Metabolism and Toxicology
Introduction
Metabolism: chemical transformations that compounds undergo in the body
= what does the body do to a compound
Toxicology: adverse effects of chemicals on organisms
= what does the compound do to the body
Paracetamol → safe painkiller (max 6 a day; 3 gram)
Deadly dose: approximate 30 gram
• Works after 30 minutes → needs to be absorbed
• Stops working after 6 hours → its metabolized then
Water-soluble compound is inefficiently absorbed and easily excreted.
Most drugs are lipophilic → to enter cells compounds need to pass membranes
• When compound reaches the bloodstream → bound to albumin (big protein)
• Clearance is slow → lipophilic drugs cannot be excreted
Compounds need to become hydrophilic (water-soluble) to can be excreted.
• Metabolism transforms a lipophilic compound into a water-soluble compound that can be
excreted
Lipophilic drugs → backbone conjugated with polar molecules → easily excreted
Toxification
Cytochrome P450 → hydroxylation on nitrogen atom of paracetamol
• Unstable intermediate
→ transforms into NAPQ: responsible for toxic reactions in the body (electrophilic + load)
→ bind easily to nucleophiles → cell damage
Dose
Once: >70 mg/kg, toxic effects 5 gram
>140mg/kg, moderate liver damage 10 gram
>200mg/kg, severe liver damage 14 gram
Chronic use: 3-4 g/day: liver damage
Toxicity enhanced by chronic alcohol use and by certain drugs such as carbamazepine, isoniazid and
barbiturates.
Antidote: N-acetylcysteine or methionine
Toxification or bioactivation → metabolites are more toxic than parent compound
Detoxification → metabolites less toxic than parent compound
ADR → adverse drug reaction
Off label use → toxic effect may sometimes be a desirable effect for other diseases
Adverse effect: any (unwanted) effect of a drug that interferes with the normal function and
adaptability of the body to (stimuli from) the environment.
,Targets of toxicity
• Receptors
• Other proteins: enzymes, transporters etc.
• Membrane-lipids
• DNA/RNA
• Ca2+ homeostasis
• Ion channels
• Mitochondria (energy)
Manifestations of toxic effects
• Local → systemic
• Rapid onset/acute → delayed/chronic
• Reversible → irreversible
• Gradual → all or nothing
• Direct → indirect
Adverse effects
• Cell death
• Pharmacological/physiological effects
• Genotoxicity
• Carcinogenicity
• Reproduction damage
• Teratogenicity
• Neurotoxicity
• Immunotoxicity, sensitization,
• Irritation (skin, eye)
Toxic effects by the immune system: immunological response to damage somewhere in the body,
e.g. sensitization, hapten formation
On-target toxicity: undesirable effect due to exaggerated pharmacological effect
› For example, by increased exposure of the target (higher blood concentrations, etc.),
interaction with the same target on/in another cell/organ
Off-target toxicity: undesirable effect via a completely different mechanism (different target) than
pharmacological effect
› dose response curve of the toxic effect ≠ intended pharmacological effect
,Dose-response curve
• Responders and no responders
• Y-axis: cumulative response %
• TD50
Hormesis: effect between dose and effect is U-shaped
E.g. effect vitamin A on embryo development.
Toxicity is more related to exposure than to the dose.
Same dose does not always give the same exposure:
• Differences in exposure routes
• Species/interindividual differences in ADME
• Saturation of effect, repair or detoxification
• Accumulation
• Time (dose x time)
• Interactions (induction, inhibition)
Toxicity is determined by:
1. Characteristics of the compound → (danger/hazard) the compound potentially has an
adverse effect
2. Amount/dose/concentration → degree of exposure to the hazard
LD50: dosage causing death in 50% of exposed animals
Chemical and physical properties
• However: The structure does not always predict activity (SAR)
• There are some (chemical)class specific effects, but most compounds have a unique toxicity
profile
Biological effects (receptor binding, affinity for active center of an enzyme, DNA binding etc)
Dose and systemic exposure
• The amount administered (mg/kg)
• Bioavailability
Exposure of the target organ/tissue/cell
• Administration route
• Metabolite activity
• Transporters in cell membrane
• Blood flow
• Time (chronic exposure)
Individual differences in sensitivity.
, TD50
The dose of a drug necessary to provoke 50% of the maximal toxic effect (in case of the dose-effect
curve)
or
The dose at which 50% of the exposed population shows the toxic effect (in case of a dose-response
curve).
TC50
The (plasma) concentration of a drug necessary to provoke 50% of the toxic effect (in case of the
dose-effect curve)
or
The (plasma) concentration necessary to provoke the toxic effect in 50% of the exposed population
(in case of the dose response curve).
ED50
The dose of a drug necessary to achieve 50% of the desired effect (in case of dose-effect curve)
or
The dose at which 50% of the treated population shows the desired effect (in the case of the dose-
response curve).
EC50
The (plasma) concentration of a drug necessary to achieve 50% of the (desired) effect (in case of
dose-effect curve)
or
The (plasma) concentration at which 50% of the treated population shows the desired effect (in the
case of the dose response curve).
LOAEL: Lowest Observed Adverse Effect Level
The first tested dose/concentration of a drug that gives a significant toxic effect (if a series of
ascending doses / concentrations is tested).
(read out parameter that appeared most sensitive)
NOAEL: No Observed Adverse Effect Level
In a tested series of ascending doses / concentrations, that dose / concentration that is just lower
than the LOAEL
From hazard to risk
Hazard = biological property/potential
Risk = the chance on toxicity
𝑁𝑂𝐴𝐸𝐿
𝐴𝐷𝐼/𝑇𝐷𝐼/𝑅𝑓𝑑 = = 𝑚𝑔/𝑘𝑔 𝑏𝑤/𝑑𝑎𝑦
𝑠𝑎𝑓𝑒𝑡𝑦 𝑓𝑎𝑐𝑡𝑜𝑟
ADI: acceptable daily intake
Safety factor: → interspecies difference = 10
→ interindividual difference = 10
10x10=100
Introduction
Metabolism: chemical transformations that compounds undergo in the body
= what does the body do to a compound
Toxicology: adverse effects of chemicals on organisms
= what does the compound do to the body
Paracetamol → safe painkiller (max 6 a day; 3 gram)
Deadly dose: approximate 30 gram
• Works after 30 minutes → needs to be absorbed
• Stops working after 6 hours → its metabolized then
Water-soluble compound is inefficiently absorbed and easily excreted.
Most drugs are lipophilic → to enter cells compounds need to pass membranes
• When compound reaches the bloodstream → bound to albumin (big protein)
• Clearance is slow → lipophilic drugs cannot be excreted
Compounds need to become hydrophilic (water-soluble) to can be excreted.
• Metabolism transforms a lipophilic compound into a water-soluble compound that can be
excreted
Lipophilic drugs → backbone conjugated with polar molecules → easily excreted
Toxification
Cytochrome P450 → hydroxylation on nitrogen atom of paracetamol
• Unstable intermediate
→ transforms into NAPQ: responsible for toxic reactions in the body (electrophilic + load)
→ bind easily to nucleophiles → cell damage
Dose
Once: >70 mg/kg, toxic effects 5 gram
>140mg/kg, moderate liver damage 10 gram
>200mg/kg, severe liver damage 14 gram
Chronic use: 3-4 g/day: liver damage
Toxicity enhanced by chronic alcohol use and by certain drugs such as carbamazepine, isoniazid and
barbiturates.
Antidote: N-acetylcysteine or methionine
Toxification or bioactivation → metabolites are more toxic than parent compound
Detoxification → metabolites less toxic than parent compound
ADR → adverse drug reaction
Off label use → toxic effect may sometimes be a desirable effect for other diseases
Adverse effect: any (unwanted) effect of a drug that interferes with the normal function and
adaptability of the body to (stimuli from) the environment.
,Targets of toxicity
• Receptors
• Other proteins: enzymes, transporters etc.
• Membrane-lipids
• DNA/RNA
• Ca2+ homeostasis
• Ion channels
• Mitochondria (energy)
Manifestations of toxic effects
• Local → systemic
• Rapid onset/acute → delayed/chronic
• Reversible → irreversible
• Gradual → all or nothing
• Direct → indirect
Adverse effects
• Cell death
• Pharmacological/physiological effects
• Genotoxicity
• Carcinogenicity
• Reproduction damage
• Teratogenicity
• Neurotoxicity
• Immunotoxicity, sensitization,
• Irritation (skin, eye)
Toxic effects by the immune system: immunological response to damage somewhere in the body,
e.g. sensitization, hapten formation
On-target toxicity: undesirable effect due to exaggerated pharmacological effect
› For example, by increased exposure of the target (higher blood concentrations, etc.),
interaction with the same target on/in another cell/organ
Off-target toxicity: undesirable effect via a completely different mechanism (different target) than
pharmacological effect
› dose response curve of the toxic effect ≠ intended pharmacological effect
,Dose-response curve
• Responders and no responders
• Y-axis: cumulative response %
• TD50
Hormesis: effect between dose and effect is U-shaped
E.g. effect vitamin A on embryo development.
Toxicity is more related to exposure than to the dose.
Same dose does not always give the same exposure:
• Differences in exposure routes
• Species/interindividual differences in ADME
• Saturation of effect, repair or detoxification
• Accumulation
• Time (dose x time)
• Interactions (induction, inhibition)
Toxicity is determined by:
1. Characteristics of the compound → (danger/hazard) the compound potentially has an
adverse effect
2. Amount/dose/concentration → degree of exposure to the hazard
LD50: dosage causing death in 50% of exposed animals
Chemical and physical properties
• However: The structure does not always predict activity (SAR)
• There are some (chemical)class specific effects, but most compounds have a unique toxicity
profile
Biological effects (receptor binding, affinity for active center of an enzyme, DNA binding etc)
Dose and systemic exposure
• The amount administered (mg/kg)
• Bioavailability
Exposure of the target organ/tissue/cell
• Administration route
• Metabolite activity
• Transporters in cell membrane
• Blood flow
• Time (chronic exposure)
Individual differences in sensitivity.
, TD50
The dose of a drug necessary to provoke 50% of the maximal toxic effect (in case of the dose-effect
curve)
or
The dose at which 50% of the exposed population shows the toxic effect (in case of a dose-response
curve).
TC50
The (plasma) concentration of a drug necessary to provoke 50% of the toxic effect (in case of the
dose-effect curve)
or
The (plasma) concentration necessary to provoke the toxic effect in 50% of the exposed population
(in case of the dose response curve).
ED50
The dose of a drug necessary to achieve 50% of the desired effect (in case of dose-effect curve)
or
The dose at which 50% of the treated population shows the desired effect (in the case of the dose-
response curve).
EC50
The (plasma) concentration of a drug necessary to achieve 50% of the (desired) effect (in case of
dose-effect curve)
or
The (plasma) concentration at which 50% of the treated population shows the desired effect (in the
case of the dose response curve).
LOAEL: Lowest Observed Adverse Effect Level
The first tested dose/concentration of a drug that gives a significant toxic effect (if a series of
ascending doses / concentrations is tested).
(read out parameter that appeared most sensitive)
NOAEL: No Observed Adverse Effect Level
In a tested series of ascending doses / concentrations, that dose / concentration that is just lower
than the LOAEL
From hazard to risk
Hazard = biological property/potential
Risk = the chance on toxicity
𝑁𝑂𝐴𝐸𝐿
𝐴𝐷𝐼/𝑇𝐷𝐼/𝑅𝑓𝑑 = = 𝑚𝑔/𝑘𝑔 𝑏𝑤/𝑑𝑎𝑦
𝑠𝑎𝑓𝑒𝑡𝑦 𝑓𝑎𝑐𝑡𝑜𝑟
ADI: acceptable daily intake
Safety factor: → interspecies difference = 10
→ interindividual difference = 10
10x10=100
