Disposition of irinotecan and SN-38 following oral and intravenous irinotecan dosing in mice

The present study was conducted to quantitate the disposition of irinotecan lactone and its active metabolite SN-38 lactone in mice following oral and intravenous administration, and to evaluate the systemic exposure of irinotecan lactone and SN-38 lactone associated with antitumor doses of irinotecan lactone in mice bearing human tumor xenografts. Nontumor-bearing mice were given a single oral or intravenous irinotecan dose (5, 10, 40, or 75 mg/kg), and serial plasma samples were subsequently obtained. Irinotecan and SN-38 lactone plasma concentrations were measured using an isocratic HPLC assay with fluorescence detection. The disposition of intravenous irinotecan lactone was modeled using a two-compartment pharmacokinetic model, and the disposition of oral irinotecan and SN-38 lactone was modeled with noncompartmental methods. Irinotecan lactone showed biphasic plasma disposition following intravenous dosing with a terminal half-life ranging between 1.1 to 3 h. Irinotecan lactone disposition was linear at lower doses (5 and 10 mg/kg), but at 40 mg/kg irinotecan lactone clearance decreased and a nonlinear increase in irinotecan lactone AUC was observed. The steady-state volume of distribution ranged from 19.1 to 48.1 l/m². After oral dosing, peak irinotecan and SN-38 lactone concentrations occurred within 1 h, and the irinotecan lactone bioavailability was 0.12 at SN-38 lactone in murine plasma at 1000 ng/ml was 3.4 ± 0.67%, whereas at 100 ng/ml the percent unbound was 1.18 ± 0.14%. Irinotecan and SN-38 lactone AUCs in micebearing human neuroblastoma xenografts were greater than in nontumour-bearing animals. Systemic exposure to unbound SN-38 lactone in nontumor- bearing animals after a single oral irinotecan dose of 40, 10, and 5 mg/kg was 28.3, 8.6, and 2.9 ng h/ml, respectively. Data from the present study provide important information for the design of phase I studies of oral irinotecan.