Cascara is the dried bark of the cascara buckthorn Rhamnus purshianus (Rhamnaceae), a small tree native to the forests of the Pacific coast of North America. Most of the drug material is gathered from wild trees in Oregon, Washington, and British Columbia. Trees are felled and the bark is stripped from the trunk and branches, then dried. The fresh bark is unsuitable for drug use, causing griping and nausea, and thus the bark is stored for at least a year before being processed. During this time, enzymic hydrolysis and oxidation modify the anthraquinone-based constituents and thus the cathartic activity. Cascara preparations are mainly formulated from extracts of the bark.
Cascara bark contains about 6-9% of anthracene derivatives, 80-90% of which are anthrone C-glycosides. The major constituents are cascarosides A and B (Figure 3.33), which contain both 0- and C-glucoside linkages, and represent a pair of optical isomers differing only in the stereochemistry of the C-glucoside bond. These have a substitution pattern analogous to aloe-emodin (Figure 3.30) and oxidative hydrolysis (e.g. aq HNO3 or H2O2/HCl) liberates aloe-emodin. Acid hydrolysis does not cleave the C-glucose linkage, and instead generates barbaloin (Figure 3.33), a mixture of two diastereoisomeric forms, which have been named aloin A and aloin B. It is likely that during any chemical manipulation, the two forms may interconvert via the anthranol tautomer (Figure 3.33). Similar components in the bark, though usually present in smaller amounts than cascarosides A and B, are cascarosides C and D (Figure 3.33). These are also a pair of diastereoisomers, and have a substitution pattern analogous to chrysophanol (Figure 3.30). Hydrolysis of the 0-glucose linkage yields chrysaloin, sometimes referred to as deoxybarbaloin. Barbaloin and chrysaloin are also found in the bark, and are thought to be breakdown products formed by enzymic hydrolysis of the cascarosides. Other compounds identified in the bark include simple anthraquinones and their 0-glycosides, and some dianthrone derivatives.
The principal purgative activity originates from the cascarosides, the C-glycosides barbaloin and chrysaloin being less active when taken orally. As with the sennosides, the actual purgative agent is produced by the action of intestinal flora, and the cascarosides are transformed into aloe-emodin anthrone (Figure 3.33). Cascara has a similar pharmacological action to senna, i.e. it stimulates peristalsis of the large intestine, and has found major use in the correction of habitual constipation. It has a stronger effect than senna, however, and its routine usage is not now recommended.
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