The use of physiological models to assist in understanding chemical exposure and dosimetry for early life stages Jeffrey Fisher FDA/NCTR

Computational Research (PBPK/PD Modeling) Extrapolation of data. Adult, infant, and fetus. Body weight Tissue volumes Blood flows Biliary and kidney excretion Metabolism

What is going on with early life stage PBPK models? Historically, environmental and food contamination safety assessments lack information about early life stages (e.g., fetus, neonate, infant). Best Pharmaceuticals for Children Act (BPCA) of 2002 and encouragement by FDA has resulted in a large number of pediatric PBPK models in the literature for drugs over the last 5 years. PBPK modeling community from drugs need to get together with others using models.

Growth in PBPK modeling Number of PBPK models published each year (Rowland et al. 2011) My first published PBPK papers

Why develop mathematical PK and PD models for life stages? Allows for predictions of internal doses or concentrations, extrapolations across species, routes of exposure and dose. Use: Exposure and Risk Assessments 1.Models can simulate the physiological and biochemical changes over gestation and lactation. 2.Need to add chemical or drug specific information.

Computational models for early life stages at FDA/NCTR ( Example 1) Bisphenol A- food and environmental contaminant. Probably most of us are excreting very small amounts of BPA in our urine today. This is one fundamental public health concerns in my opinion.

A large set of BPA pharmacokinetics studies conducted at NCTR with mice, rats, and monkeys including life stages. Relatively low experimental dose (100 µg/kg) Use deuterated BPA to avoid contamination issue Use modern analytical methods

Model Schematic Serum Liver Fat Gonad Slow Rich Skin Gut Vbody Stomach Small Intestine Urine excretion Oral BPA BPA BPAG EHR as BPA EHR as BPAG Gut glucuronidation Hepatic glucuronidation Urine excretion Brain Vbody Urine excretion BPAS Oral uptake EHR: enterohepatic recirculation Hepatic sulfation Gastric emptying Gut glucuronidation & biliary excretion Hepatic glucuronidation & biliary excretion Dermal exposure

PND3PND10PND21AdultPND5PND35PND70AdultNewbornAdult Rats MonkeysHumans Theoretical: Repeated daily oral dosing with 50 µg/kg of BPA Daily AUC –BPA in serum Daily peak conc-BPA in serum Sparse data

Modeling of Infants: Simulations of BPA ingestion in food (  g/kg bw/d) 6 meals, 0.3 (mean) and 0.6 (90 th )  g/kg bw/d. BPA or BPAg in serum ( nM ) BPAg BPA Infant 2µg/L (ppb) 0.2µg/L 0.2/L () 0.02 BPA or BPAg in serum ( nM ) BPAg BPA Infant Time (days) BPA or BPAg in serum ( nM BPAg BPA BPA or BPAg in serum ( nM BPA-G BPA

Teeguarden et al. 2013

Computational efforts at FDA/NCTR for early life stages (Example 2) Biologically Based Dose Response (BBDR) Modeling– Endocrine Disruption (hypothalamic-pituitary-thyroid axis) Prediction of hypothyroxinemia and hypothyroidism in pregnant mother and nursing and bottle fed infant. Thyroid hormone model with iodine and food contaminant perchlorate.

Dose- Response for the HPT axis Traditional dose response PBPK and PD model Administered dose Internal dose /MOA HPT axis homeostatic controls Adverse outcome ? Dose-Response

Thyroid Axis Perturbations Iodide Deficiency –Substrate and iodide stores depletion Exposure to Thyroid Active Chemicals –Perchlorate (ClO 4 - ) –Thiocyanate (SCN - ) –Nitrate (NO 3 - ) Mode of Action Inhibition to NIS-mediated uptake of iodide *NIS – Sodium Iodide Symporter UptakeOrganification, Biosynthesis and Distribution Elimination (ClO 4 -, SCN -, NO 3 - )

Schematic of Deterministic BBDR-HPT Axis Near-Term Pregnancy Model Lumen et al. 2013

Estimates of Iodide Status in the U.S. Population FDA Total Diet Study (Murray et al. 2008) –Women of reproductive age: 145 to 197 µg/day of iodide. Biomonitoring (Caldwell et al. 2011) –Median urinary concentration of iodide in pregnant women: 125 µg/L. –Of which 57% of the pregnant women’s urinary iodide concentrations <150 µg/L. Estimates of Perchlorate Exposure in the U.S. Population FDA Total Diet Study (Murray et al. 2008) –Women of reproductive age: 0.08 – 0.11 µg/kg/day of perchlorate Biomonitoring (Huber et al. 2011) –Mean perchlorate dose in the U.S.: µg/kg/day, including a potential drinking water component. –Pregnant women mean food intake dose: µg/kg/day of perchlorate For the total population of the United States, the perchlorate contributions was estimated to be 80% from food and remaining 20% from drinking water (Huber et al. 2011)

Application of BBDR-HPT Axis Near-Term Pregnancy Model Evaluation of the effects of iodide nutrition and perchlorate exposure on maternal thyroid hormone levels Lumen et al *HPT, Hypothalamus Pituitary Thyroid

How much of perchlorate exposure does it take to be associated With hypothyroxinemia and onset of sub-clinical hypothyroidism? Lumen et al. 2013

Probabilistic Analysis Model predicted maternal thyroid hormone levels for a population of pregnant women Total T4 (nmol/L) Free T4 (pmol/L)

Estimates of Perchlorate Exposure in the U.S. Population FDA Total Diet Study (Murray et al. 2008) – lower and upper bound average of perchlorate intakes for all age groups spans from 0.08 – 0.39 µg/kg/day ( ) – For women years and years the range was 0.08 – 0.11 µg/kg/day ( ). NHANES ( ) and UCMR ( ) (Huber et al. 2011) – Mean food perchlorate intake in the U.S. is µg/kg/day and µg/kg/day including drinking water. – Pregnant women had a mean perchlorate food intake of µg/kg/day In the United States, the perchlorate contribution from food is 80% and from drinking water 20% (Huber et al. 2011)

Exposure Scenarios Maternal fT4 (pmol/L) Geometric Mean 95% Confidence Interval (CI) LowerUpper Iodine intake (75 to 250 µg/day) without perchlorate exposure Iodine intake with perchlorate exposure from food intake (0.08 – 0.39 µg/kg/day) (Huber et al and Murray et al. 2008) th percentile food intake of perchlorate (0.278 µg/kg/day) and iodine intake (Huber et al. 2011) Perchlorate intake of 3.4 µg/kg/day associated with non-overlapping CI compared to without perchlorate exposure and iodine intake Preliminary Analysis with Perchlorate Exposure

Lactating mom and nursing infant and bottle fed infant Currently we are working on thyroid hormone models and iodine model to predict perchlorate induced changes in serum thyroid hormones as a function of iodine intake and perchlorate exposure. Predict serum thyroid hormones in newborn to 90 days of age.

Infant HPT axis Revved up, high through-put. Many comparisons to thyroid hormones or iodine stores in young compared to adults. Young very sensitive compared to adults. Relative to functioning of the HPT axis, the young appear more resistant to insult than adults (not for radioactive iodines).

Regulatory science for early life stages and perchlorate Publication of models in peer reviewed literature. Peer review of model code by EPA. Does the model have merit for an intended purpose? If the model has merit does it provide important information for regulatory science? What are the major uncertainties, gaps in data or gaps in knowledge.

Contributors NCTR - Nysia George, Annie Lumen, library staff US EPA-Eva McLanahan, Paul Schlosser, Santhini Ramasamy Contractors: Abt Associates, Teresa Leavens