Simultaneous determination of the lactone and carboxylate forms of the camptothecin derivative CPT-11 and its metabolite SN-38 in plasma by high-performance liquid chromatography

CPT-11 (irinotecan) and mainly its metabolite SN-38 are potent antitumor derivatives of camptothecin. As the active lactone forms of both CPT-11 and SN-38 exist in pH-dependent equilibrium with their respective less potent open-ring hydroxy acid species, the simultaneous monitoring of both forms of both compounds is relevant. CPT-11 and SN-38 derivatives have quite different fluorescence responses. In order to avoid any compromise on the wavelength setting, we developed chromatographic conditions allowing simple automated wavelength setting changes which have been prevented using existing methods involving conventional C₁₈ columns. This was achieved by means of a Symmetry C₁₈ column combined to a gradient elution program using acetonitrile and 75 mM ammonium acetate plus 7.5 mM tetrabutylammonium bromide at pH 6.4. The developed conditions allowed an elution order suitable for a simple automated wavelength change in respect to reliable peak integration. CPT-11 and SN-38 derivatives were detected at λ_ex=362 nm/λ_em=425 nm and λ_ex=375 nm/λ_em=560 nm, respectively. The developed method allowed the detection of amounts less than 3 pg of each derivative injected on column. The method was successfully applied to pharmacokinetic and toxicokinetic studies in rat and dog.