Excoecaria (PROSEA Medicinal plants)

Plant Resources of South-East Asia Introduction

List of species

Excoecaria L.

Protologue: Syst. nat., ed. 10, 2: 1288 (1759).

Family: Euphorbiaceae

Chromosome number: x= unknown; E. agallocha: 2n= about 70, 140, 168, E. cochinchinensis: 2n= 22, E. indica: 2n= 64

Major species

• Excoecaria agallocha L.,
• E. cochinchinensis Lour.

Origin and geographic distribution

Excoecaria comprises about 40 species occurring in tropical Africa, Madagascar, and from India and Sri Lanka eastwards to Indo-China, southern China, Taiwan, the Ryukyu Islands, Thailand, throughout the Malesian region, northern Australia and the Pacific Islands. Most species are found in the Asiatic tropics, including 12 in Malesia.

Uses

Various parts of most Excoecaria species contain highly irritating and poisonous substances, most prominent in the latex. Trees of E. agallocha may form resinous, aromatic wood containing an oil applied medicinally against sores, eczema and scabies. Various parts of E. agallocha, E. cochinchinensis and E. indica, most likely substances from the latex, are used as fish poison. The bark of E. agallocha is also used as an ingredient for dart poison and as a purgative. In Sarawak the leaves of E. borneensis Pax & Hoffm. are heated over a fire and applied as a poultice to relieve rheumatism. Its leaves are poisonous to cattle. E. bantamensis Müll. Arg. (synonym E. macrophylla J.J. Smith) from Borneo, the Philippines and Java is known for its irritating latex. E. oppositifolia Griffith from Burma (Myanmar), Thailand and Indo-China is also known to cause skin irritation. The wood of various Excoecaria species has limited applications as a timber, but yields good quality charcoal and can be used as raw material for the production of kraft pulp. Pieces of the resinous, aromatic wood of E. agallocha are called "garu laut" or "garu mata buta" in Ternate and "menengan" in Bali. It serves as a substitute for "gaharu" (incense) from Aquilaria spp. However, the low likelihood of its occurrence and the amounts available render it insignificant. The leaves of E. agallocha and E. indica contain tannin and are used to prepare a dye.

Properties

The extremely irritant and caustic latex of E. agallocha is known to contain a complex mixture of polyfunctional diterpene esters: C-20 compounds, esterified with aliphatic saturated homologous n-carboxylic acids (even numbered, C22-C30). The mixture can be further divided into 3 groups of compounds, which are also known as cryptic irritants. Based on their ground skeletons, the largest group (group I) contains analogues of aliphatic, polyunsaturated 9,13,14-ortho-esters of the daphnane-type parent alcohol 5β-hydroxyresiniferonol-6α,7α-epoxide. Group II contains analogue structures of 5β,12β-dihydroxyresiniferonol-6α,7α-epoxide, and group III contains analogues of aliphatic 13-polyunsaturated, 20-saturated diesters of the tigliane-type parent alcohol 12-deoxyphorbol. In general, the 3 groups exhibit practically no irritant activity in the well-known mouse ear irritation test. However, alkaline trans-esterification of groups I-III releases the corresponding highly irritant multicomponent mixtures of Excoecaria factors A (OH-20 deacylated). They comprise a mixture of the well-known Excoecaria factors A1/A2/A3 and mixtures of Excoecaria factors A4/A5, A6/A7 and A8/A9. The highly irritant mixture A1/A2/A3 was obtained directly (0.2% of the latex) using an alternative, extremely mild separation procedure. These represent the so-called free Excoecaria factors, the natural constituents of the latex responsible for its bioactivity.

In contrast to its irritant effects, latex from E. agallocha has been successfully applied for wound healing in both humans and animals. Despite the toxic constituents in the leaf, protein extracted from mature leaves can be used for feeding fish.

Several labdane-type diterpenes have been isolated from the wood of E. agallocha.

Leaf extracts of E. agallocha show significant antiviral activity against tobacco mosaic virus (>70% inhibition), and the phorbol ester 12-deoxyphorbol 13-(3E,5E-decadienoate) isolated from leaves and stems of E. agallocha shows anti-HIV activity. The latter compound is also a potent displacer of H³-phorbol dibutyrate from rat brain membranes.

To search for possible antineoplastic agents, 17 diterpenes isolated from the resinous wood of E. agallocha (grown in Okinawa, Japan) were screened using an in vitro synergistic assay system. Of these diterpenes, ent-16-hydroxy-3-oxo-13-epi-manoyloxide, (13R,14R)-ent-8α,13,14,15-diepoxy-13-epi-labda-3β-ol, ent-3β-hydroxy-15-beyeren-2-one and ent-15-hydroxy-labda-8(17),13E-dien-3-one exhibited significant inhibitory effects on Epstein-Barr virus activation induced by the tumour promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). In a 2-stage in vivo carcinogenesis test on mouse tumours using 7,12-dimethylbenz-α-anthracene as initiator and TPA as promoter, ent-3β-hydroxy-15-beyeren-2-one exhibited remarkable antitumour-promoting activity.

Using juvenile Nile tilapia (Sarotherodon niloticus) as test organism, 96 hour bioassays were run from the highest concentration in which no death occurred to the lowest concentrations in which mortality was 100%. LC₅₀ values were 0.002-0.003 ppt for E. agallocha sap and 0.05 ppt for E. agallocha root.

Leaves of E. oppositifolia yielded the irritant Excoecaria factor O1, an ester of 5Œ≤-hydroxyresiniferonol-6α,7α-epoxide. Its structure was shown to be identical with that of a factor obtained by transesterification of the cryptic Excoecaria factor group Oz from the latex of the same plant, and with Excoecaria factor B4 from E. cochinchinensis. Furthermore, the latex of E. oppositifolia contained Excoecaria factors O2 (an ester of 5β-hydroxyresiniferonol-6α,7α-epoxide) and O3 (an ester of 5β, 12β-dihydroxyresiniferonol-6α,7α-epoxide). Excoecaria factor O3 is identical with Excoecaria factor A7 from E. agallocha , and Excoecaria factor B6 from E. cochinchinensis. From the non-irritant ethyl acetate fractions of E. agallocha, E. oppositifolia and E. cochinchinensis latices, three thin layer chromatography (TLC)-homogenous non-irritant mixtures, A'z, O'z and B'z, were isolated.

The latex of E. cochinchinensis is also very rich in diterpene esters of the same 3 structural groups as found in E. agallocha latex. After alkaline trans-esterification, the corresponding highly irritant multicomponent mixtures of Excoecaria factors B1-B8 were isolated. From the first group, Excoecaria factors B1/B2/B3 are identical with A1/A2/A3 from E. agallocha, and B4 is identical to O1 from E. oppositifolia. From the second group B5/B6 is identical with A6/A7 from E. agallocha, and B6 also with O3 from E. oppositifolia. Finally, from the third group, Excoecaria factor B7 is identical with A8 from E. agallocha. Furthermore, 4 TLC-homogeneous fractions (F1-F4), highly enriched in diterpene esters were obtained from the latex of E. cochinchinensis, collected from Thailand. These fractions were analysed by GC-MS, and were found to consist of mixtures of highly unsaturated esters of both daphnane and tigliane type parent alcohols.

E. cochinchinensis is used in traditional medicine for its uterotonic properties. Also, daphnane type esters are currently used routinely in China for fertility regulation. Similarly, daphnane and tigliane type Excoecaria factors present in the latex of E. cochinchinensis may be responsible for the uterotonic activity of its leaves utilized in Thai herbal medicine.

The screening of pharmacological activities of alcoholic extracts of E. cochinchinensis was performed using different test systems. In the hippocratic screening (rat), signs and symptoms such as decrease of motor activity, depression of respiration, loss of screen grip, enophthalmos, blanching of ears and oral mucosa, and hypothermia were observed following the intraperitoneal injection of E. cochinchinensis. Effects on the cardiovascular system were hypotension (pentobarbital anaesthetized rat), and the depression of myocardial contractility (isolated atrial strips of guinea-pig and rat). A marked stimulant activity of E. cochinchinensis on smooth muscles was observed with the smooth muscle of ileum (guinea-pig and rat) and uterus (rat), but not with that of the trachea (tracheal chain preparation of guinea-pig).

Several diester diterpenes, based upon phorbol, 4-deoxyphorbol, 4α-deoxyphorbol, 4-deoxy-5-hydroxyphorbol and 4,20-dideoxy-5-hydroxyphorbol were isolated from the fruit oil of E. indica , e.g. sapatoxin A (12-0-[n-deca-2,4,6-trienoyl]-4-deoxyphorbol-13-acetate), B (12-0-[n-deca-2,4,6-trienoyl]-4-deoxy-5-hydroxyphorbol-13-acetate), C (12-0-[n-deca-2,4,6-trienoyl]-4,20-dideoxy-5-hydroxyphorbol-13-acetate), and sapintoxins B, C and D.

Corresponding tri- and tetra-esters were produced by acetylation and mono-esters by selective hydrolysis. Twenty-six of these diterpenes were tested for production of erythema in vivo, and induction of human and rabbit platelet aggregation in vitro. The flatter shape of the AB-ring-trans compounds is necessary for interaction of phorbolesters with their receptor; the cis analogues were inactive. The tertiary C-4 hydroxy group of phorbol was not necessary for activity, although the 4-deoxy derivatives were less potent than the 4-hydroxy diterpenes. A primary hydroxy group at C-20 was essential for biological activity because the methyl and aldehyde derivatives of this position were inactive. The C-20 acetates were also inactive on platelets, but they did produce erythema, possibly because of the removal of the ester due to lipase activity in the skin. 5-Hydroxy-analogues which had undergone intramolecular hydrogen bonding had greatly reduced activities in both systems. Membrane stabilisers, phospholipase A2 and calmodulin inhibitors were antagonists for phorbol esters in platelet aggregation tests, whilst cyclo-oxygenase inhibitors and free radical scavengers had no inhibitory effects. Consequently, one electron withdrawal and free radical formation seem to play no part in the biological activity of these compounds. Furthermore, sapintoxin D exhibited a weak tumour promoting activity in the Sencar mouse skin assay.

The antihypertensive activity of geraniin and the phenolic glycoside 6-O-galloyl-D-glucose isolated from the leaves of Triadica sebiferum (L.) Small (synonyms Sapium sebiferum (L.) Roxb., Excoecaria sebifera (L.) Müll. Arg.,) has been confirmed in various test systems. Methyl gallate, methyl-3,4,5-trihydroxybenzoate, also isolated from the leaves, showed strong in vitro antiherpetic activity in the plaque reduction assay using herpes simplex virus 2.

Description

• Evergreen or briefly deciduous, usually dioecious, small to medium-sized trees up to 25(-40) m tall, occasionally shrubs; bole cylindrical, sometimes poorly shaped, up to 60(-100) cm in diameter, without buttresses; bark exuding abundant dirty white to pale yellow latex.
• Leaves arranged spirally or opposite, simple, entire or crenulate, petiole short, with two glands at the base of the blade; stipules small.
• Flowers unisexual, small, in an axillary or terminal, unisexual or bisexual raceme or spike; bracts small, biglandular; sepals (2-)3, free or shortly connate; petals absent; disk absent. Male flowers 1-3 together; stamens (2-)3; pistillode absent. Female flowers at the base of the raceme or on separate inflorescences; ovary superior, 3-locular with 1 ovule in each cell, styles simple, connate at base, recurved.
• Fruit a small, smooth, green to dark brown, dehiscing capsule with 3 bivalved parts.
• Seed without caruncle.
• Seedling with epigeal germination; cotyledons emergent; hypocotyl elongated; all leaves arranged spirally, conduplicate.

Growth and development

In Bangladesh, the mean annual diameter increment of E. agallocha in different plots in mangrove forest ranges from 0.1-0.2 cm. Inflorescences develop on younger shoots, usually from the upper of the axillary bud pair and seemingly independent of the time of shoot extension. They may remain unextended for a long time, but the final extension at anthesis is rapid. Bees are particularly common flower visitors and may be the chief pollinators. They are attracted by the yellow nectar-secreting glands at the margin of the catkin bracts. Old trees of E. indica have branches drooping to the ground. The seeds are probably dispersed by water.

Other botanical information

The generic boundaries between Excoecaria and related genera of the section Hippomaneae (e.g. Sapium) are not very clear, and various taxonomic publications treat some of the species mentioned here in various other genera. The delimitation followed here represents a conservative view. Triadica sebiferum (L.) Small (synonyms Sapium sebiferum (L.) Roxb., Excoecaria sebifera (L.) Müll. Arg.), cultivated and locally naturalized throughout the tropics, is well known for its fruits yielding tallow and oil. The root bark is used in folk medicine for its diuretic properties and reported successful in the treatment of schistosomiasis. The leaves are traditionally used in Chinese medicine for shingles.

Ecology

E. agallocha is frequently found in the drier parts of mangrove swamps and along rocky shores, and may occur in pure stands. The other species occur in primary or occasionally secondary evergreen rain forest, up to 800 m altitude. E. indica is additionally found in sago swamps, gallery forest and on the inland edge of mangrove swamps.

Propagation and planting

Excoecaria can be propagated by seed. Sown fruits of E. indica show only 5% germination in 318-413 days. In India direct sowing by broadcasting the viable seeds of E. agallocha has proven very satisfactory. The species has become more common in Indian mangrove forest because the trees coppice well, can survive repeated felling and are disliked by browsing deer. Rooting of stem cuttings of E. agallocha is promoted by application of indole butyric acid (IBA) (2500 ppm) and naphthalene acetic acid (NAA) (500 ppm). Cuttings should preferably be taken from the middle part of the stem. Treatments with 1000 ppm indole acetic acid or indole butyric acid give good growth and rooting. In vitro propagation using nodal segments is also possible. The best axillary sprouting was seen on a medium containing benzyladenine, zeatin and indole butyric acid in concentrations of 13.3, 4.65 and 1.23μM, respectively. Nodal segments from rooted cuttings and seedlings responded better than those of mature tree explants. Multiple shoot induction was complemented with efficient shoot elongation, and repeated subculture of binodal segments from axillary shoots resulted in 10-12 shoots per explant in 3 months. Rooting was achieved by growing shoots in the medium with 0.23μM indole butyric acid. Regenerated plants showed 85% survival under ex vitro conditions. E. cochinchinensis is commonly propagated by cuttings.

Diseases and pests

In North Sumatra and Peninsular Malaysia, localized mass defoliation of E. agallocha by caterpillars of the noctuid Achaea janata has been observed, but trees survived the attack.

Yield

The yield of sapintoxin D, present in unripe fruits of E. indica amounts to 5 mg per kg dried fruit.

Genetic resources and breeding

Apart from a few individuals in botanical gardens, there are no records of ex situ conservation of Excoecaria.

Prospects

The presence of substantial quantities of extremely irritant and caustic diterpene derivatives in Excoecaria will strongly limit their use as a medicinal plant. Some compounds, however, may serve as models in research.

Literature

• Blaschek, W., H√§nsel, R., Keller, K., Reichling, J., Rimpler, H. & Schneider, G. (Editors), 1998. Hagers Handbuch der Pharmazeutischen Praxis. Folgeband 2: Drogen A-K [Hagers handbook of the practice of pharmacology. Suppl. 2: drugs A-K]. 5th Edition. Springer Verlag, Berlin, Germany. pp. 676-682.
• Cheng, J.T., Chang, S.S. & Hsu, F.L., 1994. Antihypertensive action of geraniin in rats. Journal of Pharmacy and Pharmacology 46(1): 46-49.
• Kanjanapothi, D., Taesotikul, T. & Panthong, A., 1986. Pharmacological activities of Excoecaria bicolor. TISTR Bibliographical Series No 9. Abstracts on Medicinal plants in Thailand No 2. TISTR, Bangkok, Thailand. p. 49.
• Karalai, C., Wiriyachitra, P., Sorg, B. & Hecker, E., 1995. Medicinal plants of Euphorbiaceae occurring and utilized in Thailand. V. Skin irritants of the daphnane and tigliane type in latex of Excoecaria bicolor and the uterotonic activity of the leaves of the tree. Phytotherapy Research 9(7): 482-488.
• Konishi, T., Takasaki, M., Tokuda, H., Kiyosawa, S. & Konoshima, T., 1998. Anti-tumor-promoting activity of diterpenes from Excoecaria agallocha. Biological and Pharmaceutical Bulletin 21(9): 993-996.
• Purwaningsih, 1998. Excoecaria L. In: Sosef, M.S.M., Hong, L.T. & Prawirohatmodjo, S. (Editors): Plant Resources of South-East Asia No 5(3). Timber trees: Lesser-known timbers. Backhuys Publishers, Leiden, the Netherlands. pp. 229-231.

Selection of species

• Excoecaria agallocha
• Excoecaria cochinchinensis
• Excoecaria indica

Authors

• J.L.C.H. van Valkenburg