Determination of unbound docetaxel and paclitaxel in plasma by ultrafiltration and liquid chromatography–tandem mass spectrometry

This work describes the development of a straightforward method for the determination of free docetaxel and paclitaxel in plasma. The separation of bound and unbound drug was performed with ultrafiltration. Different ultrafiltration devices were evaluated, especially regarding non-specific binding to the device. The most appropriate device for this application was selected and a procedure to counteract non-specific binding to the ultrafiltrate collection cup was developed. This consisted of a wash procedure with methyl t-butyl ether. A liquid/liquid extraction with methyl t-butyl ether was performed and samples were analysed with a previously developed liquid chromatography–tandem mass spectrometry procedure. The method used a Merck Purospher Star RP-18 column (55 mm × 2.0 mm, 3-μm particle size) and electrospray in the positive mode. A triple quadrupole instrument was used to monitor MRM transitions. Small modifications to this procedure were made to ensure adequate sensitivity. Within- and between-day reproducibility did not exceed 15% and accuracy ranged between 94.4 and 102.5%. The calibration range of the method was from 0.4 to 100 ng/ml both for paclitaxel and docetaxel. Finally, a fast and relatively simple method could be developed.