Oxytocin Pharmacokinetics and Pharmacodynamics

Oxytocin is the first-line drug to promote contraction of the uterus and prevent atony immediately after delivery. Nonetheless, unpredictable uterine atony refractory to oxytocin affects roughly 250,000 parturients annually in the U.S. and rates are increasing. This two-part study will measure the action of oxytocin at cesarean delivery. The first part will measure the pharmacokinetics of a single intravenous (IV) dose of deuterium-labeled oxytocin. The second part will measure the pharmacodynamics of all plasma oxytocin to see how concentrations correspond to the contractile effect on the uterus. After delivery of the fetus, study subjects will receive a bolus of IV deuterated oxytocin followed by an unlabeled oxytocin infusion. Venous blood samples drawn at multiple time points (within 1 hour after delivery) will be analyzed for plasma concentrations of labeled and unlabeled (endogenous + exogenous infused) oxytocin over time. Plasma concentrations will be compared with 0-10 uterine tone scores measuring uterine contraction strength, to describe the concentration-effect relationship. The goal of this study is to define both the pharmacokinetics and pharmacodynamics of oxytocin in parturients to help identify the cause(s) of failed first-line oxytocin therapy.

Parturients aged 18-50 with an intrauterine term pregnancy (37 weeks or greater) undergoing non-emergent cesarean delivery at the University of Chicago (UC) and Stanford University will be recruited. Methods: All subjects will receive standard pre-, intra- and postoperative care for their cesarean delivery with additional study interventions. Study subjects will have a second intravenous (IV) catheter inserted in the contralateral arm to their first IV for intra-operative study-related blood draws. A single, 1 International Unit (IU) bolus of deuterated oxytocin (d5OT) will be given to the subject, followed by an infusion of standard oxytocin at 2.5 IU/hour immediately after umbilical cord clamping. IV samples will be drawn from the second IV at 0, 1, 2.5, 5, 10, 20, 30, 45, 60 minutes post cord clamping. The samples will be processed, batched and shipped to the central lab being used in this study. Uterine tone scores will be measured simultaneously with blood draws until accurate palpation is no longer possible. Uterine tone score is a 0 to 10 numeric rating scale with 0 to describe unsatisfactory or no uterine tone and 10 to describe perfect uterine tone.

Oxytocin, pharmacokinetics and Pharmacodynamics, Uterine Tone, Postpartum, Elective C-Section, Parturients

All subjects will receive a 1 IU bolus of deuterated oxytocin (d5OT) intraoperatively during their cesarean delivery.

Eligible subjects will receive a single, 1 IU bolus of deuterated oxytocin (d5OT) intra-operatively, followed by an infusion of standard therapeutic d0 oxytocin immediately after umbilical cord clamping.

Subjects will receive a 1 IU bolus of deuterated oxytocin (d5OT) intraoperatively during their elective cesarean delivery.

Uterine tone will be ranked by the obstetrician on a 0-10 scale by palpation of the uterus. A score of 0 will represent 'no tone' and a score of 10 will represent 'perfect tone'. The relationship between plasma concentrations and clinical effect will be determined for pharmacodynamic modeling.

Uterine tone ranked by the obstetrician on a 0-10 scale by palpation of the uterus. A score of 0 will represent 'no tone' and a score of 10 will represent 'perfect tone'. The relationship between plasma concentrations and clinical effect will be determined for pharmacodynamic modeling.

Uterine tone ranked by the obstetrician on a 0-10 scale by palpation of the uterus. A score of 0 will represent 'no tone' and a score of 10 will represent 'perfect tone'. The relationship between plasma concentrations and clinical effect will be determined for pharmacodynamic modeling.

Uterine tone ranked by the obstetrician on a 0-10 scale by palpation of the uterus. A score of 0 will represent 'no tone' and a score of 10 will represent 'perfect tone'. The relationship between plasma concentrations and clinical effect will be determined for pharmacodynamic modeling.

Uterine tone ranked by the obstetrician on a 0-10 scale by palpation of the uterus. A score of 0 will represent 'no tone' and a score of 10 will represent 'perfect tone'. The relationship between plasma concentrations and clinical effect will be determined for pharmacodynamic modeling.

Uterine tone ranked by the obstetrician on a 0-10 scale by palpation of the uterus. A score of 0 will represent 'no tone' and a score of 10 will represent 'perfect tone'. The relationship between plasma concentrations and clinical effect will be determined for pharmacodynamic modeling.

Uterine tone ranked by the obstetrician on a 0-10 scale by palpation of the uterus. A score of 0 will represent 'no tone' and a score of 10 will represent 'perfect tone'. The relationship between plasma concentrations and clinical effect will be determined for pharmacodynamic modeling.

Uterine tone ranked by the obstetrician on a 0-10 scale by palpation of the uterus. A score of 0 will represent 'no tone' and a score of 10 will represent 'perfect tone'. The relationship between plasma concentrations and clinical effect will be determined for pharmacodynamic modeling.

Uterine tone ranked by the obstetrician on a 0-10 scale by palpation of the uterus. A score of 0 will represent 'no tone' and a score of 10 will represent 'perfect tone'. The relationship between plasma concentrations and clinical effect will be determined for pharmacodynamic modeling.

Uterine tone ranked by the obstetrician on a 0-10 scale by palpation of the uterus. A score of 0 will represent 'no tone' and a score of 10 will represent 'perfect tone'. The relationship between plasma concentrations and clinical effect will be determined for pharmacodynamic modeling.

Inclusion Criteria 18-50 years old intrauterine pregnancy term pregnancy (>/= 37 weeks gestation) scheduled or unscheduled non-emergent cesarean delivery Exclusion Criteria: emergent cesarean delivery allergy to oxytocin inability to provide informed consent