Transcript Dose-Response Concept

Dose-Response Concept
Assumptions in Deriving the
Dose-Response Relationship
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The response is due to the chemical
administered
There is a molecular site(s) with which the
chemical interacts to produce the response
The response is a function of the [ ] of the
compound at the site of action
The [ ] of the compound at the site of action
is related to the dose of the compound
Assumptions Continued
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There exist both a quantifiable method
of measuring and a precise means of
expressing the effect of the compound
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A chemical that produces cancer through
effects on DNA, liver damage through
inhibition of a specific enzyme, and CNS
effects through ion channel blockage will
have three distinct dose-response
relationships, one for each endpoint
Molecular Targets of Chemical
Compounds
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Receptors
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Ion Channel Receptors
Carrier Proteins
G-Protein Coupled Receptors
Tyrosine-Kinase Receptors
Ah Receptors
Steroid Hormone Receptors
Receptors
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Binding of a chemical to a receptor
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Can initiate a cellular response similar to,
or identical to, an endogenous chemical –
This is termed an agonistic action and
the chemical is termed an agonist for the
endogenous substance
Example of an Agonist Binding
to Receptor
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Buspirone – attaches to the serotonin
IA receptor and activates it, mimicking
serotonin action on the receptor, which
results in the antianxiety action of
clinical significance
Receptors
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Binding of a chemical near the binding
site for an endogenous chemical can
facilitate the binding of the endogenous
chemical – this is also an agonistic
action
Example of an Agonist Binding
Near the Receptor
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Benzodiazepines bind to a site near the
GABA-binding site and facilitate the
action of GABA. This action allows flow
of chloride ions into the neuron,
hyperpolarizing the neuron and
inhibiting neuronal function.
Benzodiazepines are used as sedative
and anti-anxiety agents
Receptors
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Binding to the receptor site normally
occupied by an endogenous chemical
blocks access of the endogenous
chemical to the binding site but does
not initiate a normal physiological
response – this is an antagonistic
action and the chemical is termed an
antagonist for the receptor site.
Example of an Antagonist
Binding to a Receptor
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Fluoxetine competes with serotonin for
the reuptake protein, blocking access of
serotonin to the receptor and
prolonging serotonin’s presence in the
synaptic cleft. This allows more
serotonin stimulation of postsynaptic
receptors, leading to down regulation in
the number of serotonin receptors and
relief of clinical depression
Molecular Targets of Chemical
Compounds - Continued
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Enzymes
Lipids
Nucleic Acids
Subcellular Organelle Targets
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Cell Membrane
Mitochondria
Endoplasmic Reticulum
Ribosomes
Promotor Regions on DNA
The Dose-Response Graph
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Classic Example
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Normal Distribution
Popcorn Example
The Normal Distribution
Cumulative Frequency
Distribution
Change to a Dose-Response
Curve
Classic Dose-Response Curve
on Log – Log Coordinates
Probit Scale
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http://wwwstat.stanford.edu/~naras/jsm/NormalDe
nsity/NormalDensity.html
68% of the observations fall within 1
standard deviation of the mean,
95% of the observations fall within 2
standard deviations of the mean,
99.7% of the observations fall within 3
standard deviations of the mean.
Classic Dose-Response Curve
on Log – Log Coordinates
Non-Normal Distributions
Types of Exposure to
Chemicals
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Exposure may be classified as
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Acute Exposure – This usually refers to a single
exposure to a chemical. If repeated exposures
are given they are given within a 24-hr period
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The chemical is usually given by injection or by dermal
application but would also include oral administration
Acute exposure by inhalation refers to continuous
exposure for less than 24 hours, usually for 4 hours
Acute Exposure - Continued
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Mouse and rat are the species most
commonly used for testing
Both sexes are used
Food is withheld the night before testing
The number of animals that reach a
prescribed endpoint at each dose are
tabulated
10 animals per dose
5 dose levels
Acute Exposure - Continued
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If larger animals are used the dose is
increased in the same animal until the
prescribed endpoint is reached
Endpoints could be
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Lethal dose (death)
Toxic dose (ex. Liver injury)
Effective dose (ex. Relief from itching)
Subchronic Testing
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90 days is the most common test
duration but 30 days to 90 days can be
used
Usually oral administration of the
chemical via food; also implant
Used to further characterize the specific
organs affected by test compound after
repeated administration of the chemical
Subchronic Exposure
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At least 3 doses
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A high dose that produces toxicity but
death in less than 10% of the animals
A low dose that does not produce apparent
toxic effects during an acute exposure
An intermediate dose
For Drugs Under Development
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Acute and Subchronic studies must be
completed before company can file an
IND (Investigate New Drug) application
with the FDA (Food and Drug
Administration).
If the application is approved then
Clinical Trials can begin. Chronic tests
can begin at the same time.
Chronic Exposure
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Exposure to a chemical for a period
longer than 3 months, usually 6 months
to 2 years in rodents
Drug Testing – 6 months
Food Additives with potential lifetime
human exposure – 2 years required
Chronic Exposure - Continued
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Designed to assess cumulative toxicity
of chemicals including consideration of
carcinogenic potential
Mice – 18 months to 2 years
Rats – 2 to 2.5 years
Start with 60 animals/sex/dose to end
up with 30 animals to survive study
Chronic Exposure - Continued
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Highest administered dose = “Estimated
Maximum Tolerable Dose (MTD) derived from
subchronic study
The National Toxicology Program defines the
MTD as “a dose that suppresses body weight
slightly (i.e. 10%) in a 90 day study
Also use ½ MTD, ¼ MTD, and a control
group
What Can Be Learned From A
Dose-Response Curve?
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LD50 – Median Lethal Dose, quantity of the
chemical that is estimated to be fatal to 50%
of the organisms
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LD50 values are the standard for comparison of
acute toxicity between chemical compounds and
between species
TD50 – Median Toxic Dose
ED50 – Median Effective Dose
LC50 – Median Lethal Concentration
What Can Be Learned From A
Dose-Response Curve?
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LD50, TD50, and ED50 values vary by:
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Species
Gender
Genetic strain
Age
Route of administration
Environmental conditions
Nutritional status
What Can Be Learned From A
Dose-Response Curve?
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NOAEL Value – No Observed Adverse
Effect Level, The highest dose of a
chemical that, in a given toxicity test,
causes no observable effect in test
animals
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The NOEL for the most sensitive test
species and the most sensitive indicator of
toxicity is usually employed for regulatory
purposes
What Can Be Learned From A
Dose-Response Curve?
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LOAEL Value – Lowest Observed
Adverse Effect Level, The lowest dose
of a chemical that, in a given toxicity
test, does cause an observable effect in
test animals
Route of Administration
Comparison of LD50 Values
Why LD50 Values Alone Are
Not Very Informative
Dose-Response Graph For A
Noncancer Causing Chemical
Relationship Between ED50,
TD50 and LD50
How Safe Is A Drug?
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Therapeutic Index
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= LD50 / ED50
Margin of Safety
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= LD1 / ED99
Therapeutic Index
Margin of Safety
Potency and Efficacy
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Potency – Is given by the position of
the dose-response curve along the xaxis; farther to the left = more potent
Efficacy – Is given by the peak of the
dose-response curve; the higher the
peak the greater the maximum effect or
efficacy
Potency and Efficacy
Carcinogenic Chemical DoseResponse Graph
Tumors Graph
Tumors
Ames Test For Mutagenicity
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Assumption – Any substance that is
mutagenic for the bacteria used in the
test may also turn out to be a
carcinogen.
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Benefits of Test – Easy to conduct, low cost
Drawbacks – Test gives some false
negatives and some false positives
Ames Test - Continued
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Test strain of bacterium used is a strain
of Salmonella typhimurium that carries
a mutant gene making it unable to
synthesize the amino acid histidine from
ingredients in its culture
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A “back mutation” to this gene will allow
the hisidine requiring strain of bacteria to
grow on histidine deficient media.
Ames Test - Continued
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The test involves placing the histidinerequiring strain of bacteria on a culture
plate along with the test chemical. If
the bacteria grow on the histidinedeficient culture medium, a mutation
has occurred
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Therefore, the test chemical is mutagenic
and possibly carcinogenic
Effects of More Than One
Chemical
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Additive Effect:the combined effect of the two
chemicals is equal to the sum of the effects of
each agent given alone. This is the most
commonly observed effect when two
chemicals are given together. (2 + 2 = 4)
Effects of More Than One
Chemical – Continued
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Synergistic Effect: occurs when the
combined effects of two chemicals are much
greater than the sum of the effects of each
agent given alone.
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CCl4 and ethanol are hepatotoxic alone but when
given together produce much more liver injury
than the mathematical sum of their individual
effects. (2 + 2 = 20).
Smoking and asbestos exposure is another
example.
Cocaine use with alcohol use is a third example.
Cocaine + Alcohol
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Metabolism of ethanol and cocaine
together produces a metabolite called
cocaethylene. This metabolite of
cocaine and ethanol has
pharmacological properties similar to
those of cocaine but with a longer
duration in the blood plasma (three to
five times as long).
Effects of More Than One
Chemical – Continued
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Potentiation: occurs when one
compound does not have a toxic effect
on a certain organ or system but when
added to another chemical makes that
chemical much more toxic.
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CCl4 is hepatotoxic, isopropanol is not
hepatotoxic, when given together the
effect of CCl4 is more than expected. (0 +
2 = 10)
Effects of More Than One
Chemical – Continued
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Antagonism: occurs when two
chemicals administered together
interfere with each other’s action.
Antagonistic interactions are very often
desirable in toxicology and are the basis
of many antidotes. (2 + (-2) = 0).
Types of Antagonism
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Functional antagonism occurs when two
chemicals counterbalance each other by
producing opposite effects on the same
physiological function.
Chemical antagonism is a chemical
reaction between two compounds that
produces a less toxic product. Example
= a chelator and a metal.
Types of AntagonismContinued
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Dispositional antagonism occurs when the
disposition of a chemical is altered so that the
concentration and/or duration of the chemical
at the target organ are diminished. Ex.
Metabolism is increased – Excretion is
increased, therefore half-life is decreased
Receptor antagonism occurs when two
chemicals that bind to the same receptor
produce less of an effect when given together
than the addition of their separate parts.
Receptor antagonists are often termed
blockers.
Types of AntagonismContinued
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Receptor antagonism occurs when two
chemicals that bind to the same
receptor produce less of an effect when
given together than the addition of their
separate parts. Receptor antagonists
are often termed blockers.