Fluoropyrimidine Bioactivation and Metabolism by the Gut Microbiome

Abstract

Despite decades of research into why populations of cancer patients differentially respond to the same treatment, variation in response to therapy regimens remains a significant challenge in the oncology field. However, our understanding of the causal mechanisms of interpatient variability remains lacking; up to 35% of patients require dose adjustments and 10% of patients are removed from the fluoropyrimidine capecitabine altogether. This research seeks to further our understanding of the microbiome’s role in fluoropyrimidine treatment by identifying gut bacterial strains responsible for capecitabine activation as well as biochemically characterizing drug inactivating enzymes from the human gut microbiome. Growth curve data show that knockouts of bacterial enzymes homologous to mammalian enzymes known to be involved in capecitabine metabolism increase bacterial resistance to capecitabine. Additionally, an unbiased transposon mutagenesis screen identifies enzymes known to increase resistance as well as enzymes both within and outside of nucleotide metabolism. These two pieces of data suggest that bacteria have the capacity to metabolize capecitabine, upending the paradigm that activation of this prodrug happens exclusively in the liver and tumor. Confirmation of these metabolites with high resolution triple quadrupole linear ion trap mass spectrometry is ongoing. Separately, previous studies indicate that gut bacteria are capable of degrading 5‐fluorouracil, however it is not clear how efficient these pathways are or to what extent they occur in the host. Enzyme kinetics of homologues of this enzyme within the microbiome show that degradative reaction for uracil is essentially irreversible. Additionally, this enzymatic characterization has identified the non‐enzymatic enantiomer of the degraded fluoropyrimidine as a potential inhibitor for this enzyme, which might allow for co‐dosing and inhibition of bacterial metabolism. Metagenomic sequencing of the degradative enzymes within patient samples is ongoing and will potentially reveal the main drivers of this activity in patients. These avenues of research will uncover new bacterial metabolism pathways of xenobiotics, compare these metabolic pathways between microbiota and host, and have the potential to lead to new advances in disrupting microbial metabolism of chemotherapeutics.