Chemotherapy-induced mucositis : the role of gastrointestinal microflora and mucins in the luminal environment.

Abstract

Mucositis manifesting as diarrhoea is a common side effect of chemotherapy which remains poorly understood. It is one of a number of manifestations of alimentary mucositis, which affects the entire gastrointestinal tract. The exact number of patients that are affected by diarrhoea as a result of treatment is uncertain, although it is believed that approximately 10% of patients with advanced cancer will be afflicted. Despite advances in the understanding of oral and small intestinal mucositis over recent years, large intestinal mucositis, including diarrhoea, has not been well defined and the underlying mechanisms of the condition are yet to be established. The majority of the literature available concerning diarrhoea is based on clinical observations, with very little basic research existing. However, from the research conducted, it is likely that the intestinal microflora and mucins play a role in the development of chemotherapy-induced diarrhoea. This thesis will examine in detail what is known about the mechanisms of chemotherapy-induced diarrhoea (CID). Furthermore it will explore the potentially important relationship between intestinal microflora, the luminal environment and the subsequent development of chemotherapy-induced mucositis and diarrhoea. 5-Fluorouracil (5-FU) is a commonly used chemotherapy agent in clinical oncology practice. Two of its major side effects are mucositis and diarrhoea. The structure of mucins offers mucosal protection, and allows maintenance of intestinal flora by providing attachment sites and preventing bacterial overgrowth and/or penetration. Following treatment with 5-FU, we showed decreases in Clostridium spp., Lactobacillus spp. and Streptococcus spp., and an increase in Escherichia spp. in the jejunum. In the colon, 5-FU caused decreases in Enterococcus spp., Lactobacillus spp. and Streptococcus spp. Real time PCR of faecal samples showed decreasing trends in Lactobacillus spp. and Bacteroides spp., and an increasing trend in E. coli. Significant increases (p<0.05) were seen in Clostridium spp. and Staphylococcus spp. at 24 h. Goblet cell numbers decreased significantly in the jejunum from 24-72 h, with a significant increase in the percentage of cavitated goblet cells, suggesting 5-FU treatment causes significant changes in intestinal flora and mucin secretion in rats. These changes could result in systemic effects, and in particular may contribute to the development of chemotherapy-induced mucositis.

Irinotecan causes cholinergic and delayed onset diarrhoea in patients, in which β-glucuronidase produced by gut bacteria is thought to be involved. Diarrhoea was observed in treated rats, as expected, following irinotecan treatment. β-glucuronidase expression increased in the jejunum and colon. Faecal flora changed quantitatively after treatment also, with increases in E. coli, Staphylococcus spp., and Clostridium spp. (all β-glucuronidase producing), and decreases in Lactobacillus spp., Bifidobacterium spp. (both beneficial bacteria), and Bacteroides spp. (β-glucuronidase producing, major component of intestinal flora), suggesting that irinotecan-induced diarrhoea may be caused by an increase in β-glucuronidase producing bacteria. However, the increase in bacteria may also be caused by irinotecan, further exaggerating the toxicity of the drug, and emphasising the need for these specific bacteria to be therapeutically targeted for successful treatment regimens to be accomplished.

Mucus production appears to be increased after irinotecan treatment, which may contribute to the development of diarrhoea. Goblet cells were demonstrated to decrease significantly after irinotecan treatment. However, mucin secretion increased. Mucin expression changed significantly after treatment. Muc2 and Muc4 decreased significantly in the villi of the jejunum after treatment, Muc2 and Muc4 decreased significantly in the crypts. Muc2 decreased significantly in the colon. This indicates that irinotecan causes an increase in mucin secretion and a net decrease in mucin-producing goblet cells, and the expression of Muc2 and Muc4 in the gastrointestinal tract is altered following treatment. Increased mucin secretion is likely to be related to altered mucin expression, and may contribute to chemotherapy-induced diarrhoea.

To determine if the changes to the intestinal microflora caused by chemotherapy could be translated to the clinic, a pilot clinical study was carried out. Sixteen patients experiencing CID were recruited to the study with two control subjects. A large proportion of patients (75%) demonstrated a reduced anaerobic component of their faecal microflora. A reduced diversity of species was also observed in patients. The majority of patients exhibited decreases in Clostridium spp., Lactobacillus spp. and Bifidobacterium spp., whilst all patients exhibited decreases in Bacteroides spp. and Enterococcus spp. Patients receiving antibiotics did not exhibit any marked differences to patients not receiving antibiotics. This indicates that the results observed in the animal studies are clinically relevant, and further research into this area should be undertaken. CID is associated with marked changes in the intestinal microflora. These changes may result in diminished bacterial functions within the gut, altering gut function and initiating intestinal damage, resulting in the onset of diarrhoea.

In conclusion, there is clear evidence demonstrating chemotherapy treatment results in changes to the intestinal microflora and mucin secretion, which may be responsible in part for the development of severe mucositis and diarrhoea. Irinotecan toxicity may be compounded by the increase in β-glucuronidase producing bacteria. The intestinal flora of cancer patients experiencing CID is also noticeably different to that of healthy subjects. Irinotecan causes changes to mucin secretion, and the specific expression of Muc2, Muc4 and Klf4, suggesting that secretory control by the enteric nervous system may also be affected by chemotherapy. This research has extended the understanding of chemotherapy-induced mucositis and diarrhoea, complex side effects of chemotherapy. However, new areas for future research have also been identified.