Drug transport across the placenta, role of the ABC drug efflux transporters

Neonatal outcomes and follow-up of children born to women with pregnancy-associated cancer: a prospective observational study

BACKGROUND

During the last decade, there has been a growing number of cases of children born from pregnancy-associated cancer (PAC), however there are currently insufficient data on the follow up to be observed in this category of newborns. Objective of the study was to evaluate the neonatal outcomes of infants born to mother with PAC, the potential adverse effect of chemotherapy during pregnancy and the risk of metastasis to the fetus.

METHODS

Maternal clinical data and neonatal outcomes of child born to mothers diagnosed with PAC were collected; infants were divided into those were and were not exposed to chemotherapy during fetal life and their outcomes were compered.

RESULTS

A total of 37 newborn infants from 36 women with PAC were analyzed. Preterm delivery occurred in 83.8% of the cases. No significant differences in neonatal outcomes were found between infants who were and were not exposed to chemotherapy during pregnancy. The median follow-up period was 12 months.

CONCLUSIONS

PAC treatment during the second or third trimester does not seem to be dangerous for the fetus, however infants born from PAC must be carefully evaluated for to rule out the consequences of chemotherapy and exclude the presence of metastasis. Long-term follow-up, especially in children exposed to chemotherapy, should be encouraged to obtain relevant data on long-term toxicity.

The Effect of Tobacco Use on the Expression of Placental Transporters in Alaska Native Women

Prenatal tobacco use among Alaska Native (AN) women has decreased substantially over the past two decades. Previous research suggests that providing AN women with feedback regarding fetal exposure to tobacco may further promote cessation. Transporters in the placenta regulate fetal exposure to nutrients and xenobiotics, including compounds associated with tobacco use. We examined whether prenatal tobacco use impacts transporter expression in the placenta, and whether this is influenced by fetal sex, degree of tobacco exposure, or transporter genotype. At delivery, we obtained placental samples from AN research participants who smoked cigarettes, used commercial chew or iqmik (oral tobacco), or did not use tobacco during pregnancy. Transporter expression was evaluated using qRT-PCR and Western blotting and tested for correlations between transcript levels and urinary biomarkers of tobacco use. The impact of BCRP/ABCG2 and OATP2B1/SLCO2B1 genotypes on protein expression was also examined. Oral tobacco use was associated with decreased P-gp and increased MRP1, MRP3, LAT1, and PMAT mRNA expression. Transcript levels of multiple transporters significantly correlated with tobacco biomarkers in maternal and fetal urine. In women carrying male fetuses, both smoking and oral tobacco were associated with decreased P-gp. Oral tobacco was also associated with decreased LAT1 in women carrying female fetuses. BCRP and OATP2B1 genotypes did not appear to impact protein expression. In conclusion, prenatal tobacco use is associated with altered expression of multiple placental transporters which differs by fetal sex. As transcript levels of multiple transporters were significantly correlated with tobacco use biomarkers, eliminating prenatal tobacco use should alleviate these changes.

Transporter Regulation in Critical Protective Barriers: Focus on Brain and Placenta

Drug transporters play an important role in the maintenance of chemical balance and homeostasis in different tissues. In addition to their physiological functions, they are crucial for the absorption, distribution, and elimination of many clinically important drugs, thereby impacting therapeutic efficacy and toxicity. Increasing evidence has demonstrated that infectious, metabolic, inflammatory, and neurodegenerative diseases alter the expression and function of drug transporters. However, the current knowledge on transporter regulation in critical protective barriers, such as the brain and placenta, is still limited and requires more research. For instance, while many studies have examined P-glycoprotein, it is evident that research on the regulation of highly expressed transporters in the blood–brain barrier and blood–placental barrier are lacking. The aim of this review is to summarize the currently available literature in order to better understand transporter regulation in these critical barriers.

Neurotoxic effect of nalufin on the histology, ultrastructure, cell cycle and apoptosis of the developing chick embryo and its amelioration by selenium

The use of opioids during pregnancy has recently dramatically increased presenting major health problems, especially on the developing neonatal nervous system development. Nalufin is considered one of the most used opioid analgesics for treatment of moderate to severe pain, especially during pregnancy. The aim of the present study was firstly to assess the possible neurotoxic effects of nalufin injection during the organogenesis period of chick embryos, and second to investigate the ameliorative effects of selenium as a supplement. Fertilized chicken eggs were in ovo injected with 0.2ml of either nalufin (20 mg/kg egg) or selenium (0.1 mg/kg egg) or both. Nalufin injection resulted in cerebral cortical layer disruption, increase of Caspase-3 immunoexpression and chromatolytic nuclei, degenerated organelles, rarefied cytoplasm and hemorrhage. On the molecular levels, nalufin induced DNA fragmentation, cell cycle arrest and increased the percentage of apoptosis of the neuronal cells. Selenium combined treatment restored the three-layered structure of the cerebral cortex, decreased caspase-3 immuno-expression, improved ultrastructure and recovered cell cycle arrest, decreased apoptosis, and DNA degradation. In conclusion, nalufin treatment during pregnancy imposes great concerns and should not be used during embryonic development, on the other hands, selenium appears to be a promising neuroprotective agent against nalufin-induced neurotoxicity.

Intrauterine growth restriction alters the activity of drug metabolising enzymes in the maternal-placental-fetal unit

PURPOSE

Ten percent of pregnancies are affected by intrauterine growth restriction (IUGR), and evidence suggests that affected neonates have reduced activity of hepatic cytochrome P450 (CYP) drug metabolising enzymes. Given that almost all pregnant individuals take medications and additional medications are often required during an IUGR pregnancy, we aimed to determine the impact of IUGR on hepatic CYP activity in sheep fetuses and pregnant ewes.

METHODS

Specific probes were used to determine the impact of IUGR on the activity of several CYP isoenzymes (CYP1A2, CYP2C19, CYP2D6 and CYP3A) in sheep fetuses and pregnant ewes. Probes were administered intravenously to the ewe at 132 days (d) gestation (term 150 d), followed by blood sampling from the maternal and fetal circulation over 24 h. Maternal and fetal liver tissue was collected at 139-140 d gestation, from which microsomes were isolated and incubated with probes. Metabolite and maternal plasma cortisol concentrations were measured using Liquid Chromatography - tandem mass spectrometry (LC-MS/MS).

RESULTS

Maternal plasma cortisol concentration and maternal hepatic CYP1A2 and CYP3A activity was significantly higher in IUGR pregnancies. Maternal hepatic CYP activity was higher than fetal hepatic CYP activity for all CYPs tested, and there was minimal CYP1A2 or CYP3A activity in the late gestation fetus when assessed using in vitro methods.

CONCLUSIONS

The physiological changes to the maternal-placental-fetal unit in an IUGR pregnancy have significant effects on maternal drug metabolism, suggesting changes in medications and/or doses may be required to optimise maternal and fetal health.

The impact of intrauterine growth restriction on cytochrome P450 enzyme expression and activity

With the increased prevalence of non-communicable disease and availability of medications to treat these and other conditions, a pregnancy free from prescribed medication exposure is rare. Up to 99% of women take at least one medication during pregnancy. These medications can be divided into those used to improve maternal health and wellbeing (e.g., analgesics, antidepressants, antidiabetics, antiasthmatics), and those used to promote the baby's wellbeing in either fetal (e.g., anti-arrhythmics) or postnatal life (e.g., antenatal glucocorticoids). These medications are needed for pre-existing or coincidental illnesses in the mother, maternal conditions induced by the pregnancy itself through to conditions that arise in the fetus or that will be encountered by the newborn. Thus, medications administered to the mother may be used to treat the mother, the fetus or both. Metabolism of medications is regulated by a range of physiological processes that change during pregnancy. Other pathological processes such as placental insufficiency can in turn have both immediate and lifelong adverse health consequences for babies. Individuals born growth restricted are more likely to require medications but may also have an altered ability to metabolise these medications in fetal and postnatal life. This review aims to determine the effect of suboptimal fetal growth on the fetal expression of the drug metabolising enzymes (DMEs) that convert medications into active or inactive metabolites, and the transporters that remove both these medications and their metabolites from the fetal compartment.

Interplay of drug transporters P-glycoprotein (MDR1), MRP1, OATP1A2 and OATP1B3 in passage of maraviroc across human placenta

Special attention is required when pharmacological treatment is indicated for a pregnant woman. P-glycoprotein (MDR1) is a well-known transporter localized in the maternal blood-facing apical membrane of placental syncytiotrophoblast and is considered to play an important role in protecting the developing fetus. Maraviroc, a MDR1 substrate that is registered for treatment of HIV infection, shows a low toxicity profile, suggesting favorable tolerability also if administered to pregnant women. Nevertheless, there is only poor understanding to date regarding the extent to which it permeates across the placental barrier and what are the transport mechanisms involved. Endeavoring to clarify the passage of maraviroc across placenta, we used in this study the method of closed-circuit perfusion of maraviroc across human placental cotyledon. The data obtained confirmed slight involvement of MDR1, but they also suggest possible interaction with other transport system(s) working in the opposite direction from that of MDR1. Complementary in vitro studies, including cellular experiments on choriocarcinoma BeWo cells as well as transporter-overexpressing MDCKII and A431 cell lines and accumulation in placental fresh villous fragments, revealed maraviroc transport by MRP1, OATP1A2, and OATP1B3 transporters. Based on mRNA expression data in the placental tissue, isolated trophoblasts, and fetal endothelial cells, especially MRP1 and OATP1A2 seem to play a crucial role in cooperatively driving maraviroc into placental tissue. By the example of maraviroc, we show here the important interplay of transporters in placental drug handling and its possibility to overcome the MDR1-mediated efflux.

Dysfunction of ABC transporters at the blood-brain barrier: Role in neurological disorders

ABC (ATP-binding cassette) transporters represent one of the largest and most diverse superfamily of proteins in living species, playing an important role in many biological processes such as cell homeostasis, cell signaling, drug metabolism and nutrient uptake. Moreover, using the energy generated from ATP hydrolysis, they mediate the efflux of endogenous and exogenous substrates from inside the cells, thereby reducing their intracellular accumulation. At present, 48 ABC transporters have been identified in humans, which were classified into 7 different subfamilies (A to G) according to their phylogenetic analysis. Nevertheless, the most studied members with importance in drug therapeutic efficacy and toxicity include P-glycoprotein (P-gp), a member of the ABCB subfamily, the multidrug-associated proteins (MPRs), members of the ABCC subfamily, and breast cancer resistance protein (BCRP), a member of the ABCG subfamily. They exhibit ubiquitous expression throughout the human body, with a special relevance in barrier tissues like the blood-brain barrier (BBB). At this level, they play a physiological function in tissue protection by reducing or limiting the brain accumulation of neurotoxins. Furthermore, dysfunction of ABC transporters, at expression and/or activity level, has been associated with many neurological diseases, including epilepsy, multiple sclerosis, Alzheimer's disease, and amyotrophic lateral sclerosis. Additionally, these transporters are strikingly associated with the pharmacoresistance to central nervous system (CNS) acting drugs, because they contribute to the decrease in drug bioavailability. This article reviews the signaling pathways that regulate the expression and activity of P-gp, BCRP and MRPs subfamilies of transporters, with particular attention at the BBB level, and their mis-regulation in neurological disorders.

Contributions of Drug Transporters to Blood-Placental Barrier

The placenta is the only organ linking two different individuals, mother and fetus, termed as blood-placental barrier. The functions of the blood-placental barrier are to regulate material transfer between the maternal and fetal circulation. The main functional units are the chorionic villi within which fetal blood is separated by only three or four cell layers (placental membrane) from maternal blood in the surrounding intervillous space. A series of drug transporters such as P-glycoprotein (P-GP), breast cancer resistance protein (BCRP), multidrug resistance-associated proteins (MRP1, MRP2, MRP3, MRP4, and MRP5), organic anion-transporting polypeptides (OATP4A1, OATP1A2, OATP1B3, and OATP3A1), organic anion transporter 4 (OAT4), organic cation transporter 3 (OCT3), organic cation/carnitine transporters (OCTN1 and OCTN2), multidrug and toxin extrusion 1 (MATE1), and equilibrative nucleoside transporters (ENT1 and ENT2) have been demonstrated on the apical membrane of syncytiotrophoblast, some of which also expressed on the basolateral membrane of syncytiotrophoblast or fetal capillary endothelium. These transporters are involved in transport of most drugs in the placenta, in turn, affecting drug distribution in fetus. Moreover, expressions of these transporters in the placenta often vary along with the gestational ages and are also affected by pathophysiological factor. This chapter will mainly illustrate function and expression of these transporters in placentas, their contribution to drug distribution in fetus, and their clinical significance.

Impact of endotoxin on the expression of drug transporters in the placenta of HIV-1 transgenic (HIV-Tg) rats

BACKGROUND

Inflammatory responses in HIV (+) patients may be exacerbated due to reports of subclinical endotoxemia and existing immune dysregulation. As inflammation has been reported to mediate changes in the expression of transporters, this could be potentiated in pregnant HIV (+) women. Similar to humans, the HIV-Tg rat model develops immune dysregulation and chronic AIDS-like conditions. Our objective was to examine the expression of placental drug transporters in HIV-Tg rats in response to low-dose endotoxin.

METHODS

Pregnant HIV-Tg rats or wild-type littermates (WT) were treated with low dose bacterial endotoxin 0.1mg/kg (n=8) or 0.25mg/kg (n=4-6) on GD18 and placentas were harvested 18h later. Placental and hepatic expression of transporters and cytokines were examined using qRT-PCR and Western blotting.

RESULTS

As compared to WT, endotoxin administration increased the hepatic and placental expression of IL-6 and TNF-α to a greater extent in HIV-Tg rats (p<0.05). The placental mRNA and protein expression of Abcb1a and Slco2b1 was significantly decreased in endotoxin-treated HIV-Tg but not WT rats and downregulation of Slco4a1 mRNA was more pronounced in the HIV-Tg group (p<0.05). These changes significantly correlated with the placental expression of pro-inflammatory cytokines. Abcc3 mRNA expression was increased in the placenta of endotoxin treated WT rats only, while placental expression of Abcc1, Abcc2 and Abcc4 was not significantly affected in both WT and HIV rats. Endotoxin imposed a pronounced downregulation in the hepatic expression of Abcb1a, Abcc2, Abcc4, Abcg2, Slco1a1, Slco1b2 and Slc29a1 in both HIV-Tg and WT rats; however, Abcb1b expression was increased in HIV but not WT rats.

CONCLUSION

Our results indicate that low-dose endotoxin resulted in an augmented inflammatory response in HIV-Tg rats accompanied with significant changes in the placental expression of several drug transporters. Our data suggests that subclinical endotoxemia and other co-existing infections may alter the placental transfer of drugs in the HIV population.

Effects of prenatal and lactation nicotine exposure on glucose homeostasis, lipogenesis and lipid metabolic profiles in mothers and offspring

There is increasing evidence suggesting that maternal nicotine (NIC) exposure alone can lead to many deleterious consequences in the fetus. In this study, we aimed to evaluate the effects of prenatal and lactation NIC exposure on glucose homeostasis, lipogenesis and lipid metabolism in mothers and pups. After maternal NIC exposure (from gestational day 9 to weaning), NIC mothers showed lower body weight, decreased parametrial white adipose tissue (pWAT) and inguinal WAT weights, lower homeostasis model assessment of beta cell function, higher serum total cholesterol (TC) and low-density lipoprotein cholesterol levels, higher Castelli index values, lower hepatic mRNA levels of sterol regulatory element binding protein-1c (SREBP1c), lipoprotein lipase, acetyl-CoA carboxylase, fatty acid synthase (FAS) and glucose transporters 4 (GLUT4), as well as lower SREBP1c, FAS, leptin and GLUT4 mRNA levels in pWAT. However, female NIC pups presented higher body weights and serum TC levels, and increased trends for high density lipoprotein-cholesterol and Castelli index I. Male NIC pups had higher body weight, serum TC levels and Castelli index I values, and lower glycemia levels. Additionally, hepatic and adipose FAS gene expression from the female NIC pups presented a decreasing trend, while the male NIC pups had lower hepatic FAS expression and higher adipose FAS expression. In conclusion, prenatal and lactation NIC exposure induced deleterious effects on the glucose homeostasis, lipogenesis and lipid metabolism in both mothers and pups, which may promote several important metabolic disorders in the progeny. Additionally, there are gender-specific effects on pups.

Bioluminescent imaging of ABCG2 efflux activity at the blood-placenta barrier

Physiologic barriers such as the blood placenta barrier (BPB) and the blood brain barrier protect the underlying parenchyma from pathogens and toxins. ATP-binding cassette (ABC) transporters are transmembrane proteins found at these barriers, and function to efflux xenobiotics and maintain chemical homeostasis. Despite the plethora of ex vivo and in vitro data showing the function and expression of ABC transporters, no imaging modality exists to study ABC transporter activity in vivo at the BPB. In the present study, we show that in vitro models of the placenta possess ABCG2 activity and can specifically transport D-luciferin, the endogenous substrate of firefly luciferase. To test ABCG2 transport activity at the BPB, we devised a breeding strategy to generate a bioluminescent pregnant mouse model to demonstrate transporter function in vivo. We found that coadministering the ABCG2 inhibitors Ko143 and gefitinib with D-luciferin increased bioluminescent signal from fetuses and placentae, whereas the control P-gp inhibitor DCPQ had no effect. We believe that our bioluminescent pregnant mouse model will facilitate greater understanding of the BPB and ABCG2 activity in health and disease.

Epirubicin: a new entry in the list of fetal cardiotoxic drugs? Intrauterine death of one fetus in a twin pregnancy. Case report and review of literature

Background

Current knowledge indicate that epirubicin administration in late pregnancy is almost devoid of any fetal cardiotoxicity. We report a twin pregnancy complicated by breast cancer in which epirubicin administration was causatively linked to the death of one twin who was small for gestational age (SGA) and in a condition of oligohydramnios and determined the onset of a transient cardiotoxicity of the surviving fetus/newborn.

Case presentation

A 38-year-old caucasic woman with a dichorionic twin pregnancy was referred to our center at 20 and 1/7 weeks for a suspected breast cancer, later confirmed by the histopathology report. At 31 and 3/7 weeks, after the second chemotherapy cycle, ultrasound examination evidenced the demise of one twin while cardiac examination revealed a monophasic diastolic ventricular filling, i.e. a diastolic dysfunction of the surviving fetus who was delivered the following day due to the occurrence of grade II placental abruption. The role of epirubicin cardiotoxicity in the death of the first twin was supported by post-mortem cardiac and placental examination and by the absence of structural or genomic abnormalities that may indicate an alternative etiology of fetal demise. The occurrence of epirubicin cardiotoxicity in the surviving newborn was confirmed by the report of high levels of troponin and transient left ventricular septal hypokinesia.

Conclusion

Based on our findings we suggest that epirubicin administration in pregnancy should be preceded by the screening of some fetal conditions like SGA and oligohydramnios that may increase its cardiotoxicity and that, during treatment, the diastolic function of the fetal right ventricle should be specifically monitored by a pediatric cardiologist; also, epirubicin and desamethasone for lung maturation should not be closely administered since placental effects of glucocorticoids may increase epirubicin toxicity.

The ABCG2 efflux transporter from rabbit placenta: Cloning and functional characterization

In human placenta, the ATP-binding cassette efflux transporter ABCG2 is highly expressed in syncytiotrophoblast cells and mediates cellular excretion of various drugs and toxins. Hence, physiological ABCG2 activity substantially contributes to the fetoprotective placenta barrier function during gestation. Developmental toxicity studies are often performed in rabbit. However, despite its toxicological relevance, there is no data so far on functional ABCG2 expression in this species. Therefore, we cloned ABCG2 from placenta tissues of chinchilla rabbit. Sequencing showed 84-86% amino acid sequence identity to the orthologues from man, rat and mouse. We transduced the rabbit ABCG2 clone (rbABCG2) in MDCKII cells and stable rbABCG2 gene and protein expression was shown by RT-PCR and Western blot analysis. The rbABCG2 efflux activity was demonstrated with the Hoechst H33342 assay using the specific ABCG2 inhibitor Ko143. We further tested the effect of established human ABCG2 (hABCG2) drug substrates including the antibiotic danofloxacin or the histamine H2-receptor antagonist cimetidine on H33342 accumulation in MDCKII-rbABCG2 or -hABCG2 cells. Human therapeutic plasma concentrations of all tested drugs caused a comparable competitive inhibition of H33342 excretion in both ABCG2 clones. Altogether, we first showed functional expression of the ABCG2 efflux transporter in rabbit placenta. Moreover, our data suggest a similar drug substrate spectrum of the rabbit and the human ABCG2 efflux transporter.

Impact of bacterial and viral challenge on multidrug resistance in first- and third-trimester human placenta

The ABC transporters P-glycoprotein (P-gp, official gene symbol ABCB1) and breast cancer resistance protein (BCRP, official gene symbol ABCG2) protect the conceptus from exposure to toxins and xenobiotics present in the maternal circulation. Viral or bacterial challenges alter expression of placental multidrug transporters in rodents. We hypothesized that exposure to lipopolysaccharide (LPS, bacterial antigen) and polyinosinic-polycytidylic acid (poly(I:C), viral antigen) would decrease P-gp and BCRP in the human placenta. Placental explants from first and third trimesters were challenged with 0.1 to 10 μg/mL LPS or 1 to 50 μg/mL poly(I:C) for 4 or 24 hours; mRNA levels, protein expression, and localization were assessed by quantitative real-time PCR, Western blot analysis, and immunohistochemistry, respectively. Toll-like receptor (TLR)-3 and TLR-4 mRNA expression increased from the first to third trimester (P < 0.01), and the receptors localized to cytotrophoblasts in the first trimester and to syncytiotrophoblasts in the third trimester. LPS exposure in first-trimester explants decreased (P < 0.001) ABCB1 and ABCG2 mRNA and protein levels. In contrast, poly(I:C) decreased (P < 0.05) ABCB1, TLR-3, and TLR-4 mRNA levels in the third trimester but not first trimester. LPS and poly(I:C) treatments increased (P < 0.01) IL-8 and chemokine ligand 2. Results suggest that bacterial infections likely alter exposure of the conceptus to toxins and drugs during early pregnancy, whereas viral infections may disrupt fetal protection in later stages of pregnancy.

Copper oxide and zinc oxide nanomaterials act as inhibitors of multidrug resistance transport in sea urchin embryos: their role as chemosensitizers

The ability of engineered nanomaterials (NMs) to act as inhibitors of ATP-binding cassette (ABC) efflux transporters in embryos of white sea urchin (Lytechinus pictus) was studied. Nanocopper oxide (nano-CuO), nanozinc oxide (nano-ZnO), and their corresponding metal ions (CuSO4 and ZnSO4) were used as target chemicals. The results showed that nano-CuO, nano-ZnO, CuSO4, and ZnSO4, even at relatively low concentrations (0.5 ppm), significantly increased calcein-AM (CAM, an indicator of ABC transporter activity) accumulation in sea urchin embryos at different stages of development. Exposure to nano-CuO, a very low solubility NM, at increasing times after fertilization (>30 min) decreased CAM accumulation, but nano-ZnO (much more soluble NM) did not, indicating that metal ions could cross the hardened fertilization envelope, but not undissolved metal oxide NMs. Moreover, nontoxic levels (0.5 ppm) of nano-CuO and nano-ZnO significantly increased developmental toxicity of vinblastine (an established ABC transporter substrate) and functioned as chemosensitizers. The multidrug resistance associated protein (MRP, one of ABC transporters) inhibitor MK571 significantly increased copper concentrations in embryos, indicating ABC transporters are important in maintaining low intracellular copper levels. We show that low concentrations of nano-CuO and nano-ZnO can make embryos more susceptible to other contaminants, representing a potent amplification of nanomaterial-related developmental toxicity.

Identification of CYP3A7 for glyburide metabolism in human fetal livers

Glyburide is commonly prescribed for the treatment of gestational diabetes mellitus; however, fetal exposure to glyburide is not well understood and may have short- and long-term consequences for the health of the child. Glyburide can cross the placenta; fetal concentrations at term are nearly comparable to maternal levels. Whether or not glyburide is metabolized in the fetus and by what mechanisms has yet to be determined. In this study, we determined the kinetic parameters for glyburide depletion by CYP3A isoenzymes; characterized glyburide metabolism by human fetal liver tissues collected during the first or early second trimester of pregnancy; and identified the major enzyme responsible for glyburide metabolism in human fetal livers. CYP3A4 had the highest metabolic capacity towards glyburide, followed by CYP3A7 and CYP3A5 (Clint,u=37.1, 13.0, and 8.7ml/min/nmol P450, respectively). M5 was the predominant metabolite generated by CYP3A7 and human fetal liver microsomes (HFLMs) with approximately 96% relative abundance. M5 was also the dominant metabolite generated by CYP3A4, CYP3A5, and adult liver microsomes; however, M1-M4 were also present, with up to 15% relative abundance. CYP3A7 protein levels in HFLMs were highly correlated with glyburide Clint, 16α-OH DHEA formation, and 4'-OH midazolam formation. Likewise, glyburide Clint was highly correlated with 16α-OH DHEA formation. Fetal demographics as well as CYP3A5 and CYP3A7 genotype did not alter CYP3A7 protein levels or glyburide Clint. These results indicate that human fetal livers metabolize glyburide predominantly to M5 and that CYP3A7 is the major enzyme responsible for glyburide metabolism in human fetal livers.

Maternal and fetal metabonomic alterations in prenatal nicotine exposure-induced rat intrauterine growth retardation

Prenatal nicotine exposure causes adverse birth outcome. However, the corresponding metabonomic alterations and underlying mechanisms of nicotine-induced developmental toxicity remain unclear. The aims of this study were to characterize the metabolic alterations in biofluids in nicotine-induced intrauterine growth retardation (IUGR) rat model. In the present study, pregnant Wistar rats were intragastrically administered with different doses of nicotine (0.5, 1.0 and 2.0 mg/kg d) from gestational day (GD) 11-20. The metabolic profiles of the biofluids, including maternal plasma, fetal plasma and amniotic fluid, were analyzed using (1)H nuclear magnetic resonance (NMR)-based metabonomic techniques. Prenatal nicotine exposure caused noticeably lower body weights, higher IUGR rates of fetal rats, and elevated maternal and fetal corticosterone (CORT) levels compared to the controls. The correlation analysis among maternal, fetal serum CORT levels and fetal bodyweight suggested that the levels of maternal and fetal serum CORT presented a positive correlation (r=0.356, n=32, P<0.05), while there was a negative correlation between fetal (r=-0.639, n=32, P<0.01) and maternal (r=-0.530, n=32, P<0.01) serum CORT level and fetal bodyweight. The fetal metabonome alterations included the stimulation of lipogenesis and the decreased levels of glucose and amino acids. The maternal metabonome alterations involved the enhanced blood glucose levels, fatty acid oxygenolysis, proteolysis and amino acid accumulation. These results suggested that prenatal nicotine exposure is associated with an altered maternal and fetal metabonome, which may be related to maternal increased glucocorticoid level induced by nicotine.

Associations between ABCG2 gene polymorphisms and isolated septal defects in a Han Chinese population

Breast cancer resistance protein (BCRP) in the placenta, encoded by the ABCG2 gene in humans, plays an essential role in regulating fetal exposure to toxicants and the maintenance of cellular folic acid homeostasis. This study aimed at exploring the associations between 421C>A and 34G>A polymorphisms within the ABCG2 gene of the children and isolated septal defects in a Han Chinese population. An age- and gender-matched case-control study involving 210 pairs was conducted. Genotyping of the ABCG2 gene polymorphisms was performed by sequencing. Forty-six placental tissues and umbilical cords from healthy Han Chinese mothers with uncomplicated pregnancy were collected to investigate the impact of these two polymorphisms on the transcription and translation activities of the ABCG2 gene. The results showed that there were no differences in the genotype distributions and allele frequencies of 421C>A polymorphism. For the 34G>A polymorphism, more cases were carriers of the GA/AA genotypes (adjusted odds ratio [OR]: 1.6, 95% confidence interval [CI]: 1.0-2.3). The ABCG2 mRNA and protein expression did not differ among the three genotypes of 421C>A polymorphism. For the 34G>A polymorphism, the ABCG2 mRNA and protein expression of the GG genotype was significantly higher than that of the AA genotype. In conclusion, 34G>A polymorphism in the ABCG2 gene of the children is associated with isolated septal defects in a Han Chinese population, presumably through regulation of BCRP expression in the placenta.

Effect of Spirulina maxima and its protein extract on micronuclei induction by hydroxyurea in pregnant mice and their fetuses

The purpose of the present report was to determine the inhibitory effect of Spirulina maxima (Sm) and its protein extract (PE), mainly consisting of C-phycocyanin, on the increase in micronuclei and bone marrow cytotoxicity induced by hydroxyurea (HU) in pregnant mice and their fetuses. The two tested antimutagenic agents were administered daily from day 10 to day 18 of pregnancy, and HU (300 mg/kg) was administered once on day 16 of the assay. The experimental design also included mice that were administered only Sm or PE (1000 and 400 mg/kg, respectively), two control groups that were administered with vehicles (water and 0.5% Tween 80), and one additional group that was treated solely with HU. Blood samples from the pregnant mice and their fetuses were examined at day 19 of pregnancy. Significant increases in the number of micronucleated polychromatic erythrocytes and in the total number of micronucleated erythrocytes were observed in all HU-treated animals. In contrast, similarly low numbers of micronuclei were observed in the two control groups and in the groups treated with Sm and PE alone. The administration of Sm (100, 500, and 1000 mg/kg) and PE (100, 200, and 400 mg/kg) to HU-treated animals conferred moderate genotoxic protection (∼30%) and some protection against the cytotoxicity induced by HU in mice. The obtained results provide new information regarding the capacity of the tested agents to confer protection to adult mice and transplacentally, as well as on a specific subclass of micronuclei.

Cost, effectiveness and environmental relevance of multidrug transporters in sea urchin embryos

ATP-binding cassette transporters protect cells via efflux of xenobiotics and endogenous byproducts of detoxification. While the cost of this ATP-dependent extrusion is known at the molecular level, i.e. the ATP used for each efflux event, the overall cost to a cell or organism of operating this defense is unclear, especially as the cost of efflux changes depending on environmental conditions. During prolonged exposure to xenobiotics, multidrug transporter activity could be costly and ineffective because effluxed substrate molecules are not modified in the process and could thus undergo repeated cycles of efflux and re-entry. Here we use embryos of the purple sea urchin, Strongylocentrotus purpuratus, as a model to determine transport costs and benefits under environmentally relevant xenobiotic concentrations. Strikingly, our results show that efflux transporter activity costs less than 0.2% of total ATP usage, as a proportion of oxygen consumption. The benefits of transport, defined as the reduction in substrate accumulation due to transporter activity, depended largely, but not entirely, on the rate of passive flux of each substrate across the plasma membrane. One of the substrates tested exhibited rapid membrane permeation coupled with high rates of efflux, thus inducing rapid and futile cycles of efflux followed by re-entry of the substrate. This combination significantly reduced transporter effectiveness as a defense and increased costs even at relatively low substrate concentrations. Despite these effects with certain substrates, our results show that efflux transporters are a remarkably effective and low-cost first line of defense against exposure to environmentally relevant concentrations of xenobiotics.

Placental HSD2 Expression and Activity Is Unaffected by Maternal Protein Consumption or Gender in C57BL/6 Mice

The placenta acts as a physiological barrier, preventing the transfer of maternal glucocorticoids to the developing fetus. This is accomplished via the oxidation, and subsequent inactivation, of endogenous glucocorticoids by the 11-β hydroxysteroid dehydrogenase type 2 enzyme (HSD2). Maternal protein restriction during pregnancy has been shown to result in a decrease in placental HSD2 expression and fetal glucocorticoid overexposure, especially late in gestation, resulting in low birth weight and “fetal programming” of the offspring. This dietary intervention impairs fetal growth and cardiovascular function in adult C57BL/6 offspring, but the impact on placental HSD2 has not been defined. The goal of the current study was to examine the effects of a maternal low-protein diet (18% versus 9% protein) on placental HSD2 gene expression and enzyme activity in mice during late gestation. In contrast to previous studies in rats, a maternal low-protein diet did not affect HSD2 protein or enzyme activity levels in the placentas of C57BL/6 mice and this was irrespective of the gender of the offspring. These data suggest that the effects of maternal protein restriction on adult phenotypes in C57BL/6 mice depend upon a mechanism that may be independent of placental HSD2 or possibly occurs earlier in gestation.

Time-dependent effect of oral morphine consumption on the development of cytotrophoblast and syncytiotrophoblast cells of the placental layers during the three different periods of pregnancy in Wistar rats

Previous studies have shown that morphine abuse during pregnancy cancause a delay in the development of the placenta and embryo and also bring about birth defects. The present study investigates the effect of the duration of maternal morphine consumption during pregnancy, as well as the impacts of morphine abuse on the development of placental layers during the three different periods of pregnancy in Wistar rats. Materials and Methodology. Female Wistar rats have been used in the present study. Experimental groups received morphine (0.05 mg/mL of drinking water) after one night of coupling with male rats for mating. On 9th, 10th, and 14th days of pregnancy, pregnant animals were killed, and placentas were removed and fixed. The cells of the placentas layers were calculated by light microscope and MOTIC and SPSS software. Results. The maternal surface thickness of the placenta was significantly increased, whereasthe fetal surface thickness of placenta was significantly decreased with morphine consumption with a time-dependent manner in experimental groups, compared to control groups. Moreover, the number of trophoblast cells increased in both maternal and fetal surfaces of placenta with respect to the duration of morphine consumption which was overt in the experimental groups compared to the control groups. Conclusion. In general, the time-dependent effects of oral morphine consumption can inhibit the development and natural functioning of cytotrophoblast and syncytiotrophoblast cells of the placental layers.

Placental transfer of maraviroc in an ex vivo human cotyledon perfusion model and influence of ABC transporter expression

Nowadays, antiretroviral therapy is recommended during pregnancy to prevent mother-to-child transmission of HIV. However, for many antiretroviral drugs, including maraviroc, a CCR5 antagonist, very little data exist regarding placental transfer. Besides, various factors may modulate this transfer, including efflux transporters belonging to the ATP-binding cassette (ABC) transporter superfamily. We investigated maraviroc placental transfer and the influence of ABC transporter expression on this transfer using the human cotyledon perfusion model. Term placentas were perfused ex vivo for 90 min with maraviroc (600 ng/ml) either in the maternal-to-fetal (n = 10 placentas) or fetal-to-maternal (n = 6 placentas) direction. Plasma concentrations were determined by ultra performance liquid chromatography coupled with tandem mass spectrometry (UPLC-MS/MS). Fetal transfer rates (FTR) and clearance indexes (CLI) were calculated as ratios of fetal to maternal concentrations at steady state (mean values between 30 and 90 min) and ratios of FTR of maraviroc to that of antipyrine, respectively. ABC transporter gene expression levels were determined by quantitative reverse transcription (RT)-PCR and ABCB1 protein expression by Western blotting. For the maternal-to-fetal direction, the mean FTR and CLI were 8.0% ± 3.0 and 0.26 ± 0.07, respectively, whereas the mean CLI was 0.52 ± 0.23 for the fetal-to-maternal direction. We showed a significant inverse correlation between maraviroc CLI and ABCC2, ABCC10, and ABCC11 placental gene expression levels (P < 0.05). To conclude, we report a low maraviroc placental transfer probably involving ABC efflux transporters and thus in all likelihood associated with a limited fetal exposition. Nevertheless, these results would need to be supported by in vivo data obtained from paired maternal and cord blood samples.

Effects of rifampicin, dexamethasone, St. John's Wort and Thyroxine on maternal and foetal expression of Abcb1 and organ distribution of talinolol in pregnant rats

It is well accepted that ABCB1 plays a critical role in absorption, distribution and elimination of many xenobiotics and drugs. Only little is known about the regulation and function of ABCB1 during pregnancy. Thus, the aim of this study is to investigate maternal, placental and foetal Abcb1 expression and function in pregnant rats after induction with rifampicin, dexamethasone, St. John's wort (SJW) or thyroxine. Wistar rats were orally treated with rifampicin (250 mg/kg), SJW (1.0 g/kg), thyroxine (9 μg/kg), dexamethasone (1 mg/kg) or 0.5% methylcellulose suspension (control) for 9 days during late pregnancy (each N = 5). Afterwards, organ mRNA expression and protein content of Abcb1a were determined. Tissue concentrations of the ABCB1 probe drug talinolol were measured after repeated administration of the drug (100 mg/kg, 9 days) and after induction with oral rifampicin (250 mg/kg, 9 days, N = 5). Abcb1 expression was substantially lower in foetal than in maternal organs. Abcb1 was significantly induced by SJW in the maternal jejunum and placenta, by dexamethasone in foetal brain and liver and by thyroxine in the placenta and maternal and foetal brain. Rifampicin induced Abcb1 in all maternal and foetal organs. However, organ distribution of talinolol was not influenced by comedication of rifampicin. In conclusion, maternal and foetal Abcb1 organ expression in pregnant rats is inducible by nuclear receptor agonists. Although rifampicin regulates maternal and foetal Abcb1 expression, organ distribution of talinolol remains unchanged most likely caused by the known inhibitory effect of rifampicin on Abcb1 function.

Barriers in the developing brain and Neurotoxicology

The brain develops and grows within a well-controlled internal environment that is provided by cellular exchange mechanisms in the interfaces between blood, cerebrospinal fluid and brain. These are generally referred to by the term "brain barriers": blood-brain barrier across the cerebral endothelial cells and blood-CSF barrier across the choroid plexus epithelial cells. An essential component of barrier mechanisms is the presence of tight junctions between the endothelial and epithelial cells of these interfaces. This review outlines historical evidence for the presence of effective barrier mechanisms in the embryo and newborn and provides an up to date description of recent morphological, biochemical and molecular data for the functional effectiveness of these barriers. Intercellular tight junctions between cerebral endothelial cells and between choroid plexus epithelial cells are functionally effective as soon as they differentiate. Many of the influx and efflux mechanisms are not only present from early in development, but the genes for some are expressed at much higher levels in the embryo than in the adult and there is physiological evidence that these transport systems are functionally more active in the developing brain. This substantial body of evidence supporting the concept of well developed barrier mechanisms in the developing brain is contrasted with the widespread belief amongst neurotoxicologists that "the" blood-brain barrier is immature or even absent in the embryo and newborn. A proper understanding of the functional capacity of the barrier mechanisms to restrict the entry of harmful substances or administered therapeutics into the developing brain is critical. This knowledge would assist the clinical management of pregnant mothers and newborn infants and development of protocols for evaluation of risks of drugs used in pregnancy and the neonatal period prior to their introduction into clinical practice.

Enhancement of placental antioxidative function and P-gp expression by sodium ferulate mediated its protective effect on rat IUGR induced by prenatal tobacco/alcohol exposure

This study was aimed to explore the therapeutic effect of sodium ferulate (SF) on rats with intrauterine growth retardation (IUGR), and then to clarify the corresponding mechanism. Pregnant rats were divided into normal group, tobacco/alcohol exposure group, and tobacco/alcohol+SF groups. Fetal developmental indices, placental weight, histological alteration, oxidative and antioxidative-function (e.g. MDA, SOD, CAT) and Mdr1 levels were assayed. Results showed exposure to tobacco/alcohol resulted in reduced fetal developmental indices and placental histological alteration, as well as the increased MDA content, decreased SOD and CAT activities and decreased Mdr1a level. After SF treatment, fetal developmental indices, and placental weight, histological alteration, oxidative and antioxidative-function and mdr1a levels were reversed. Our study indicated SF may be effective in reversing IUGR production, and its underlying mechanism may be due to enhanced placental antioxidative function and P-gp expression, which may be related to IUGR formation by tobacco/alcohol exposure.

ATP-binding cassette efflux transporters in human placenta

Pregnant women are often complicated with diseases including viral or bacterial infections, epilepsy, hypertension, or pregnancy-induced conditions such as depression and gestational diabetes that require treatment with medication. In addition, substance abuse during pregnancy remains a major public health problem. Many drugs used by pregnant women are off label without the necessary dose, efficacy, and safety data required for rational dosing regimens of these drugs. Thus, a major concern arising from the widespread use of drugs by pregnant women is the transfer of drugs across the placental barrier, leading to potential toxicity to the developing fetus. Knowledge regarding the ATP-binding cassette (ABC) efflux transporters, which play an important role in drug transfer across the placental barrier, is absolutely critical for optimizing the therapeutic strategy to treat the mother while protecting the fetus during pregnancy. Such transporters include P-glycoprotein (P-gp, gene symbol ABCB1), the breast cancer resistance protein (BCRP, gene symbol ABCG2), and the multidrug resistance proteins (MRPs, gene symbol ABCCs). In this review, we summarize the current knowledge with respect to developmental expression and regulation, membrane localization, functional significance, and genetic polymorphisms of these ABC transporters in the placenta and their relevance to fetal drug exposure and toxicity.

Clinical therapeutics in pregnancy

Most drugs are not tested for use during pregnancy, consequently, labeling, which may include information about fetal safety, includes nothing about dosing, efficacy, or maternal safety. Yet these are concerns of health care providers considering treatment of disease during pregnancy. Therefore, the practitioner treats the pregnant woman with the same dose recommended for use in adults (typically men) or may decide not to treat the disease at all. However, is the choice of not treating a woman during pregnancy better than dealing with the challenges which accompany treatment? This paper, which summarizes metabolic and physiologic changes induced by pregnancy, illustrates that standard adult dosing is likely to be incorrect during pregnancy.

P-glycoprotein: tissue distribution, substrates, and functional consequences of genetic variations

P-glycoprotein (ABCB1, MDR1) belongs to the ABC transporter family transporting a wide range of drugs and xenobiotics from intra- to extracellular at many biological interfaces such as the intestine, liver, blood-brain barrier, and kidney. The ABCB1 gene is highly polymorphic. Starting with the observation of lower duodenal protein expression and elevated digoxin bioavailability in relation to the 3435C>T single nucleotide polymorphism, hundreds of pharmacokinetic and outcome studies have been performed, mostly genotyping 1236C>T, 2677G>T/A, and 3435C>T. Though some studies pointed out that intracellular concentrations of anticancer drugs, for example, within lymphocytes, might be affected by ABCB1 variants resulting in differential outcome, current knowledge of the functional significance genetic variants of ABC membrane transporters does not allow selection of a particular SNP to predict an individual's pharmacokinetics.

[Contributions of the ex vivo human perfused placenta in the study of placental transfer of drugs]

Perfused human placental lobule was developed during the 1970s. Only this model respects the anatomical features of the human placenta. This approach allows different technical conditions (concentrations of drugs…) without ethical problems. Limitations of this ex vivo model are detailed in this review, also its recent contributions in better understanding of placental passage of drugs.

Modulation of endocrine and transport functions in human trophoblasts by saquinavir and nelfinavir

OBJECTIVES

The distribution of drugs to the maternal-fetal interface is influenced by the expression of various efflux transporters. Among these transporters, P-glycoprotein (P-gp) is responsible for the efflux of a great number of drugs such as protease inhibitors of the human immunodeficiency virus, thus reducing the chemical exposure of the fetus.

STUDY DESIGN

The effects of saquinavir and nelfinavir were evaluated on human trophoblast functions and integrity by investigating their effect on human chorionic gonadotropin (hCG) secretion and on P-gp expression and functionality.

RESULTS

Nelfinavir significantly reduced hCG secretion by 30% after a 48-h treatment but it had no effect on syncytia formation. Saquinavir had no effect on hCG secretion but significantly increased both expression (to a 2-fold extent) and functionality (by 17.9%) of P-gp, whereas nelfinavir only increased functionality (by 23.1%) with a dissociation of P-gp from caveolin-1.

CONCLUSION

These results suggest that the effects of saquinavir and nelfinavir differ on trophoblast functions.

Drug transporters in the human blood-placental barrier

Studies on the increasing number of transporters found in the placental barrier are gaining momentum, because of their tissue-specific expression, significance in physiology and disease, and the possible utilization of the emerging knowledge in pharmacology. In the placenta, both syncytiotrophoblast and fetal capillary endothelium express transporters. Fetal exposure is determined by the net effect of combination of transporters, their nature and localization in relation to placental cells and their substrate specificity. Although the significance of placental transporters on human fetal drug exposure is almost an unstudied field so far, their potential use to design drugs that do not cross the placenta is already being pursued. It is thus of interest to review the existing knowledge of human placental transporters. Transporters in all groups which take part in drug transport are found in human placenta. Especially, ATP-binding cassette transporters ABCG2/breast cancer resistance protein, ABCB1/P-glycoprotein and ABCC2/MRP2 are all expressed at the apical surface of syncytiotrophoblast facing maternal blood and are putatively important protective proteins both for placental tissue and the fetus, because they are efflux transporters and their substrates include many drugs and also environmental chemicals. Such protective effect has been shown in animals, but these results cannot be directly extrapolated to humans due to interspecies differences in placental structure and function. Experimental models utilizing human placental tissue, especially human placental perfusion, offer valuable possibilities, which have been insufficiently studied so far.

Nuclear receptors CAR and PXR: Molecular, functional, and biomedical aspects

Nuclear receptors (NRs) are ligand-activated transcription factors sharing a common evolutionary history and having similar sequence features at the protein level. Selective ligand(s) for some NRs is not known, therefore these NRs have been named "orphan receptors". Whenever ligands have been recognized for any of the orphan receptor, it has been categorized and grouped as "adopted" orphan receptor. This group includes the constitutive androstane receptor (CAR) and the pregnane X receptor (PXR). They function as sensors of toxic byproducts derived from endogenous metabolites and of exogenous chemicals, in order to enhance their elimination. This unique function of CAR and PXR sets them apart from the steroid hormone receptors. The broad response profile has established that CAR and PXR are xenobiotic sensors that coordinately regulate xenobiotic clearance in the liver and intestine via induction of genes involved in drug and xenobiotic metabolism. In the past few years, research has revealed new and mostly unsuspected roles for CAR and PXR in modulating hormone, lipid, and energy homeostasis as well as cancer and liver steatosis. The purpose of this review is to highlight the structural and molecular bases of CAR and PXR impact on human health, providing information on mechanisms through which diet, chemical exposure, and environment ultimately impact health and disease.

Tissue distribution, gender-divergent expression, ontogeny, and chemical induction of multidrug resistance transporter genes (Mdr1a, Mdr1b, Mdr2) in mice

Multidrug resistance (Mdr) transporters are ATP-binding cassette transporters that efflux amphipathic cations from cells and protect tissues from xenobiotics. Unfortunately, Mdr transporters also efflux anticancer drugs from some tumor cells, resulting in multidrug resistance. There are two groups of Mdrs in mice: group I includes Mdr1a and Mdr1b that transport xenobiotics, whereas group II is Mdr2, a flipase that facilitates phospholipid excretion into bile. Little is known about the regulation of Mdr genes in vivo. The purpose of this study was to determine tissue distribution, gender differences, ontogeny, and chemical induction of Mdrs in mice. The mRNA of Mdr1a is highest in gastrointestinal tract, Mdr1b in ovary and placenta, and Mdr2 in liver. Both Mdr1a and Mdr1b in kidney show female-predominant expression patterns due to repression by androgens. The ontogeny of mouse Mdr1a in duodenum and brain as well as Mdr1b in brain, kidney, and liver all share a similar developmental pattern: low expression at birth, followed by a gradual increase to mature levels at approximately 30 days of age. In contrast, Mdr2 mRNA in liver is markedly up-regulated at birth, which returns to low levels by 5 days of age and then gradually increases to mature levels. None of the Mdrs in liver are readily inducible by any class of microsomal enzyme inducers. In conclusion, the three Mdr transporters in mice are expressed in a tissue-specific and age-dependent pattern, there are gender differences in expression, and Mdr transporters are inducible by only a few microsomal enzyme inducers.

Tissue distribution of YM758, a novel If channel inhibitor, in pregnant and lactating rats

In this study the tissue distribution of radioactivity in pregnant and lactating rats was investigated by quantitatively determining radioactivity concentrations and by whole-body autoradioluminograms after a single oral administration of 14C-YM758. In addition, the transfer of radioactivity into the reproductive tissues, foetus, and milk is discussed in terms of the localization of transporters in syncytiotrophoblast and mammary gland. The radioactivity concentrations in the liver were the highest of all the tissues and organs tested at all the sampling times. The radioactivity in main tissues (liver and kidney), including reproductive tissues (amniotic fluid, placenta, ovary, and uterus), was not retained for a long time, as in the plasma. The tissue/plasma (T/P) ratio of radioactivity in the foetus was below 1.0, which might be due to Mdr1-mediated export of YM758 into blood via the blood-placenta barrier since YM758 is a substrate for hMDR1, not for hBCRP/rBcrp. The T/P ratio of radioactivity in the maternal milk 1 and 4 h after oral administration of 14C-YM758 was 7.2 and 11.0, respectively. To understand better the distribution of new drugs into the reproductive tissues/milk, and to interpret further the results of reproductive safety studies for drug development, the contribution of transporters expressed in the blood-placenta barrier and mammary gland to the drug-transfer into placenta and milk should be considered.

BCRP/ABCG2 in the placenta: expression, function and regulation

Knowledge concerning transport of maternally administered drugs across the placental barrier is essential for determining potential toxicity of drugs to the fetus and the value of drug therapy during pregnancy. An important determinant for fetal drug exposure is the expression of efflux transporters in the placenta. Among human tissues, the ATP-binding cassette efflux transporter BCRP (gene symbol ABCG2) is most abundantly expressed in the apical membrane of placental syncytiotrophoblasts. Although the precise physiological role of BCRP in the placenta is still unclear, existing data strongly suggest that BCRP plays an important role in protecting the fetus against the potential toxicity of drugs, xenobiotics, and metabolites by expelling them across the placental barrier. In this review, we summarize the current knowledge with respect to the expression, function, and polymorphisms of BCRP, as well as transcriptional and posttranscriptional regulation of the transporter in the placenta. Finally, clinical significance of BCRP in the placenta for drug therapy in pregnant women is discussed.