Drug Transfer and Metabolism by the Human Placenta

Pharmacologic and nonpharmacologic options for the management of HIV infection during pregnancy

Over the past decade, significant advances have been made in the treatment of HIV-1 infection using both pharmacologic and nonpharmacologic strategies to prevent mother-to-child transmission (MTCT). Optimal prevention of the MTCT of HIV requires antiretroviral drugs (ARV) during pregnancy, during labor, and to the infant. ARVs reduce viral replication, lowering maternal plasma viral load and thus the likelihood of MTCT. Postexposure prophylaxis of ARV agents in newborns protect against infection following potential exposure to maternal HIV during birth. In general, the choice of an ARV for treatment of HIV-infected women during pregnancy is complicated by the need to consider the effectiveness of the therapy for the maternal disease as well as the teratogenic or teratotoxic potential of these drugs. Clinicians managing HIV in pregnancy need to discuss the potential risks and benefits of available therapy options so that mothers can make informed decisions in choosing the best treatment regimen for themselves and for their children.

Hydroxylated metabolites of polybrominated diphenyl ethers in human blood samples from the United States

BACKGROUND

A previous study from our laboratory showed that polybrominated diphenyl ethers (PBDEs) were metabolized to hydroxylated PBDEs (HO-PBDEs) in mice and that para-HO-PBDEs were the most abundant and, potentially, the most toxic metabolites.

OBJECTIVE

The goal of this study was to determine the concentrations of HO-PBDEs in blood from pregnant women, who had not been intentionally or occupationally exposed to these flame retardants, and from their newborn babies.

METHODS

Twenty human blood samples were obtained from a hospital in Indianapolis, Indiana, and analyzed for both PBDEs and HO-PBDEs using electron-capture negative-ionization gas chromatographic mass spectrometry.

RESULTS

The metabolite pattern of HO-PBDEs in human blood was quite different from that found in mice; 5-HO-BDE-47 and 6-HO-BDE-47 were the most abundant metabolites of BDE-47, and 5'-HO-BDE-99 and 6'-HO-BDE-99 were the most abundant metabolites of BDE-99. The relative concentrations between precursor and corresponding metabolites indicated that BDE-99 was more likely to be metabolized than BDE-47 and BDE-100. In addition, three bromophenols were also detected as products of the cleavage of the diphenyl ether bond. The ratio of total hydroxylated metabolites relative to their PBDE precursors ranged from 0.10 to 2.8, indicating that hydroxylated metabolites of PBDEs were accumulated in human blood.

CONCLUSIONS

The quite different PBDE metabolite pattern observed in humans versus mice indicates that different enzymes might be involved in the metabolic process. Although the levels of HO-PBDE metabolites found in human blood were low, these metabolites seemed to be accumulating.

In Vitro Models Using the Human Placenta to Study Fetal Exposure to Drugs

Over the recent years there has been a gradual rise in the use of pharmaceuticals during pregnancy. Knowledge on placental drug transfer and metabolism has increased during the past decades as well. Investigation of the transplacental transfer of any therapeutically useful drug is essential to the understanding of its metabolic processes and is a prerequisite for its use during pregnancy. The purpose of this review is to give insight on the various techniques that have been developed to evaluate transplacental transfer of drugs and xenobiotics.

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.

Accumulation of CVIET Pfcrt allele of Plasmodium falciparum in placenta of pregnant women living in an urban area of Dakar, Senegal

OBJECTIVES

Chemoprophylaxis is recommended during pregnancy to reduce the risk of placental infection. However, in areas with increasing drug resistance, it can trigger selection of resistant parasites in the placenta and increase the frequency of placental malaria. The objective of this study was to analyse the selection of drug-resistant parasites in the placenta in an area where chloroquine was still recommended as prophylaxis.

PATIENTS AND METHODS

We analysed the polymorphism of parasites from matched placental and venous blood samples at the time of delivery from women in Dakar. Polymorphism of the isolates was studied using nested PCR typing of MSA1 and MSA2 genes, and full sequence of PfCRT exon 2.

RESULTS

Of 692 women recruited at delivery, 72 had placental malaria. Two Pfcrt exon 2 genotypes were found, and 86% of the placentas had monoallelelic CVIET infection compared with 39% that had peripheral blood infection. Mixed parasite populations of CVIET/CVMNK occurred in 53% of the peripheral blood samples but only in 7% of the infected placentas. This selection of CVIET in placenta was not related to a decreased polymorphism of the parasites, as a large diversity of MSA1 and MSA2 was found in both placenta and venous blood. This diversity confirms that a multiplicity of circulation isolates can occur at low parasite transmission. msp1 and msp2 genotyping revealed mostly distinct populations of parasites in venous and placental blood.

CONCLUSIONS

These data suggest that, even in low transmission areas, diverse parasite populations can accumulate in the placenta during pregnancy despite strong selection at the PfCRT locus due to chemoprophylaxis with chloroquine.

ABCG2/BCRP decreases the transfer of a food-born chemical carcinogen, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in perfused term human placenta

We have studied the role of ATP binding cassette (ABC) transporters in fetal exposure to carcinogens using 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) a known substrate for ABC transporters as a model compound. In perfusion of human term placenta, transfer of (14)C-PhIP (2 microM) through the placenta resulted in fetal-to-maternal concentration ratio (FM ratio) of 0.72+/-0.09 at 6 h. The specific ABCG2 inhibitor KO143 increased the transfer of (14)C-PhIP from maternal to fetal circulation (FM ratio 0.90+/-0.08 at 6 h, p<0.05) while the ABCC1/ABCC2 inhibitor probenecid had no effect (FM ratio at 6 h 0.75+/-0.10, p=0.84). There was a negative correlation between the expression of ABCG2 protein in perfused tissue and the FM ratio of (14)C-PhIP (R=-0.81, p<0.01) at the end of the perfusion. The expression of ABCC2 protein did not correlate with FM ratio of PhIP (R: -0.11, p=0.76). In addition, PhIP induced the expression of ABC transporters in BeWo cells at mRNA level. In conclusion, our data indicates that ABCG2 decreases placental transfer of (14)C-PhIP in perfused human placenta. Also, PhIP may modify ABC transporter expression in choriocarcinoma cells.

A reproductive screening test of hawthorn

AIM OF STUDY

Hawthorn (Crataegus) has a long history as a medicine. The current clinical use of hawthorn as a heart medicine dates back to the late 19th century. It is well tolerated clinically yet contraindicated in pregnancy.

MATERIALS AND METHODS

To determine the safety of hawthorn to the developing fetus, pregnant rats were dosed daily by gavage using 56 times the human dose of hawthorn on either gestation days (GD) 1-8 or GD 8-15. On GD 20, fetuses were weighed and examined for signs of external, internal or skeletal malformations. Rat fetuses were also explanted on GD 10.5 and cultured with hawthorn extract for 26 h.

RESULTS

Hawthorn did not have an adverse effect on embryonic development in vivo or in vitro.

CONCLUSIONS

While the results suggest that hawthorn, taken at the recommended dose would have no adverse effects on embryonic development this may be due to the low bioavailability of some hawthorn constituents when taken orally. Pharmacokinetic studies are required to determine the extent of absorption of hawthorn from the small intestine in healthy adults in order to verify its safety.

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

The placenta serves an important role both as a protective barrier as well as in normal fetal development. The ATP-binding cassette (ABC) proteins perform crucial functions in the distribution of nutrients and exchange of waste metabolites across the placenta. They also protect the developing fetus from xenobiotics to which the pregnant mother is exposed. Recent studies in P-glycoprotein (P-gp) deficient mdr1a and mdr1b (-/-) CF-1 mice have shown pronounced increases in fetal exposure to P-gp substrates due to increased transplacental penetration demonstrating the important protective role of P-gp to the developing fetus. The role of placental ABC transporter proteins in protecting the fetus against maternal exposure to drugs, toxins and other xenobiotics is discussed. Overall, the paucity of information available on the transplacental transfer of drugs emphasizes the need to further employ preclinical in vivo models for drug development in order to best predict fetal outcomes of drug administration to pregnant mothers.

Changes of multiple biotransformation phase I and phase II enzyme activities in human fetal adrenals during fetal development

AIM

Fetal adrenal, which synthesizes steroid hormones, is critical to fetal growth and development. Our recent research showed that some xenobiotics could interfere with steroidogenesis and induce intrauterine growth retardation in rats. The study on the characteristics of biotransformation enzymes in fetal adrenals still seems to be important with respect to possible significance in xenobiotic-induced fetal development toxicity. In this study, the activities of several important xenobiotic-related phase I and phase II enzymes in human fetal adrenals were examined and compared with those in fetal livers.

METHODS

The activity and mRNA expression were determined by enzymatic analysis and RT-PCR.

RESULTS

The levels of cytochrome (CYP)2A6, CYP2E1, and CYP3A7 isozymes in fetal adrenals were 82%, 92%, and 33% of those in fetal livers, respectively. There was a good positive correlation between adrenal CYP2A6 activity and gestational time. The values of alpha glutathione S-transferase (GST), pi-GST, and microGST in adrenals were 0.5, 4.4, and 8.3-fold of those in the livers, respectively, and the activity of adrenal pi-GST was negatively correlated with gestational time. The uridine diphosphoglucuronyl transferase activities, which were measured using p-hydroxy-biphenyl and 7-hydroxy-4-methylcoumarin as substrates, were 9% and 3%, respectively, of those in the fetal livers.

CONCLUSION

Our investigation suggested that adrenal could be an important xenobiotic-metabolizing organ in fetal development and may play a potential role in xenobiotic-induced fetal development toxicity.

The emerging importance of transporter proteins in the psychopharmacological treatment of the pregnant patient

P-glycoprotein, breast cancer resistance protein, and multidrug resistance proteins have physiological functions in placental tissue. Several antidepressants, antipsychotics, and anti-epileptic drugs have been found to be substrates of P-glycoprotein and other transporters. The extent that drugs pass through the placental barrier is likely influenced by drug transporters. The rational choice of psychoactive drugs to treat mental illness in women of child-bearing age should incorporate knowledge of both drug disposition as well as expected pharmacologic effects. This review summarizes the current data on drug transporters in the placental passage of medications, with a focus on medications used in clinical psychopharmacology.

Immunosuppressant therapy in pregnant organ transplant recipients

Transplant recipients are becoming pregnant with increasing frequency, and successful pregnancy outcomes have now been reported for women with all types of solid organ transplants. To prevent rejection of the transplanted organ, these patients are maintained on a life-long immunosuppressive regimen that must also be continued through pregnancy. Controlled human studies of the safety of these drugs have not been conducted, and knowledge regarding the pharmacokinetics of these medications in pregnancy is limited. Significant experience and safety data regarding the use of some of the more common immunosuppressants in pregnancy have, however, been accumulated from large case series and national registries. These observational studies suggest that successful pregnancy outcomes are possible in female organ transplant recipients, although sporadic adverse outcomes have been reported after immunosuppressant use in pregnancy. In this chapter, we will outline the information available regarding the use of immunosuppressive medications in pregnant transplant recipients as well as general concepts regarding fetal exposure to immunosuppressants.

The human placenta – An alternative for studying foetal exposure

Pregnant women are daily exposed to a wide selection of foreign substances. Sources are as different as lifestyle factors (smoking, daily care products, alcohol consumption, etc.), maternal medication or occupational/environmental exposures. The placenta provides the link between mother and foetus, and though its main task is to act as a barrier and transport nutrients and oxygen to the foetus, many foreign compounds are transported across the placenta to some degree and may therefore influence the unborn child. Foetal exposures to environmental and medicinal products may have impact on the growth of the foetus (e.g. cigarette smoke) and development of the foetal organs (e.g. methylmercury and thalidomide). The scope of this review is to give insight to the placental anatomy, development and function. Furthermore, the compounds physical properties and the transfer mechanism across the placental barrier are evaluated. In order to determine the actual foetal risk from exposure to a chemical many studies regarding the topic are necessary, including means of transportation, toxicological targets and effects. For this purpose several in vivo and in vitro models including the placental perfusion system are models of choice.

Examination of Glucocorticoid Receptor α-Mediated Transcriptional Regulation of P-glycoprotein, CYP3A4, and CYP2C9 Genes in Placental Trophoblast Cell Lines

The placental trophoblast at different stages of pregnancy contains some drug transporters and xenobiotic-metabolising enzymes, as well as ligand-activated nuclear receptors, which control their inducible transcriptional regulation. Glucocorticoid receptor alpha (GRalpha) is expressed in both placental syncytiotrophoblast and cytotrophoblast. GRalpha was shown to control inducible expression of several enzymes of the cytochrome P-450 family (CYP) and the drug transporter P-glycoprotein in the liver. However, GRalpha-mediated transcriptional regulation of drug transporters and CYPs has not been studied in the placental trophoblast. In this study, we examined the expression and activity of GRalpha in the transcriptional regulation of P-glycoprotein, CYP3A4, and CYP2C9 in placental trophoblast cell lines. Employing RT-PCR, Western blotting, and luciferase gene reporter assay, we detected the expression and activity of GRalpha in JEG3 and BeWo cell lines. However, we observed that only MDR1 mRNA was up-regulated after treatment of placental cells with dexamethasone. Accordingly, only the promoter of the MDR1 gene was activated by dexamethasone in gene reporter assays in placental cells and the activation was abolished by RU486, an antagonist of GRalpha. CYP3A4 and CYP2C9 promoters were activated in placental cells only after co-transfection with hepatocyte nuclear factor 4alpha (HNF4alpha), which indicates the hepatocyte-specific character of GRalpha-mediated regulation of the genes. On the other hand, coexpression of HNF4alpha had no effect on the activation of the MDR1 gene promoter, suggesting HNF4alpha-independent regulation via GRalpha. We conclude that GRalpha may be involved in the transcriptional regulation of P-glycoprotein in the placental trophoblast. We also indicate that the CYP3A4 and CYP2C9 genes are not inducible through GRalpha in placental cell lines, due to the lack of HNF4alpha expression and possibly some additional hepatocyte-specific transcriptional factors.

The fate and effects of xenobiotics in human placenta

During past decades, knowledge on placental drug metabolism and mechanisms of placental transfer has increased significantly. Most pharmaceutical drugs administered during pregnancy cross the placenta to some extent. The important properties determining the placental transfer by passive diffusion are molecular weight, pK(a), lipid solubility and protein binding. In addition to passive diffusion, compounds may cross the placenta via active transfer, facilitated diffusion, phagocytosis and pinocytosis. This review gives an update of efflux transporter proteins and xenobiotic-metabolizing enzymes that modify the fate and effects of drugs in the placenta.

Using pharmacokinetics to predict the effects of pregnancy and maternal-infant transfer of drugs during lactation

Knowledge of pharmacokinetics and the use of a mechanistic-based approach can improve our ability to predict the effects of pregnancy for medications when data are limited. Despite the many physiological changes that occur during pregnancy that could theoretically affect absorption, bioavailability does not appear to be altered. Decreased albumin and alpha(1)-acid glycoprotein concentrations during pregnancy will result in decreased protein binding for highly bound drugs. For drugs metabolised by the liver, this can result in misinterpretation of total plasma concentrations of low extraction ratio drugs and overdosing of high extraction ratio drugs administered by non-oral routes. Renal clearance and the activity of the CYP isozymes, CYP3A4, 2D6 and 2C9, and uridine 5'-diphosphate glucuronosyltransferase are increased during pregnancy. In contrast, CYP1A2 and 2C19 activity is decreased. The dose of a drug an infant receives during breastfeeding is dependent on the amount excreted into the breast milk, the daily volume of milk ingested and the average plasma concentration of the mother. The lipophilicity, protein binding and ionisation properties of a drug will determine how much is excreted into the breast milk. The milk to plasma concentration ratio has large inter- and intrasubject variability and is often not known. In contrast, protein binding is usually known. An extensive literature review was done to identify case reports including infant concentrations from breast-fed infants exposed to maternal drugs. For drugs that were at least 85% protein bound, measurable concentrations of drug in the infant did not occur if there was no placental exposure immediately prior to or during delivery. Knowledge of the protein binding properties of a drug can provide a quick and easy tool to estimate exposure of an infant to medication from breastfeeding.

Nicardipine in pre-eclamptic patients: placental transfer and disposition in breast milk

To assess the safety risks to the fetus and neonate caused by maternal use of nicardipine in pre-eclamptic patients, we evaluated the placental transfer and the transfer to breast milk after maternal intravenous administration of nicardipine. In ten pre-eclamptic subjects, nicardipine concentrations of maternal blood (P) and both arterial and venous umbilical cord blood samples (Uarterial and Uvenous) were assessed, and the U/P ratio was calculated as an indication of placental transfer. We found a median transfer of 0.15 (Uarterial/P, range 0.05-0.22) and 0.17 (Uvenous/P, range 0.023-0.22). The highest umbilical cord concentration found after maternal dosage of 4.5 mg/hour was 18 ng/ml, which can be considered as subtherapeutic. Therefore, adverse fetal reactions caused by a direct pharmacological effect of nicardipine are unlikely to occur. Nicardipine levels were determined in 34 breast milk samples of seven women, and were found to be undetectable in 82% of the samples. In six breast milk samples of four different women, nicardipine levels (ranging from 5.1 to 18.5 ng/ml) were detectable during maternal nicardipine dosages ranging from 1 to 6.5 mg/hour. The maximum possible exposure of a neonate to nicardipine was calculated to be less than 300 ng/day, which is an insignificant fraction of therapeutic dosages used in neonates. In conclusion, the exposure of a fetus and neonate to nicardipine through placental transfer and disposition in breast milk expression is low.

The effect of opiates on the activity of human placental aromatase/CYP19

Aromatase, cytochrome P450 19, is a key enzyme in the biosynthesis of estrogens by the human placenta. It is also the major placental enzyme that metabolizes the opiates L-acetylmethadol (LAAM), methadone, and buprenorphine (BUP). Methadone and BUP are used in treatment of the opiate addict and are competitive inhibitors of testosterone conversion to estradiol (E(2)) and 16alpha-hydroxytestosterone (16-OHT) to estriol (E(3)) by aromatase. The aim of this investigation is to determine the effect of 20 opiates, which can be administered to pregnant patients for therapeutic indications or abused, on E(2) and E(3) formation by placental aromatase. Data obtained indicated that the opiates increased, inhibited, or had no effect on aromatase activity. Their effect on E(3) formation was more pronounced than that on E(2) due to the lower affinity of 16-OHT than testosterone to aromatase. The K(i) values for the opiates that inhibited E(3) formation were sufentanil, 7 +/- 1 microM; LAAM, 13 +/- 8 microM; fentanyl, 25 +/- 5 microM; oxycodone, 92 +/- 22 microM; codeine, 218 +/- 69 microM; (+)-pentazocine, 225 +/- 73 microM. The agonists morphine, heroin, hydromorphone, oxymorphone, hydrocodone, propoxyphene, meperidine, levorphanol, dextrorphan, and (-)-pentazocine and the antagonists naloxone and naltrexone caused an increase in E(3) formation by 124-160% of control but had no effect on E(2) formation. Moreover, oxycodone and codeine did not inhibit E(2) formation and the IC(50) values for fentanyl, sufentanil, and (+)-pentazocine were >1000 microM. It is unlikely that the acute administration of the opiates that inhibit estrogen formation would affect maternal and/or neonatal outcome. However, the effects of abusing any of them during the entire pregnancy are unclear at this time.

P-glycoprotein in the placenta: Expression, localization, regulation and function

Detailed understanding of the mechanisms employed in transfer of drugs across the placenta is essential for optimization of pharmacotherapy during pregnancy. Disclosure of drug efflux transporters as an "active component" of the placental barrier has brought new important insights into the field of transplacental pharmacokinetics. P-glycoprotein (P-gp, MDR1) is the first discovered and so far the best characterized of drug efflux transporters, whose role in the regulation of drug disposition to the fetus has been extensively studied. Expression of P-gp in the placental trophoblast layer was confirmed at the mRNA and protein levels in all phases of pregnancy, and several in vitro and in vivo studies demonstrated functional activity of the transporter in materno-fetal drug transport. P-gp is able to actively pump drugs and other xenobiotics from trophoblast cells back to the maternal circulation, providing thus protection to the fetus. This review summarizes the current knowledge on the expression, localization and function of P-gp in the placenta. In addition, we include the latest data concerning transcriptional regulation of placental P-gp expression and polymorphisms of the MDR1 gene. Clinical significance of placental P-gp and its future perspectives for pharmacotherapy during pregnancy are also discussed.

Differential protein binding of indinavir and saquinavir in matched maternal and umbilical cord plasma

AIMS

To determine whether lower umbilical cord than maternal binding of indinavir and saquinavir contributed to the low cord : maternal (C : M) total concentration ratios reported previously.

METHODS

Indinavir and saquinavir unbound fraction (fu) was determined using equilibrium dialysis. Buffer solutions of human serum albumin (HSA) (20.0, 30.0, 40.0 g l(-1)) and alpha(1)-acid glycoprotein (AAG) (0.20, 0.60, 2.00 g l(-1)) were spiked with indinavir (1.00 and 8.00 mg l(-1)) or saquinavir (0.15 and 1.50 mg l(-1)). Matched maternal and umbilical cord plasma was spiked with 1.00 mg l(-1) indinavir (n = 12) or 0.15 mg l(-1) saquinavir (n = 20). Spiked protein/plasma solutions were dialyzed against isotonic phosphate buffer, at 37 degrees C. At equilibrium, indinavir and saquinavir concentrations were quantified, and the f(u) determined.

RESULTS

Indinavir and saquinavir demonstrated protein concentration-dependent binding in buffer solutions of HSA and AAG. Indinavir f(u) was significantly higher in umbilical cord (0.53 +/- 0.12) compared with maternal (0.36 +/- 0.11) plasma (95% CI of the difference -0.26, -0.097). Similarly, saquinavir fu was different between umbilical cord (0.0090 +/- 0.0046) and maternal plasma (0.0066 +/- 0.0039) (95% CI of the difference -0.0032, -0.0016). The transplacental AAG concentration gradient contributed significantly to the binding differential of both drugs.

CONCLUSIONS

The differential plasma binding of both drugs, which was largely the result of the transplacental AAG concentration gradient, would contribute to the low C : M total plasma concentration ratios observed previously. Unbound concentrations of indinavir and saquinavir are likely to be substantially lower in umbilical cord than maternal plasma.

Metabolic and genetic factors contributing to alcohol induced effects and fetal alcohol syndrome

Alcohol-related damages on newborns and infants include a wide variety of complications from facial anomalies to neurodevelopmental delay, known as fetal alcohol syndrome (FAS). However, only less than 10% of women drinking alcohol during pregnancy have children with FAS. Understanding the risk factors increasing the probability for newborn exposed in utero to alcohol to develop FAS is therefore a key issue. The involvement of genetics as a one risk factor in FAS has been suggested by animal models and by molecular epidemiological studies on different populations, bearing allelic variants for those enzymes, such as ADH e CYP2E1, involved in ethanol metabolism. Indeed, one of the major factors determining the peak blood alcohol exposure to the fetus is the metabolic activity of the mother, in addition to placental and fetal metabolism, explaining, at least partially, the risk of FAS. The different rates of ethanol metabolism may be the result of genetic polymorphisms, the most relevant of which have been described in the paper.

Enzymes involved in the bioconversion of ester-based prodrugs

Enzymes are essential for the activation of many prodrugs. In this review, the most important enzymes (e.g., paraoxonase, carboxylesterase, acetylcholinesterase, cholinesterase) involved in the bioconversion of ester-based prodrugs will be discussed in terms of their biology and biochemistry. Most of these enzymes fall into the category of hydrolytic enzymes. However, nonhydrolytic enzymes, including cytochrome P450s, can also catalyze the bioconversion of ester prodrugs and thus will be discussed here. Other factors influencing the ability of these enzymes to catalyze the bioconversion of ester-based prodrugs, particularly species and interindividual differences and stereochemical and structural features of the prodrugs, will be discussed.

Experimental methods to study human transplacental exposure to genotoxic agents

Human placenta differs more than any other organ between species. This is the primary reason to develop models utilizing human tissue to study placental functions. There are no major ethical restrictions using human placenta for scientific studies. Also, the size of human placenta enables a great number of different parameters to be studied in one placenta. The most important cell types considering transplacental transfer, are the trophoblasts differentiating into syncytiotrophoblasts facing maternal circulation, and endothelial cells of fetal vessels. Primary trophoblasts are difficult to culture and do not grow in monolayer thus inhibiting studies on the polarized functions of transport. Several cell lines originating from trophoblasts have been developed, of which BeWo cells seem most useful for transport studies, because they grow in a tight monolayer. Placental tissue can also be retained as explant cultures, although the trophoblast viability is very restricted despite of culture conditions. Cotyledons of human placenta can be retained viable in an isolated organ perfusion. Perfused placental tissue stays viable longer than placental tissue in tissue culture. Although human placental perfusion is the most tedious experimental method to study placental functions, there are several good reasons to develop it further: transplacental transfer and molecular mechanisms of genotoxic compounds can be studied. Placental perfusion is the only experimental method that retains fully the structure of placenta for polarized transport. Furthermore, perfusion of placentas from mothers, who smoke, use illegal drugs or have a disease, allows studies on the impact of such factors on fetal exposure to genotoxic agents.

Expression and Transport Activity of Breast Cancer Resistance Protein (Bcrp/Abcg2) in Dually Perfused Rat Placenta and HRP-1 Cell Line

Breast cancer resistance protein (BCRP/ABCG2) is a member of the ATP-binding cassette transporter family that recognizes a variety of chemically unrelated compounds. Its expression has been revealed in many mammal tissues, including placenta. The purpose of this study was to describe its role in transplacental pharmacokinetics using rat placental HRP-1 cell line and dually perfused rat placenta. In HRP-1 cells, expression of Bcrp, but not P-glycoprotein, was revealed at mRNA and protein levels. Cell accumulation studies confirmed Bcrp-dependent uptake of BODIPY FL prazosin. In the placental perfusion studies, a pharmacokinetic model was applied to distinguish between passive and Bcrp-mediated transplacental passage of cimetidine as a model substrate. Bcrp was shown to act in a concentration-dependent manner and to hinder maternal-to-fetal transport of the drug. Fetal-to-maternal clearance of cimetidine was found to be 25 times higher than that in the opposite direction; this asymmetry was partly eliminated by BCRP inhibitors fumitremorgin C (2 microM) or N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide (GF120918; 2 microM) and abolished at high cimetidine concentrations (1000 microM). When fetal perfusate was recirculated, Bcrp was found to actively remove cimetidine from the fetal compartment to the maternal compartment even against a concentration gradient and to establish a 2-fold maternal-to-fetal concentration ratio. Based on our results, we propose a two-level defensive role of Bcrp in the rat placenta in which the transporter 1) reduces passage of its substrates from mother to fetus but also 2) removes the drug already present in the fetal circulation.

Drugs and the fetus

PURPOSE OF REVIEW

Few drugs are licensed in pregnancy, and data on drug use in pregnancy are mainly retrospective and uncontrolled. Pregnancy exposure has increased recently to new classes of drugs, as they have been shown to be effective and well tolerated outside of pregnancy.

RECENT FINDINGS

Anti-nausea therapies, H2-receptor and proton pump inhibitors appear to be safe. Metformin is being trialed for treatment in gestational diabetes and initial reports appear encouraging. Concern has been raised about statins in early pregnancy and should be avoided. New antiepileptic medication appears effective with low risk of abnormality. However, when combined with valproate, risks are still high. Selective serotonin reuptake inhibitors for depression appear to be effective, with likely low risk for teratogenesis, although neonatal behavioural syndrome following their use in the third trimester is a concern. Angiotensin II inhibitors should be avoided in the second and third trimesters of pregnancy. Smoking cessation programmes need to be reevaluated.

SUMMARY

Information about safety of drugs and caution about prescribing in pregnancy should continue. Until large clinical trials are performed, the risk/benefit ratio of drugs during pregnancy will remain uncertain.

Implications of genetic polymorphisms in drug transporters for pharmacotherapy

Drug transporters are increasingly recognized as a key determinant of drug disposition and response. It is now widely appreciated that expression of the ATP-dependent efflux transporter, MDR1 (ABCB1, P-glycoprotein), in organs such as the gastrointestinal tract, liver and kidney significantly alters the extent of drug absorption and excretion. Moreover, expression of MDR1 at the level of the blood-brain barrier limits the entry of many drugs into the central nervous system. Given such an important role of MDR1 in the drug disposition process, it is not surprising to see increasing focus on the role of single nucleotide polymorphisms (SNPs) in this transporter as a potential determinant of interindividual variability in drug disposition and pharmacological response. However, drug transport is often the result of the concerted action of efflux and uptake pumps located both in the basolateral and apical membranes of epithelial cells. A growing list of membrane-spanning proteins involved in the in- or outward transport of a large variety of drugs has been recognized and characterized over the past few years in almost all tissues, including organic anion and cation transporters (OAT, OCT, solute carrier family SLC22A), organic anion transport proteins (OATP, solute carrier family SLCO, formerly SLC21A), and MRPs (ABCCs), other members of the ATP-binding cassette family. We are just beginning to appreciate their role for drug delivery and disposition and the contribution of genetic polymorphisms in these transport proteins to interindividual variability in the efficacy and safety for pharmacotherapy. This review summarizes the consequences of inherited differences in drug transport for pharmacotherapy. With the main focus on ABCB1, an update of recent advances is given and clinically relevant examples are used to illustrate how heritable differential drug transport can help to explain individual variability in drug response. The pharmacogenetics of other transporters is briefly introduced.

Population Pharmacokinetics of Metformin in Late Pregnancy

The pharmacokinetic disposition of metformin in late pregnancy was studied together with the level of fetal exposure at birth. Blood samples were obtained in the third trimester of pregnancy from women with gestational diabetes or type 2 diabetes; 5 had a previous diagnosis of polycystic ovary syndrome. A cord blood sample also was obtained at the delivery of some of these women, and also at delivery of others who had been taking metformin during pregnancy but from whom no blood had been taken. Plasma metformin concentrations were assayed by a new, validated, reverse-phase HPLC method. A 2-compartment, extravascular maternal model with transplacental partitioning of drug to a fetal compartment was fitted to the data. Nonlinear mixed-effects modeling was performed in NONMEM using FOCE with INTERACTION. Variability was estimated using logarithmic interindividual and additive residual variance models; the covariance between clearance and volume was modeled simultaneously. Mean (range) metformin concentrations in cord plasma and in maternal plasma were 0.81 (range, 0.1-2.6) mg/L and 1.2 (range, 0.1-2.9) mg/L, respectively. Typical population values (interindividual variability, CV%) for allometrically scaled maternal clearance and volume of distribution were 28 L/h/70 kg (17.1%) and 190 L/70 kg (46.3%), giving a derived population-wide half-life of 5.1 hours. The placental partition coefficient for metformin was 1.07 (36.3%). Neither maternal age nor weight significantly influenced the pharmacokinetics. The variability (SD) of observed concentrations about model-predicted concentrations was 0.32 mg/L. The pharmacokinetics were similar to those in nonpregnant patients and, therefore, no dosage adjustment is warranted. Metformin readily crosses the placenta, exposing the fetus to concentrations approaching those in the maternal circulation. The sequelae to such exposure, eg, effects on neonatal obesity and insulin resistance, remain unknown.

Pregnancy-induced changes in pharmacokinetics: a mechanistic-based approach

Observational studies have documented that women take a variety of medications during pregnancy. It is well known that pregnancy can induce changes in the plasma concentrations of some drugs. The use of mechanistic-based approaches to drug interactions has significantly increased our ability to predict clinically significant drug interactions and improve clinical care. This same method can also be used to improve our understanding regarding the effect of pregnancy on pharmacokinetics of drugs. Limited studies suggest bioavailability of drugs is not altered during pregnancy. Increased plasma volume and protein binding changes can alter the apparent volume of distribution (Vd) of drugs. Through changes in Vd and clearance, pregnancy can cause increases or decreases in the terminal elimination half-life of drugs. Depending on whether a drug is excreted unchanged by the kidneys or which metabolic isoenzyme is involved in the metabolism of a drug can determine whether or not a change in dosage is needed during pregnancy. The renal excretion of unchanged drugs is increased during pregnancy. The metabolism of drugs catalysed by select cytochrome P450 (CYP) isoenzymes (i.e. CYP3A4, CYP2D6 and CYP2C9) and uridine diphosphate glucuronosyltransferase (UGT) isoenzymes (i.e. UGT1A4 and UGT2B7) are increased during pregnancy. Dosages of drugs predominantly metabolised by these isoenzymes or excreted by the kidneys unchanged may need to be increased during pregnancy in order to avoid loss of efficacy. In contrast, CYP1A2 and CYP2C19 activity is decreased during pregnancy, suggesting that dosage reductions may be needed to minimise potential toxicity of their substrates. There are limitations to the available data. This analysis is based primarily on observational studies, many including small numbers of women. For some isoenzymes, the effect of pregnancy on only one drug has been evaluated. The full-time course of pharmacokinetic changes during pregnancy is often not studied. The effect of pregnancy on transport proteins is unknown. Drugs eliminated by non-CYP or non-UGT pathways or multiple pathways will need to be evaluated individually. In conclusion, by evaluating the pharmacokinetic data of a variety of drugs during pregnancy and using a mechanistic-based approach, we can start to predict the effect of pregnancy for a large number of clinically used drugs. However, because of the limitations, more clinical, evidence-based studies are needed to fully elucidate the effects of pregnancy on the pharmacokinetics of drugs.

Alterations of placental cytochrome P450 1A1 and P-glycoprotein in tobacco-induced intrauterine growth retardation in rats

AIM

To investigate the alterations of placental P-glycoprotein (P-gp) and cytochrome P450 1A1 (CYP1A1) at different gestational days (GD), and to explore the possible significance of placental P-gp and CYP1A1 in tobacco smoke-induced intrauterine growth retardation (IUGR) in rats.

METHODS

An IUGR model was produced by passive tobacco smoking from GD7 to parturition (GD21) and predicted using fetal development parameters. Placental structure and function were monitored by observing pathological alteration and antioxidative function, including the content of malondialdehyde and the activities of superoxide dismutase and catalase (CAT). The expressions of CYP1A1 and P-gp (mdr 1a and mdr 1b) were detected using a reverse transcription polymerase chain reaction and immunohistochemistry.

RESULTS

Placental pathological changes occurred and the malondialdehyde content increased, whereas the activities of superoxide dismutase and CAT lowered, when compared to their controls. In the rat placenta of the tobacco group, the level of CYP1A1 mRNA increased significantly; the level of mdr1a mRNA increased significantly at GD21 but not at GD14, whereas the level of mdr1b mRNA in different term remained stable; the expression of P-gp increased significantly only in full-term placenta.

CONCLUSION

The expression of placental CYP1A1 and P-gp increased in tobacco-induced IUGR. Overexpression of placental CYP1A1 can attribute to the metabolism of tobacco and the generation of reactive metabolites, which can trigger IUGR. As a compulsory mechanism, upregulation of P-gp might decrease tobacco exposure to a developing fetus with IUGR.

Risk of Testicular Germ Cell Cancer in Relation to Variation in Maternal and Offspring Cytochrome P450 Genes Involved in Catechol Estrogen Metabolism

The incidence of testicular germ cell carcinoma (TGCC) is highest among men ages 20 to 44 years. Exposure to relatively high circulating maternal estrogen levels during pregnancy has long been suspected as being a risk factor for TGCC. Catechol (hydroxylated) estrogens have carcinogenic potential, thought to arise from reactive catechol intermediates with enhanced capability of forming mutation-inducing DNA adducts. Polymorphisms in maternal or offspring genes encoding estrogen-metabolizing enzymes may influence prenatal catechol estrogen levels and could therefore be biomarkers of TGCC risk. We conducted a population-based, case-parent triad study to evaluate TGCC risk in relation to maternal and/or offspring polymorphisms in CYP1A2, CYP1B1, CYP3A4, and CYP3A5. We identified 18- to 44-year-old men diagnosed with invasive TGCC from 1999 to 2004 through a population-based cancer registry in Washington State and recruited cases and their parents (110 case-parent triads, 50 case-parent dyads). Maternal or offspring carriage of CYP1A2 -163A was associated with reduced risk of TGCC [maternal heterozygote relative risk (RR), 0.6; 95% confidence interval (95% CI), 0.2-1.7; offspring heterozygote RR, 0.7; 95% CI, 0.3-1.5)]. Maternal CYP1B1 (48)Gly homozygosity was associated with a 2.7-fold increased risk of TGCC (95% CI, 0.9-7.9), with little evidence that Leu(432)Val or Asn(453)Ser genotypes were related to risk. Men were also at increased risk of TGCC if they carried the CYP3A4 -392G (RR, 7.0; 95% CI, 1.6-31) or CYP3A5 6986G (RR, 2.4; 95% CI, 1.1-5.6) alleles. These results support the hypothesis that maternal and/or offspring catechol estrogen activity may influence sons' risk of TGCC.

Variable expression of MRP2 (ABCC2) in human placenta: influence of gestational age and cellular differentiation

MRP2 (ABCC2) is an ATP-binding cassette (ABC)-type membrane protein involved in transport of conjugates of various drugs and endogenous compounds. MRP2 has been localized to the apical membrane of syncytiotrophoblasts and is assumed to be involved in diaplacental transfer of the above substances. It has been shown that both genetic and environmental factors can influence MRP2 expression. We therefore investigated whether gestational age, cellular differentiation, and genetic polymorphisms influence expression and localization of MRP2 in 58 human placenta samples. We detected a significant increase of transporter-mRNA with gestational age by quantitative real-time polymerase chain reaction (MRP2 mRNA/18S rRNA ratio x 1000 +/- S.D.; 0.43 +/- 0.13 in early preterms versus 1.18 +/- 0.44 in late preterms versus 2.1 +/- 0.63 in terms; p < 0.05). MRP2 protein followed the mRNA amount as shown by Western blotting (mean relative band intensity +/- S.D.; 0.56 +/- 0.1 versus 0.7 +/- 0.18 versus 0.92 +/- 0.19; early preterms versus terms p < 0.05). In cultured cytotrophoblasts, MRP2 expression increased with differentiation to syncytiotrophoblasts, with a peak on day 2 (MRP2 mRNA/18S rRNA ratio x 1000 +/- S.D.; 0.06 +/- 0.01 versus 0.88 +/- 0.27 versus 0.24 +/- 0.02 on days 0, 2, and 4). Moreover, we studied the effect of single nucleotide polymorphisms (C-24T; G1249A, and C3972T) in the MRP2 gene on placental expression. One of these polymorphisms (G1249A) resulted in a significantly reduced expression of MRP2 mRNA in preterms. In summary, the expression of MRP2 in human placenta is influenced by gestational age, cellular differentiation, and genetic factors.

Corticosteroids, pregnancy, and HELLP syndrome: a review

Corticosteroids are potent antiinflammatory and immunosuppressive drugs, which are used in the treatment of a wide range of medical disorders. During pregnancy, several corticosteroids are administered for maternal as well as fetal reasons. Prednisone and prednisolone show limited transplacental passage and are thus used for treatment of maternal disease. Dexamethasone and betamethasone, drugs that can easily cross the placenta, are more suitable for fetal indications. During the last decade, administration of corticosteroids was introduced in the treatment of hemolysis, elevated liver enzymes, and low platelets (HELLP syndrome), a severe form of preeclampsia unique to human pregnancy. Several randomized, controlled trials as well as other prospective and retrospective studies have been performed to investigate this beneficial effect of corticosteroids on biochemical measures and clinical signs. This review discusses the characteristics of corticosteroids in humans and details the use of corticosteroids during pregnancy. A review of literature on the effect of corticosteroids on HELLP syndrome is given and possible mechanisms of action are discussed.