Pharmacokinetic studies on fenoterol in maternal and cord blood

Fenoterol plasma concentrations were measured by radioimmunoassay in 38 pregnant women at different stages of preterm labor and in cord blood. Eight women were treated intravenously until delivery with 1.0 to 4 micrograms/min of fenoterol for periods ranging from 27 hours to 27 days; blood samples were taken at the same time as cord blood. In these women the fenoterol concentrations in cord blood ranged from 18 to 53% of the maternal concentrations. In eight women treated intravenously with 1.2 to 4.0 micrograms/min for 2 to 15 days, the infusion was stopped 1.3 to 38 hours before delivery. In these instances the concentrations in cord blood reached as much as 90% of the maternal, meaning that the rate of elimination from fetal plasma is lower than that from maternal plasma. Five women were treated daily with 20 to 30 mg per os for 3 to 17 days (three of these women had also had intravenous treatment before). The ratio of cord to maternal blood concentrations was higher than in women receiving the drug intravenously, the relative times of sampling being the same. The findings suggest that: (1) the placental transfer of fenoterol is higher than that found in previous studies in humans and animals with tritium-labeled substances; (2) the rate of fenoterol elimination from fetal plasma after intravenous and oral long-term therapy is lower than that from maternal plasma; (3) after oral administration, the ratios of fetal to maternal fenoterol concentrations are higher than after intravenous infusion.