Induction of cytochrome P450 isozymes by phenobarbital in pregnant rat and fetal livers and placenta

Prenatal test cohort of a modified rat comparative thyroid assay adding brain thyroid hormone measurements and histology but lowering group size appears able to detect disruption by sodium phenobarbital

The Comparative Thyroid Assay (CTA, USEPA) is a screening test for thyroid hormone (TH) disruption in peripheral blood of dams and offspring. Recently, we began investigating feasible improvements to the CTA by adding examination of offspring brain TH concentrations and brain histopathology. In addition, we hypothesize that the number of animals required could be reduced by 50 % while still maintaining sensitivity to characterize treatment related changes in THs. Previously, we showed that the prenatal test cohort of the modified CTA could detect 1000 ppm sodium phenobarbital (NaPB)-induced suppression of brain T3 (by 9 %) and T4 (by 33 %) with no significant changes in serum T3 and T4 (less than 8 %). In the current study we expanded the dose response in a prenatal test cohort. Pregnant SD rats (N = 10/group) were exposed to 0, 1000 or 1500 ppm NaPB in the diet from gestational days (GD) 6 to GD20. Serum THs concentrations in GD20 dams together with serum/brain THs concentrations and brain histopathology in the GD20 fetuses were examined. NaPB dose-dependently suppressed serum T3 (up to -26 %) and T4 (up to -44 %) in dams, with suppression of T3 in serum (up to -26 %) and brain (up to -18 %) and T4 in serum (up to -26 %) and brain (up to -29 %) of fetuses but without clear dose dependency. There were no remarkable findings that deviated significantly from controls in GD20 fetal brain by qualitative histopathology. Overall, the present study suggests that the prenatal test cohort of this modified CTA is able to detect the expected fetal TH disruptions by prenatal exposure to NaPB, while also reducing the number of animals used by 50 %, consistent with the results of our previous study. These findings add to the suggestion that lowering group sizes and adding endpoints may be a useful alternative to the original CTA design.

Induction by Phenobarbital of Phase I and II Xenobiotic-Metabolizing Enzymes in Bovine Liver: An Overall Catalytic and Immunochemical Characterization

In cattle, phenobarbital (PB) upregulates target drug-metabolizing enzyme (DME) mRNA levels. However, few data about PB's post-transcriptional effects are actually available. This work provides the first, and an almost complete, characterization of PB-dependent changes in DME catalytic activities in bovine liver using common probe substrates and confirmatory immunoblotting investigations. As expected, PB increased the total cytochrome P450 (CYP) content and the extent of metyrapone binding; moreover, an augmentation of protein amounts and related enzyme activities was observed for known PB targets such as CYP2B, 2C, and 3A, but also CYP2E1. However, contradictory results were obtained for CYP1A, while a decreased catalytic activity was observed for flavin-containing monooxygenases 1 and 3. The barbiturate had no effect on the chosen hydrolytic and conjugative DMEs. For the first time, we also measured the 26S proteasome activity, and the increase observed in PB-treated cattle would suggest this post-translational event might contribute to cattle DME regulation. Overall, this study increased the knowledge of cattle hepatic drug metabolism, and further confirmed the presence of species differences in DME expression and activity between cattle, humans, and rodents. This reinforced the need for an extensive characterization and understanding of comparative molecular mechanisms involved in expression, regulation, and function of DMEs.

Morphology and physiology of rat placenta for toxicological evaluation

The placenta plays a pivotal role in fetal growth, and placental dysfunction and injury are associated with embryo/fetal toxicity. Histological examination of the rat placenta for safety evaluation provides valuable clues to the mechanisms of this toxicity. However, the placenta has specific and complex biological features unlike those of other organs, and placental structure dramatically changes depending on the time during the gestation period. Thus, time-dependent histopathological examination of the rat placenta should be performed based on the understanding of normal developmental changes in morphology and function. The placentas of rats and humans are both anatomically classified as discoid and hemochorial types. However, there are differences between rats and humans in terms of placental histological structure, the fetal-maternal interface, and the function of the yolk sac. Therefore, extrapolation of placental toxicity from rats to humans should be done cautiously in the evaluation of risk factors. This review describes the development, morphology, physiology, and toxicological features of the rat placenta and the differences between the rat and human placenta to enable accurate evaluation of reproductive and developmental toxicity in studies.

Maternal Pharmacokinetics and Fetal Disposition of (±)-Citalopram during Mouse Pregnancy

While selective-serotonin reuptake inhibitor (SSRI) antidepressants are commonly prescribed in the treatment of depression, their use during pregnancy leads to fetal drug exposures. According to recent reports, such exposures could affect fetal development and long-term offspring health. A central question is how pregnancy-induced physical and physiological changes in mothers, fetuses, and the placenta influence fetal SSRI exposures during gestation. In this study, we examined the effects of gestational stage on the maternal pharmacokinetics and fetal disposition of the SSRI (±)-citalopram (CIT) in a mouse model. We determined the maternal and fetal CIT serum concentration-time profiles following acute maternal administration on gestational days (GD)14 and GD18, as well as the fetal brain drug disposition. The results show that pregnancy affects the pharmacokinetics of CIT and that maternal drug clearance increases as gestation progresses. The data further show that CIT and its primary metabolite desmethylcitalopram (DCIT) readily cross the placenta into the fetal compartment, and fetal exposure to CIT exceeds that of the mother during gestation 2 h after maternal administration. Enzymatic activity assays revealed that fetal drug metabolic capacity develops in late gestation, resulting in elevated circulating and brain concentrations of DCIT at embryonic day (E)18. Fetal exposure to the SSRI CIT in murine pregnancy is therefore influenced by both maternal gestational stage and embryonic development, suggesting potential time-dependent effects on fetal brain development.

Ontogeny of mammalian metabolizing enzymes in humans and animals used in toxicological studies

It is well recognized that expression of enzymes varies during development and growth. However, an in-depth review of this acquired knowledge is needed to translate the understanding of enzyme expression and activity into the prediction of change in effects (e.g. kinetics and toxicity) of xenobiotics with age. Age-related changes in metabolic capacity are critical for understanding and predicting the potential differences resulting from exposure. Such information may be especially useful in the evaluation of the risk of exposure to very low (µg/kg/day or ng/kg/day) levels of environmental chemicals. This review is to better understand the ontogeny of metabolizing enzymes in converting chemicals to either less-toxic metabolite(s) or more toxic products (e.g. reactive intermediate[s]) during stages before birth and during early development (neonate/infant/child). In this review, we evaluated the ontogeny of major "phase I" and "phase II" metabolizing enzymes in humans and commonly used experimental animals (e.g. mouse, rat, and others) in order to fill the information gap.

Drug Interactions at the Human Placenta: What is the Evidence?

Pregnant women (and their fetuses) are treated with a significant number of prescription and non-prescription medications. Interactions among those drugs may affect their efficacy and toxicity in both mother and fetus. Whereas interactions that result in altered drug concentrations in maternal plasma are detectable, those involving modulation of placental transfer mechanisms are rarely reflected by altered drug concentrations in maternal plasma. Therefore, they are often overlooked. Placental-mediated interactions are possible because the placenta is not only a passive diffusional barrier, but also expresses a variety of influx and efflux transporters and drug-metabolizing enzymes. Current data on placental-mediated drug interactions are limited. In rodents, pharmacological or genetic manipulations of placental transporters significantly affect fetal drug exposure. In contrast, studies in human placentae suggest that the magnitude of such interactions is modest in most cases. Nevertheless, under certain circumstances, such interactions may be of clinical significance. This review describes currently known mechanisms of placental-mediated drug interactions and the potential implications of such interactions in humans. Better understanding of those mechanisms is important for minimizing fetal toxicity from drugs while improving their efficacy when directed to treat the fetus.

Transplacental benzene exposure increases tumor incidence in mouse offspring: possible role of fetal benzene metabolism

Childhood cancer is the leading cause of disease-related death in children aged 1-14 years in Canada and the USA and it has been hypothesized that transplacental exposure to environmental carcinogens such as benzene may contribute to the etiology of these cancers. Our objectives were to determine if transplacental benzene exposure increased tumor incidence in mouse offspring and assess fetal benzene metabolism capability. Pregnant CD-1 and C57Bl/6N mice were given intraperitoneal injections of corn oil, 200 mg/kg, or 400 mg/kg benzene on gestational days 8, 10, 12 and 14. A significant increase in tumor incidence was observed in CD-1, but not C57BL/6N, 1-year-old offspring exposed transplacentally to 200 mg/kg benzene. Hepatic and hematopoietic tumors were predominantly observed in male and female CD-1 offspring, respectively. Female CD-1 offspring exposed transplacentally to 200 mg/kg benzene had significantly suppressed bone marrow CD11b(+) cells 1 year after birth, correlating with reduced colony-forming unit granulocyte/macrophage numbers in 2-day-old pups. CD-1 and C57Bl/6N maternal blood benzene levels and fetal liver benzene, t, t-muconic acid, hydroquinone and catechol levels were analyzed by gas chromatography/mass spectrometry. Significant strain-, gender- and dose-related differences were observed. Male CD-1 fetuses had high hydroquinone levels, whereas females had high catechol levels after maternal exposure to 200 mg/kg benzene. This is the first demonstration that transplacental benzene exposure can induce hepatic and hematopoietic tumors in mice, which may be dependent on fetal benzene metabolism capability.

Phenobarbital (PB)-induced changes in blood coagulationrelated parameters in pregnant rats, lactating rats and pups

Effects of repeated administration of phenobarbital (PB) on blood coagulation-related parameters were examined in non-pregnant, pregnant and lactating rats, and also in pups born to PB-treated lactating dams. PB was orally administered at a dose level of 80 mg/kg/day to pregnant (from gestation day (GD) 13), postpartum (from postpartum day (PPD) 7) and non-pregnant rats (from 13 weeks of age) for 7 days. Blood was collected on GD20 or PPD14 to perform blood coagulation examination. Concurrently, the blood coagulation parameters were examined in the pups. Increases in liver weight and/or hepatic cytochrome P450 content were observed in the PB-treated non-pregnant, pregnant and lactating rats. Activated partial thromboplastin time (APTT) was prolonged and anti-thrombin III (ATIII) concentration was increased in the lactating rats, while there were no changes in prothrombin time (PT) or APTT in the non-pregnant and pregnant rats. Moreover, prolongation of PT and APTT and decreases in factors VII and IX activities were observed in their pups. Thus, prolongation of blood coagulation time was confirmed in both dams and their pups following PB-administration to lactating dams. Effects of vitamin K(2) (VK(2)) on PB-induced changes in blood coagulation-related parameters of both dams and their pups were examined by co-administration with PB and VK(2) to lactating dams. PT and APTT were comparable to the control and PB-induced prolongation of blood coagulation time was improved in the pups while APTT was prolonged in dams, suggesting that VK(2) was beneficial to pups but not to dams.

Histopathological effect of ketoconazole on rat placenta

In order to investigate the morphological effects of ketoconazole on hypertrophied placentas, we examined the sequential histopathological changes in the placenta from rats exposed to ketoconazole. Ketoconazole was administered orally at 0 and 25 mg/kg/day during gestation days (GDs) 12 to 14, and the placentas were sampled on GDs 15, 17 and 21. All dams showed neither effect on body weight nor any abnormal clinical signs during the experimental period. In the treated group, the placentas appeared more hypertrophic with increases in the weight, diameter and thickness on GD 21. Histopathologically, increased thickness was noted in the labyrinth zone and basal zone on GDs 17 and 21, while on GD 15 the change had been already evident in the former zone. In the labyrinth zone, the mitotic figures of the trophoblasts were significantly elevated on GD 15. A multiple cystic dilatation of maternal sinusoids was observed in some placentas on GDs 15, 17 and 21. In the basal zone, an increase in spongiotrophoblasts and clusters of glycogen cells were detected on GDs 17 and 21. In the decidua basalis, there were no significant changes in either histology or thickness between the control and treated group during GDs 15 to 21. In conclusion, ketoconazole increased the population of composed cells in the labyrinth and basal zone, leading to placental hypertrophy in pregnant rats.

Immunohistochemical analysis of expressions of hepatic cytochrome P450 in F344 rats following oral treatment with kava extract

Kava (Piper methysticum), used for relaxation and pain relief, has been one of the leading dietary supplements and several reports linking hepatic functional disturbances and liver failure to kava have resulted in a ban on sales in Europe and Canada and the issuance of warnings by the US FDA. The National Toxicology Program conducted 14-week rat studies to characterize the toxicology of kava exposure in Fischer 344 rats [National Toxicity Program. 90 day gavage toxicity studies of KAVA KAVA EXTRACT in Fischer rats and B6C3F1 mice. Research Triangle Park, NC; 2005a; National Toxicity Program. Testing status of agents at NTP (KAVA KAVA EXTRACT M990058). Research Triangle Park, NC; 2005b. (http://ntp.niehs.nih.gov/index.cfm?objectid=071516E-C6E1-7AAA-C90C751E23D14C1B)]. Groups of 10 male and 10 female rats were administered kava extract by gavage at 0, 0.125, 0.25, 0.5, 1.0, and 2.0 g/kg/day. Increased gamma-glutamyl-transpeptidase (GGT) activities were observed in the 2.0 g/kg males and 1.0 and 2.0 g/kg females, as well as increased serum cholesterol levels in males and females at 0.5 g/kg and higher. Increases in incidence and severity of hepatocellular hypertrophy (HP) were noted in males at 1.0 g/kg and females at 0.5 g/kg and higher, as well as increased liver weights. Immunohistochemical analyses of the expression of cytochrome-P450 (CYP) enzymes in liver of the control and 1.0- and 2.0-g/kg-treated groups indicated decreased expression of CYP2D1 (human CYP2D6 homolog) in 2.0 g/kg females and increased expression of CYP1A2, 2B1, and 3A1 in 1.0 and 2.0 g/kg groups of both sexes. The no observed adverse effect levels were decided as 0.25 g/kg in both genders, based on neurotoxic effects, increases in GGT, cholesterol, liver weight, and HP and decreases in body weight. Kava-induced hepatic functional changes in the F344 rat might be relevant to human clinical cases of hepatotoxicity following exposure.

Ontogeny of hepatic CYP1A2 and CYP2E1 expression in rat

We report a comprehensive examination of rat hepatic CYP1A2 and CYP2E1 ontogeny. We compare the data to human data to determine the rat's capacity as a model to identify CYP-mediated mechanisms underlying age-dependent differences in susceptibility to toxicity. We evaluated CYP expression using real-time RT-PCR, immunoblot and immunohistochemistry, and specific probe activity in male rat livers (n = 4) at critical developmental life stages. CYP2E1 mRNA expression was low at birth, then increased rapidly to peak prior to weaning. CYP1A2 transcript levels remained very low postnatally and then increased dramatically to reach peak expression during weaning. Immunoreactive CYP1A2 and CYP2E1 was first detected at postnatal day 3 (PD3), and reached 50% of adult levels after weaning, and adult levels by puberty. CYP1A2 and CYP2E1 probe activity (pmol/(min mg)) was detected at PD3 and peaked during weaning and late neonatal period, respectively. CYP activity fell to adult values by puberty, a pattern that closely mirrored the temporal changes in mRNA but not protein. An increasing preferential localization of CYP1A2 and CYP2E1 immunoreactivity in perivenous hepatocytes was observed with maturation to adulthood. Although differences in CYP1A2 and CYP2E1 ontogeny between rats and humans exist, knowledge of these differences will aid interspecies extrapolation of developmental toxicokinetic data.

Lack of effect on rat testicular organogenesis after in utero exposure to 3-monochloropropane-1,2-diol (3-MCPD)

3-Monochloropropane-1,2-diol (3-MCPD) is a food-born contaminant known to display toxic effects on male reproduction, producing infertility in rats and humans. Using the rat as a model, we investigated whether or not testicular organogenesis, which, in the rat species, occurs during the second half of gestation, was at particular risk regarding 3-MCPD toxicity. Pregnant rats were given daily doses of 5, 10 or 25 mg/kg BW of 3-MCPD from days 11.5-18.5 postcoitum (dpc). On 19.5 dpc, testes were removed from fetuses for histological examination and testosterone analysis. Eight genes were selected among the differentiation markers of testicular cell lineages, and their expression was studied by RT-PCR. The levels of 3-MCPD and its main metabolite, beta-chlorolactic acid, were assayed in fetal tissues and dam plasma. Our results show a statistically significant decrease in the mean body weight gain of pregnant rats treated with 10 and 25 mg/kg BW of 3-MCPD. Fetal testes exposed to 3-MCPD exhibited normal histology and produced testosterone at levels that were similar to controls. In addition, 3-MCPD did not alter gene expression in the fetal testes. This lack of effect occurred under conditions where 3-MCPD and beta-chlorolactic acid were found to readily cross the placental barrier and diffuse throughout the fetal tissues. Our findings indicate that 3-MCPD has minimal effect on rat testicular organogenesis.

Microarray analysis on Phase II drug metabolizing enzymes expression in pregnant rats after treatment with pregnenolone-16α-carbonitrile or phenobarbital

We previously reported the expression profiles of 9 cytochrome P450 isozymes (CYPs) proteins and those of 40 CYPs genes in pregnant rat's liver, placenta and fetal liver after treatment with pregnenolone-16alpha-carbonitrile (PCN) or phenobarbital (PB). This study was carried out focusing on the gene expression profiles of Phase II drug metabolizing enzymes, Glutathione S-transferase isozymes (GSTs) and UDP-glycosyltransferase isozymes (UDPGTs). Fischer 344 (F344) pregnant rats were daily treated intraperitoneally with 50 mg/kg of PCN or 80 mg/kg of PB from 13 to 16 days of gestation (DG). They were sacrificed on 17 DG, and microarray analysis using Affymetrix Rat Expression Array 230 A was performed. Among 16 GSTs genes examined in this study, 7 genes were significantly induced in dam's liver and 3 genes in fetal liver, respectively, in the PCN-group, while 8 genes were significantly induced in dam's liver and 1 gene in fetal liver, respectively, in the PB-group. On the other hand, among 11 UDPGTs genes examined, 5 genes were significantly induced in dam's liver and 3 genes in fetal liver, respectively, in the PCN-group, while 5 genes were significantly induced in dam's liver and 1 gene in fetal liver, respectively, in the PB-group. There were no significant changes in the placenta of all groups. This is the first report of the gene expression profiles of Phase II drug metabolizing enzymes in pregnant rat and fetal livers and placenta after treatment with typical inducers of drug metabolizing enzymes.