VARIANCES IN PHARMACOKINETIC PARAMETERS DUE TO ASSAY METHODS FOR β-METHYLDIGOXIN

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A digoxin radioimmunoassay (RIA) or fluorescence polarization immunoassay (FPIA) kit is frequently used in routine therapeutic drug monitoring (TDM) of β-methyldigoxin (MD) by applying a calibration curve made using the corresponding digoxin calibrators. The variances in the plasma levels (61 samples) and pharmacokinetics (5 patients) due to these two different assay methods for MD were examined in our patients with congestive heart failure. Although the plasma levels of MD measured by these methods were well correlated (r=0.956,p<0.001) to each other over a wide range, RIA showed significantly lower values (p<0.01) in the subtherapeutic range (<0.80 ng/ml), but significantly higher values (p<0.002) in the therapeutic and toxic ranges (0.80-2.00 and 2.00<ng/ml), respectively than FPIA. This trend occurred with increasing concentrations. When MD samples, spiked in normal human plasma, were analyzed by these methods, RIA showed almost true MD values and gave larger values than FPIA with a mean ratio of FPIA to RIA of 0.83. In contrast, normal plasma samples, each spiked with a MD metabolite such as digoxigenin-bisdigitoxide or digoxigenin-monodigitoxide, showed higher values by 10 to 22% in FPIA. These observations are in good agreement with the findings obtained in a pharmacokinetic study that RIA gave significantly higher levels than FPIA, only in the early stage after MD administration, resulting in a smaller total volume of distribution and a larger β value in the elimination phase, as compared with FPIA. These data suggested that the application of FPIA for pharmacokinetic studies of MD should be considered, especially for the calculation of FPIA for pharmacokinetic studies of MD should be considered, especially for the calculation of loading dose and that FPIA may be useful for the routine assay of plasma samples which are collected during the elimination phase in TDM of MD.
社団法人日本薬学会の論文

VARIANCES IN PHARMACOKINETIC PARAMETERS DUE TO ASSAY METHODS FOR β-METHYLDIGOXIN

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