Transplacental and nonplacental clearances of diphenhydramine in the chronically instrumented pregnant sheep

Bisphenol S instead of Bisphenol A: Toxicokinetic investigations in the ovine materno-feto-placental unit

Bisphenol S (BPS) is widely used as a substitute for Bisphenol A in consumer products. Despite its potential endocrine-disrupting effects and widespread exposure, toxicokinetic data, particularly during the critical period of pregnancy, are not available for BPS. The objectives of our study were to evaluate the mechanisms determining fetal exposure to BPS and to BPS glucuronide (BPSG) and to compare them with those prevailing for BPA. The disposition of BPS and BPSG was evaluated in the materno-fetal unit of the catheterized pregnant ewe model, following intravenous administrations of BPS and BPSG to mothers and their fetuses. In a second experiment, the rate of BPS accumulation in the fetal compartment was determined under steady-state conditions after repeated intravenous BPS administrations to the mother. In the maternal compartment, BPS was mainly metabolized into BPSG and totally eliminated in urine. Only 0.40% of the maternal dose was transferred to the fetus. However, once in the fetal compartment, 26% of the fetal dose was rapidly eliminated through placental transfer, while 46% of BPS was metabolized into BPSG which remained trapped in the fetal compartment. Thus, the elimination of BPSG from the fetal compartment required its back-conversion into bioactive BPS, leading to an 87% enhancement of the fetal BPS exposure. Our findings demonstrate that, despite the low materno-fetal placental transfer of BPS, this substitute for BPA is able to accumulate in the fetal compartment after repeated maternal exposure, leading to chronic fetal exposure to BPS in a range of concentrations similar to those of BPA.

Bisphenol A disposition in the sheep maternal-placental-fetal unit: mechanisms determining fetal internal exposure

The widespread human exposure to bisphenol A (BPA), a xenoestrogen interfering with developmental processes, raises the question of the mechanisms determining fetal exposure to BPA. A physiological model was developed in ewes to determine whether the pregnancy-associated physiological changes and the metabolic specificities of the fetal-placental unit can influence BPA toxicokinetics (TK) and fetal exposure to BPA. In a first longitudinal study, BPA was infused (2 mg/[kg·day] i.v. for 1 day) into ewes before breeding, at early and late stages of gestation, and after lambing. In a second study, BPA and BPA-glucuronide (BPA-G) were infused intravenously into pregnant ewes or into fetuses at 4 mo of gestation. BPA and its metabolites were assayed in maternal and fetal plasma and amniotic fluid sampled at steady state and after the end of the infusion. The pregnancy status did not modify the TK parameters of BPA and of BPA-G. Five percent of the BPA dose infused into the pregnant ewe was transferred across the placenta to the fetus. The fetal-placental unit was very efficient in metabolizing BPA into conjugated compounds; those metabolites remained trapped in the fetal-placental compartment, leading to a high fetal exposure to BPA conjugates. Taking into account a body weight adjustment, the ovine fetus in late pregnancy is exposed to a BPA dose similar to that of its mother. In contrast to its mother, the fetus exhibits much higher and sustained exposure to BPA metabolites without evidence of their hydrolysis.

The Use of Microdialysis for the Study of Drug Kinetics: Central Nervous System Pharmacokinetics of Diphenhydramine in Fetal, Newborn, and Adult Sheep

The central nervous system (CNS) pharmacokinetics of the H(1) receptor antagonist diphenhydramine (DPHM) were studied in 100- and 120-day-old fetuses, 10- and 30-day-old newborn lambs, and adult sheep using in vivo microdialysis. DPHM was administered i.v. at five infusion rates, with each step lasting 7 h. In all ages, cerebrospinal fluid (CSF) and extracellular fluid (ECF) concentrations were very similar to each other, which suggests that DPHM between these two compartments is transferred by passive diffusion. In addition, the brain-to-plasma concentration ratios were >or=3 in all age groups, suggesting the existence of a transport process for DPHM into the brain. Both brain and plasma DPHM concentrations increased in a linear fashion over the dose range studied. However, the ECF/unbound plasma and CSF/unbound plasma DPHM concentration ratios were significantly higher in the fetus and lambs (approximately 5 to 6) than in the adult (approximately 3). The factors f(CSF) and f(ECF), the ratios of DPHM areas under the curves (AUCs) in CSF and ECF to the plasma DPHM AUC, respectively, decreased with age, indicating that DPHM is more efficiently removed from the brain with increasing age. The extent of plasma protein binding of the drug increased with age. This study provides evidence for a transporter-mediated mechanism for the influx of DPHM into the brain and also for an efflux transporter for the drug, whose activity increases with age. Moreover, the higher brain DPHM levels in the fetus and lamb compared with the adult may explain the greater CNS effects of the drug at these ages.

Pharmacokinetics and renal excretion of diphenhydramine and its metabolites, diphenylmethoxyacetic acid and diphenhydramine-N-oxide, in developing lambs

The developmental disposition of diphenhydramine (DPHM) and its metabolites, diphenylmethoxyacetic acid (DPMA) and DPHM-N-oxide (DPHMNO), was investigated in postnatal lambs. Lambs received a DPHM intravenous (iv) bolus 15 days (Group A; n = 5) or 2 months (Group B; n = 6) after birth. Total body clearance of DPHM in postnatal lambs (Group A = 138.7 +/- 80.5 mL/min/kg; Group B = 165.7 +/- 51.3 mL/min/kg) was similar to the nonplacental clearance values (i.e., the component of fetal total body clearance that is not due to elimination via the placenta) estimated for fetal lamb (116.3 +/- 49. 6 mL/min/kg), and significantly greater than estimates in adult sheep (38.5 +/- 12.3 mL/min/kg). In addition, Group A DPHM renal clearance (CL(r), >1.80 +/- 1.24 mL/min/kg) was similar to that of the fetus (2.06 +/- 0.24 mL/min/kg), and significantly greater than that for Group B (0.26 +/- 0.17 mL/min/kg) and the adult (0.012 +/- 0.005 mL/min/kg). In contrast, similar to the fetal situation, postnatal DPMA CL(r) (Group A = 0.02 +/- 0.02 mL/min/kg; Group B = 0.05 +/- 0.01 mL/min/kg) was significantly less than adult values (0. 53 +/- 0.19 mL/min/kg). Because DPMA is not sequentially metabolized in sheep, the lower CL(r) in postnatal lambs results in longer apparent elimination half-lives of this metabolite (Group A = 90.4 +/- 32.2 h; Group B = 13.13 +/- 11.0 h) compared with that in the adult (2.9 +/- 1.6 h). No age-related difference in DPHMNO CL(R) was observed. Alterations in the CL(r) of DPHM and DPMA are likely related to differences in the rate of development of mechanisms (i.e. , tubular secretion and reabsorption and glomerular filtration rate) involved in the urinary drug excretion of organic acids and bases.

Diphenhydramine disposition in the sheep maternal-placental-fetal unit: determinants of plasma drug concentrations in the mother and the fetus

The objective of this study was to identify the important factors that determine plasma concentrations of diphenhydramine (DPHM) in the mother and the fetus after maternal as well as fetal steady-state drug administration. Inter-relationships were evaluated between maternal and fetal placental and nonplacental clearances, plasma protein binding, and steady-state plasma concentrations of DPHM among data obtained from 18 pregnant sheep during late gestation. The major determinant of plasma DPHM concentrations in the mother after maternal as well as fetal administration appears to be maternal plasma protein binding and maternal nonplacental clearance. In contrast, the major determinant of fetal plasma DPHM concentrations after maternal drug administration was the extent of fetal first-pass hepatic drug uptake from the umbilical vein. However, after fetal drug administration, the fetal plasma concentrations were related to the extent of fetal plasma protein binding and fetal placental and nonplacental clearances. The index of fetal-to-maternal placental drug transfer after fetal drug administration (steady-state maternal-to-fetal plasma concentration ratio) was related to steady-state fetal plasma unbound fraction and fetal placental and nonplacental clearance. However, this index was not related to the magnitude of the factors operating on the maternal side of the placenta such as maternal plasma protein binding and maternal nonplacental clearance. This might indicate a lack of complete equilibration of the unbound drug concentrations on the two sides of the placenta at the exchange site.

Fetal hepatic drug elimination

The majority of studies of fetal hepatic elimination have concentrated on the expression and activity of the metabolizing enzymes, but the unique physiologic milieu of the fetal liver should also be considered. The basic structure of the liver is formed by the end of the first trimester. The fetal hepatic circulation differs substantially from that of the adult in that there is an extra input vessel, the umbilical vein, and there is shunting of 30-70% of hepatic blood flow via the ductus venosus. The left and right lobes of the fetal liver seem to function independently with respect to a variety of biochemical parameters, due at least in part to the lower oxygen supply to the right lobe. The zonation of drug-metabolizing enzymes along the hepatic acinus, which is prominent in the adult liver, is absent in the fetal liver. Unlike rodent species, the human fetal liver has a significant capacity for drug metabolism. Of the oxidative enzymes, CYP3A7 accounts for up to 50% of total fetal hepatic cytochrome P450 content. Expression of this enzyme decreases dramatically after birth. CYP1A1 and CYP2D6 have also been detected in human fetal liver, but whether CYP2E1 is expressed remains controversial. Several other cytochrome P450s have been identified and await characterization. Fetal hepatic drug conjugation may prolong fetal exposure to the metabolites produced, which, being more water soluble, do not readily cross the placenta back to the mother and, if excreted in fetal urine, can be recycled in the fetus via amniotic fluid and fetal swallowing. Limited activity of glucuronidation enzymes has been demonstrated in human fetal liver in contrast to the activity of sulfation enzymes, which is significant. Limited in vivo studies in fetal sheep have demonstrated significant fetal hepatic drug elimination, and this has been confirmed in studies of the isolated perfused fetal sheep liver. Our understanding of fetal hepatic elimination processes has advanced steadily over the years. Future developments, however, should consider more fully the influence of the unique physiological milieu of the fetal liver, in addition to the expression and activity of drug metabolizing enzymes.

Placental transfer and fetal metabolism of zidovudine in the baboon

Zidovudine (azidothymidine, AZT) is used in pregnancy to reduce mother to infant transmission of HIV. Understanding the disposition of AZT in the fetus is necessary to optimize therapeutic regimens directed toward the fetus. Recent studies in primates found similar steady-state levels of the glucuronide metabolite of AZT (AZT-glu) in the fetus to those in the mother, raising the question of whether the metabolite was of fetal or maternal origin. The objective of this study was to determine whether glucuronidation occurred in the fetal compartment and to quantify the placental and fetal clearances of AZT using the two-compartment model at steady state. Steady-state concentrations were obtained after paired maternal and fetal infusions of AZT in chronically catheterized pregnant baboons. During maternal infusion, the mean (+/-SE) fetal to maternal ratio of AZT was < 1 (0.84 +/- 0.06, p < 0.02), suggesting clearance of AZT in the fetus. Mean total maternal clearance of AZT was 725 +/- 49 mL/min and placental clearance was 36 +/- 4 mL/min, or approximately 5% of maternal clearance. Fetal clearance of AZT was estimated at approximately 15% of placental clearance. This suggests fetal nonplacental clearance is minimal compared with that in the mother, but does not preclude the fetus from actively contributing to the metabolite in the fetal circulation. During infusion of AZT to the fetus, the concentration of AZT-glu in the fetus was 7.0 +/- 0.8 times that in the mother. This is compelling evidence that glucuronide can be formed in the fetal compartment. Thus, fetal metabolism has an impact on the concentration of both AZT and AZT-glu in the fetal circulation.

Pharmacology of drug transfer across the placenta

The effect of drugs in the fetus can be thought of in terms of the dose-response based on a knowledge of the general principles by which drug is transferred from the mother to the fetus. Fetal drug concentration, which determines the fetal response, is a function of the maternal concentration, the placental permeability, the fetal drug clearance, and differences in protein binding and ionization between the maternal and fetal plasma.

Simultaneous analysis of diphenhydramine and a stable isotope analog (2H10)diphenhydramine using capillary gas chromatography with mass selective detection in biological fluids from chronically instrumented pregnant ewes

This report describes both the synthesis of a stable isotope analog of the H1 receptor antagonist diphenhydramine (DPHM), and the simultaneous quantitation of DPHM and a deuterated stable isotope analog of DPHM, viz. (2H10)DPHM in biological fluids from the chronically instrumented pregnant ewe. (2H10)DPHM was synthesized and purified, and both its structure and purity were verified. Biological samples were prepared for analysis using liquid-liquid extraction prior to capillary gas chromatography/mass spectrometry. The method employed electron impact ionization with selective ion monitoring of ions with m/z 165 for DPHM and m/z 173 for (2H10)DPHM. The minimal quantifiable concentration of DPHM and (2H10)DPHM from a 1.0 ml sample was 2.0 ng ml-1 in fetal and maternal plasma, fetal tracheal fluid and amniotic fluid. The method was validated from 2.0 ng ml-1 to 200.0 ng ml-1 for both DPHM and (2H10)DPHM in plasma, fetal tracheal fluid and amniotic fluid. Differences in the disposition between DPHM and (2H10)DPHM were not apparent during a control experiment in which both labeled and unlabeled DPHM were administered to a chronically instrumented fetal lamb. This method provides the required sensitivity and selectivity for the simultaneous quantitation of unlabeled and labeled DPHM during pharmacokinetic experiments conducted in near-term pregnant sheep.