Differential transplacental binding of diazepam: causes and implications

Rivaroxaban transfer across the dually perfused isolated human placental cotyledon

OBJECTIVE

The purpose of this study was to determine the rate and extent of rivaroxaban transfer across the term human placenta and determine whether passive diffusion was the primary mechanism involved in this transfer.

STUDY DESIGN

The transplacental pharmacokinetics of rivaroxaban was determined with the ex-vivo placenta perfusion model. Rivaroxaban was added to the maternal or fetal circulation only (250 ng/mL). Additional experiments were conducted under equilibrative conditions with the addition of rivaroxaban to both the maternal and fetal circulations (250 ng/mL). Rivaroxaban concentrations were measured with the use of liquid chromatography-tandem mass spectrometry.

RESULTS

There was rapid transfer of rivaroxaban across the human placenta in both the maternal-to-fetal and fetal-to-maternal directions, as evidenced by transfer ratios of 0.69 (interquartile range, 0.58-0.73; n = 5) and 0.69 (interquartile range, 0.67-0.71; n = 2), respectively, after 3 hours. Under equilibrative conditions (n = 2), rivaroxaban concentrations remained relatively constant, which suggests that rivaroxaban crosses the placenta down a concentration gradient.

CONCLUSION

This is the first direct evidence of rivaroxaban transfer across the term human placenta from both the mother-to-fetus and fetus-to-mother directions. Our results document that unbound rivaroxaban rapidly crosses the placental barrier via passive diffusion. However, because rivaroxaban is highly bound to plasma proteins (up to 95%), this suggests that the amount of unbound drug that may reach the fetus is likely much lower. Additional studies will need to explore its safety before administering rivaroxaban to a pregnant woman.

The human placental perfusion model: a systematic review and development of a model to predict in vivo transfer of therapeutic drugs

Dual perfusion of a single placental lobule is the only experimental model to study human placental transfer of substances in organized placental tissue. To date, there has not been any attempt at a systematic evaluation of this model. The aim of this study was to systematically evaluate the perfusion model in predicting placental drug transfer and to develop a pharmacokinetic model to account for nonplacental pharmacokinetic parameters in the perfusion results. In general, the fetal-to-maternal drug concentration ratios matched well between placental perfusion experiments and in vivo samples taken at the time of delivery of the infant. After modeling for differences in maternal and fetal/neonatal protein binding and blood pH, the perfusion results were able to accurately predict in vivo transfer at steady state (R² = 0.85, P < 0.0001). Placental perfusion experiments can be used to predict placental drug transfer when adjusting for extra parameters and can be useful for assessing drug therapy risks and benefits in pregnancy.

Drug Transfer and Metabolism by the Human Placenta

The major function of the placenta is to transfer nutrients and oxygen from the mother to the foetus and to assist in the removal of waste products from the foetus to the mother. In addition, it plays an important role in the synthesis of hormones, peptides and steroids that are vital for a successful pregnancy. The placenta provides a link between the circulations of two distinct individuals but also acts as a barrier to protect the foetus from xenobiotics in the maternal blood. However, the impression that the placenta forms an impenetrable obstacle against most drugs is now widely regarded as false. It has been shown that that nearly all drugs that are administered during pregnancy will enter, to some degree, the circulation of the foetus via passive diffusion. In addition, some drugs are pumped across the placenta by various active transporters located on both the fetal and maternal side of the trophoblast layer. It is only in recent years that the impact of active transporters such as P-glycoprotein on the disposition of drugs has been demonstrated. Facilitated diffusion appears to be a minor transfer mechanism for some drugs, and pinocytosis and phagocytosis are considered too slow to have any significant effect on fetal drug concentrations. The extent to which drugs cross the placenta is also modulated by the actions of placental phase I and II drug-metabolising enzymes, which are present at levels that fluctuate throughout gestation. Cytochrome P450 (CYP) enzymes in particular have been well characterised in the placenta at the level of mRNA, protein, and enzyme activity. CYP1A1, 2E1, 3A4, 3A5, 3A7 and 4B1 have been detected in the term placenta. While much less is known about phase II enzymes in the placenta, some enzymes, in particular uridine diphosphate glucuronosyltransferases, have been detected and shown to have specific activity towards marker substrates, suggesting a significant role of this enzyme in placental drug detoxification. The increasing experimental data on placental drug transfer has enabled clinicians to make better informed decisions about which drugs significantly cross the placenta and develop dosage regimens that minimise fetal exposure to potentially toxic concentrations. Indeed, the foetus has now become the object of intended drug treatment. Extensive research on the placental transfer of drugs such as digoxin and zidovudine has assisted with the safe treatment of the foetus with these drugs in utero. Improved knowledge regarding transplacental drug transfer and metabolism will result in further expansion of pharmacological treatment of fetal conditions.

The effects of benzodiazepine use during pregnancy and lactation

Although there are a number of studies and individual case reports concerning the use of benzodiazepines in human pregnancy, the data concerning teratogenicity and effects on postnatal development and behaviour are inconsistent. There is evidence from studies in the 1970s that first trimester exposure to benzodiazepines in utero has resulted in the birth of some infants with facial clefts, cardiac malformations, and other multiple malformations, but no syndrome of defects. Diazepam and chlordiazepoxide are amongst the drugs most frequently implicated in the earlier studies. However, data from later studies provide no clear evidence of significant increase in either the overall incidence of malformations or of any particular type of defect. Many of the women included in these studies has psychiatric illnesses, epilepsy, or diabetes all of which have an intrinsic risk in pregnancy, and some were on multidrug therapy. Medical-obstetric histories and family history of malformations were not always presented in the publications, which makes assessment of risk associated with benzodiazepine use per se difficult. Nevertheless, in most of the studies involving first trimester use of benzodiazepines, the majority of infants were normal at birth and had normal postnatal development. Late third trimester use and exposure during labour seems to be associated with much greater risks to the fetus/neonate. Some, but by no means all infants exposed at this time, exhibit either the floppy infant syndrome, or marked neonatal withdrawal symptoms. Symptoms vary from mild sedation, hypotonia, and reluctance to suck, to apnoeic spells, cyanosis, and impaired metabolic responses to cold stress. These symptoms have been reported to persist for periods from hours to months after birth. This correlates well with the pharmacokinetic and placental transfer of the benzodiazepines and their disposition in the neonate. However, there has been no significant increase in the incidence of neonatal jaundice and kernicterus in term infants. The prolonged use of benzodiazepines throughout pregnancy raised the concern that there may be altered transmitter synthesis and function, leading to neurobehavioural problems in the children. In approximately 550 children who were followed up for various times up to four years of age, there is no increase in either the malformation rate or adverse effects on neurobehavioural development and IQ. Although some of the data indicate that a small number of children were slower to develop during the first year or so, they did exhibit catch up growth and most had developed normally by four years of age. Where developmental deficits persisted, it was not possible to prove a cause-effect relationship with benzodiazepine exposure. These children were often from families where there was maternal illness requiring prolonged drug therapy or where there were social problems. It is important to consider poor environmental and social factors when assessing the possible prenatal influence of the benzodiazepines on the postnatal health and development of the child. There is evidence that clonazepam, clorazepate, diazepam, lorazepam, midazolam, nitrazepam, and oxazepam are excreted into breast milk. The published data indicate that the levels detected in breast milk are low; therefore, the suckling infant is unlikely to ingest significant amounts of the drug in this way. Problems may arise if the infant is premature or has been exposed to high concentrations of drug either during pregnancy or at delivery.

Differential transplacental binding of valproic acid: influence of free fatty acids

The unbound fraction of valproic acid (VPA) was found to be significantly lower in cord serum (6.0 +/- 0.8%) than in maternal serum collected before oxytocin (12.2 +/- 2.7%) or after delivery (9.9 +/- 2.3%). The difference was probably due to the concentration of free fatty acids (acting as displacing agents) being higher in maternal serum. The transplacental binding gradient explains the clinical observation that total VPA levels at delivery are higher in the newborn than in the mother.

Naloxone protein binding in adult and foetal plasma

Binding of naloxone hydrochloride was determined at 37 degrees C, by equilibrium dialysis against 0.067 M phosphate buffer, pH 7.4, in plasma obtained from 18 healthy adults, and 18 samples of umbilical cord venous (foetal) plasma. The percentage free fraction (% free) in plasma was independent of naloxone concentration (9 ng/ml to 2.5 micrograms/ml). Percent free naloxone in adult (means = 54.0) was lower (p less than 0.01) than in foetal (means = 61.5) plasma. In buffered solutions of purified HSA, %free naloxone (means = 68.7) was independent of HSA concentration over the range 3.0 g/dl to 5.5 g/dl. Adult plasma concentrations of alpha 1-acid glycoprotein (alpha 1-AGP) and beta-lipoprotein were higher (p less than 0.01) than foetal concentrations. Furthermore %free naloxone in foetal plasma decreased with the in-vitro addition of purified alpha 1-AGP. It is suggested that qualitative differences in adult and foetal albumin and quantitative differences in plasma levels of alpha 1-AGP and perhaps beta-lipoprotein are responsible for naloxone plasma binding differences between adults and the newborn.