Fetal and neonatal excretion of free and conjugated ritodrine

The ability of the human fetus and neonate to conjugate and excrete ritodrine, a beta 2-sympathomimetic drug, was investigated. Free and conjugated ritodrine concentrations in the plasma, amniotic fluid and urine were measured in 11 mother-infant pairs, to whom intravenous ritodrine had been administered before elective cesarean section at term. Ritodrine was determined by HPLC with electrochemical detection. At delivery, conjugated ritodrine values were significantly higher than those for the free form in maternal and fetal plasma. There were significant positive correlations between the concentrations in the maternal and umbilical vein plasma for both free and conjugated ritodrine. In the amniotic fluid, the total ritodrine concentrations were much higher than those in the fetal plasma, the conjugated form accounting for 90.2% of the total. Furthermore, the percentages of conjugated ritodrine in the amniotic fluid and neonatal urine were significantly higher than the percentage in the maternal urine on the day of birth. In the neonatal urine, the concentrations of free and conjugated ritodrine decreased rapidly after birth as did those in the maternal urine, on day 3 postpartum being less than 2% of the values on the day of parturition. These results indicate that the fetus at term is capable of forming conjugated metabolites of ritodrine and of excreting free and conjugated ritodrine in its urine.