Disposition of morphine in tissues of the pregnant rat and foetus following single and continuous intraperitoneal administration to the mother

Foetal exposure to maternally administered opiates such as morphine represent a serious human health problem but disposition studies in man are difficult to perform. Morphine disposition was therefore investigated in pregnant rats and their foetuses near term as a model. Disposition was examined either following intraperitoneal dosing as a single dose or continuous infusion. A high-pressure liquid chromatography assay for morphine in plasma and tissue was developed and validated. Following the single morphine dose, foetal distribution was rapid and concentrations in foetal and placental tissue were from 2 . 6 (whole foetus) to 27 . 6 (placenta) times higher compared with maternal plasma. The rank order of the area under the concentration vs time curve (AUC) of morphine in tissues was: placenta greater than or equal to foetal liver > foetal brain > whole foetus > maternal brain. The foetal brain to maternal brain AUC ratio for morphine was 9 . 5, suggesting large differences in their blood-brain barrier permeability. Following continuous administration of morphine there were significant linear relationships between maternal plasma and tissue concentrations with the same rank order as the single dose study. However, following continuous administration the relative amount of morphine in placenta and foetal liver was reduced by half and one-third, respectively, compared with the single dose study. These results document why the rat foetus is particularly susceptible to the pharmacodynamic effects of morphine following maternal administration.