Lack of Evidence that Ursodeoxycholic Acid’s Effects on the Gut Microbiome Influence Colorectal Adenoma Risk

Objective We previously reported that Ursodeoxycholic acid (UDCA), a therapeutic bile acid, reduces risk for advanced colorectal adenoma in men but not women. Interactions between the gut microbiome and fecal bile acid composition as a factor in colon cancer neoplasia have been postulated but evidence is limited to small cohorts and animal studies. Design Using banked stool samples collected as part of a phase III randomized clinical trial of UDCA for the prevention of colorectal neoplasia, we compared change in the microbiome composition after 3 years intervention in a subset of participants randomized to 8–10 mg/kg of body weight UDCA (n=198) to placebo (n=203). UDCA effects on the microbiome, sex and adenoma outcome were investigated. Results Study participants randomized to UDCA experienced compositional changes in their microbiome that were statistically more similar to other individuals in the UDCA arm than to those in the placebo arm. This change reflected an UDCA-associated shift in microbial community distance metrics (P <0.001), independent of sex, with no evidence of UDCA effect on microbial richness (P > 0.05). These UDCA-associated shifts in microbial community distance metrics from baseline to end-of-study were not associated with risk of any or advanced adenoma (all P> 0.05) in men or women. Conclusion Despite a large sampling of randomized clinical trial participants, daily UDCA use only modestly influenced the relative abundance of microbial species in stool with no evidence for effects of UDCA on stool microbial community composition as a modifier of colorectal adenoma risk. SUMMARY What is already known about this subject? Ursodeoxycholic acid (UDCA) is a therapeutic bile acid used in the treatment of primary biliary cirrhosis (PBC) and investigated for anti-cancer activity in the colon In humans, UDCA is produced in the colon from the conjugation of primary bile acids by intestinal bacteria Intestinal bacteria play a critical role in human intestinal health and disease including a hypothesized role in the development of colorectal cancer. UDCA was found to reduce the risk of more advanced colorectal adenoma with effects present in men but not women. Therapeutic UDCA was recently shown to reduce the extent of bacterial dysbiosis in patients with PBC What are the new findings? Among a population of patients with colorectal adenoma, low dose oral UDCA taken daily produced modest changes in fecal bacterial composition UDCA associated changes in the gut microbiome were similar in men and women. UDCA associated changes in the gut micobiome were not associated with risk of any or advanced colorectal adenoma in the patient population. How might it impact on clinical practice in the foreseeable future? These findings confirm effects of oral UDCA on the microbiome that may be beneficial for patients with PBC. These findings suggest that the anti-cancer effects of UDCA for colorectal adenoma prevention are not due to major effects of UDCA on the gut microbiome.