dendronephthya hemprichi
224	Erylosides K [62]	Erylus lendenfeldi
225	Erylosides L [62]	E. lendenfeldi
226	Erylosides B [63]	E. lendenfeldi

Figure 13

Steriod structures (178–226).

Moderate growth inhibition for a human colon tumor cell line was observed with 180 [58]. Compounds 184 and 186 exhibited activity against three human tumor cell lines including the lung non-small cell line A549, the glioblastoma line U373 and the prostate line PC-3 [59]. Compound 187 showed an IC50 of 6.1 and 8.2 μg/mL against the human cancer cell lines HepG2 and HCT, respectively [24].

Compounds 204–207 show antibacterial activity against Gram-positive (Bacillus subtilis) and Gram-negative bacteria (Pseudomonas aeruginosa), at a 1 μg/ml concentration [64]. Compounds 208–210 exhibited antitumor activity based on four cancer panels: HepG2, WI 38, VERO, and MCF-7 [65]. Compounds 215–220 exhibited cytotoxic effects in the tumor cell lines, HepG2 and MCF-7 with IC50 in the range of 20-500 μM. Interestingly, 217 showed the highest affinity to DNA with IC50 30 μg/mL [66]. Compounds 223 and 224 showed antifungal activity against Candida tropicalis, with petri dish inhibition at 10 μg/disc [67].

7. Drug Leads

Even though terpenes are the largest group of natural products with over 25,000 structures thus far reported, a small subset of these metabolites have been investigated for biological function and/or activity. Basic biological constituents such as membrane components, hormones, antioxidants and chemical defenses require the isoprenoid building module. Future chemical studies of marine organisms are expected to generate an ensemble of novel terpenes based on progressive knowledge on enzymatic machinery and selective pressures under which such organisms have evolved. The expanding chemo-diversity of marine terpenes is being assisted in part by advanced analytical chemistry methods for structure determination and sophisticated diving techniques for sample collection.

Methods for assaying for in vitro biological activity can be more variable in terms of stardardized protocols. The same positive or native chemical controls are not always utilized making direct comparisons of biological activity between different testing laboratories unreliable or at least not reproducible. Moreover, with the paucity of ethnomedical knowledge from marine sources, the basis for selecting the most promising bioassay can be more of an art than a science. The screening for anti-cancer activity in facilities such as the National Cancer Institute (NCI) Chemotherapeutic Agents Repository operated by Fisher BioServices [71] can provide invaluable, cost-free, sensitive screening of hits against multiple-target 60 cancer cell line panels, broadening the opportunity to conduct more comprehensive and mechanistic studies. In the case where a set of metabolites has already been identified possessing a given biological activity, computational, in silico, and pharmacophore modeling can guide future design of druggable analogues with better biological activity, without expected toxicity, even if the structural characterization of the biological target(s) is/are not feasible. Such virtual models utilizes steric and electronic descriptors to identify pharmacophoric features such as hydrophobic centroids, aromatic rings, hydrogen bonding acceptors/donors and cation/anion interactions to match optimal supramolecular interactions with a specific biological target that triggers or blocks a response. Functional group properties can also be identified for the rational semi-synthetic design of biologically active marine natural scaffolds. Strategies such as the Topliss scheme designate a series of substituents based on lipophilic, electronic and steric properties to generate multiple analogues with slight controlled chemical property differences that can be used for comprehensive structure-activity studies to obtain superior biological activity relative to the parent natural product. While these techniques and tools are not distinct or exclusive for exploring marine sources and marine-derived natural products, such methods can be effective for enhancing biological activity. For example, sipholenol A is a noteworthy example of developing a marine metabolite using medicinal chemistry approaches to generate biologically active analogue libraries [49]. These natural product examples with exceptional biological potency outcomes (IC50 in the low μM range for invasive breast cancer) demonstrate the potential of marine natural products for the discovery of future novel druggable entities useful for the control and management of human diseases.

8. Conclusions

Terpenoids provide a vast array of molecular architectures with the coral community of the Red Sea having added significantly to the structure database over the last thirty years. While marine invertebrates in this ecosystem are still being discovered, interest in both the chemistry and biological activity of Red Sea terpenes has generated many novel structures with promising biological activities.