Oxycodone pharmacokinetics and fetal exposure after intravenous or epidural administration to the ewe

There are limited data on oxycodone pharmacokinetics during pregnancy and on fetal exposure after maternal administration. The present study describes the pharmacokinetics of intravenous (i.v.) oxycodone in pregnant sheep and fetal exposure after intravenous and epidural administration.

Ten pregnant sheep received 0.1 mg·kg^-1 oxycodone intravenously, and blood samples were collected up to 24 hours. Seven days later, the ewes were randomized to receive 0.5 mg·kg^-1 oxycodone intravenously (n = 5) or epidurally (n = 5) as a single bolus, before laparotomy for placement of catheters into the fetal superior vena cava and carotid artery. Paired maternal and fetal blood samples were taken when the fetal arterial catheter was in place and at the end of surgery. Maternal blood samples were taken up to 24 hours.

After 0.1 mg·kg^-1 oxycodone intravenously, the median clearance was 5.2 L·h^-1 ·kg^-1 (range 4.6-6.2), but the volume of distribution varied between 1.5 and 4.7 L·kg^-1 . The area under the curve was 17 h·ng·mL^-1 (range 14-19) and the plasma concentration at 2 minutes 60 ng·mL^-1 (range 50-74). Following administration of 0.5 mg·kg^-1 intravenously or epidurally, oxycodone concentrations were similar in the maternal and the fetal plasma. Accumulation of oxymorphone in the fetus occurred; fetal-to-maternal ratios were 1.3-3.5 (median 2.1) in the i.v.-group and 0.9-3.0 (1.3) in the Epidural-group.

We determined the pharmacokinetics of oxycodone in pregnant sheep. We showed accumulation of oxymorphone, which an active metabolite of oxycodone, in the fetus. Further studies in human pregnancies are required to evaluate the safety of oxycodone.