Pharmacokinetics and pharmacodynamics of irinotecan during a phase II clinical trial in colorectal cancer. Pharmacology and Molecular Mechanisms Group of the European Organization for Research and Treatment of Cancer.

PURPOSE

A pharmacokinetic study was performed during a phase II clinical trial of irinotecan (CPT-11) to confirm the pharmacokinetic profile of this drug and its metabolite and to investigate interpatient and intrapatient pharmacokinetic variations and pharmacokinetic-pharmacodynamic relationships.

PATIENTS AND METHODS

Twenty-six men and 21 women (mean age, 61 years) with metastatic colorectal cancer, performance status less than 3 (World Health Organization [WHO] scale), and normal renal and hepatic function were administered CPT-11 (350 mg/m2) by 30-minute intravenous (IV) infusion every 21 days. CPT-11 and its metabolites SN-38 and SN-38 glucuronide (SN-38G) were determined by high-performance liquid chromatography (HPLC) using fluorimetric detection.

RESULTS

The mean CPT-11 clearance and area under the concentration-time curve (AUC) were 15.2 L/h. m2 and 24,769ng. h/mL, respectively. The large difference in SN-38 and SN-38G AUCs (559 v 2,283 ng. h/mL) was suggestive of extensive glucuronidation of SN-38. Interindividual variation in the metabolic ratio ([AUCSN-38 + AUCSN-38Gl/AUCCPT-11) was marked (coefficient of variation [CV] = 51.6%] compared with intrapatient variation in this variable (CV = 32.6%). A significant relationship existed between percentage reduction in neutrophil count and the AUC of CPT-11 (r = .597, P < .001) and SN-38 (r = .559, P < .001). No relationship was identified between any pharmacokinetic parameter and delayed diarrhea or therapeutic outcome.

CONCLUSION

Interindividual variations in the metabolic ratio suggest interpatient variation in carboxylesterase activity. Furthermore, glucuronidation of SN-38 may also be in part responsible for the large interpatient variability in the total SN-38 AUC. Conversely, low intrapatient variation of this parameter was observed in this study, which indicates a lack of autoinduction of the carboxylesterase system. The relationship between neutropenia and both CPT-11 and SN-38 pharmacokinetic parameters confirms the results of previous studies.