Assay for the quinocarmycin analog DX-52-1 in human plasma using high-performance liquid chromatography with automated column switching and low wavelength ultraviolet detection

The hydrocyanated derivative of the antitumor antibiotic quinocarmycin, DX-52-1 (I), exhibits impressive activity against human melanoma xenograft models in vivo. Phase I clinical trials to evaluate this compound as an antineoplastic agent have been initiated by the US National Cancer Institute. We have developed an HPLC assay for the determination of I in human plasma involving automated column switching and UV detection at 210 nm. The preparation of samples for chromatographic analysis entails the preliminary removal of plasma proteins by precipitation with acetonitrile, acidifying the clear supernatant to pH 4.5, then extracting twice with tert .-butyl methyl ether to recover the drug. A heart-cutting procedure employing two HPLC columns with contrasting retention characteristics under isocratic reversed-phase conditions was used to achieve the selectivity required for low wavelength UV detection of the analyte. The sample extract was initially loaded onto a column packed with a cyanopropyl stationary phase. During the predetermined time interval that I eluted from this column, a fully automated six-position switching valve was used to direct the effluent onto an octadecylsilane analytical column. The assay has been thoroughly validated with regard to linearity, inter- and intra-day accuracy and precision, recovery, selectivity and specificity. Using a sample volume of 1.0 ml, the lowest concentration of I quantified with acceptable day-to-day reproducibility was found to be 2.56 ng/ml (R.S.D. 18.9%, n =21, 4 months). This proved to be sufficiently sensitive for pharmacokinetic drug level monitoring in cancer patients treated with a 6-h continuous intravenous infusion of I, even at the starting dose of 3 mg/m 2 . The successful performance and reliability of the assay has been demonstrated through extensive application to the routine analysis of plasma specimens acquired during a phase I clinical trial of the drug.