Quantitative analysis of the novel depsipeptide anticancer drug Kahalalide F in human plasma by high‐performance liquid chromatography under basic conditions coupled to electrospray ionization tandem mass spectrometry

Kahalalide F (KF) is a novel cyclic depsipeptide anticancer drug, which has shown anticancer activity both in vitro and in vivo especially against human prostate cancer cell lines. To characterize the pharmacokinetics of KF during a phase I clinical trial in patients with androgen refractory prostate cancer, a method was developed and validated for the quantitative analysis of KF in human plasma using high‐performance liquid chromatography (HPLC) coupled to positive electrospray ionization tandem mass spectrometry (ESI‐MS/MS). Microbore reversed‐phase liquid chromatography (LC) performed with mobile phases containing trifluoroacetic acid, an additive commonly used for separating peptides, resulted in substantial suppression of the signal for KF on ESI‐MS/MS. An alternative approach employing a basic mobile phase provided an excellent response for KF when detected in the positive ion mode. Plasma samples were prepared for LC MS/MS by solid‐phase extraction on C 18 cartridges. The LC separation was performed on a Zorbax Extend C 18 column (150 × 2.1 mm i.d., particle size 5 µm) with acetonitrile –10 m M aqueous ammonia (85 : 15, v/v) as the mobile phase, at a flow‐rate of 0.20 ml min −1 . A butyric acid analogue of KF was used as the internal standard. The lower limit of quantitation (LLQ) using a 500 µl sample volume was 1 ng ml −1 and the linear dynamic range extended to 1000 ng ml −1 . The inter‐assay accuracy of the assay was −15.1% at the LLQ and between −2.68 and −9.05% for quality control solutions ranging in concentration from 2.24 to 715 ng ml −1 . The inter‐assay precision was 9.91% or better at these concentrations. The analyte was stable in plasma under all relevant conditions evaluated and for a period of 16 h after reconstituting plasma extracts for LC analysis at ambient temperature. Copyright © 2002 John Wiley & Sons, Ltd.