Serum level of ritodrine in man.

A sensitive radioimmunoassay has been developed for the determination of ritodrine in serum and plasma. Interference by ritodrine metabolites, viz. the sulphate and glucuronide conjugates, was negligible. The sensitivity was 0.3 ng/ml in 0.1 ml plasma or serum. The method was used to determine the serum level of ritodrine in man after oral or parenteral administration. In healthy volunteers intravenous infusion of 9 mg in 1 h resulted in a peak serum level of about 45 ng/ml. In every subject the heart rate varied with time in approximately the same way as the serum level. Intramuscular injection of 10 mg gave a peak level of 20 ng/ml, and after oral administration of 10 mg the maximum peak concentration was 10 ng/ml. In all instances there was a detectable level in serum for up to 25 h after dosing. The bioavailability of the oral formulation was 30% of the parenteral preparations. The effective half-life was 1.3--2 h. Women in pre-term labour were treated by intravenous infusion of ritodrine hydrochloride, and after parturition, ritodrine was determined in serum from the mother and from umbilical cord blood. The results showed that ritodrine crossed the placental barrier and entered the fetal circulation.