Ritodrine pharmacokinetics.

The purpose of this study was to add to the limited information available regarding the pharmacology of ritodrine in the peripartum period when treatment fails and labor is not inhibited. Plasma or urine samples from eight parturients and 13 infants were studied; in addition plasma samples at delivery were obtained from a total of 26 mothers and infants. All the mothers received ritodrine in the 24 hours before delivery. Plasma and urine ritodrine (free and conjugated) were determined with HPLC by electrochemical detection. In maternal plasma, an apparent rapid distribution phase with a t1/2 of 32 +/- 21 minutes was followed by a prolonged equilibrium phase with a t1/2 of 17 +/- 10 hours. Seventy-six percent of the ritodrine excreted by the mother was in the form of a conjugate. Ninety percent of the ritodrine excreted by the neonate was also excreted in the form of a conjugate.