Placental transfer of clenbuterol early in human pregnancy

After administration of clenbuterol 80 microgram p.o., a beta-adrenoceptor agonist, the concentrations of (14C-labelled) clenbuterol in fetus, placenta and maternal plasma in 9 patients at 9-12 weeks gestation were measured during therapeutic abortion. The time interval between the administration and abortion ranged from 120 to 280 min. The mean concentrations of clenbuterol in maternal plasma, fetus and placenta were 0.37 (range 0.22-0.56), 0.32 (0.14-0.48) and 0.91 (0.12-1.73) ng equivalents per ml or per gram of tissue wet weight. The mean concentration ratio of clenbuterol between fetus and maternal plasma was 0.84 (6 cases); it did not vary with time. The concentration of clenbuterol in three amniotic fluid samples ranged from 0.06 to 0.16 ng/ml (mean 0.11). Maternal plasma concentrations showed wide variability of the pharmacokinetic phase at the time of abortion. The studies indicate that clenbuterol crosses the placenta early in human pregnancy and that it accumulates in the placenta.