Estimation of placental and lactational transfer and tissue distribution of atrazine and its main metabolites in rodent dams, fetuses, and neonates with physiologically based pharmacokinetic modeling

https://doi.org/10.1016/j.taap.2013.08.010Get rights and content

Highlights

  • We developed PBPK models for atrazine in rat dams, fetuses, and neonates.

  • We conducted pharmacokinetic (PK) study with atrazine in pregnant mice.

  • Model predictions were in good agreement with experimental rat and mouse PK data.

  • The fetus is exposed to atrazine/its main metabolite at levels similar to the dam.

  • The nursing neonate is exposed primarily to atrazine's main metabolite DACT.

Abstract

Atrazine (ATR) is a widely used chlorotriazine herbicide, a ubiquitous environmental contaminant, and a potential developmental toxicant. To quantitatively evaluate placental/lactational transfer and fetal/neonatal tissue dosimetry of ATR and its major metabolites, physiologically based pharmacokinetic models were developed for rat dams, fetuses and neonates. These models were calibrated using pharmacokinetic data from rat dams repeatedly exposed (oral gavage; 5 mg/kg) to ATR followed by model evaluation against other available rat data. Model simulations corresponded well to the majority of available experimental data and suggest that: (1) the fetus is exposed to both ATR and its major metabolite didealkylatrazine (DACT) at levels similar to maternal plasma levels, (2) the neonate is exposed mostly to DACT at levels two-thirds lower than maternal plasma or fetal levels, while lactational exposure to ATR is minimal, and (3) gestational carryover of DACT greatly affects its neonatal dosimetry up until mid-lactation. To test the model's cross-species extrapolation capability, a pharmacokinetic study was conducted with pregnant C57BL/6 mice exposed (oral gavage; 5 mg/kg) to ATR from gestational day 12 to 18. By using mouse-specific parameters, the model predictions fitted well with the measured data, including placental ATR/DACT levels. However, fetal concentrations of DACT were overestimated by the model (10-fold). This overestimation suggests that only around 10% of the DACT that reaches the fetus is tissue-bound. These rodent models could be used in fetal/neonatal tissue dosimetry predictions to help design/interpret early life toxicity/pharmacokinetic studies with ATR and as a foundation for scaling to humans.

Introduction

Atrazine [ATR; 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine, CAS# 1912-24-9] is a chlorotriazine herbicide used extensively on crops to control broadleaf weeds (EPA, 2003). Due to its widespread use, relative persistence in water (ATSDR, 2003) and extreme persistence in soil (Jablonowski et al., 2009), ATR is ubiquitous in the environment (Battaglin et al., 2009).

Possible exposure sources for ATR include contaminated air (dust), food, and drinking water (García et al., 2012, Lozier et al., 2012, Mosquin et al., 2012). Surface and drinking water ATR concentrations (up to 224 and 34 μg/L, respectively) in places with heavy ATR use, such as the Midwestern U.S., substantially exceed current maximum contaminant levels (MCL), i.e., 3 and 0.1 μg/L in U.S. and Europe, respectively (ATSDR, 2003, Mosquin et al., 2012). According to the EPA guidelines for acute exposure risk assessment of ATR, the lowest observed adverse effect level (LOAEL), no observed adverse effect level (NOAEL), reference dose (RfD) and population adjusted dose (PAD) are 70,000, 10,000, 100 and 10 μg/kg/day, respectively (EPA, 2003). For the general population in the U.S., the estimated acute and chronic dietary exposures to ATR are 0.234–0.857 and 0.046–0.286 μg/kg/day, respectively, which is relatively low (Gammon et al., 2005). On the other hand, the exposure levels could reach up 151,000 μg per work shift for ATR manufacturing workers, indicating a much higher occupational overexposure risk (Catenacci et al., 1993).

ATR and/or its metabolites have been frequently detected in spot urine samples from pesticide applicators (ATR equivalents [ATR and up to 8 identifiable metabolites, of which DE, DIP and DACT, abbreviations defined below, account for > 85%]: 100–510 μg/L; detected in every sample; Barr et al., 2007), their families (ATR mercapturate: 0.024–4.9 μg/L, 27% positive samples) and the general population (ATR mercapturate: ≤ 3.8 μg/L [creatinine normalized]; 14% positive samples, Curwin et al., 2007), including pregnant women and young children (Chevrier et al., 2011, Curwin et al., 2007). Quantifiable levels of ATR (> 0.05 μg/L) or one of its metabolites (ATR mercapturate: > 0.02 μg/L) in pregnant women's first-morning-void urine have been associated with adverse birth outcomes, such as fetal growth restriction (Chevrier et al., 2011).

ATR and its metabolites have also been detected in plasma, urine and multiple tissues (including the brain, liver and kidney) of ATR-treated rodents (Fraites et al., 2011, Ross et al., 2009), in fetuses and in the milk of orally exposed rat dams (Fraites et al., 2011), in human umbilical cord plasma samples from residentially exposed, low risk, urban population (Whyatt et al., 2003), and in breast milk samples collected from a general population in France (Balduini et al., 2003). Thus, in utero and lactational exposures may be important routes for ATR to reach the developing fetus or neonate.

In the body, ATR is metabolized by several hepatic P450s (e.g., CYP2B1, CYP2D1, and CYP2E1; Hanioka et al., 1998a) fairly rapidly to desethylatrazine (DE; 2-chloro-4-amino-6-isopropylamino-s-triazine, CAS# 6190-65-4) and desisopropylatrazine (DIP; 2-amino-4-chloro-6-ethylamino-s-triazine, CAS# 1007-28-9), which, in turn, are metabolized to didealkylatrazine (DACT; 2-chloro-4,6-diamino-1,3,5-triazine, CAS# 3397-62-4), the major in vivo metabolite of ATR in mice (Ross and Filipov, 2006, Ross et al., 2009), rats (Brzezicki et al., 2003), and, apparently, humans (Barr et al., 2007; Fig. 1). During gestational and/or lactational stages, ATR is also extensively metabolized following a similar pattern (Fraites et al., 2011). Emerging evidence suggests that the metabolism of ATR is auto-inducing and is physiological stage-independent, i.e., short-term ATR exposure increases its own metabolism and/or the expression of ATR-metabolizing P450 isoforms in peripubertal (Pogrmic-Majkic et al., 2012), adult (Hanioka et al., 1998b), and pregnant and/or lactating rats (Fraites et al., 2011). However, there is still much unknown about the pharmacokinetic behavior of ATR in dams, fetuses, and neonates. Of note, while some such pharmacokinetic data are available in the rat (Fraites et al., 2011), there is no gestational of lactational pharmacokinetic study in the mouse, which is important for species comparison and extrapolation.

Developmental exposure of laboratory animals to higher levels of ATR (35–200 mg/kg) results in various adverse effects ranging from suppression of postnatal development to full-litter resorption (Narotsky et al., 2001, Rayner et al., 2005, Rooney et al., 2003). Of note, perinatal exposure of rodents to environmentally-relevant low doses of ATR causes neurobehavioral deficits (≥ 1 μg/kg) and structural brain changes (100 μg/kg) in the offspring (Belloni et al., 2011, Giusi et al., 2006), suggesting that the developing brain might be particularly sensitive to ATR. In terms of the effects of ATR's main metabolite DACT on the developing nervous system, in vivo studies do not exist at this point. However, our recent in vitro study suggested that DACT is less potent than ATR. Nevertheless, high concentrations of DACT disrupt dopaminergic neuron morphological differentiation (Lin et al., 2013). The studies described above highlight the potential of adverse effects of ATR overexposure on the developing fetus and neonate, the brain in particular. However, these studies do not correlate adverse effects with estimations of fetal or neonatal target tissue concentrations of ATR or its metabolites.

Risk assessment of ATR in sensitive subpopulations, including fetuses and infants, is limited by the scarcity of human pharmacokinetic data. Physiologically based pharmacokinetic (PBPK) models in rodents are useful tools that can aid the process because they can perform route-to-route, species, and dose extrapolations, as well as dose–response analysis. Fetal and/or neonatal rodent PBPK models have been developed for several other xenobiotics (Corley et al., 2003, Lu et al., 2012); these models facilitate the risk assessment of developmental exposure to these chemicals. At present, PBPK models for ATR are available for adult male rats (McMullin et al., 2003, McMullin et al., 2007, Timchalk et al., 1990) and mice (Lin et al., 2011), but not for rodent dams, fetuses, or neonates. In order to improve our understanding of potential adverse effects due to developmental ATR exposure by providing fetal and neonatal tissue dosimetry for ATR and its metabolites, while taking advantage of very recent pharmacokinetic data from rat dams exposed to ATR during gestation and/or lactation (Fraites et al., 2011), we sat out to develop PBPK models for ATR describing its kinetic behavior in rodent dams, fetuses, and neonates.

Section snippets

Data source for model calibration

The data used to calibrate the gestational and lactational models for ATR are from two independent studies, performed by the same group (Fraites et al., 2011). In study 1, pregnant Sprague Dawley rats were treated with ATR (5 or 25 mg/kg) by daily oral gavage from gestational day (GD) 14 to GD20. Two hours after the last dosing on GD20, maternal plasma, tissues (the brain and 4th mammary gland), and fetuses were collected. Fetuses were analyzed on a per whole fetus basis. In study 2, Sprague

Gestational model

Model predictions of maternal plasma, maternal tissue and fetal concentrations of ATR and its metabolites at 2 h after the last dosing on GD20 were compared to measured data in pregnant rats dosed orally with 5 mg/kg ATR from GD14 to GD20 (Fraites et al., 2011). Results for ATR and its major metabolite DACT are shown in Fig. 3A, while results for the intermediate metabolites (DE and DIP) are provided in Appendix A (Fig. A2). Overall, the model slightly overestimated (within a factor of 2)

Discussion

The present PBPK models properly describe placental/lactational transfer and tissue distribution of ATR and its metabolites in pregnant, lactating, fetal and neonatal rats; rat-based models also extrapolate well to mice. Model predictions indicate that (1) the fetus is exposed to both ATR and its major metabolite DACT at levels similar to their maternal plasma levels, with DACT's levels being much higher than ATR, (2) the nursing neonate is exposed primarily to DACT at levels around one-third

Conflict of interest statement

The authors declare that there are no conflicts of interest.

Disclaimer

The findings and conclusions in this report are those of the author(s) and do not necessarily represent the official position of the U.S. Food and Drug Administration. Mention of trade names is not an endorsement of any commercial product.

Acknowledgments

The experimental data used for model calibration was based on Fraites et al. (2011). We thank all the authors because this work was essential for our publication. We also wish to acknowledge Drs. K. Barry Delclos, Xiaoxia Yang, and Frederick Beland, National Center for Toxicological Research, Food and Drug Administration, Jefferson, AR, for critically reviewing this manuscript. Our interactions with Dr. Sheppard A. Martin, Neurotoxicology Branch, Toxicity Assessment Division, National Health

References (129)

  • N. Ejiri et al.

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

    Exp. Mol. Pathol.

    (2005)
  • J.W. Fisher et al.

    Physiologically based pharmacokinetic modeling of the pregnant rat: a multiroute exposure model for trichloroethylene and its metabolite, trichloroacetic acid

    Toxicol. Appl. Pharmacol.

    (1989)
  • J.W. Fisher et al.

    Physiologically based pharmacokinetic modeling of the lactating rat and nursing pup: a multiroute exposure model for trichloroethylene and its metabolite, trichloroacetic acid

    Toxicol. Appl. Pharmacol.

    (1990)
  • J.W. Fisher et al.

    Pharmacokinetic modeling: prediction and evaluation of route dependent dosimetry of bisphenol A in monkeys with extrapolation to humans

    Toxicol. Appl. Pharmacol.

    (2011)
  • M.J. Fraites et al.

    Gestational atrazine exposure: effects on male reproductive development and metabolite distribution in the dam, fetus, and neonate

    Reprod. Toxicol.

    (2011)
  • M.L. Gargas et al.

    A toxicokinetic study of inhaled ethylene glycol ethyl ether acetate and validation of a physiologically based pharmacokinetic model for rat and human

    Toxicol. Appl. Pharmacol.

    (2000)
  • M.L. Gargas et al.

    A toxicokinetic study of inhaled ethylene glycol monomethyl ether (2-ME) and validation of a physiologically based pharmacokinetic model for the pregnant rat and human

    Toxicol. Appl. Pharmacol.

    (2000)
  • M.E. Hahn

    The aryl hydrocarbon receptor: a comparative perspective

    Comp. Biochem. Physiol. C Pharmacol. Toxicol. Endocrinol.

    (1998)
  • N. Hanioka et al.

    In vitro biotransformation of atrazine by rat liver microsomal cytochrome P450 enzymes

    Chem. Biol. Interact.

    (1998)
  • N. Hanioka et al.

    In vitro metabolism of chlorotriazines: characterization of simazine, atrazine, and propazine metabolism using liver microsomes from rats treated with various cytochrome P450 inducers

    Toxicol. Appl. Pharmacol.

    (1999)
  • X.J. He et al.

    Changes in cytochrome P450 isozymes (CYPs) protein levels during lactation in rat liver

    Exp. Mol. Pathol.

    (2005)
  • X.J. He et al.

    Gene expression profiles of drug-metabolizing enzymes (DMEs) in rat liver during pregnancy and lactation

    Exp. Mol. Pathol.

    (2007)
  • P.T. Henderson

    Metabolism of drugs in rat liver during the perinatal period

    Biochem. Pharmacol.

    (1971)
  • S. Imaoka et al.

    Age-dependent expression of cytochrome P-450s in rat liver

    Biochim. Biophys. Acta

    (1991)
  • M.O. Islam et al.

    Induction of P-glycoprotein, glutathione-S-transferase and cytochrome P450 in rat liver by atrazine

    Environ. Toxicol. Pharmacol.

    (2002)
  • N.D. Jablonowski et al.

    Persistence of 14C-labeled atrazine and its residues in a field lysimeter soil after 22 years

    Environ. Pollut.

    (2009)
  • M.E. Jonsson et al.

    Basal and 3,3′,4,4′,5-pentachlorobiphenyl-induced expression of cytochrome P450 1A, 1B and 1C genes in zebrafish

    Toxicol. Appl. Pharmacol.

    (2007)
  • A. Kaune et al.

    High-performance liquid chromatographic measurement of the 1-octanol–water partition coefficient of s-triazine herbicides and some of their degradation products

    J. Chromatogr. A

    (1998)
  • Z. Lin et al.

    A physiologically based pharmacokinetic model for atrazine and its main metabolites in the adult male C57BL/6 mouse

    Toxicol. Appl. Pharmacol.

    (2011)
  • Z. Lin et al.

    Differentiation state-dependent effects of in vitro exposure to atrazine or its metabolite diaminochlorotriazine in a dopaminergic cell line

    Life Sci.

    (2013)
  • J.C. Lipscomb et al.

    In vitro to in vivo extrapolation for trichloroethylene metabolism in humans

    Toxicol. Appl. Pharmacol.

    (1998)
  • A.E. Loccisano et al.

    Evaluation of placental and lactational pharmacokinetics of PFOA and PFOS in the pregnant, lactating, fetal and neonatal rat using a physiologically based pharmacokinetic model

    Reprod. Toxicol.

    (2012)
  • T.S. McMullin et al.

    Oral absorption and oxidative metabolism of atrazine in rats evaluated by physiological modeling approaches

    Toxicology

    (2007)
  • A. Noble

    Partition coefficients (n-octanol–water) for pesticides

    J. Chromatogr. A

    (1993)
  • E.J. O'Flaherty et al.

    A physiologically based kinetic model of rat and mouse gestation: disposition of a weak acid

    Toxicol. Appl. Pharmacol.

    (1992)
  • K. Pogrmic-Majkic et al.

    Atrazine effects on antioxidant status and xenobiotic metabolizing enzymes after oral administration in peripubertal male rat

    Environ. Toxicol. Pharmacol.

    (2012)
  • P. Poulin et al.

    A priori prediction of tissue:plasma partition coefficients of drugs to facilitate the use of physiologically-based pharmacokinetic models in drug discovery

    J. Pharm. Sci.

    (2000)
  • M.S. Poulsen et al.

    Modeling placental transport: correlation of in vitro BeWo cell permeability and ex vivo human placental perfusion

    Toxicol. In Vitro

    (2009)
  • A.A. Abdel-Rahman et al.

    Pharmacokinetic profile and placental transfer of a single intravenous injection of [(14)C]chlorpyrifos in pregnant rats

    Arch. Toxicol.

    (2002)
  • P.L. Altman et al.

    Volume of blood in tissue: vertebrates

  • S.K. Arthur et al.

    Renal function during lactation in the rat

    J. Physiol.

    (1983)
  • J.C. Atherton et al.

    Changes in water and electrolyte balance, plasma volume and composition during pregnancy in the rat

    J. Physiol.

    (1982)
  • ATSDR

    Toxicological Profile for Atrazine

  • J.E. Bakke et al.

    Metabolism of atrazine and 2-hydroxyatrazine by the rat

    J. Agric. Food Chem.

    (1972)
  • D.B. Barr et al.

    Assessing exposure to atrazine and its metabolites using biomonitoring

    Environ. Health Perspect.

    (2007)
  • W.A. Battaglin et al.

    The occurrence of glyphosate, atrazine, and other pesticides in vernal pools and adjacent streams in Washington, DC, Maryland, Iowa, and Wyoming, 2005–2006

    Environ. Monit. Assess.

    (2009)
  • S.C. Brake et al.

    The role of intraoral and gastrointestinal cues in the control of sucking and milk consumption in rat pups

    Dev. Psychobiol.

    (1982)
  • R.P. Brown et al.

    Physiological parameter values for physiologically based pharmacokinetic models

    Toxicol. Ind. Health

    (1997)
  • J.M. Brzezicki et al.

    Quantitative identification of atrazine and its chlorinated metabolites in plasma

    J. Anal. Toxicol.

    (2003)
  • K. Capek et al.

    The development of the control of water metabolism. I. The excretion of urine in young rats

    Physiol. Bohemoslov.

    (1956)
  • Cited by (0)

    View full text