Chemical constituents and biological activities of the soft corals of genus Cladiella: A review

Biochemical Systematics and Ecology 34 (2006) 781e789 www.elsevier.com/locate/biochemsyseco

Chemical constituents and biological activities of the soft corals of genus Cladiella: A review Parvataneni Radhika* Andhra University, University College of Pharmaceutical Sciences, Pharmaceutical & Medicinal Chemistry Laboratory, Visakhapatnam 530 003, Andhra Pradesh, India Received 25 October 2005; accepted 15 May 2006

Dedicated to the late Dr. D. John Faulkner,1 the late Dr. Paul J. Scheuer,2 the late Dr. Kenneth L. Rinehart3 and the late Prof. Dr. C. Bheemasankara Rao4 for their pioneering work on bioactive marine natural products.

Abstract In this review, structures of natural products isolated from the soft corals of genus Cladiella and their biological activities are described. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Soft coral; Cladiella; Sesquiterpenes; Diterpenes; Ceramides; Cerebrosides; Sterols; Sterol glycosides; Lipid glycosides; Biological activity

A number of reviews on the chemical and biological perspectives of natural products of marine origin have appeared in Chemistry of Marine Compounds (Scheuer, 1973), Marine Toxins (Scheuer, 1977), Interesting Aspects of Marine Natural Products Chemistry (Faulkner and Fenical, 1977), Marine Natural Products: Chemical and Biological Perspectives (Scheuer, 1978e1983), Bioorganic Marine Chemistry (Scheuer, 1987e1992), and Marine Natural Products (Faulkner, 2002; Blunt et al., 2006). The work of Rinehart at the University of Illinois at ChampaigneUrbana and Pettit at Arizona State University opened a new horizon in discovering new biologically active molecules from the marine environment. The specific biological activity of marine natural products was also well reviewed and published in different journals (Faulkner, 2000; El Sayed et al., 2000; Blunden, 2001; Beutler and McKee, 2002; Proksch et al., 2002; Gochfeld et al., 2003; Haefner, 2003; Newman and Cragg, 2004).

* Tel.: þ91 891 2575781; fax: þ91 891 2755580. E-mail address: [email protected] 1 Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0212, USA. 2 University of Hawaii, Manoa, Department of Chemistry, 2545 The Mall, Honolulu, Hawaii 96822, USA. 3 Department of Chemistry, University of Illinois, 454B Roger Adams Lab, 600 South Mathews Avenue, Urbana, IL 61801, USA. 4 Department of Organic Chemistry, School of Chemistry, Andhra University, Visakhapatnam 530 003, Andhra Pradesh, India. 0305-1978/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.bse.2006.05.011

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Soft corals are a rich source of bioactive molecules such as terpenes, steroids and steroidal glycosides (Faulkner and Fenical, 1977). It was reported that 50% of the soft coral extracts exhibited ichthyotoxic characteristics (Sammarco and Coll, 1988). Cnidarianealgal symbiotic associations are common in marine environment and are of great ecological importance. Many of the secondary metabolites of cembranoid type diterpenes from the soft corals may be involved in ecological interactions (Coll, 1992), whereas other metabolites have biological activities such as antifungal, antineoplastic, ichthyotoxic (Kashman and Groweiss, 1977), cytotoxic (Duh et al., 2000), HIV-inhibitory (Rashid et al., 2000) and anti-inflammatory activity (Radhika et al., 2005). Soft corals are widely distributed but have marked preference for tropical waters of a depth between 5 and 30 m than temperate reefs. The genera of Sinularia, Lobophytum and Sarcophyton are the most prolific soft corals, whereas species of Cladiella (Phylum: Coelenterata, Family: Alcyoniidae, Class: Anthozoa, Order: Alcyonacea) are found on reefs in the Indo-Pacific region. The chemical constituents and different types of biological activities of 14 species of this genus have been studied by various research groups. The structure and biological activities of these metabolites are described and presented in this review. 1. Sesquiterpenes and diterpenes Studies on the chemical constituents of soft corals of Cladiella were initiated in 1977 with the discovery of two eunicellin type of diterpene metabolites, cladiellin 1 and acetoxycladiellin 2 (Kazlauskas et al., 1977) from a species of Cladiella, collected on the Great Barrier Reef near Townsville. They were structurally related to eunicellin, a metabolite isolated from the gorgonian Eunicella stricta. The structure of the m-chlorobenzoate derivative of cladiellin was determined by X-ray analysis. A diterpene 3 was reported from an unidentified pacific soft coral related to cladiellin 1 by chemical interconversion (Hochlowski and Faulkner, 1980). The soft coral has now been identified as a species of Cladiella (identified by Dr. J.C. Coll). An eunicellin-based diterpene 4 has been isolated from a Cladiella sp. (Uchio et al., 1989). The structure and relative stereochemistry of metabolite 4 were determined on the basis of spectroscopic (IR, 1H and 13C NMR) and chemical evidences. Its structure was confirmed by a single-crystal X-ray analysis. Co-occurring with metabolite 4 were three other eunicellin-based diterpenoids 5, 6 and 7 isolated from an Okinawan Cladiella species of soft coral (Uchio et al., 1992). Their structures have been determined by spectral evidence and confirmed by chemical interconversion. The diterpenoid cladiellisin 8 was isolated from the soft coral Cladiella similis (Liu et al., 1992). Its structure was determined by IR, NMR, and 2D-COSY. The compounds isoneocembreneA 9 and 11,12-epoxy isoneocembrene-A 10 were two cembrane diterpenoids reported from the soft coral, Cladiella klugzingeri (Hari Babu, 1992), on the coasts of the Andaman and Nicobar Islands. A cembranoid diterpene was isolated from the soft coral, Cladiella krempfi (Sarma et al., 1993) from Minicoy Island (India). Its structure was established by X-ray crystallography as sclerophytin F methyl ether 11 with the R absolute configuration at all six epimeric centers, assuming a configuration similar to that of sclerophytin C. Seven cladiellane diterpenes, 12, 13, 14, 15, 16, 17 and 18 were isolated from the soft coral Cladiella australis (Bheemasankara Rao et al., 1994; Sreenivasa Rao et al., 1994) collected on the coasts of the Andaman and Nicobar Islands of the Indian Ocean. The structures of these metabolites were elucidated on the basis of high resolution spectral data and chemical studies. In addition, sclerophytins C 19 and E 20 reported earlier from Sclerophytum capitalis, were also isolated from this species. It was reported that compound 11 may be an artifact of the isolation process (Sarma et al., 1995). Cladiella sphaeroides (Yamada et al., 1997), a species found in Japan, contained a new bioactive diterpenoid, cladiellaperoxide 21, together with the eunicellin-type diterpenoid which had the same relative stereostructure as that of the diterpenoid cladiellisin 8. All the structures were characterized by NMR studies and chemical conversion. The absolute configurations of 8 and 21 have been elucidated by modified Mosher method. Compound 21 showed toxicity in the brine shrimp lethality bioassay at a 30-ppm concentration, while cladiellisin 8 was inactive in this bioassay. Three cembrane diterpenes namely flaccidoxide 22 (1Z,3E,7E,11S,12S,14S)-11,12-epoxy cembra-1,3,7-trien-14-ol 23 and flaccidoxide-13-acetate 24 (Gray et al., 2000) were isolated from the southern African soft coral, Cladiella kashmani, collected off Ponto do Oura, Mozambique. Application of the modified Mosher’s method established the previously unassigned absolute configuration of 22 as (1Z,3E,7E,11S,12S,13S,14R)-14-acetoxy-11,12-epoxy cembra-1,3,7-trien-13-ol. Acetylation of 22 yielded 24 and confirmed the structure of 24 as (1Z,3E,7E,11S,12R,13S,14R)-13,14-diacetoxy-11,12-epoxy cembra-1,3,7-triene. The isolation of cembranoids from C. kashmani was reported for the first time. All the three diterpenes (22, 23 and 24) were toxic to the brine shrimp Artemia salina.

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The sesquiterpenes, cladioxazole 25 (Ata et al., 2003) and cladidiol 26 (Ata et al., 2004) were reported for the first time from the soft coral genus Cladiella collected from the Andaman and Nicobar Islands, India. The structures of 25 and 26 were established with the aid of extensive spectral studies. Apart from the compounds 25 and 26, three diterpenes, cladiellaperoxide 27 (6E)-2a,9a-epoxyeunicella-6,11(12)-dien-3b-ol 28 and polyanthellin A 29 were also isolated from the same species. Compounds 26e29 were tested for antibacterial activity against Streptococcus pyogenes (ATCC 19615), Staphylococcus aureus (ATCC 25923), Enterococcus faecalis (ATCC 19433), Escherichia coli (ATCC 25933) and Pseudomonas aeruginosa (ATCC 27853) at a concentration of 30 mg/mL, using the disc-agar diffusion method. Compounds 27e29 exhibited antibacterial activity against S. pyogenes, E. coli and P. aeruginosa, while cladidiol 26 was inactive in this bioassay. Compound 26 exhibited modest acetylcholinesterase (AchE) inhibitory activity with an IC50 value (inhibition concentration to lower enzyme activity by 50%) of 67 mM. Compounds 27e29 were also screened, but were inactive against acetylcholinesterase in a AchE-inhibition assay. The ethyl acetate extract of C. australis (Ahmed et al., 2005), a soft coral collected from the southern Taiwanese waters, yielded four new eunicellin-based diterpenoids, australins AeD (30e33). The molecular structures of these metabolites, including their relative stereochemistries were established on the basis of extensive spectroscopic methods (including 1D and 2D NMR) and by comparison of their NMR spectral data with those of related metabolites. Metabolite 31 has been shown to exhibit moderate cytotoxic activity against MCF-7, MDA-MB-231 (human breast cancer), and HepaG2/DMEM-12 (human liver cancer) cell lines. C. kashmani collected in Kenya afforded four eunicellane-based diterpenes, epoxycladines 34e37 (Chill et al., 2005). The structures of the compounds were elucidated by interpretation of MS, COSY, HMQC, HMBC and NOESY data. Sclerophytin F methyl ether 11 (Sarma and Ramakrishna Rao, 2005) when tested for anti-HIV and anti-cancer activities showed moderate anti-HIV activity.

2. Ceramides, cerebrosides, lipids and glycolipids The ceramide, humesamide (38), was isolated from the soft coral Cladiella humesi Verseveldt (Wang et al., 2000) collected from Linshui County of Hainan Province. Its structure was established by spectroscopic analysis and chemical degradation methods. N-hexadecanol-nonadecasphinga-4(E)-ene (39), a marine ceramide, was isolated from the soft coral Cladiella species (Zhang et al., 2003) collected from Sanya Bay in Hainan Province, China. The chemical structure of ceramide 39 was determined on the basis of spectral studies and chemical degradation methods. A mixture of cerebroside homologues identified as 1-(b-D-glucopyranosyloxy)-2-N-(2-hydroxyacyl)-erythro-2amino-9-methyloctadeca-4E,8E-dien-3-ol (40) was isolated from a species of soft coral Cladiella (Dmitrenok et al., 2001) collected on the seaboard of the Andaman and Nicobar Islands (Indian Ocean). The compound 40 was tested for antibacterial and antifungal activities (Radhika et al., 2003) at different concentrations against Bacillus subtilis, E. coli, Bacillus pumilus, P. aeruginosa, Rhizopus oryzae and Aspergillus niger using cup plate and diffusion plate methods. It showed significant antibacterial and antifungal activity against some of the bacteria and fungi tested at a concentration of 200 mg/mL. The batyl alcohol (41) was isolated from the soft corals C. similis (Liu et al., 1993), Cladiella densa (Wang et al., 1995) and Cladiella species (Zhang et al., 2003). A crystalline compound isolated from the soft coral C. densa (Wang et al., 1995) was identified as hentriacontanol (42) by spectroscopic methods. A lipid glycoside (43) was obtained from more polar fractions of ethyl acetate solubles of Cladiella species (Anjaneyulu et al., 2001) of Andaman and Nicobar Islands. The structure of the glycolipid has been deduced as 2-hydroxy-3-(octadecyloxy)-propyl-b-D-arabinopyranoside (43) from its spectral data. The compound 43 showed

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significant antibacterial and antifungal activity (Radhika et al., 2003) at different concentrations against B. subtilis, E. coli, B. pumilus, P. aeruginosa, R. oryzae and A. niger using cup plate and diffusion plate methods.

3. Sterols and sterol glycosides The compounds (20R, 24S)-24-methyl-3b-hydroxycholest-5-ene-21-oic acid (44) and 3b, 4a-dihydroxypregn20-ene-4-O-b-D-arabinopyranoside (45) were isolated from C. klugzingeri off the coasts of Andaman and Nicobar Islands (Hari Babu, 1992). Compound 44 was a monohydroxy C28 sterol containing carboxylic acid group at C-21. The structure was established by using high resolution 1H NMR, DEPT, 13C NMR, mass spectral data and a series of chemical conversions. The structure of compound 45 and its aglycone was deduced by high resolution 1H NMR, 13 C NMR, 2D-HMQC, HMBC and mass spectral data. The compounds ergost-5-en-3b-ol (46) and gorgosterol (47) were isolated from the soft coral C. similis (Liu et al., 1993). They were identified by means of spectroscopy and GCeMS methods. Compound 47 was also isolated from soft coral C. densa (Wang et al., 1995). The ethyl acetate extract of the soft coral C. krempfi (Sarma et al., 1995) afforded a ring-D oxidized sterol which was shown to be 3b, 16b-dihydroxy-24(S)-methylcholestenol (48). The less polar fractions of hexaneeethyl acetate solubles of a species of Cladiella (Anjaneyulu et al., 2001) yielded a monohydroxy sterol (49). The structure of compound 49 was deduced mainly from its mass spectrum. Two steroidal glycosides 50 and 51 were isolated from the ethanol extract of Chinese soft coral, C. krempfi (Lan et al., 2003). Based on spectral data the structures were established as 3-O-[-Darabinopyranosyl]-pregn-5,20-dien-3b-ol (50) and 4-O-b-D-arabinopyranosyl-pregn-20-en-3b, 4a-diol (51). Bioassay showed that compound 51 displayed cytotoxicity against human Hep-G2 and Cne2 cell lines in vitro with IC50 values 27.9, 14.3 mg/mL, respectively. Pregn-1,20-dien-3-one (52), pregn-1,4,20-trien-3-one (53), and ergost-5,24(28)-dien-3b-ol (54) were isolated from a Cladiella species (Zhang et al., 2003). The chemical structures of compounds 52, 53 and 54 were determined on the basis of spectroscopic data and chemical degradation. The compound 24-methylenecholest-5-ene-3b, 16b-diol-3-O-a-L-fucopyranoside (55) was isolated from a soft coral species of Cladiella collected on the coasts of Andaman and Nicobar Islands. The 5a-reductase inhibitory activity (Radhika et al., 2004) of compound 55 was evaluated by measuring the conversion of [3H]T to [3H]DHT in Penicillium crustosum broths.

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4. Conclusion Fifty-five secondary metabolites (1e55) have been isolated from soft corals of the genus Cladiella. They represent four major skeletal types of diterpenes namely, bioactive eunicellin (1e8, 27e33), cladiellin (12e21), epoxycladines(34e37), and cembrane- (9e11, 22e24). So far there are only two reports of sesquiterpenes, cladioxazole (25) that possess a unique skeleton having an oxazole ring in its structure and cladidiol (26) from this genus. The presence of other compounds was also reported from this genus including ceramides (38e39), cerebroside (40), batyl alcohol (41), hentriacontanol (42), glycolipid (43), polar sterols (44, 46e49, 52e54) and steroidal glycosides (45, 50e51, 55). Acknowledgements I profoundly thank the Council of Scientific and Industrial Research (CSIR), New Delhi, Government of India for financial support. Special thanks to Prof. B.S. Sastry and Dr. Y. Rajendra Prasad, Associate Professor, Department of Pharmaceutical Sciences, Andhra University, Visakhapatnam for their valuable suggestions. References Ahmed, A.F., Wu, M.-H., Wang, G.-H., Wu, Y.-C., Sheu, J.-H., 2005. Eunicellin-based diterpenoids, Australins AeD, isolated from the soft coral Cladiella australis. J. Nat. Prod. 68, 1051e1055. Anjaneyulu, V., Subba Rao, P.V., Radhika, P., Laatsch, H., Asolkar, R.N., 2001. A new glycolipid and a new monohydroxy sterol from Cladiella species of Andaman and Nicobar Islands. Indian J. Chem. 40B, 405e409. Ata, A., Ackerman, J., Radhika, P., 2003. Cladioxazole: a novel sesquiterpene from a marine soft coral of genus Cladiella. Tetrahedron Lett. 44, 6951e6953. Ata, A., Ackerman, J., Bayoud, A., Radhika, P., 2004. Bioactive chemical constituents of Cladiella species. Helv. Chim. Acta 87, 592e597. Beutler, J.A., McKee, T.C., 2002. Novel marine and microbial natural product inhibitors of vacuolar ATPase. Curr. Med. Chem. 9, 1241e1253. 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