Effects of inflammation on irinotecan pharmacokinetics and development of a best-fit PK model

Development of a physiologically based pharmacokinetic model to predict irinotecan disposition during inflammation

Irinotecan, a first-line chemotherapy for gastrointestinal (GI) cancers has been causing fatal toxicities like bloody diarrhea and steatohepatitis for years. Irinotecan goes through multiple-step drug metabolism after injection and one of its intermediates 7-ethyl-10-hydroxy-camptothecin (SN-38) is responsible for irinotecan side effect. However, it is unclear what is the disposition kinetics of SN-38 in the organs subjected to toxicity. No studies ever quantified the effect of each enzyme or transporter on SN-38 distribution. In current study, we established a new physiologically based pharmacokinetic (PBPK) model to predict the disposition kinetics of irinotecan. The PBPK model was calibrated with in-house mouse pharmacokinetic data and evaluated with external datasets from the literature. We separated the contribution of each parameters in irinotecan pharmacokinetics by calculating the normalized sensitivity coefficient (NSC). The model gave robust prediction of SN-38 distribution in GI tract, the site of injury. We identified that bile excretion and UDP-glucuronosyltransferases (UGT) played more important roles than fecal excretion and renal clearance in SN-38 pharmacokinetics. Our NSC showed that the impact of enzyme and transporter on irinotecan and SN-38 pharmacokinetics evolved when time continued. Additionally, we mapped out the effect of inflammation on irinotecan metabolic pathways with PBPK modelling. We discovered that inflammation significantly increased the blood and liver exposure of irinotecan and SN-38 in the mice receiving bacterial endotoxin. Inflammation suppressed UGT, microbial metabolism but increased fecal excretion. The present PBPK model can serve as an efficacious and versatile tool to quantitively assess the risk of irinotecan toxicity.

Amentoflavone from Selaginella tamariscina as a potent inhibitor of gut bacterial β-glucuronidase: Inhibition kinetics and molecular dynamics stimulation

Gut bacterial β-glucuronidase (GUS) plays a pivotal role in the metabolism and reactivation of a vast of glucuronide conjugates of both endogenous and xenobiotic compounds in the gastrointestinal tract of human, which has been implicated in certain drug-induced gastrointestinal tract (GI) toxicity in clinic. Inhibitors of gut microbial GUS exhibited great potentials in relieving the drug-induced GI toxicity. In this study, Selaginella tamariscina and its major biflavonoid amentoflavone (AMF) were evaluated for their inhibitory activity against Escherichia coli GUS. Two selective probe substrates for GUS (a specific fluorescent probe substrate for GUS, DDAOG and a classical drug substrate for GUS, SN38G) were used in parallel for charactering the inhibition behaviors. Both the extract of S. tamariscina and its major biflavonoid AMF displayed evident inhibitory effects on GUS, and the IC₅₀ values of AMF against GUS mediated DDAOG and SN-38G hydrolysis were 0.62 and 0.49 μM, respectively. Inhibition kinetics studies indicated that AMF showed mixed type inhibition for GUS-mediated DDAOG hydrolysis, while displayed competitive type inhibition against GUS-mediated SN-38G hydrolysis, with the K_i values of 0.24 and 1.25 μM, respectively. Molecular docking studies and molecular dynamics stimulation results clarified the role of amino acid residues Leu361, Ile363, and Glu413 in the inhibition of AMF on GUS. These results provided some foundations for the potential clinical utility of S. tamariscina and its major biflavonoid AMF for treating drug-induced enteropathy.