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 : –LD 50 Dose that is Lethal in 50% of individuals tested; –LC 50 Concentration that is Lethal in 50% of individuals tested; estimated by graphical or computation means –EC 50 Concentration that has a measurable Effect on 50% of individual tested. This is the parameter that is used estimate effects that are not lethal. –IC 50 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 (LC 50 ) is different

Problem with the LD 50 method –Quoting LC 50 only can be misleading because the slope of two plots can be very different, but result in the same LC 50 (ignores kinetic differences) –LC 50 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 ? LD 50 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: –NOECNo Observed Effect Concentration; determined by statistical hypothesis testing –NOELNo Observed Effect Level; determined by statistical hypothesis testing; reported as a dose –NOAECNo Observed Adverse Effect Concentration; chosen for its’ impact on a species –NOAELNo Observed Adverse Effect Level; chosen for its’ impact on a species – reported as dose –LOECLowest Observed Effect Concentration –LOELLowest Observed Effect Level (dose) –MTCMinimum Threshold Concentration –MATCMaximum 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 : type of study temporal/spatial scale immediatereproductionInterspecies interaction Community abiotic factors ecosystemsuccession acute chronic microcosm mesocosm field study

Types of tests: –Single species or community –Aquatic Usually whole body exposure Design variable usually relate to test chamber –Static tests –Static renewal –Recirculation –Continuous or intermittent flow-through –Terrestrial 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 –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