Transcript Lecture 2 - Rutgers University

• Dose response relationships
– A graph describing the response of an organism, population, or
biological community to a range of concentrations of a xenobiotic
– Usually shown as a curve or histogram describing enzyme
inhibition, DNA damage, death, behavioral changes or any other
measurable/quantifiable parameter
• dose ≠ concentration
– Concentration is the relative concentration of a chemical with
respect to water/air e.g. concentration of DDE in Lake Apopca
water
– Dose is the amount of a chemical that actually gets into an
organism / actually reaches the active site e.g. concentration of
DDE in Alligator tissue
• Additional mortality is seen as dose is increased
• The first derivative of this dose response curve
is often a normal distribution
– This implies that there is a concentration range that
has a greater effect on mortality than the range
above and below
– Typically observed for multigenic responses
• Other distributions can be observed. For
example a bimodal distributions would result, if
the toxicant had a greater effect on male than
on females
• There are several parameters that are commonly estimated
by graphical or computation means from dose response
plots :
– LD50
Dose that is Lethal in 50% of individuals tested;
– LC50
Concentration that is Lethal in 50% of individuals
tested; estimated by graphical or computation
means
– EC50
Concentration that has a measurable Effect on
50% of individual tested. This is the parameter
that is used estimate effects that are not lethal.
– IC50
Concentration that Inhibits 50% of a particular
process e.g. enzyme activity, growth rate in
bacteria
• How do we rank the toxic effect of chemical ? /
How do we compare the toxicity of two
chemicals ?
– Run an experiment with both chemicals through the
same concentration range and using the same tester
species (all else being equal)
– Plot the data together on the same Percent mortality
plot
• Slopes of lines are similar
• Midpoint (LC50) is different
• Problem with the LD50 method
– Quoting LC50 only can be misleading because the
slope of two plots can be very different, but result in
the same LC50 (ignores kinetic differences)
– LC50 Experiments are typically done for short periods
of time (typically 96 hours). The risk of slow acting
chemicals, or chemicals that move into tissue slowly
may be underestimated
– chronic exposure
– how to you measure dose ?
• LD50 plots with similar slopes may suggest a
similar mode of action
• To describe the effect of chemicals that do not have an immediate
lethal effect other terminology is used:
– NOEC
No Observed Effect Concentration;
determined by statistical hypothesis testing
– NOEL
No Observed Effect Level; determined by
statistical hypothesis testing; reported as a
dose
– NOAEC
No Observed Adverse Effect Concentration;
chosen for its’ impact on a species
– NOAEL
No Observed Adverse Effect Level; chosen
for its’ impact on a species – reported as dose
– LOEC
Lowest Observed Effect Concentration
– LOEL
Lowest Observed Effect Level (dose)
– MTC
Minimum Threshold Concentration
– MATC
Maximum Allowable Toxicant Concentration
• Threshold concentration :
– concentration below which no effect is observed
– A : no threshold
– B : threshold
– C : hormetic response
• Over the years test methods have been standardized.
Protocols are available from
– American Society for Testing and Materials (ASTM)
– Organization for Economic Cooperation and Development
(OECD)
– National Toxicology Program (NTP)
• Protocols are available as
– U.S. EPA publications
– The federal register
– From researchers that pioneer and develop standard
methodology
• Assumptions of standard methods:
– The response observed is due to the chemical administered
– The magnitude of the response is related/proportional to the
amount of chemical administered
• Assumes there is a molecular target / receptor for the chemical in an
organism
• Assumes that the concentration of the chemical at the receptor site is
related to the dose administered
– There is a method for accurately and precisely quantifying the
response/toxicity
– Endpoints are known i.e. toxicity assay needs to be run within the
range of the toxic effect of a chemical – too little, no effect; too
much, 100% mortality at lowest dose
• Advantages of standard methods
– Results are comparable between different labs
– Results can be reproduced by other labs
– Data can be compiled from the literature and comparisons drawn
– Provides criteria for decision making
– Logistically simplified – hire technicians that can perform many
assays with little training
– Standard ASTM methods handbook
– Methods can be critically investigated and changed based on
best available science
– Provides guidelines on how to collect data and perform statistical
analysis
• Types of toxicity tests :
field study
type of
study
mesocosm
microcosm
chronic
acute
immediate reproduction Interspecies Community ecosystem succession
interaction abiotic factors
temporal/spatial scale
• Types of tests:
– Single species or community
– Aquatic
• Usually whole body exposure
• Design variable usually relate to test chamber
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Static tests
Static renewal
Recirculation
Continuous or intermittent flow-through
– Terrestrial
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Intravenous (mouse, rat, rabbit, cat, dog)
Intromusuclar(“”)
Intraperitoneal(“”)
Gavage (tube into stomach)
Feeding studies
Inhalation
Dermal (rabbit)
Spraying (community/field study)
Soil addition
• Standard test organisms
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Should be widely available
Easily maintained in the lab
History of genetic composition (known mutations)
Must be sensitive to a toxicant
Must be representative of a particular taxonomic class
• Mayer et. Ellersieck 1986
– Examined 5000 end-point tests on 66 species for 410 chemicals
– Found that 88% of test on Daphnia, Gammarus, and rainbow trout
provided the lowest indication of toxicity