# Multiple Exposure Route 

# Manganese PBPK Models for Monkeys and Humans

Jeff Schroeter

Mn TAP Meeting
October 31, 2013• Mn is an essential nutrient that is required for numerous physiological functions

• The primary source of Mn exposure is through the diet

• Human consumption ranges from 1

- 10 mg/day with a small fraction absorbed by the GI tract

• Homeostatic controls regulate absorption and excretion to maintain stable tissue levels despite dietary fluctuations

• Neurotoxicity can occur when Mn intake exceeds

elimination  accumulation in midbrain regions (e.g., globus pallidus 

## Introduction• Concerns exist as to whether inhalation exposure to high

Mn levels leads to enhanced brain accumulation due to deposited particles bypassing the GI tract

• PBPK models were developed to describe Mn tissue dosimetry resulting from dietary and inhalation exposure

• Models were based on a multitude of Mn pharmacokinetic

studies and 54 Mn tracer clearance studies in rats, monkeys,

and humans 

## Introduction• A series of pharmacokinetic modeling studies described Mn kinetics using compartmental models (Teeguarden et al., 2007)

• PBPK model of the adult rat (Nong et al., 2008)

• PBPK models of adult rats and monkeys (Nong et al., 2009)

• PBPK models of monkeys and humans (Schroeter et al., 2011) 

## Manganese PBPK Model History• Multi

- dose route capability: inhalation, diet, and oral exposure; iv, ip, and subq injection

• Homeostatic control processes (inducible biliary excretion)

• Saturable tissue binding capacities

• Preferential fluxes in various tissues

 Can be used to predict exposure scenarios that result in

increased brain Mn concentrations 

## Manganese PBPK Model HighlightsMonkey

• Anatomical parameters from the literature

• Basal Mn tissue concentrations from Dorman et al. (2006)

• 54 Mn tracer kinetic studies were used to calibrate fractional dietary uptake ( Fdietup ) and the biliary excretion rate constant ( kbilec

)

• Diffusion parameters, dissociation rate constants, and tissue binding capacities were calibrated to the pharmacokinetic data from Dorman et al. (2006) 

## Manganese PBPK Model DevelopmentHuman

• Anatomical parameters were either scaled from the monkey or obtained from the literature

• An average body weight of 70 kg for adults was used

• Basal Mn tissue concentrations from cadaver studies

• Dietary intake was set to 3 mg Mn/day

• Dietary absorption was calibrated to whole

- body

elimination kinetics of 54 Mn

• Rate constants were scaled from the monkey model 

## Manganese PBPK Model Development• IP injection (Dastur et al., 1971)

- 200 μCi 54 Mn given ip

- 12 monkeys, average BW=2.5 kg

- whole

- body retention measured for 278 days

• IV injection (Furchner et al., 1966)

- 0.6 μCi 54 Mn given iv

- 3 monkeys, average BW = 8.5 kg

- whole

- body retention measured for one year

• Oral exposure (Furchner et al., 1966)

- 0.6 μCi 54 Mn given orally

- 3 monkeys, average BW = 7 kg

- whole

- body retention measured for 60 days 54 

## Mn Tracer Studies in Monkeys• Subcutaneous injection (Newland et al., 1987)

- 6

- week continuous exposure to 200 μ Ci 54 Mn and 400 mg Mn (MnCl 2 solution) administered subcutaneously with osmotic pump

- one monkey, BW = 5 kg

- fecal activity was measured for ~150 days

• Inhalation exposure (Newland et al., 1987)

- Monkeys were endotracheally exposed to nebulized MnCl 2 aerosol for 30 minutes; Monkey A: 24 μ Ci 54 Mn, Monkey B: 60 μ Ci

54 Mn

- Monkey A: BW = 3.1, Monkey B: BW = 2.5 kg

- fecal activity was reported for ~250 days post

- exposure 54 

## Mn Tracer Studies in Monkeys

## IP results

## IV results

## Oral results

## Subcutaneous exposure

## Inhalation exposure

## Summary of 54 

## Mn Tracer Studies in Monkeys

• Small changes were made to fractional dietary uptake and the biliary excretion rate constant to account for changes in Mn diet and monkey body weights in the

different studies

• All other parameters in the PBPK model were unchanged to maintain a consistent description of Mn kinetics

• Biphasic elimination curves were observed for all
exposure routes

• The PBPK model accurately replicated the elimination curves for all exposure routes• Mahoney and Small (1968): 3 normal subjects were given an iv dose of 2.5 μ Ci 54 Mn

• Mena et al. (1967): 8 healthy individuals were given an iv

dose of 20 μ Ci 54 Mn

• No information on gender, BW, or Mn in diet

• Whole

- body retention was measured for > 30 days

• Elimination curves were used to calibrate fractional dietary uptake ( Fdietup = 0.06) and the biliary excretion rate constant ( kbilec = 0.051) 54 

## Mn Tracer Studies in Humans54 

## Mn Tracer Studies in HumansMahoney and Small (1968):

• One subject on a reduced calorie diet (800 cal/day) for 6 months prior to exposure

– assume 1 mg/day Mn in diet

• Reduced calorie diet was continued throughout the study (~ 90 days)

• 60 days after iv dose of 2.5 μCi 54 Mn, the subject ingested 800 mg/day supplemental Mn for 35 days

 Fdietup reduced from 0.062 to 0.00046 to account for decreased absorption of high oral Mn dose 54 

## Mn Tracer Studies in Humans54 

## Mn Tracer Studies in Humans54 

## Mn Tracer Studies in Humans

Mahoney and Small (1968):

• One subject on a normal diet was pre

- loaded with 300 mg/day Mn 10 days prior to iv dose of 2.5 μCi 54 Mn

• This regimen continued throughout the experiment for 40 more days

 Fdietup was reduced by 28% per day for 15 days starting with the high

- dose Mn ingestion. Final Fdietup

value was 0.0004354 

## Mn Tracer Studies in Humans54 

## Mn Tracer Studies in Humans

Mahoney and Small (1968):

• Iron

- deficient subject: 50 days after iv dose of 2.5 μCi

54 Mn, iron therapy was initiated with 400 mg/day Fe

 Fdietup was increased during the iron

- deficient phase ( Fdietup = 0.075) and decreased ( Fdietup = 0.035) after initiation of iron therapy54 

## Mn Tracer Studies in Humans• Six women ingested a meal labeled with 0.1 MBq 54 Mn (Davidsson et al., 1988)

• Whole

- body retention was measured for 30 days

• Oral 54 Mn exposures were used to calibrate GI parameters in the PBPK model 54 

## Mn Tracer Studies in Humans54 

## Mn Tracer Studies in Humans• Plasma Mn concentrations were measured in men consuming different levels of dietary Mn (Freeland

- Graves, 1994)

Days mg/day Mn

1

- 21 2.36

22

- 42 2.89

43

- 63 2.06

64

- 101 1.21

102

- 112 3.79

113

- 125 2.65 54 

## Mn Tracer Studies in Humans54 

## Mn Tracer Studies in Humans

## Monkey Inhalation Exposures

Dorman et al. (2006):

• Monkeys were exposed to 0, 0.06, 0.3, or 1.5 Mg Mn/m 3

of MnSO 4 for 6 hours/day, 5 days/week for 13 weeks

• Monkeys were maintained on a 133 ppm Mn diet

PBPK model parameters were calibrated using tissue Mn concentrations during and after the exposure period

## Monkey Inhalation Exposures

1.5 mg Mn/m 3

## Monkey Inhalation Exposures

## Human Inhalation Exposures

• Dietary absorption and biliary excretion were calibrated

to 54 Mn whole

- body retention data

• Rate constants were scaled from the monkey and were further adjusted to fit basal Mn tissue levels

• 3 mg/day Mn in diet

• Exposure scenario: 1.5 Mg Mn/m 3 of MnSO 4 for 8 hours/day, 5 days/week for 13 weeks

## Human Inhalation Exposures

1.5 mg Mn/m 3

## Human Inhalation Exposures

Globus pallidus

## Human Inhalation Exposures

iv dose of 54 Mn given on day 0

## Interspecies Comparison

Total/free/bound Mn

## Interspecies Comparison

## Sensitivity Analysis

## Summary

• The PBPK models provide consistent multi

- species descriptions of Mn tracer kinetics and tissue Mn concentrations among multiple dose routes

• The PBPK models account for background tissue levels associated with normal dietary Mn exposure and simulate simultaneous exposure by ingestion and inhalation

• Simulation results recapitulate the rapid increase in tissue Mn from high inhalation concentrations and the subsequent

return to basal levels

• The models can be used to evaluate exposure conditions

that result in increased brain Mn tissue concentrations