Alleviating Cancer Drug Toxicity by Inhibiting a Bacterial Enzyme

TLDR

CPT‑11 causes severe diarrhea in colon cancer patients due to reactivation by symbiotic bacterial β‑glucuronidases in the gut. The study aimed to inhibit these enzymes while preserving commensal bacteria essential for human health. Crystal structures revealed that selectivity arises from a loop unique to bacterial β‑glucuronidases. High‑throughput screening identified potent bacterial β‑glucuronidase inhibitors that selectively block the bacterial enzyme without affecting the mammalian ortholog, do not kill bacteria or harm mammalian cells, and when orally administered protect mice from CPT‑11‑induced toxicity, showing that targeting microbial enzymes can enhance chemotherapeutic efficacy.

Abstract

The dose-limiting side effect of the common colon cancer chemotherapeutic CPT-11 is severe diarrhea caused by symbiotic bacterial β-glucuronidases that reactivate the drug in the gut. We sought to target these enzymes without killing the commensal bacteria essential for human health. Potent bacterial β-glucuronidase inhibitors were identified by high-throughput screening and shown to have no effect on the orthologous mammalian enzyme. Crystal structures established that selectivity was based on a loop unique to bacterial β-glucuronidases. Inhibitors were highly effective against the enzyme target in living aerobic and anaerobic bacteria, but did not kill the bacteria or harm mammalian cells. Finally, oral administration of an inhibitor protected mice from CPT-11-induced toxicity. Thus, drugs may be designed to inhibit undesirable enzyme activities in essential microbial symbiotes to enhance chemotherapeutic efficacy.