Kleibeuker Department of Gastroenterology and Hepatology, University Hospital, PO Box 30.001, 9700 RB Groningen, the Netherlands. E-mail: email@example.com
Anthranoid laxatives are widely used laxatives of natural origin. Because of their chemical structure they are carried unabsorbed to the large bowel, where metabolism to the active aglycones takes place. These aglycones exert their laxative effect by damaging epithelial cells, which leads directly and indirectly to changes in absorption, secretion and motility. Damaged epithelial cells can be found as apoptotic bodies in the pigmented colonic mucosa, characteristic for pseudomelanosis coli. Pseudomelanosis coli is a condition caused by chronic (ab)use of anthranoid laxatives and has recently been associated with an increased risk of colorectal carcinoma. In vitro and animal studies have shown a potential role of anthranoid laxatives in both the initiation and promotion of tumorigenesis. Studies in humans have also suggested tumour promoting activities for these laxatives. Although the short-term use of these substances is generally safe, long-term use cannot be recommended.
Ever since their first introduction by the Arabs in the 19th century, anthranoids have been widely used as laxative agents. Because of their natural origin, they are generally considered harmless and are therefore a popular remedy for constipation.
The chronic use of anthranoid laxatives however, has been associated with the development of pseudomelanosis coli and more recently with a possibly increased risk of colonic carcinoma.
This review aims to give an overview of anthranoid laxatives in general and their presumed relation to colon carcinogenesis.
Anthranoid laxatives are a group of substances generally described as herbal laxatives because of their natural origin. Sennosides, the most well-known members of the anthranoid family, are obtained from the dried leaflets and pods of senna plants. These plants, Cassia acutifolia and Cassia angustifolia are mainly cultivated in Egypt and India.1 Plants such as Frangulae cortex (Rhamnus frangula), Cascara sagrada (Rhamnus purshiana) and also rhubarb (Rheum palmatum) contain other anthranoids such as frangula, aloe-emodin, chrysophanol and rhein.2,3
The basic structure for all anthranoid laxatives is an anthracene ring ( Figure 1), to which a hydroxyl or carbonyl function is substituted at C-9 and hydroxygroups at C-8 for the laxative function.3 Based on the group present at C-10, anthranoids can be divided into three groups: anthrones, anthraquinones and dianthrones (Table 1). Each of these three groups can be transformed into another by oxidative or reductive reactions ( Figure 2). Some of the most common anthranoids are shown in Figure 2. In plants, anthranoids are mostly present as sugar derivatives—the glycosides.4 One or more sugar molecules, mostly glucose or rhamnose, are bound by a β-glycosidic linkage to OH groups at position 8 and occasionally at position 1 (O-glycosides) or sometimes directly to C-10 (C-glycosides). Combinations of O- and C-glycosidic linkages within one molecule can also occur.
Table 1. . Groups of anthranoid laxatives
Due to the β-glycosidic linkage between the sugar and anthranoid structure, the molecule is protected against both hydrolysation by acid in the stomach and α-glucosidase activity in the small intestine.5 The glycosidic forms of anthranoid laxatives are therefore carried unabsorbed into the large intestine, where their metabolism takes place by the intestinal bacterial flora.6 Bacterial β-glucosidase and reductase activity transform the anthranoid laxative, which can be regarded as an inactive prodrug, into its pharmacologically active aglycon anthrone.7 The first laxative effect is seen after 4–5 h, which is the time required for transport of the anthranoid to the large bowel and its subsequent metabolization.8Figure 3 demonstrates the metabolism of sennosides A and B to rhein anthrone7 and rhein.9 Part of these substances are subsequently absorbed and converted into their corresponding glucuronide and sulphate derivatives, which are excreted in urine and bile.10 Glycosidic forms of anthranoid laxatives are not absorbed, but when they are metabolized to aglycons, these can be absorbed from the large intestine. The absorption of orally ingested aglycon forms however, also takes place elsewhere in the gastrointestinal tract,10,11 leading to a lower concentration of anthranoids in the colon. As laxative activity is dependent upon the local action of anthranoids on the colonic epithelium, higher dosages of aglycones than of glycosides are necessary to obtain the same laxative effect.12
MECHANISM OF ACTION
The laxative effect induced by anthranoids is caused by two independent mechanisms, namely, changes in colonic motility leading to an accelerated large intestinal transit, and alterations in colonic absorption and secretion resulting in fluid accumulation.13–15 Both mechanisms are dependent on an interaction of the laxative with the colonic epithelium.
Motility changes are caused indirectly by epithelial cell damage. The disruption of mucosal integrity triggers the release of cytokines, which activate immune competent cells, such as monocytes, lymphocytes and mast cells, to the release of histamine and serotonin. Histamine has been shown to elicit a contractile response in the colonic muscles of both rabbits and humans;16 and in patients with elevated serotonin concentrations due to carcinoid syndrome, colonic transit is accelerated.17 Additionally, histamine and serotonin mediate the biosynthesis of prostaglandin E2,18,19 which, in turn, also accelerates large intestinal transit,20 whereas ileal transit time is not affected.21
Besides their effect on large intestinal transit, prostaglandins may also be involved in the second mechanism of action of anthranoid laxatives.22,23 However, not all studies agree on whether prostaglandins actually affect colonic fluid absorption and secretion.24 The effect of anthranoids on secretion and absorption is principally induced by a direct interaction between the laxative and the epithelial cells.25 Anthranoids uncouple mitochondrial oxidative phosphorylation,26–28 resulting in a decreased ATP-production.26 Lower intracellular concentrations of ATP, combined with a direct inhibition of the Na+/K+ ATPase system15,28–30 on the basolateral membrane of the cell,31 leads to a breakdown of the ion gradient over the epithelial cell membrane. This prevents the absorption of sodium and water from the bowel lumen to the circulation.14
However, this can only explain the decrease in absorption and does not account for the observed secretory effect of anthranoid laxatives. The net secretion of fluid14,15 and electrolytes, including chloride,32,33 could be caused by the disruption of tight junctions between colonic epithelial cells, resulting in an enhanced permeability of the epithelium.15,34,35 The normal tissue pressure of the subepithelial space could then act as a driving force for the filtration of fluid and electrolytes through the leaky mucosa of the colon, leading to net secretion.15,36
Anthranoids thus induce a watery diarrhoea associated with bowel cramps.
Anthranoid laxatives are used in medical practice as a bowel preparation for diagnostic procedures such as a barium enema, which requires a clean colon, and sometimes for the short-term treatment of constipation. Recently, preparation for colonoscopy with sennosides was advised against, as sennosides caused an increase in mononuclear infiltration in the lamina propria of colonic mucosa which could interfere with the diagnostic interpretation of biopsies.37
The (ab)use of anthranoid laxatives, both as prescribable and as over-the-counter preparations, is probably quite high. In the UK in 1993, 2% of middle aged people (40–59 years) took laxatives more than once a week, whereas in 1981, 20–30% of people over the age of 60 used them more than once a week.38 In Germany, about 80% of people chronically abusing laxatives use anthranoid-containing drugs.39
These figures probably do not even provide the complete picture of laxative use, as many people take health products, such as herbal teas and `natural' weight-reducing products, who are unaware of their anthranoid laxative contents. Another group, predominantly women with an associated psychiatric disorder, is taking laxatives excessively and will go to great lengths to conceal their chronic laxative abuse.40
Chronic use or abuse of laxatives can lead to a number of symptoms and signs, such as abdominal pain, diarrhoea, nausea, vomiting and hypokalaemia.40 It has also been associated with the development of a cathartic colon,41–43 a condition in which the colon becomes atonic and dilated. This has been attributed by Smith to damage and loss of the myenteric plexus of the large bowel,44,45 but this could not be confirmed by others.46,47
Another condition which is often associated with chronic anthranoid laxative (ab)use is pseudomelanosis coli.48–50 In three studies which together covered about 1000 patients with pseudomelanosis coli, 95% admitted habitual laxative use.49,51,52 Of patients using anthranoid laxatives chronically, 73.4% developed pseudomelanosis coli.53 It generally evolves after 4–12 months of laxative intake and disappears 6–11 months after stopping it.48,54
Although pseudomelanosis coli can probably also be caused by other factors55,56 or other laxatives,57 the association between chronic anthranoid laxative use and pseudomelanosis coli is further supported by the fact that pseudomelanosis coli could be induced in guinea-pigs by administration of the anthraquinone danthron.58
Pseudomelanosis coli is a brownish pigmentation of the colon caused by the accumulation of dark brown pigment in macrophages of the lamina propria.59 Some authors claim that the pigmentation to be most pronounced in the rectum and decreasing toward the caecum, stopping abruptly at the ileocaecal valve,59 whereas others have stated that pseudomelanosis coli is most common in the proximal colon and much rarer in the distal parts.60 Originally, the pigment was considered to be melanin or a melanin-like substance61,62 and therefore the name melanosis coli was introduced. This was later changed into pseudomelanosis coli as the pigment proved to be more characteristic, both histochemically and ultrastructurally, for lipofuscin.50,63 The pigment was believed to originate either from macrophages64 or from the organelles of epithelial cells63 damaged by anthranoid laxatives. The damage induced by these laxatives would cause the cells to die by means of apoptosis. Apoptosis, from the Greek words apo, `off', and ptosis, `falling', is programmed cell death, which occurs normally under physiological circumstances, but can also be activated by pathological stimuli.65,66 In guinea-pigs, Walker et al. demonstrated, that anthraquinones induced the apoptosis of colonic epithelial cells, resulting in the appearance of apoptotic bodies.58 These apoptotic bodies were subsequently transformed into the lipofuscin pigment present in lysosomes of macrophages.58,67
Although pseudomelanosis coli has long been regarded as a harmless pigmentation of the colonic epithelium, more recently Siegers et al. have suggested a possible relationship between the presence of pseudomelanosis coli—and thus of chronic anthranoid laxative use—and an increased risk for the development of colonic neoplasms.68 The observed differences in the incidence of pseudomelanosis coli between patients without abnormality, patients with adenoma and patients with carcinoma in their study, might partially be due to confounding factors such as chronic constipation or dietary composition. However, the markedly increased incidence of pseudomelanosis coli in patients with colorectal carcinoma, together with the results of several in vitro and in vivo studies, makes it plausible that the chronic use of anthranoid laxatives possibly enhances the risk of colonic neoplasms.
ANTHRANOID LAXATIVES AND CARCINOGENESIS
In vitro studies
Anthranoid laxatives have been shown to influence the carcinogenic process by either initiating or promoting tumorigenesis. During initiation, mutations arise in the genetic material of the cell, for instance by the interaction of a chemical with DNA, resulting in the formation of a potentially neoplastic cell. Anthranoids have been shown to bind to DNA,69 and in vitro studies have demonstrated the mutagenic and genotoxic effects of several anthranoids.70,71 The promotion of tumorigenesis, during which a potentially neoplastic cell develops into a neoplasm, may also be influenced by anthranoids. Danthron, rhein and chrysophanol all enhance the malignant transformation of mouse fibroblasts and stimulate the growth of primary rat hepatocytes.71
Many studies have investigated the mutagenic and genotoxic effect of anthranoids in bacterial strains. Rhein, the aglycon form of sennosides,2,70 aloe emodin,73 emodin,74–76 chrysophanic acid70,76 and danthron70,76 all induced mutations in several bacterial strains. Strains TA1537, TA2637 and TA97 of Salmonella typhimurium were the most frequently mutated.73,77 These particular strains are particuarly sensitive to frame-shifting mutagens,78 which could indicate that anthranoids may act by intercalation.
Sennosides A and B failed to show mutagenicity in a bacterial test system,79 whereas crude senna leaves80 and senna81 were able to induce mutations in strains TA98 and TA102. Crude senna leaves and senna itself also contain, apart from sennosides, aglycon forms of anthranoids which can be responsible for the observed mutagenic effect. In mammalian cell test systems however, neither rhein, chrysophanic acid and danthron, nor sennosides A or B induced mutations.70,79
Senna extract did not show any carcinogenic potential when it was administered to rats over a 2-year period.82 However, tumour induction was demonstrated in rats by Mori et al., both for danthron and 1-hydroxyanthraquinone.83–85 Twenty-five large bowel, 12 liver and five gastric carcinomas developed in 29 male rats after 480 days of a diet supplemented with 1% 1-hydroxyanthraquinone.83 Only 112 days were necessary to induce four adenocarcinomas and five adenomas in the colon of 12 rats on a diet contaning 1% danthron.84 In 17 mice, nine hepatocellular adenomas and four large intestinal adenocarcinomas developed after 500 days of a diet with 0.2% danthron.85
In these studies the tumorigenic potential of anthranoid laxatives was only seen after a long-term exposure to high dosages, which is characteristic of carcinogens acting via tumour promotion. Several other studies also demonstrated a tumour-promoting effect of anthranoids. Both sennosides and 1,8-dihydroxyanthraquinone induce a dose-dependent increase in cell proliferative activity in the intestinal epithelium of rats86 and cell proliferation has been shown to be important for intestinal carcinogenesis by 1-hydroxyanthraquinone.87 1-Hydroxyanthraquinone acts synergistically with methylazoxymethanol acetate on the induction of large bowel and liver carcinogenesis in rats.88 Furthermore, sennosides, cascara and danthron all stimulate the cell proliferation of 1,2-dimehtylhydrazine-induced aberrant crypt foci, thus increasing the average number of crypts per focus in the rat colon.89 The number of aberrant crypts per focus is regarded as a predictive index of tumour incidence.90
The process of carcinogenesis is not only determined by the degree of proliferation but also by apoptosis. Apoptosis is an efficient means of deleting cells suffering from DNA damage. Any inbalance between proliferation and apoptosis therefore disturbs the regulation of cell division and can enhance the development of tumours.66 DNA damage caused by chemical carcinogens induces apoptosis at particular sites and this has been shown to correlate with subsequent tumour formation at those locations in the gut.91,92 Senna, sennosides and danthron have been demonstrated to induce apoptosis in the colonic epithelium of the guinea-pig.58,93 Only 4–6 h after the oral administration of danthron, apoptotic bodies appeared in the lamina propria.58
Few studies evaluating the possible carcinogenic effects of anthranoid laxatives have been performed in humans. Several studies could not find a relationship between anthranoid laxative use and the development of (colorectal) cancer.94–97 However, some results might be confounded by the fact that no distinction was made between anthranoids and other laxatives.95,97 Additionally, constipation itself has been mentioned as a risk factor for colorectal cancer.98,99 In these studies, no adequate distinction was made between the constipation itself and the use of laxatives as the risk factor involved. Other studies deny the relationship between prolonged bowel transit time or number of bowel evacuations and colorectal cancer risk.100,101
One report described the case of an 18-year-old woman, who had been treated for 5 years with a danthron-containing laxative, who died from a leiomyosarcoma of the small intestine.102 Nusko et al. could not find an increased incidence of colonic carcinoma in patients with pseudomelanosis coli retrospectively, but he did observe that colonic adenomas occurred more frequently in these patients.103 This is in agreement with the results of the retrospective part of the study of Siegers et al., who demonstrated an increased incidence of pseudomelanosis coli in patients with colonic adenomas but not in patients with colorectal carcinomas.68 In the prospective part of their study however, an increased incidence of pseudomelanosis coli was found in the colorectal cancer patients. Their hypothesis concerning this discrepancy between their retrospective and prospective findings was, that it could be due to the fact that pseudomelanosis coli had been incompletely documented in cases of detection of a colorectal carcinoma in the population from the retrospective part of their study.68 The possible association between pseudomelanosis coli and the development of colorectal carcinoma was also proposed by Steward et al.,104 and was further supported by the fact that an increased incidence of synchronous invasive colonic cancers has been reported in patients with pseudomelanosis coli.59
With respect to the possible mechanism of carcinogenesis, Kleibeuker et al. demonstrated that a single high dose of sennosides induced an excessive increase of proliferative activity in the epithelium of the sigmoid colon.105 These findings were confirmed in a controlled study, which showed a markedly increased proliferative activity in the entire colon.106 Increased proliferation, together with the observed decreased expression of bcl-2—indicating an inhibition of apoptosis—could both be compensatory mechanisms for a massive cell loss induced by anthranoids, possibly by apoptosis. In the colonic biopsies of patients with pseudomelanosis coli, apoptosis was indeed demonstrated.58
These results could be an indication of the tumour promoting activities of anthranoid laxatives in humans. In a recent case report, a patient was described with both pseudomelanosis coli, two metachronous colon cancers and the presence of aberrant crypt foci in the colon.107 The crypts in most of the aberrant crypt foci of this patient were large, and proliferating cells were observed in the luminal side of the crypts, which are normally seen only in colorectal adenomas. Furthermore, severe dysplasia was present in multiple aberrant crypt foci. The patient had used a variety of laxatives. The presence of pseudomelanosis coli strongly suggests that anthranoids were among them, although those reported by the patient did not seem to include them.
Anthranoid laxatives are a group of naturally occurring laxatives which are commonly used in clinical practice and as a self-medication for chronic constipation. Although the short-term use of these laxatives is generally safe, results from in vitro and animal studies suggest that they have a tumorigenic potential. Obviously, care should be taken to extrapolate the findings of animal studies to the human situation as these results were encountered in an experimental setting using very high dosages for a relatively long period of time compared to the lifespan of the animals. However, several human studies have also implicated possible carcinogenic effects after long-term administration. Therefore, these substances should be used carefully and should not be applied chronically.
Future investigations should focus on the association between anthranoid laxatives and colorectal cancer in large population based studies and on the mechanism of their possible carcinogenic effect. This latter could give us more insight into the process of colonic carcinogenesis in general.
Supported by grant RUG 94-785 of the Dutch Cancer Society and grant WS 96-64 of the Dutch Digestive Diseases Foundation.