PHARMACOKINETICS OF FENOTEROL IN PREGNANT-WOMEN

The beta(2)-sympathomimetic drug (fenoterol hydrobromide, CAS 1944-12- 3, Partusisten(R)) is routinely used to inhibit uterine contractions ( tocolysis). Investigations of plasma concentrations of those receiving i.v. or oral tocolysis often show different results both within parti cular groups of pregnant women and in comparison with non-pregnant per sons. The aim of this study was to determine the pharmacokinetics of f enoterol in pregnant women, an important factor which so far had not b een known. Four healthy pregnant women with similar weight and gestati onal age and all with premature labor were administered a continuous i ntravenous infusion of 4 mu fenoterollmin. During and up to 24 hours a fter the end of the infusion, venous blood samples were taken in order to determine the fenoterol plasma concentrations by radioimmunoassay. From a steady state concentration (c(SS)) of 2242 +/- 391 pg/ml (x +/ - S.E.), a non-linear two-phased plasma elimination was seen with half -lives t(1/2) of 11.40 min and 4.87 h. The area under the plasma conce ntration-time curve (AUC(0-12h)) was 6.27 ng/ml ml h. The total cleara nce (Cl-tot) was 114.8 l/h. These data are nearly the same as the data already known for healthy non-pregnant (male) volunteers. The deviati ons which are seen in the plasma concentrations in pregnant women in c omparison to non-pregnant persons during or after continuous i.v. infu sion can therefore not be caused by differences in the pharmacokinetic s. Other factors, however, such as body weight and/or gestational age, might influence the results.