Population pharmacokinetics and pharmacodynamics of irinotecan (CPT-11) and active metabolite SN-38 during phase I trials.

BACKGROUND

Irinotecan (CPT-11) is a novel water-soluble camptothecin derivative selected for clinical testing based on its good in vitro and in vivo activity in various experimental systems, including pleiotropic drug-resistant tumors. Its mechanism of action appears mediated through topoisomerase I inhibition. The purpose of this study was to describe CPT-11 and active metabolite SN-38 population pharmacokinetics, examine patient characteristics that may influence pharmacokinetics, and to investigate pharmacokinetic-pharmacodynamic relationships that may prove useful in the future clinical management of this drug.

PATIENTS AND METHODS

As part of 3 Phase I studies including 235 patients, pharmacokinetics of CPT-11 and metabolite SN-38 were determined in 107 patients. CPT-11 was administered as a 30-min i.v. infusion according to 3 different schedules: daily for 3 consecutive days every 3 weeks, weekly for 3 weeks, and once every 3 weeks. Patients characteristics were the following: median age 53 years; 62 men, 45 women; 105 caucasians, 2 blacks; performance status was 0-1 in 96 patients; tumor sites were predominantly colon, rectum, head and neck, lung, ovary and breast; with the exception of 6 patients, all had been previously treated with surgery, chemotherapy and/or radiotherapy. CPT-11 and metabolite SN-38 were simultaneously determined by HPLC using fluorescence detection. Pharmacokinetic parameters were determined using model-independent and model-dependent analyses.

RESULTS

168 pharmacokinetic data sets were obtained in 107 patients (97 first courses, 43 second courses, 23 third courses, 4 fourth courses, and 1 fifth course). Rebound concentrations of CPT-11 were frequently observed at about 0.5 to 1 h following the end of the i.v. infusion, which is suggestive of enterohepatic recycling of the drug. Model-independent analysis yielded the following mean population pharmacokinetic parameters for CPT-11: a terminal half-life of 10.8 h, a mean residence time (MRT) of 10.7 h, a volume of distribution at steady state (Vdss) of 150 L/m2, and a total body clearance of 14.3 L/m2/h. Model-dependent analysis disclosed a CPT-11 plasma disposition as either biphasic or triphasic with a mean terminal half-life of 12.0 h. The volume of distribution Vdss (150 L/m2) and total body clearance (14.8 L/m2/h) yielded almost identical values to the above model-independent analysis. The active metabolite SN-38 presented rebound concentrations in many courses at about 1 h following the end of the i.v. infusion which is suggestive of enterohepatic recycling. The mean time at which SN-38 maximum concentrations was reached was at 1 h since the beginning of the 0.5 h infusion (i.e., 0.5 h post i.v.). SN-38 plasma decay followed closely that of the parent compound with a mean apparent terminal half-life of 10.6 h. Mean 24 h CPT-11 urinary excretion represented 16.7% of the administered dose, whereas metabolite SN-38 recovery in urine was minimal (0.23% of the CPT-11 dose). The number of CPT-11 treatments did not influence pharmacokinetic parameters of either the parent compound or metabolite SN-38. Although CPT-11 pharmacokinetics presented an important interpatient variability, both CPT-11 maximum concentrations (Cmax) and the CPT-11 area under the plasma concentration versus time curves (AUC) increased proportionally and linearly with dosage (Cmax, r = 0.78, p < 0.001); CPT-11 AUC, r = 0.88, p < 0.001). An increase in half-life and MRT was observed at higher dosages, although this did not influence the linear increase in AUC as a function of dose. The volume of distribution at steady state (Vdss) and the total body clearance (CL) were not affected by the CPT-11 dose. Metabolite SN-38 AUC increased proportionally to the CPT-11 dose (r = 0.67, p < 0.001) and also with the parent compound AUC (r = 0.75, p < 0.001) (ABSTRACT TRUNCATED)