Toxicity Testing Flashcards
What is toxicology?
- “the science of poisons”
- The study of the adverse effects of chemicals or physical agents on living organisms
What are the adverse effects of chemicals or physical agents on living organisms?
- Death
- More subtle effects that reduce an organism’s fitness, lifespan or reproductive output
Who was the first person to use toxicology?
- Hippocrates (≈400 BCE): first physician to use toxicology principles
Who is often referred to as the father of Toxicology?
- Paracelsus (1493-1541): “the dose makes the poison”
What are the four stages in the development of toxicity?
- Delivery to the target site
- Interaction with target molecule and alteration of biological environment
- Cellular dysfunction
- Exceedance of repair mechanism
How do the four stages of toxicity fit into the process in developing toxicity?
- The first thing is that you need to have the toxicant
- Then stage one (delivery to target site) needs to be completed
- Then stage 2a: Interaction with target molecule
- Or stage 2b: Alteration of biological environment
- This then leads to stage 3 (Cellular dysfunction)
- After stage three there are many repair mechanisms which can be used to overcome cellular dysfunction
- If the repair mechanisms are exceeded by the toxicant in stage four then Toxicity can or has been reached
What is a therapeutic window
- The space between a healthy dose of medicine and a toxic dose of medicine.
What is a dose-response curve?
- The dose–response curve describes the magnitude of the response of an organism, as a function of exposure to a stimulus or stress after a certain exposure time
What is on the X and Y axis of a dose-response curve?
- X = dose
- Y = % of population affected
What is the point of Departure (“Threshold of toxicity”)
- The point on a dose-response curve where the dosage begins to affect the population
What is LD50?
- LD50 stands for Lethal dose which affects 50% of the population
- It is a point on the dose-response curve where the lethal dose is most likely to be
What is generally affected as the dose or exposure increases on a dose-response curve?
- Bio-molecule will be affected first (e.g.enzyme). However this is not fatal because of the repair mechanisms which are still able to fix them
- Cell will be affected as the dosage or exposure increases slightly
- Then organ
- Then the entire organism
What is the Hill Equation?
- Commonly used to estimate the number of ligand molecules that are required to bind to the receptor in order to produce a functional effect.
effect (%) = 0 + (100 – 0) / 1+10 ^ [(logLD50 – logx) * slope]
How do you find the Toxic Equivalency Factor when comparing two chemicals?
- The TEF = the LD50 of the less toxic chemical divided by the LD50 of the other
Why is the dose-response relationship important?
- Establishes causality
- Determines threshold of toxicity (point of departure) = safe dose
- Slope determines the variability in sensitivity between individuals
What does it mean when a dose-response curve goes into a U shape instead of an S shape?
- The middle section is the healthy range where the dosage is good
- The left hand side is a deficient dosage
- The right hand side is a Toxic dosage
Why is the PoD important in toxicology?
- The Point of Departure determines the threshold of toxicity
- Separates “safe” vs. “not safe”
What are NOAEL and LOAEL?
- Until recently, toxicologists used NOAEL and LOAEL to express PoD
What does NOAEL stand for?
- No Observable Adverse Effect Level
- The highest dose tested that did not have an adverse affect
What does LOAEL stand for?
- Lowest Observable Adverse Effect Level
- The lowest dose that produced an adverse effect
What was the problem with NOAEL and LOAEL?
- Can be significantly affected by experimental design, for example: Dose selection, sample size
- Makes comparison of toxicity between studies problematic
What is now being used instead of NOAEL and LOAEL?
- Benchmark Dose (BMD)
- BMD takes into account the whole D/R, it’s much less influenced by dose selection
- Allows standardised comparison of toxicity between studies
- For example you could calculate a BMD that affects 10% of the population. First you would find 10% on the y-axis, then find where that intersects with the relationship on the x-axis and that would be you BMD10 (10 is for 10%)
What are the other things that toxicity depends on?
- “The dose makes the poison” but toxicity also depends on:
- Species
- Age, gender
- Sensitive windows of exposure (e.g., embryonic development)
- Exposure duration (acute vs. chronic; continuous vs. pulse)
- Presence of other toxicants (“mixture toxicity”)
- Exposure route (dermal, respiratory, oral …)
- Toxicokinetics (absorption, distribution, metabolism and excretion)
What is Selective Toxicity?
- Species differences usually attributable to differences in metabolism, although some may be due to physiological differences
» E.g., rats cannot vomit ≠ humans and dogs - Selective toxicity refers to species differences in toxicity
» E.g., insecticide lethal to insects but relatively nontoxic to mammals
What is Mixture toxicity?
- The presence of other chemicals may:
» Decrease toxicity (antagonism),
» Add to toxicity (additivity), or
» Increase toxicity (synergism or potentiation)
What are one of the Concepts of mixture toxicity with the abbreviation “CA”?
- Concentration addition (CA)
- One of the concepts used for mixture toxicity modelling
- If you have no interaction between chemicals in the mixture and they target the same site with the same mode of action then there will be CA
What are one of the Concepts of mixture toxicity with the abbreviation “IA”?
- Independent action (IA)
- If you have no interaction between chemicals in the mixture and they target different sites with the different modes of action then there will be IA
What are the “complex action” and “dependent action” concepts of mixture toxicity?
- Complex action (synergism / antagonism) occurs when you have Interaction between chemicals in mixture with the same target site and the same mode of action.
- Dependent action (synergism / antagonism) occurs when you have Interaction between chemicals in mixture with different target sites and different modes of action
Are complex interaction between chemicals in a mixture common?
- When there are many compounds in a mixture, all at low concentration, interaction between mixture components is rare (ie, relevant in drug development, but less so in environmental science)
- For example, there can be millions of chemicals at really low concentration in a water sample and you’re most likely going to be dealing with CA or IA.
What is Toxicokinetics (TK)?
- Movement of toxicant within living organism
What are the four main process of Toxicokinetics?
- Absorption
- Distribution
- Metabolism (aka biotransformation)
- Excretion
What parts of a living organism make up the absorption process?
- The Gastrointestinal Tract
- The skin and lungs
What parts of a living organism make up the distribution and metabolism process?
- Liver -> Bile
- Blood and lymph circulation
- Storage -> Organs, bones and fatty tissues
- Kidney
- lung
- Extracellular fluids
- Metabolism -> metabolites
What parts of a living organism make up the elimination process?
- Feces
- Urine
- Expired air
Why is absorption important?
- Essential for toxicity to occur
- Varies depending on route of exposure and chemical properties, for example:
» Injection»_space;> oral > respiratory»_space;> dermal
» Lipophilic or small substances (very easily absorbed)»_space;> hydrophilic or large substances (harder to absorbe)
What does a chemical need to be able to pass through the phospholipid bilayer?
- Main barrier is the cell membrane = phospholipid bilayer
- To pass through it, the chemical or compound needs to be Lipophilic, small and have no charge.
- It can’t be either:
» Hydrophilic + charged phosphate head
» Hydrophobic – charged fatty acid tail
What are the different types of transport across the cell membrane?
- Passive transport (with concentration gradient)
- Active transport (against concentration gradient)
WHat is the major determinant of toxicity?
- A major determinant of toxicity is the lipid solubility of the toxicant
- Lipid soluble toxicants readily penetrates cell membranes
Is there a way to measure how “lipid loving” a chemical is?
- Octanol / water partition coefficient (Kow) is a measure of lipophilicity (“lipid loving”)
» Generally expressed in log unit (logKow) - logKow = 3 → compound is 1,000× more concentrated in octanol than in water
What determines where a chemical will be distributed?
- Once absorbed, where the chemical goes in the body (distribution) depends on route of exposure and chemical properties
» Oral → biotransformation in the liver (“first pass effect”)
» Respiratory, dermal or injection: no first pass effect, so intact chemical can reach any organ - Some chemicals can
accumulate in tissues
» Lipophilic chemicals will accumulate in fat
» Some metals (e.g., Pb, Sr) will accumulate in bone
