Pharmacology And Therapeutic Applications Constipation

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Aloe latex possesses laxative properties and has been used traditionally to treat constipation (Benigni etal., 1962). The old practice of using aloe as a laxative drug is based on its content of anthraquinones like barbaloin, which is metabolised to the laxative aloe-emodin, isobarbaloin and chrysophanic acid (Leung, 1980; Blumenthal, 1998). The term 'aloe' (or 'aloin') refers to a crystalline, concentrated form of the dried aloe latex. In addition, aloe latex contains large amounts of a resinous material. Following oral administration the stomach is quickly reached and the time required for passage into the intestine is determined by stomach content and gastric emptying rate. Glycosides are probably chemically stable in the stomach (pH 1—3) and the sugar moiety prevents their absorption into the upper part of the gastrointestinal tract and subsequent detoxification in the liver, which protects them from breakdown in the intestine before they reach their site of action in the colon and rectum (Breimer and Baars, 1976; van Os, 1976). Once they have reached the large intestine the glycosides behave like pro-drugs, liberating the aglycones (aloe-emodin, rhein-emodin, chyrosophanol, etc.) that act as the laxatives (Figure 9.3).

The metabolism takes place in the colon, where bacterial glycosidases are able to cleave the C-glycosidic bond of glycosides (de Witte and Lemli, 1990). Their transformation into the active aglycones is carried out by the Eubacterium species of the human intestinal flora (Che etal, 1991). Aloe-emodin is metabolised quickly; therefore its bioavailability is low, perhaps <10%, and the half-life is approximately 48—50 hours. Aloe-emodin and rhein are poorly absorbed. In spite of this, rhein has been recovered in urine, in breast milk, in brown fat, muscle, bones and the gonads, although in very low concentrations. In human volunteers, aloe was as active as aloin. Since aloe contains only about 20% aloin, it looks as though the latter is five times more active than aloes. This means that solubility or resorption differences as influenced by accompanying substances can always play a role. It is also important to remember that the resins in aloes are as active as anthraquinones (Ramstad, 1995); like aloe, the resins require the presence of bile acids in order to act. Aglycones like aloe-emodin and rhein act syner-gistically (Yagi and Yamauchi, 1999), evoking secretory and motility changes in the gut.

Although there is no doubt that aloe exerts its action on the colonic mucosa, its mechanism of action is still unclear. It is believed that aloe, or its active ingredient aloe-emodin-9-anthrone, acts by disturbing the equilibrium between the absorption of water from the intestinal lumen via active sodium transport (Ishii etal, 1990) and the

Aloe Vera Mechanism Action
Figure 9.3 An exemplification of aloe constituents metabolism.

secretion of water into the lumen by a prostaglandin-dependent mechanism (Collier etal., 1976; Capasso etal, 1983). Other probable mechanisms could be an increased mucosal permeability of the epithelial cells of the colon, a rise of the level of cAMP in the enterocytes and a stimulation of nerve endings (Canigueral and Vila, 1993; Capasso and Gaginella, 1997; Capasso etal, 2000). Platelet-activating factor (PAF) could also contribute to the laxative effect, as aloe-emodin stimulates the release of PAF in human ileal and colonic mucosa (Tavares etal, 1996) (Table 9.1).

Furthermore, aloe, in contrast to other antraquinones drugs (cascara and senna), inhibits calcium-dependent nitric oxide synthase (NOS) activity in the rat colon, aloin being the active ingredient responsible of this activity (Izzo etal, 1999). Taken in doses of 0.25 mg, aloe causes laxative action after 6—12 hours with loose bowel movements accompanied by abdominal pain. Among the anthraquinone drugs, aloe possesses the most potent action (Table 9.2) and has an effect persisting over several days (Grindlay and Reynolds, 1986). Excessive doses may cause gas and cramps.

These adverse effects, occurring after an overdosage, have reduced its use as a laxative in recent years in some countries while in others it is still widely used. Aloe is also occa-sionally used in association with coleretic and cholagogue drugs to solve atonic

Table 9.1 Mechanisms of action of aloe and its components.

Properties

Mechanisms

Laxative

Increase mucosol permeability.

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