Clinical Pharmacokinetics of Irinotecan and Its Metabolites: A Population Analysis

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Abstract
PURPOSE: To build population pharmacokinetic (PK) models for irinotecan (CPT-11) and its currently identified metabolites. PATIENTS AND METHODS: Seventy cancer patients (24 women and 46 men) received 90-minute intravenous infusions of CPT-11 in the dose range of 175 to 300 mg/m2. The PK models were developed to describe plasma concentration profiles of the lactone and carboxylate forms of CPT-11 and 7-ethyl-10-hydroxycamptothecin (SN-38) and the total forms of SN-38 glucuronide (SN-38G), 7-ethyl-10-[4-N-(5-aminopentanoic acid)-1-piperidino]-carbonyloxycamptothecin (APC), and 7-ethyl-10-[4-amino-1-piperidino]-carbonyloxycamptothecin (NPC) by using NONMEM. RESULTS: The interconversion between the lactone and carboxylate forms of CPT-11 was relatively rapid, with an equilibration half-life of 14 minutes in the central compartment and hydrolysis occurring at a rate five times faster than lactonization. The same interconversion also occurred in peripheral compartments. CPT-11 lactone had extensive tissue distribution (steady-state volume of distribution [Vss], 445 L) compared with the carboxylate form (Vss, 78 L, excluding peripherally formed CPT-11 carboxylate). Clearance (CL) was higher for the lactone form (74.3 L/h) compared with the carboxylate form (12.3 L/h). During metabolite data modeling, goodness of fit indicated a preference of SN-38 and NPC to be formed out of the lactone form of CPT-11, whereas APC could be modeled best by presuming formation from CPT-11 carboxylate. The interconversion between SN-38 lactone and carboxylate was slower than that of CPT-11, with the lactone form dominating at equilibrium. The CLs for SN-38 lactone and carboxylate were similar, but the lactone form had more extensive tissue distribution. CONCLUSION: Plasma data of CPT-11 and metabolites could be adequately described by this compartmental model, which may be useful in predicting the time courses, including interindividual variability, of all characterized substances after intravenous administrations of CPT-11.