Pharmacokinetics of nalbuphine during parturition

Nalbuphine hydrochloride (Nubain) is a relatively new agonist-antagonist analgesic, chemically related to both oxymorphone and naloxone, which can be used as a systemic obstetric analgesic. After intravenous administration to six patients in labor at term, serial maternal serum specimens were obtained and assayed for their nalbuphine concentration. The maternal disposition of nalbuphine followed a two-compartment pharmacokinetic model, with an initial distribution phase of 4 to 20 minutes and a terminal elimination half-life of 2.4 +/- 0.4 hours. Measurement of umbilical cord blood nalbuphine concentrations demonstrated that nalbuphine crossed the placenta and entered the fetal circulation. Newborn concentrations varied substantially, ranging from one third to six times the simultaneous maternal concentration.