A receptor protein-based bioassay for quantitative determination of paclitaxel

A novel receptor-based bioassay for the quantitative measurement of Taxol was developed. The assay was based on the well-investigated and established finding that Taxol, its active analogs, and active metabolites bind reversibly to the receptor protein tubulin, a process similar to antibody and antigen interaction. The assay was performed in a competitive format by allowing a mixture of horseradish peroxidase-labeled Taxol and Taxol in the analyte sample to compete for the Taxol binding site of a polystyrene microtiter plate wall coated with purified tubulin and subsequently measuring the tubulin-Taxol complex by determining the activity of the horseradish peroxidase label. Using this method, Taxol was measured very sensitively, linear range of 0.0001-1 nM, and selectively, without interference from non-tumor-active compounds such as baccatin III, cephalomaninne, and 10-deacetyl taxol. The method was applied for the determination of picomolar concentrations of Taxol in human plasma.