Pharmacokinetics and transplacental transfer of codeine and codeine metabolites from Papaver somniferum L

Papaveris Pericarpium, which is the dried husk of Papaver somniferum L., has been used as a phytomedicine to relieve cough, diarrhea and pain. The alkaloid codeine contained therein via biotransformation converts to morphine and potentially produces addictive and toxic effects. Due to the healthy concern for a pregnant woman, our hypothesis is that codeine and its metabolites can penetrate the placental barrier to reach the foetus and amniotic fluid, and these processes may be modulated by the transporter. Because codeine is also considered a prodrug of morphine, it has a good analgesic effect. It is often used by pregnant women but may expose the foetus to the risk of morphine harm. The aim of this study is to investigate the metabolic rate, distribution and transplacental transfer mechanism of codeine and its metabolites morphine and morphine-3-glucuronide (M3G) in pregnant rats and to assess the risk of medication for pregnant women. Ultrahigh-performance liquid chromatography–tandem mass spectrometry (UHPLC–MS/MS) combined with a microdialysis system was developed to monitor codeine, morphine and M3G in multiple sites of maternal blood, placenta, foetus and amniotic fluid after codeine administration. A compartmental model was used to calculate the pharmacokinetic parameters of codeine in blood after codeine administration (10 mg/kg, i.v.). The area under the concentration (AUC) ratio of AUCₘₑₜₐbₒₗᵢₜₑ/AUCcₒdₑᵢₙₑ and AUCₜᵢₛₛᵤₑ/AUCbₗₒₒd was used to represent the metabolic biotransformation ratio and the drug from blood-to-tissue transfer ratio, respectively. The pharmacokinetic results demonstrated that codeine fit well with a two-compartment model and went through rapid metabolism to morphine and M3G in pregnant rats after codeine administration (10 mg/kg, i.v.). The biotransformation ratios of AUCₘₒᵣₚₕᵢₙₑ/AUCcₒdₑᵢₙₑ, AUCM₃G/AUCₘₒᵣₚₕᵢₙₑ and AUCM₃G/AUCcₒdₑᵢₙₑ were 0.12 ± 0.03, 54.45 ± 20.61 and 6.53 ± 2.47, respectively, after codeine administration (10 mg/kg, i.v.), which suggested that codeine was easily metabolized into M3G through morphine. The tissue distribution results demonstrated that all of the analytes penetrated into the foetus through the placenta; however, the blood-to-tissue transfer ratio (AUCₜᵢₛₛᵤₑ/AUCbₗₒₒd) of morphine and M3G was relatively lower than that of codeine after codeine administration (10 mg/kg, i.v.), which suggested that the blood-placenta barrier blocks the penetration of morphine and M3G into the foetus. Thus, the tissue transfer of morphine in the placenta and foetus was significantly enhanced by treatment with corticosterone, an inhibitor of organic cation transporter (OCT). Based on microdialysis coupled to a validated UHPLC–MS/MS system, the pharmacokinetics and metabolic biotransformation of codeine and its metabolites were analyzed and clarified. The potential mechanism of morphine placental transfer was modulated by OCT transporters.