A simple method for the separate estimation of labelled drugs and their metabolites by liquid scintillation counting. Some practical examples

Summary

A simple method is described for the separate determination in plasma of lipophilic drugs and their more hydrophilic metabolites by liquid scintillation counting. This is done by measuring the radioactivity after adding biological specimens to a scintillation liquid which takes up both the unchanged drug and its hydrophilic metabolites (Bray's solution), and by mixing further aliquots of the specimen with a scintillation fluid which only takes up unchanged drug (Toluene+PPO=TSC). The amount of hydrophilic substances dissolved in TSC, as well as irradiation from the aqueous phase, were shown to be less than 13% for both of the isotopes³H and¹⁴C, employing 0.5 ml of aqueous phase added to 10 ml toluene scintillant. Checking the method by thin layer chromatography showed that the concentrations of fentanyl and haloperidol in plasma were determined accurately. After taking account of a special procedure used to dissolve a greater proportion of morphine in toluene, the ratio of TSC and BSC measurements was in agreement with data obtained by thin layer chromatography. Experiments with chlorpromazine showed that lipophilic degradation products were formed which could not be separated from the original drug by liquid-liquid partition. In such cases, therefore, the method cannot be employed. Etorphine is an example of how reliable results were obtained despite low dosage and relatively low specific activity of this drug.