Analysis of camptothecins in biologic media is hampered by chemical hydrolysis of the parent lactone (form I) to an inactive hydroxy acid (form II). A solid-phase extraction (SPE) method utilizing C2-bonded silica particles (100 mg, 1 ml) is presented for simultaneous determination of forms I and II of camptothecin (CPT) and SN-38 (active metabolite of clinically used CPT-11) in culture media and cell lysates. A new HPLC separation is described that efficiently resolves all four compounds employing gradient elution with 10 mM ammonium acetate, increasing methanol (20–80% over 15 min), and a 15-cm by 3-mm Symmetry Shield (RP8) column. Components were detected by fluorescence at an excitation wavelength of 380 nm and emission wavelength of 423 nm. Lactones were shown to be unstable at alkaline pH and hydroxy acids unstable at alkaline pH while the following conditions preserved the chemical equilibrium in specimens: samples kept on ice, final pH of eluates 7.4, autosampler temperature 4°C, and analysis cycle <4 h. Quantitative recovery of lactones was achieved from RPMI culture medium over a wide concentration range (93.5–111.6% for 1–400 ng/ml) although greater variability was noted with the hydroxy acids (59.6–110.3%, 1–400 ng/ml). Limit of quantitation (precision and accuracy <20%) was 0.2 ng/ml for CPT lactone, 0.5 ng/ml for SN-38 lactone, and 2 ng/ml for the two hydroxy acids. The method was applied to quantitate the accumulation of SN-38 and CPT (form I and II) in HT29 and HCT116 human colon cancer cells.
- solid-phase extraction
- colon cancer cells