Development of two radioimmunoassays to detect paclitaxel in sera and in cerebrospinal, ascitic, and pleural fluids

BACKGROUND. Paclitaxel is an antimiotic agent isolated from the Pacific yew tree. It has demonstrated antitumor activity in several cancers and is the first of a new class of antineoplastic agents containing a taxane ring system. Its levels in serum and urine have been measured previously by high performance liquid chromatography (HPLC). In this study, the authors developed two competitive radioimmunoassay methods to determine whether they could reliably be used to measure levels of paclitaxel in sera and in cerebrospinal, ascitic, and pleural fluids. METHODS. A monoclonal antibody prepared against paclitaxel was employed in an immunoradiometric assay (IRMA), in which ¹²⁵I-labeled antibody was used, and in a more conventional tritiated radioimmunoassay (RIA), in which ³H-paclitaxel was used. RESULTS. Both radioimmunoassays detected levels of paclitaxel in sera that were comparable to those observed with HPLC. However, the IRMA was the most sensitive. Only the IRMA was able to detect low levels of paclitaxel in cerebrospinal fluid after paclitaxel infusion and in sera 3 weeks after infusion. Both the IRMA and RIA methods were able to detect paclitaxel in ascitic and pleural fluids. CONCLUSIONS. Monitoring paclitaxel levels reliably in sera and other bodily fluids is possible with these radioimmunoassays and may be of value in predicting and preventing toxicity and optimizing paclitaxel treatments.