Pharmacokinetics of morphine and its surrogates I: comparisons of sensitive assays of morphine in biological fluids and application to morphine pharmacokinetics in the dog.

A sensitive isotope derivatization assay was developed to quantify morphine in biological fluids in the nanogram per milliliter range. Morphine, derivatized with 3H-dansyl chloride, was separated from the reaction products by TLC. The spots were scraped from the plate, and the eluted radioactivity was determined by liquid scintillation. The standard deviations of this morphine assay were +/- 18.6 ng/ml in 100 microliter of plasma and +/- 1.86 ng/ml in 1 ml of plasma. The GLC analysis of pentafluoropropionated morphine in the range of 0--5 ng of morphine/ml of plasma had a standard deviation of +/- 0.46 ng/ml when 1 ml of plasma was taken. Liquid scintillation spectrometric analysis of 14C-morphine had a sensitivity of 1.5 ng/ml of plasma at double the background. There were no significant differences among the liquid scintillation, electron-capture GLC, and radioisotpoe derivatization methods for morphine obtained from the plasma of a dog given 14.00 mg iv of morphine. Morphine conjugates were assayed as morphine after the acid hydrolysis of plasma and urine preextracted to remove unconjugated morphine, and the equivalence of various methods was demonstrated to monitor plasma and urine pharmacokinetics in a dog.