Molecular Insights into Microbial β-Glucuronidase Inhibition to Abrogate CPT-11 Toxicity

Bacterial beta-glucuronidases expressed by the symbiotic intestinal microbiota appear to play important roles in drug-induced epithelial cell toxicity in the gastrointestinal (GI) tract. For the anticancer drug CPT-11 (irinotecan) and the nonsteroidal anti-inflammatory drug diclofenac, it has been shown that removal of the glucuronide moieties from drug metabolites by bacterial beta-glucuronidases in the GI lumen can significantly damage the intestinal epithelium. Furthermore, selective disruption of bacterial beta-glucuronidases by small molecule inhibitors alleviates these side effects, which, for CPT-11 {7-ethyl-10-[4-(1-piperidino)-1-piperidino]}, can be dose limiting. Here we characterize novel microbial beta-glucuronidase inhibitors that inhibit Escherichia coli beta-glucuronidase in vitro with K-i values between 180 nM and 2 mu M, and disrupt the enzyme in E. coli cells, with EC50 values as low as 300 nM. All compounds are selective for E. coli beta-glucuronidase without inhibiting purified mammalian beta-glucuronidase, and they do not impact the survival of either bacterial or mammalian cells. The 2.8 angstrom resolution crystal structure of one inhibitor bound to E. coli beta-glucuronidase demonstrates that it contacts and orders only a portion of the bacterial loop present in microbial, but not mammalian, beta-glucuronidases. The most potent compound examined in this group was found to protect mice against CPT-11-induced diarrhea. Taken together, these data advance our understanding of the chemical and structural basis of selective microbial beta-glucuronidase inhibition, which may improve human drug efficacy and toxicity.