Determination of Plasma Dexamethasone in the Mother and the Newborn after Administration of the Hormone in a Clinical Trial

A RIA method is described for the measurement of dexamethasone in maternal, cord, and neonate plasma in a collaborative multicenter clinical trial to evaluate the efficacy of antenatal steroid therapy in the prevention of respiratory distress syndrome. The antiserum raised against dexamethasone-3-carboxymethyloxime-BSA conjugate was highly specific in that the endogenous steroids and 11-dehydrodexamethasone, a metabolite of dexamethasone, had a cross-reaction of less than 2.0%. Both pregnancy and cord plasma had to be purified by either gel filtration and/or paper chromatography. The overall recoveries of dexamethasone were 75.3 +/- 6.7% and 48.9 +/-6.7% for maternal and cord plasma samples, respectively. The intra- and interassay coefficients of variance for maternal plasma were 8.4% ad 9.1%, respectively, and for cord plasma were 11.9% and 10.9%, respectively. There was a good correlation between the dexamethasone values obtained by RIA when compared with those obtained by performance liquid chromatography in some cord plasma specimens. The recoveries of added dexamethasone and its metabolite by high performance liquid chromatography were also found to be good. The average maternal plasma dexamethasone level was 37 ng/ml 2 h after im injections of 5 mg dexamethasone phosphate every 12 h. The half-life of dexamethasone measured after the discontinuation of the drug was 216 min. Dexamethasone appeared too be cleared very rapidly from the circulation of the fetus and neonate. Because 11-dehydrodexamethasone was not present in significant amounts in the neonate, it was concluded that other factors in addition to the conversion of dexamethasone to its 11 dehydro metabolite were responsible for the rapid clearance of dexamethasone from fetal and neonatal circulation.