integrating neurotoxicology and epidemiology in … · pyrolusite (mno2) photo courtesy aram...

AUGUST 31, 2016

MEDICHEM ANNUAL CONFERENCE 2016

INTEGRATING NEUROTOXICOLOGY AND EPIDEMIOLOGY IN EVALUATING HUMAN HEALTH EFFECTS:

MANGANESE AS A CASE STUDY

Kenneth Mundt, PhD Ramboll Environ US Corp. [email protected]

AUGUST 31, 2016


The chemistry and biology of manganese (Mn)

Biology: Uptake, Distribution and Metabolism

Physiological roles: Essentiality and Neurotoxicity

Health effects of Mn exposure

Dietary insufficiency

Low grade exposure effects

Manganism

Quantitative risk assessment: challenges and opportunities

How much Mn is too much Mn?

Current perspectives and opportunities for quantifying risks associated Mn

OVERVIEW

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Mn is a transition metal and the 12th most abundant (0.1%) mineral in the Earth’s crust: most commonly as pyrolusite (MnO2)

Normal constituent of air, soil, water, and food.

Reaches the atmosphere and waterways through

erosion of rocks and soils.

Anthropogenic sources include mining and refining activities; industrial wastes, disposal of consumer products (e.g., dry-cell batteries), etc.

In some countries, automobile exhaust

contributes (Methylcyclopentadienyl manganese tricarbonyl (MMT) (CH3C5H4)Mn(CO)3 used as fuel additive

Manganese in the Environment

Pyrolusite (MnO2) Photo courtesy Aram Dulyan.

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Production and Common Uses of Mn

South Africa contributed 6.2 of the 18 million metric tonnes produced globally in 2015

Common uses for Mn:

Manufacturing steel and other alloys

Welding rods

Dry-cell batteries

Fertilizers (e.g., Maneb, Mancozeb)

Paints

As gasoline additive to improve octane rating (MMT)

Medical imaging agent (intravenous contrast agent in MRIs)

Cosmetics

http://minerals.usgs.gov/minerals/pubs/commodity/manganese/mcs-2016-manga.pdf







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Sources of Human Exposure

Dietary: fruits, nuts, oatmeal, cereals and beans, tea, etc.

Environmental:

Air – Auto exhaust, mining, steel and alloy production and other industrial operations

Water – very low level exposure

Soil – very low level exposure

Occupational: manganese miners miller and refiners, welders, steel and other metal workers

In the US, 6185 tons of Mn and 73,644 tons of Mn compounds released by industries

“The inhalation of air contaminated with particulate matter containing manganese is the primary source of excess manganese exposure for the general population in the United States” ATSDR 2012

http://lpi.oregonstate.edu/mic/minerals/manganese#deficiency


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The Biology of Manganese

Commonly enters body via ingestion and inhalation

Low absorption from the gut (3-5%) but relatively high bioavailability (~40%)

Ingested Mn: extensive metabolism in bile

Absorption and excretion controlled in the intestine to maintain homeostasis; Fe affects Mn absorption inversely

Primary route of elimination: hepatobiliary excretion

Mn also can travel along olfactory and trigeminal nerves and accumulate in the brain

Excess Mn starts accumulating in the liver, brain and bone

O’Neal, S.L. and Zheng, W., 2015. Manganese toxicity upon overexposure: a decade in review.Current environmental health reports, 2(3), pp.315-328.

Neurotoxicity

Excretion

Exposure

Gut Lung

Liver

Bile

Enterohepatic circulation

Plasma

Blood brain barrier

CSF

Basal ganglia (globus pallidus)

Damaged BG neurons

Metal transporters

Olfactory

nerve

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Adequate Intake (AI) of Manganese

Subgroup AI of Mn (mg/d)

Tolerated upper limit

(TUL)

Men 2.3 11

Women 1.8 11

Pregnant women

2.0 -

Breastfeeding women

2.6 -

Children 4 - 8 1.5 3

Infants 1 - 3 1.2 2

Infants < 1yr 0.6 -

Daily adult intake ranges from 1 -10 mg/m3

Insufficient data to set a Recommended Daily

Allowance (RDA)

Adequate Intake (AI) based on the average dietary

intake of Mn in the Total Diet Study*

No overt deficiency state of Mn documented in

humans eating natural diets

Tolerated upper limits, set by FNB, are conservative

Dietary exposure above TULs may not directly

neurotoxic, but may potentiate inhalational exposure

*Food and Nutrition Board, Institute of Medicine. Manganese. Dietary reference intakes for vitamin A, vitamin K, boron, chromium, copper, iodine, iron, manganese, molybdenum, nickel, silicon, vanadium, and zinc. Washington, D.C.: National Academy Press; 2001:394-419.

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Physiologic Role of Manganese

Normal ranges of manganese in body fluids:

4–15 μg/L blood

1–8 μg/L urine

0.4–0.85 μg/L serum

Mn is an essential cofactor for enzymes involved in critical processes:

Mitochondrial and cytosolic respiration – Enolase, PEP kinase, etc

Carbohydrate , protein and fat metabolism

Bone mineralization and skeletal growth

Antioxidants – Superoxide Dismutase

Wound healing - Prolidase

Susceptible groups include patients on TPN, IV drug users, IV contrast recipients, premature babies and those with renal or hepatic insufficiency



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Risk Assessment: To determine, quantitatively, exposures at which hazards pose risks to human populations

Integrating evidence to understand the whole:

Exposure science (critical to hazard and risk)

Epidemiology (human studies)

Toxicology (animal studies)

Studies of disease mechanism and mode of action

Full value of epidemiology for risk assessment has not been realized

Each line of evidence contributes to understanding; reduces uncertainties

Integrating Epidemiological And Toxicological Evidence

Epidemiology Toxicology

Health risk assessment

Public Health

Mechanism / MOA

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Inhalation Exposure Standards and Guidelines

AGENCY

LIMIT

LAST UPDATED

OSHA Permissible Exposure Limit (PEL)

5 mg/m3 Ceiling 2007

NIOSH Recommended Exposure Limit (REL)

1 mg/m3 TWA 3 mg/m3 STEL

2005

ACGIH Threshold Limit Value (TLV)

0.02 mg/m3 (respirable) 0.1 mg/m3 (inhalable)

2007

ATSDR Inhalation Minimal Risk Level (MRL)

0.04 μg/m3 (chronic exposure)

2012

https://www.osha.gov/dts/chemicalsampling/data/CH_250200.html



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Health Effects of Mn Exposure

The brain is the target organ for Mn toxicity in humans

Regardless of route of exposure, neurotoxicity is the culmination of manganese

accumulation in the body, whether due to high exposure or impaired clearance

Therefore, a large proportion of studies to understand Mn toxicity have center

around CNS toxicity

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Neurotoxicity – Mode of Action (MoA)

The mode of action (MoA) for Mn neurotoxicity has

been well-characterized based on sufficient data

Physiologically based Pharmacokinetic (PBPK)

models have been developed based on animal and

human data

Various exposure states can be modelled

However, these are highly complex (see example!)

Validation in Mn exposed populations needed

Schroeter, et al. "Application of a Multi-Route Physiologically-Based Pharmacokinetic Model for Manganese to Evaluate Dose-Dependent Neurological Effects in Monkeys."Toxicological Sciences (2012)

Schematic of the Mn PBPK model for nonhuman primates

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Mn Homeostasis in the CNS

Chen, Pan, et al. "Manganese homeostasis in the nervous system." Journal of neurochemistry 134.4 (2015): 601-610.

Half-life of Mn longer in the CSF than in blood

Physiological levels influenced by increased uptake or reduced elimination

Mn(II) tightly bound to albumin, Mn(III) to transferrin

Mn transport in and out of neurons is tightly regulated by several metal transporters in the CNS

Once homeostasis in plasma is upset, transport increases to accumulation in neurons, leading to neurotoxicity

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Molecular effects of Mn in the brain

Mn transport into CNS

Disruption of ATP synthesis

Impaired apoptosis

Dopamine (DA) auto-oxidation

Impaired DA release in the globus pallidus and putamen

Pre-synaptic

neurons of the

basal ganglia

Neal, April P., and Tomás R. Guilarte. "Mechanisms of heavy metal neurotoxicity: Lead and manganese." Journal of Drug Metabolism & Toxicology 2012 (2015).

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Neurotoxicity and Mn Exposure Concentration

Respirable dust

g/m3 Total g/g in

Globus pallidus

0.01 0.4

0.1 0.4

1 0.4

10 0.5

20 0.6

50 0.7

70 0.8

120 0.9

150 1.0

Schroeter, Jeffry, et al. "Application of a Multi-Route Physiologically-Based Pharmacokinetic Model for Manganese to Evaluate Dose-Dependent Neurological Effects in Monkeys." Toxicological Sciences (2012): kfs212. Table adapted from Dr RG’s presentation, with permission

Dr Robinan Gentry’s team adapted Schroeter et al.

(2011) primate MoA model to calculate dose to the

brain in humans occupationally exposed:

Compared NOAELs in the occupational cohorts to

ambient human exposures

Estimated the magnitude of the gap between

ambient exposures and concentrations associated

with health effects.

Margins of Safety in target tissues: 2500 to 5000 (eye-hand coordination) 6000 to 12000 (hand steadiness)

Small ambient concentrations of inhaled Mn are

not expected to lead to CNS accumulation

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Range of Occupational Exposures and Observed Effects

Study Exp duration

(years) LOAEL (mg/m3 of

inhalable air) Outcome

Mergler 1994 16.7 0.032 decreased motor function

Blond and Netterstrom 2007

24 0.07 impaired fast pronation/supination; impaired fast finger tapping

Lucchini 1999 11.5 0.0967 decreased performance on neuro-behavioural exams

Iregren 1990 1-35 0.14 decreased reaction time, finger tapping

Lucchini 1995 1-28 0.149 decreased NB performance, finger tapping, symbol digit, digit span, etc.

Roels 1992 5.3 0.179 impaired visual time, eye-hand coordination, hand steadiness

Bouchard 2005 19.3 0.23 impairment in 3/12 cognitive tests, 1/19 neuro-motor tests 1/4 sensory tests

Bouchard 2007a 15.7 0.23 significantly higher scores for depression, anxiety

Bouchard 2007b 15.3 0.23 impaired performance on 1/5 NM tests

Roels 1987 1-19 0.97 altered reaction time, short term memory, decreased hand steadiness

Chia 1995 1.1-15.7 1.59 postural sway with eyes closed

Williams, Malcolm, et al. "Toxicological profile for manganese." (2012).

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Manganese Epidemiology: Exposure Response

Since mid 1950’s, numerous occupational epidemiology studies of Mn exposures among welders, miners, ship builders, steel workers, battery workers, etc. have been published

Studies of low to moderate level Mn exposure suggest a continuum of exposure effects, many of which may be arrested or reversed

Possible outcomes include a range of subtle neurological defects:

cognitive (memory loss, personality changes, neuropsychological test performance)

motor (decrease in complex motor activities)

Mn exposure concentrations ranged from 0.07 – 0.97 mg/m3, while duration of exposure varied between 1 – 24 years

The precise level(s) and duration(s) of exposure to Mn at which risk of subtle neurotoxic effects increases remain elusive!

Williams, Malcolm, et al. "Toxicological profile for manganese." (2012).

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Manganese Epidemiology: Manganism

Managanism is a neuropsychiatric condition that manifests after long-term, high level Mn exposure with rapid onset, stable course and occasionally, partial remission

Similar to, but distinct from, Parkinson’s (poor or no sustained response to levodopa)

Early subjective symptoms include fatigue, anorexia, muscle pain, stiffness, personality changes and emotional lability

Progression:

Slow speech with loss of tone or inflection, mask-like facies, slow and uncoordinated movements

Hypertonic muscles, tremors and dystonia, characteristic staggering gait (“cock-walk”)

Simultaneous bilateral and symmetric motor impairment; low amplitude, rapid tremor

Usually irreversible, and clinical symptoms may be progressive, even after exposure ceases

Jankovic, Joseph. "Searching for a relationship between manganese and welding and Parkinson’s disease." Neurology 64.12 (2005): 2021-2028.

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Integration of Epidemiology and Neurotoxicology Data

Mode of Action

Dose-Response

Exposure-Response

Risk characterization

MoA’s for Mn neurotoxicity have been reasonably identified, based on in-vitro

and animal data and PBPK models

Data available from epidemiology studies establish subtle effects at low exposures,

and frank neurotoxicity and manganism at high exposures

? “An actual threshold level at which

manganese exposure produces neurological

effects in humans has not been

established.” (ATSDR 2012)

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Significant Gaps and Challenges Remain

Toxicology:

Multiple routes of exposure – inhalation and ingestion

Essentiality vs. neurotoxicity

Determining role of dietary levels intake variability on biologically active target tissue dose remains a challenge

Epidemiology:

Significant methodological limitations, especially in early studies

Cross-sectional studies do not allow estimation of time- and dose-dependent responses

Exposure assessment inconsistent across studies and time-periods (especially retrospectively)

Health outcomes may be subjective, non-specific, with no clear diagnostic criteria

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Challenges (continued)

Lack of directed therapeutic protocols:

No sensitive and specific screening protocol, no therapeutic targets identified

Few reliable biomarkers

Blood levels are good in the short term (previous days to weeks), while hair and nails only suggest exposure in previous months or year

Bone – reliable indicator or long term accumulation and toxicity, but difficult to sample

Non-interventional test biomarkers:

fDOPA PET to identify dopaminergic dysfunction not satisfactory

Relaxation rate of T1-weighted images are new approaches to identifying CNS accumulation, but have not been validated

Neurofunctional tests – inconsistent sensitivity, specificity and reproducibility

Hyperintense signal in the

globus pallidus on

T1-weighted MRI

Hernandez, Elena Herrero, et al. "Follow-up of patients affected by manganese-induced Parkinsonism after treatment with CaNa 2 EDTA."Neurotoxicology 27.3 (2006): 333-339.

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Opportunities

Quantitative evaluation of human health risks associated with moderate to low concentrations of Mn appears to be possible

Improved indicators – including biomarkers – are increasingly available

In light of major advancements in neurotoxicolgy, effects of specific pathways can be isolated and relevant outcomes examined epidemiologically

Reliable detection of increasingly subtle neurological impacts will strengthen epidemiological validation of mathematical risk models

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Concluding Remarks

Much recent work has helped elucidate pathophysiology of manganese in humans

This has paved the way for future investigations of Mn as well as other neurotoxicants

In spite of the volume of investigations available on manganese toxicity and human

health effects, inconsistencies in the integration and application of this evidence to

regulatory policy persist

Future directions:

Apply updated methods in exposure characterization (including biomarkers of exposure and

enhanced imaging technologies) and sensitive but objectives indicators of neurological response

(including biomarkers of exposure) to new epidemiological studies

More fully integrate multiple lines of evidence to span remaining gaps and provide a reliable

and scientifically sound basis for exposure regulation and controls

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Acknowledgements

Dr Robinan Gentry, PhD, DABT ([email protected])

Dr Sandra Sulsky, PhD ([email protected])

Dr Sonja Sax, ScD ([email protected])

Krishna Vemuri, MBBS, MPH ([email protected])

Manganese health effects on neurodevelopment and neurodegenerative diseases

Division of Occupational and Environmental Medicine, Mount Sinai School of Medicine, NY, USA

Sept 25-28 2016

http://events.mountsinaihealth.org/event/manganese2016

Talk by Dr Gentry: “A tissue dose-based comparative exposure assessment for manganese: The

importance of homeostatic control for an essential metal”



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Selected references - I

Chen, Pan, Megan Culbreth, and Michael Aschner. "Exposure, epidemiology, and mechanism of the environmental toxicant manganese." Environmental Science and Pollution Research (2016): 1-9. Chen, Pan, et al. "Manganese homeostasis in the nervous system." Journal of neurochemistry 134.4 (2015): 601-610. Aschner, Judy L., and Michael Aschner. "Nutritional aspects of manganese homeostasis." Molecular aspects of medicine 26.4 (2005): 353-362. O’Neal, Stefanie L., and Wei Zheng. "Manganese toxicity upon overexposure: a decade in review." Current environmental health reports 2.3 (2015): 315-328. Park, Robert M. "Neurobehavioral deficits and parkinsonism in occupations with manganese exposure: a review of methodological issues in the epidemiological literature." Safety and health at work 4.3 (2013): 123-135. Andruska, Kristin M., and Brad A. Racette. "Neuromythology of Manganism." Current epidemiology reports 2.2 (2015): 143-148. Neal, April P., and Tomás R. Guilarte. "Mechanisms of heavy metal neurotoxicity: Lead and manganese." Journal of Drug Metabolism & Toxicology 2012 (2015). Wright, Robert O., and Andrea Baccarelli. "Metals and neurotoxicology." The Journal of nutrition 137.12 (2007): 2809-2813. Olanow, C. W. "Manganese‐induced parkinsonism and Parkinson's disease." Annals of the New York Academy of Sciences 1012.1 (2004): 209-223.

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Selected references - II

Laohaudomchok, Wisanti, et al. "Neuropsychological effects of low-level manganese exposure in welders." Neurotoxicology 32.2 (2011): 171-179.

Meyer-Baron, Monika, et al. "The neurobehavioral impact of manganese: results and challenges obtained by a meta-analysis of individual participant data." Neurotoxicology 36 (2013): 1-9. Melvin E. Andersen , David C. Dorman , Harvey J. Clewell III , Michael D. Taylor & Andy Nong (2010) Multi-Dose-Route, Multi-Species Pharmacokinetic Models for Manganese and Their Use in Risk Assessment, Journal of Toxicology and Environmental Health, Part A, 73:2-3, 217-234, DOI: 10.1080/15287390903340849 Baker, Marissa G., et al. "Blood manganese as an exposure biomarker: state of the evidence." Journal of occupational and environmental hygiene 11.4 (2014): 210-217. Reiss, Boris, et al. "Hair manganese as an exposure biomarker among welders." Annals of Occupational Hygiene (2015): mev064. Criswell, Susan R., et al. "Ex vivo magnetic resonance imaging in South African manganese mine workers." Neurotoxicology 49 (2015): 8-14. Park, Robert M., et al. "Airborne manganese as dust vs. fume determining blood levels in workers at a manganese alloy production plant." Neurotoxicology 45 (2014): 267-275. Li S-J, Jiang L, Fu X, Huang S, Huang Y-N, et al. (2014) Pallidal Index as Biomarker of Manganese Brain Accumulation and Associated with Manganese Levels in Blood: A Meta-Analysis. PLoS ONE 9(4): e93900. doi:10.1371/journal.pone.0093900

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Selected references - III

Lucchini, Roberto, and Neil Zimmerman. "Lifetime cumulative exposure as a threat for neurodegeneration: need for prevention strategies on a global scale." Neurotoxicology 30.6 (2009): 1144-1148. Zoni, Silvia, Giulia Bonetti, and Roberto Lucchini. "Olfactory functions at the intersection between environmental exposure to manganese and Parkinsonism." Journal of Trace Elements in Medicine and Biology 26.2 (2012): 179-182. Huang, Chu-Yun, et al. "Chronic manganism: A long-term follow-up study with a new dopamine terminal biomarker of 18 F-FP-(+)-DTBZ (18 F-AV-133) brain PET scan." Journal of the neurological sciences 353.1 (2015): 102-106. Zoni, Silvia, Elisa Albini, and Roberto Lucchini. "Neuropsychological testing for the assessment of manganese neurotoxicity: a review and a proposal." American journal of industrial medicine 50.11 (2007): 812-830.

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integrating neurotoxicology and epidemiology in … · pyrolusite (mno2) photo courtesy aram...

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