THE DEVELOPMENT AND CROSS-VALIDATION OF METHODS BASED ON RADIOIMMUNOASSAY AND LC MS-MS FOR THE QUANTIFICATION OF THE CLASS-III ANTIARRHYTHMIC AGENT, MK-0499, IN HUMAN PLASMA AND URINE/

An analytical method based on radioimmunoassay (RIA) has been develope d for the determination of the antiarrhythmic agent, MK-0499, in plasm a and urine. Owing to the potency of the drug, the specificity of this assay in human plasma could not be adequately determined using conven tional RIA procedures. A highly specific procedure, based on LC/MS-MS, was developed to cross-validate the RIA. The lower quantifiable limit s of the RIA and LC/MS-MS-based methods were 0.05 and 0.013 ng ml(-1), respectively. Cross-validation data, compared using paired student's t-test regression analysis, showed excellent correlation between metho ds. The mass spectrometric assay was also used to simultaneously measu re plasma concentrations of unlabeled and C-14-labeled MK-0499 followi ng administration of the drug at high specific activity to volunteers.