Aloes and rhubarb have found considerable use as purgatives in the past, but they both have a rather drastic action and their use for this purpose has largely been abandoned.
Aloes consists of the dried juice from the leaves of various Aloe species (Liliaceae/ Aloeaceae), including A. ferox (Cape aloes), A. barbadensis (Curacao aloes), and A. perryi (Socotrine aloes). The dark brown-black solid extract is extremely bitter, and contains 10-30% anthracene derivatives, the main component of which is barbaloin (Figure 3.33). Aloinosides A and B (Figure 3.33) are present in some varieties. Large amounts of resinous material form the bulk of the extract. Aloes is still used as a pharmaceutical aid in Compound Benzoin Tincture. The fresh mucilaginous gel obtained from Aloe species, particularly Aloe vera (= A. barbadensis), is held to assist wound healing, and is also widely used in skin cosmetics for its moisturizing and emollient properties. This material, mainly carbohydrate in nature (pectins and glucomannans), does not contain anthraquinone derivatives.
Rhubarb consists of the dried rhizome and root of Rheum officinale, R. palmatum, and other Rheum species (Polygonaceae). This contains 3-7.5% anthracene derivatives, mainly in the form of mono- and di-O-glucosides of rhein, physcion, and chrysophanol. Aglycones, especially rhein, are also present, and dianthrone derivatives have also been characterized. A high proportion of tannin-like materials gives rhubarb astringent as well as purgative properties. The common rhubarb cultivated for culinary use is Rheum rhaponticum, a species containing similar anthraquinone derivatives to the drug material, but which was not officially acceptable. In common with other Rheum species, this plant is considered poisonous due to the high concentration of oxalic acid present in the leaf (though not in the stem, which is edible). Toxic effects result from hypocalcaemia caused by removal of calcium from the bloodstream by formation of the insoluble calcium oxalate.
Dantron (danthron; 1,8-dihydroxyanthraquinone) (Figure 3.34) is known as a natural product, but for drug use is produced synthetically. It is prescribed to relieve constipation in geriatric and terminally ill patients. Dithranol (1,8-dihydroxyanthrone) is used as topical agent to treat troublesome cases of psoriasis. Diacetylrhein is marketed in some countries for the treatment of osteoarthritis.
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