A Physiologically Based Pharmacokinetic-Pharmacodynamic Model for Capecitabine in Colorectal Cancer Rats: Simulation of Antitumor Efficacy at Various Administration Schedules.

BACKGROUND AND OBJECTIVES Capecitabine is an oral prodrug of 5-fluorouracil and is widely used for colorectal cancer (CRC) treatment. However, knowledge of its antitumor efficacy after modification of the dosing schedule is insufficient. The aim of this study was to predict the antitumor efficacy of capecitabine using a physiologically based pharmacokinetic-pharmacodynamic (PBPK-PD) model based on metabolic enzyme activities. METHODS CRC model rats were administrated 180 mg/kg of capecitabine for 2 weeks. Blood samples were collected at 0, 0.25, 0.5, 0.75, 1, 1.5, 2, 3, 4, 6, and 8 h following capecitabine administration. Plasma concentrations of capecitabine and its metabolites were measured on days 1, 7, and 14. Metabolic enzyme activities were determined in vitro using the liver and small intestine of the CRC model rats. A PBPK-PD model was developed based on metabolic enzyme activities. The antitumor efficacy of capecitabine after regimen modification was simulated using the PBPK-PD model. RESULTS Capecitabine antitumor efficacy was dose-dependent. A dose of > 500 μmol/kg was needed to inhibit tumor growth. After capecitabine regimen modification, a 1-week postponement of capecitabine administration was more efficacious than a reduction in the dosage to 80%. CONCLUSIONS The PBPK-PD model could simulate the antitumor efficacy at various capecitabine administration schedules. PBPK-PD models can contribute to the development of an appropriate CRC chemotherapy regimen with capecitabine.