Sensitive Determination of the Carboxylate and Lactone Forms of the Novel Antitumor Drug Irinotecan and Its Active Metabolite in Plasma by HPLC

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Abstract
A sensitive high-performance liquid chromatographic (HPLC) method has been developed and validated for the determination of the lactone and carboxylate forms of the novel antitumor drug irinotecan (CPT-11) and its active metabolite SN-38 in plasma. The instability of the compounds required immediate bed-side protein precipitation of plasma samples with an ice-cold mixture of methanol and acetonitrile. These methanolic extracts could be stored at −70°C for at least 3 months without degradation of the analytes. Separation of the lactone and carboxylate forms of CPT-11 and SN-38 was achieved on a C18 reversed phase column with a mobile phase composed of a mixture of 0.1 M ammonium acetate, triethylamine, and acetonitrile (800:1:156, w/v/w) and 5 mM tetra-butyl ammonium phosphate. Detection was performed fluorimetrically. Within-run and between-run precision was always less than 11% in the concentration ranges of interest (1.0–100 ng/mL and 0.5–25 ng/mL, for CPT-11 and SN-38, respectively). The method was successfully implemented in a phase I clinical and pharmacokinetic trial in patients treated with a 14-days low dose continuous infusion of CPT-11.

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