Simultaneous determination of the lactone and carboxylate forms of 7-ethyl-10-hydroxycamptothecin (SN-38), the active metabolite of irinotecan (CPT-11), in rat plasma by high performance liquid chromatography

We established a high performance liquid chromatography (HPLC) method for the simultaneous determination of the lactone and carboxylate forms of 7-ethyl-10-hydroxycamptothecin (SN-38), the active metabolite of the antitumor drug irinotecan (CPT-11), in rat plasma. Plasma samples were pretreated with chilled MeOH and zinc sulfate to precipitate protein, and were then directly injected into the HPLC system. Chromatography was carried out with a Puresil C18 column (particle size 5 microns), and the mobile phase consisted of 0.1 M ammonium acetate buffer (pH 5.5) and acetonitrile (70/30, v/v) containing 20 mM of tetra-n-pentylammonium bromide. The column effluent was monitored with a spectrofluorometer (excitation wavelength 380 nm, emission wavelength 540 nm). The method was valid for SN-38 lactone (5-2500 ng/ml) and carboxylate (5-1000 ng/ml).