Irinotecan is a first-line treatment for colorectal cancer and the prodrug of 7-ethyl-10-hydroxy-camptothecin (SN-38). However, its fatal gastrointestinal (GI) toxicity raises serious concern. In liver, irinotecan generates its inactive metabolite, SN-38G via UDP-glucuronosyltransferase (UGT)1A1. Subsequently, SN-38G is excreted into GI tract where it is reactivated by microbiome to yield the toxic metabolite, SN-38. Activation of toll-like receptor (TLR)/myeloid differentiation primary response 88 (MyD88) by bacterial endotoxin decreases drug-metabolizing enzymes. In this study, we treated C57BL6/J mice with 50 mg/kg irinotecan once daily until observing grade 4 diarrhea. Mice were sacrificed on day0, day2 and day8. Based on the finding in C57BL6/J mice, we repeated the treatment in Tlr2-/-, Tlr4-/- and Myd88-/- mice to determine the impact of inflammation on UGT metabolism. Our toxicity study in C57BL6/J mice showed that mice started bloody diarrhea after 6 days' injection of irinotecan. Ugt1a1 expression in GI tract started decreasing after 24h since first dose, before the onset of diarrhea. In Tlr4-/- and Myd88-/- mice, no Ugt1a1 reduction was observed in distal GI tract after irinotecan injection. In Tlr2-/- mice, intestinal Ugt1a1 expression was down-regulated. Our results indicate that after two doses of irinotecan, mice started losing capability of detoxifying SN-38. TLR4 plays more important role in Ugt1a1 reduction than TLR2, despite that TLR2 and TLR4 share MyD88 as common adaptor protein. We concluded that irinotecan reduced intestinal Ugt1a1 via TLR4/MyD88 pathway, which eventually triggers the onset of diarrhea. Our finding unveils a novel mechanism underlying irinotecan-induced diarrhea and provides a new direction to prevent chemotherapy side effect.
Keywords: Drug metabolism; Inflammation; Irinotecan; Toll-like receptor; Toxicity; UDP-glucuronosyltransferase1A1.
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