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       Siamese Neem Bark is the dried stem bark of Azadirachta indica A. Juss. var. siamensis Valeton (Family Meliaceae), Herbarium Specimen Number: DMSC 5311, Crude Drug Number: DMSc 1231.

Constituents Siamese Neem Bark contains tetranortriterpenoids (e.g., nimbin and 6-deacetylnimbin) and phenolics (e.g., catechin and epicatechin).

Description of the plant (Fig. 1) Evergreen tree, up to 20 m tall; bark greyish brown, vertically striated. Leaves pinnately compound, spirally arranged, young leaves usually reddish; exstipulate; rachis 20 to 40 cm long; petiole 6 to 14 cm long, slender, swollen at base; leaflets 6 to 12 pairs, opposite or subopposite, leaflet at tip sometimes reduced; exstipellate; petiolule 4 to 7 mm long, slender, glabrous; lamina oblanceolate or broadly falcate, 6.5 to 12.5 cm long, 2.1 to 5.2 cm wide, apex acuminate, base oblique, margin serrate or serrateundulate, coriaceous, glabrous, lateral nerves 8 to 13 pairs, slender, prominent, veinlets reticulate, fainted. Inflorescence panicle, axillary or terminal, up to 30 cm long, appear before leaves. Flower white, bisexual, 1 to 1.5 cm in diameter; bracteole scaly; pedicel about 2 mm long; sepals 5, free, ovate; petals 5, free, obovate-oblong, hairy, imbricate in bud; stamens 10, connated into cylindrical staminal tube, about 6 mm long, terminally 10-lobed, glabrous outside, inside pubescent, anthers 10, attached at base of lobes inside, slightly exserted; ovary superior, globose, 3-loculed, ovules 2 per locule, placentation axile, style slender, elongate, stigma slightly 3-lobed. Fruit a drupe, oblong-ovoid, 1.5 to 2 cm long, 0.5 to 1 cm wide, green when young, yellow when ripe, shiny. Seeds 1 or 2.

Description Odour, unpleasantly characteristic; taste, bitter.

       Macroscopical (Fig. 1) Irregular pieces of stem bark, varying in shape and size, mostly curved, hard; externally greyish brown to dark brown, rough, irregularly cracked; internally yellowish brown to reddish brown, with fine longitudinal striations.

       Microscopical (Figs. 2a, 2b) Transverse section of the stem bark shows periderm and phloem tissue. Periderm: lenticels, rectangular cork cells with brown substance and groups of sclereids some containing reddish brown to brown substance. Phloem tissue: polygonal parenchyma some containing starch grains or prismatic crystals, groups of sclereids some containing reddish brown to brown substance, bast fibres some containing prismatic crystals, and phloem rays.
       Siamese Neem Bark in powder possesses the diagnostic microscopical characters of the unground drug. Rectangular cork cells with reddish brown to brown substance, sclereids containing reddish brown to brown substance, phloem rays containing starch grains, and fibres with prismatic sheath are characteristic.

Packaging and storage Siamese Neem Bark shall be kept in well-closed containers, protected from light, and stored in a dry place.

Identification

       A. Reflux 2 g of the sample, in fine powder, with 20 mL of ethyl acetate for 1 hour and filter (solution 1). Evaporate 5 mL of solution 1 to dryness and dissolve the residue in a few drops of acetic anhydride. Add slowly a few drops of sulfuric acid: a reddish brown colour is produced.

       B. Carry out the test as described in the “Thin-Layer Chromatography” (Appendix 3.1), using silica gel 60 F254 as the coating substance and a mixture of 49 volumes of hexane, 49 volumes of ethyl acetate, and 2 volumes of methanol as the mobile phase and allowing the solvent front to ascend 10 cm above the line of application. Apply separately to the plate as bands of 8 mm, 10 µL each of solutions (A) and (B). Prepare solution (A) by evaporating 5 mL of solution 1 to dryness and dissolve the residue in 1 mL of dichloromethane. For solution (B) dissolve 1 mg of 6-deacetylnimbin in 1 mL of dichloromethane. After removal of the plate, allow it to dry in air and spray with a 10 per cent v/v solution of sulfuric acid in ethanol and heat at 110° for 5 minutes; the band due to 6-deacetylnimbin is brownish black (hR_f value about 40). One pink and five brown bands are observed (Fig. 3).

Loss on drying Not more than 10.0 per cent w/w after drying at 105° to constant weight (Appendix 4.15).

Foreign matter Not more than 2.0 per cent w/w (Appendix 7.2).

Total ash Not more than 12.0 per cent w/w (Appendix 7.7).

Ethanol-soluble extractive Not less than 3.0 per cent w/w (Appendix 7.12).

Water-soluble extractive Not less than 5.0 per cent w/w (Appendix 7.12)