Determination of 5-S-cysteinyldopa in plasma and urine using a fully automated solid-phase extraction-high-performance liquid chromatographic method for an improvement of specificity and sensitivity of this prognostic marker of malignant melanoma

5-S-Cysteinyldopa (5-SCD) in plasma and urine was determined by means of a newly developed method. This method incorporates optimized conditions for blood collection and storage, as well as a new extraction and separation technique, required for the strong oxidation and light sensitive 5-SCD. The new aspects of the method are the following: immediate centrifugation and freezing of the samples after blood collection, fully automatical solid-phase extraction (SPE) with phenylboronic acid (PBA) cartridges and immediate HPLC injection of the eluate, nearly complete exclusion of light and air-oxygen during extraction, constant sample cooling, use of the more suitable internal standard 5-S-D-cysteinyldopa and easy, sensitive and selective HPLC conditions (RP18-column with isocratic separation and electrochemical detection). The method has a linear range from 0.25 to 50 mu g l(-1) and 25 to 5000 mu g l(-1) for plasma and urine samples, respectively, a limit of detection of 0.17 mu g l(-1), intra-assay variabilities from 1.7 to 3.6%, inter-assay variabilities from 4.0 to 18.3% and an average relative recovery of 103.5% for plasma and 105.4% for urine samples. In our study the measured 5-SCD concentrations of patients with melanomas at various stages correlated better with their clinical pictures than described in literature up to date. The results were obtained in comparison to patients with other skin tumors and in comparison to healthy control persons. (C) 1999 Published by Elsevier Science B.V. All rights reserved.