Toxicity and Assessment of Chemical Mixtures

Toxicity and Assessment of
Chemical Mixtures
Monday, December 12 2011
Herman Autrup
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 Exposure is often to more than one
chemical, either through co-exposures
or exposure to mixtures
 Each mixture of chemicals can have
unique toxicology
 Few mixtures in the real world are
the same
 Mixed exposures of chemicals and
non-chemical stressors
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EFFECTS OF MIXTURES
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Is there scientific evidence that when
organisms are exposed to a number
of different chemical substances,
that these substances may act jointly
in a way (addition, antagonism,
potentiation, synergies, etc.) that
affects the overall level of toxicity?
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• Substances that are mixtures themselves (multi-
constituent substances, MCS; materials of
unknown or variable composition,
• Complex reaction products or biological
materials, UVCB)
• Products that contain more than one chemical

e.g. cosmetics, plant protection products;
• Chemicals jointly emitted from production sites,
during transport processes and consumption or
recycling processes;
• Several chemicals that might occur together in
environmental media (water, soil, air), food items,
biota and humans as a result of emission from
various sources and via multiple pathways.
EU DG SANCO DEFINITION (2011)
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CUMULATIVE RISK
Cumulative risk means the risk of a common
toxic effect associated with concurrent
exposure by all relevant pathways and routes of
exposure to a group of chemicals that share a
common mechanism of toxicity
”Dose addition”
Cumulative risk means the combined risks
from aggregate exposures to multiple agents or
stressors (US EPA)
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INTERACTION TERMINOLOGY
Type of
combined effect
Subtype Synonyms Effect observed
Non-interactive Simple similar
action
Additivity Dose addition
Simple dissimilar
action

Independent
action
Response addition
Interactive Potentiation Synergy Greater than dose
additive effect
Antagonism Less than dose
additive effects
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Potentiation
Antagonism
POTENTIATION AND ANTAGONISM
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INTERACTIONS - KINETIC
• Effects on absorption, distribution,
metabolism or excretion
• Induction of ”drug” metabolizing enzymes
• Reversible inhibition of ”drug”
metabolizing enzymes (competetive or non-
competitive)
• Suicide inhibition of enzymes
• Alteration in levels of co-factors
Do also apply to transporter proteins
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INTERACTIONS - DYNAMIC
• Toxicodynamic
• Changes in structure or amount of target
•Up-regulation
• Down-regulation
• Competition
Alteration in level of cofactors/coenzymes

Effect on protective or repair systems
Non-specific mechanisms (cell death and damage
to membrane
)
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INTERACTIONS
 Synergism or potentiation
 Could be relevant, even at exposures
below the respective NOAEL
 Evidence is against this being common
when all of the compounds are below
their respective NOAELs
 Antagonism (of toxicological effect)
 Although this might occur is some cases,
there is no public health concern
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SIMPLE SIMILAR ACTION
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SIMPLE SIMILAR ACTION
 Dose/concentration additivity
 It is likely to occur when the chemicals in
the mixture act:
 In the same way
 By the same mechanism(s) – possibly at
the same macromolecule
 Differ only in their potencies
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SIMPLE SIMILAR ACTION
 Effect is obtained by summing the doses of
the individual compounds, having adjusted

for differences in their potencies
Effecttotal = PotencyA x DoseA + PotencyB x
Dose B + + PotencyN x DoseN
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POTENCY FACTORS
 Toxicity equivalence factors (TEF)
 Toxicity is calculated relative to an index
compound
 The intake of residues is the multiplied
for each member of the common
mechanism group (CMG)
 Thus the residue is ”normalised” in term
of the reference compound
 Developed for risk assessment of
mixtures of structurally related dioxines
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GROUPING COMPOUNDS
 Common Mechanism Group
Group of compounds having the same toxicological
endpoint
 Show dose addition
 Easily defined with some groups, w.g.
Anticholinesterase OPs
 More difficult with e.g. Endocrine disruptors, where
the effect may be similar but the mechanism different
 Common assessment group
 Group of compounds assumed to act by the same
mode of action on bais of preliminary evaluation
(e.g. Common target organ
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CONCENTRATION ADDITION
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SIMPLE DISSIMILAR ACTION
 Results in response addition
 It occurs when:
The modes, nature and/or sites of action
differ among the chemicals in the mixture
The constituents do not modulate the
effect of other constituents of the mixture
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SIMPLE DISSIMILAR ACTION
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DISSIMILAR ACTIONS
Christiansen S (2009) EHP 117: 1839-
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PROBLEM FORMULATION
 Is there environmental release
 Are the identity of at least some of the
components known
 Is there significant systemic exposure
 Is there a pausible biological hypothesis to
consider the possibility of combined
effects/interactions
 Basis to consider components in ”common
mechanism group
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RISK ASSESSMENT PROCESS
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TIERED APPROACH
 Zero tier

 Does exposure exceed relevant TTC
 First tier (e.g. HI or PODI)
 Do components act on the same tissue/cell
 Same mode of action
 Plausible hypothesis for interaction
 Default factors to allow for potential synergy
 Second tier
 Mechanism of action
 Relative potency
 Third tier
 PBPK/PD
 Probabilistic assessment of exposure and/or hazard
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HAZARD INDEX
 Hazard index (HI) is the sum of the
exposures divided by their reference doses
HI = Exposure
1/ADI1 + Exposure2/ADI2 + +
Exposure
n/ADIn
 HI inappropriate as it is based on the ADI,
which in turn is based upon critical NOELs
 An uncertainty factor, which may be different
for the different compounds
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HAZARD INDENTIFICATION
 Determine chemical composition
Chemicals in a mixture may vary
 Burning conditions in a wood stove
 Predict toxicology

 Test the toxicity of source-specific mixtures
in animal studies
Compare toxicity testing results among the various
mixtures to determine which component contribute
most to toxicity
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