Metabolism and Toxicology
Lecture 1: Introduction (van der Graaf)
Metabolism: chemical transformation that compounds undergo in the body body do compound.
Toxicology: adverse effects of chemicals on organisms compound to body.
Paracetamol is a very lipophilic compound. It is a save painkiller but deadly with a high dose (ong. 10
times the maximum daily dose = 30 gram)
Absorption and elimination
- Lipophilic compounds pass membranes and be absorbed.
- Ionized compounds do not pass membranes easy anymore.
- Water-soluble compounds difficult to absorb but are easily excreted.
- Fat-soluble compounds absorbed easy but can keep circulating in the body for a long
time.
Lipophilic compounds can bind to proteins, to get soluble protein too big to pass filter slows
clearance.
- Albumin: plasma protein, not filtered in glomerulus.
Compounds need to be hydrophilic/water-soluble to be excreted. Lipophilic drugs need to be
metabolised to make it water-soluble in order to be eliminated.
Toxicity: metabolism leads to reactive metabolites.
Metabolism of paracetamol
- Normal pathway: Paracetamol is converted to
hydrophilic compound which is easy to excrete.
- Toxic pathway: Paracetamol is metabolized to an N-
hydroxyl metabolite unstable, so it changes to NAPQI
toxic.
Normal pathway is faster will be chosen first. When all
routes are saturated toxicity.
Toxification / bioactivation: metabolite more toxic than parent compound.
Detoxification: metabolite less toxic than parent compound.
Adverse effect: any unwanted effect of a drug that interferes with the normal function and
adaptability of the body to the environment.
Undesirable effect: on/off target
- On-target toxicity: due to exaggerated pharmacological effect or on same receptor but on
different tissue. On same receptor.
- Off-target toxicity: via completely different mechanism (different target) than the
pharmacological effect. Effect on the incorrect receptor.
1
,Targets for toxins:
- Receptors
- Proteins (enzymes/transporters)
- Membrane-lipids
- DNA/RNA (can lead to tumors)
- Ca2+ homeostasis
- Ion channels
- Mitochondria (damage no energy)
Immunotoxicity: toxic effects by immune system, response to damage somewhere in the body.
- Due to toxic effects on immune system itself: toxic effect on B or T lymphocytes.
- Due to suppression/stimulation of the immune system.
Same dose ≠ same exposure
These can all affect the exposure, even in the dose is not different.
- Differences in exposure routes.
- Species/interindividual differences in ADME.
- Accumulation.
- Time (dose*time).
- Interactions (induction/inhibition).
Toxicity is determined by:
These determine the risks and the safety of the compound.
- Characteristics of the compound: hazard Compound potentially has an adverse effect.
- Amount/dose/concentration: degree of exposure to the hazard.
Toxicity depends on the exposure (= AUC of the concentration-time curve).
Parameters
- LD50: dose needed to kill 50% of population. Low = toxic.
- TD50: dose needed for 50% of max toxic effect / where 50% of population shows toxic effect.
- TC50: plasma concentration for 50% of max toxic effect / where 50% of pop shows toxic
effect.
- ED50: dose for 50% of desired effect / when 50% of population shows desired effect.
- EC50: plasma concentration for 50% of desired effect / when 50% of population shows
desired effect.
LOAEL: Lowest Observed Adverse Effect Level first tested dose/concentration that gives toxic
effect.
NOAEL: No Observed Adverse Effect Level: just below LOAEL.
ADI: acceptable daily intake.
Risk assessment
- Hazard: anything that can cause harm (biological property/potential).
- Risk: greatness of chance that hazard will harm someone (chance on toxicity.
Measures for safety
- Therapeutic index: TD50 / ED50.
- Margin of safety MOS: TD1 / ED99
2
,Drug testing
Toxicity is tested first in vitro on animals on healthy people on patients
Toxicology in drug development
- Research phase: screening for toxicology/metabolism (in vitro).
- Development phase: ADME (in vitro + in vivo).
- Clinical phase: human PK/toxicity/safety.
- Post-marketing: side effects, pharmacovigilance, PMS.
ADR’s are a major clinical problem.
When a drug is put on the market, the safety data is limited.
Lareb: the central collection point for all side effects of pharmaceutical drugs in the Netherlands.
REACH: Registration, Evaluation and Authorization of CHemical substances. Data about safe use of
chemical substances. European community regulations.
3
, Lecture 2: ADME & Toxicokinetics (Olinga)
ADME determines the exposure to the final toxic compound.
Toxicity
- Local toxicity: harmful without being absorbed.
- Systemic toxicity: harmful after uptake in the systemic circulation.
Bioavailability: fraction of dose that reaches the systemic circulation. Decreased by the first pass
effect (metabolism in the intestine and liver).
- Bioavailability = AUCoral/AUCi.v.
Absorption takes place in the epithelial cells.
Membrane transport
Membranes are lipophilic hydrophobic compounds are taken up easily. Hydrophilic compounds
are taken up via transporters.
- Passive transport: due to concentration difference. Via simple diffusion, a channel, or a
carrier. Lipophilic drugs can diffuse, hydrophilic drugs need to be carrier mediated.
- Active transport: against electrochemical gradient energy required. Transport through a
channel.
Absorption in GI
Depends on:
- pH (varies from 2-7).
- Blood flow.
- Metabolism in intestinal epithelial.
- Transporters in intestinal epithelium.
- Nutritional status.
- Intestinal contractions.
- Bacteria (metabolism, bioactivation, detoxification).
Surface small intestine = 250 m2.
Compound properties that affect uptake
- Lipophilicity
- Pka
- Affinity for uptake carriers
- Size
- Solubility
- Affinity for metabolic enzymes.
- Affinity for excretion carriers (P-gp).
Factors influencing intestinal toxicity (exposure)
- High concentrations in intestinal lumen after oral administration.
- Degree of uptake/excretion in intestinal epithelium.
- Metabolism (bacteria, through wall epithelial cells, pH)
Enterohepatic circulation: compound is metabolised in the liver excreted back into GI tract
taken up by the liver again.
4
Lecture 1: Introduction (van der Graaf)
Metabolism: chemical transformation that compounds undergo in the body body do compound.
Toxicology: adverse effects of chemicals on organisms compound to body.
Paracetamol is a very lipophilic compound. It is a save painkiller but deadly with a high dose (ong. 10
times the maximum daily dose = 30 gram)
Absorption and elimination
- Lipophilic compounds pass membranes and be absorbed.
- Ionized compounds do not pass membranes easy anymore.
- Water-soluble compounds difficult to absorb but are easily excreted.
- Fat-soluble compounds absorbed easy but can keep circulating in the body for a long
time.
Lipophilic compounds can bind to proteins, to get soluble protein too big to pass filter slows
clearance.
- Albumin: plasma protein, not filtered in glomerulus.
Compounds need to be hydrophilic/water-soluble to be excreted. Lipophilic drugs need to be
metabolised to make it water-soluble in order to be eliminated.
Toxicity: metabolism leads to reactive metabolites.
Metabolism of paracetamol
- Normal pathway: Paracetamol is converted to
hydrophilic compound which is easy to excrete.
- Toxic pathway: Paracetamol is metabolized to an N-
hydroxyl metabolite unstable, so it changes to NAPQI
toxic.
Normal pathway is faster will be chosen first. When all
routes are saturated toxicity.
Toxification / bioactivation: metabolite more toxic than parent compound.
Detoxification: metabolite less toxic than parent compound.
Adverse effect: any unwanted effect of a drug that interferes with the normal function and
adaptability of the body to the environment.
Undesirable effect: on/off target
- On-target toxicity: due to exaggerated pharmacological effect or on same receptor but on
different tissue. On same receptor.
- Off-target toxicity: via completely different mechanism (different target) than the
pharmacological effect. Effect on the incorrect receptor.
1
,Targets for toxins:
- Receptors
- Proteins (enzymes/transporters)
- Membrane-lipids
- DNA/RNA (can lead to tumors)
- Ca2+ homeostasis
- Ion channels
- Mitochondria (damage no energy)
Immunotoxicity: toxic effects by immune system, response to damage somewhere in the body.
- Due to toxic effects on immune system itself: toxic effect on B or T lymphocytes.
- Due to suppression/stimulation of the immune system.
Same dose ≠ same exposure
These can all affect the exposure, even in the dose is not different.
- Differences in exposure routes.
- Species/interindividual differences in ADME.
- Accumulation.
- Time (dose*time).
- Interactions (induction/inhibition).
Toxicity is determined by:
These determine the risks and the safety of the compound.
- Characteristics of the compound: hazard Compound potentially has an adverse effect.
- Amount/dose/concentration: degree of exposure to the hazard.
Toxicity depends on the exposure (= AUC of the concentration-time curve).
Parameters
- LD50: dose needed to kill 50% of population. Low = toxic.
- TD50: dose needed for 50% of max toxic effect / where 50% of population shows toxic effect.
- TC50: plasma concentration for 50% of max toxic effect / where 50% of pop shows toxic
effect.
- ED50: dose for 50% of desired effect / when 50% of population shows desired effect.
- EC50: plasma concentration for 50% of desired effect / when 50% of population shows
desired effect.
LOAEL: Lowest Observed Adverse Effect Level first tested dose/concentration that gives toxic
effect.
NOAEL: No Observed Adverse Effect Level: just below LOAEL.
ADI: acceptable daily intake.
Risk assessment
- Hazard: anything that can cause harm (biological property/potential).
- Risk: greatness of chance that hazard will harm someone (chance on toxicity.
Measures for safety
- Therapeutic index: TD50 / ED50.
- Margin of safety MOS: TD1 / ED99
2
,Drug testing
Toxicity is tested first in vitro on animals on healthy people on patients
Toxicology in drug development
- Research phase: screening for toxicology/metabolism (in vitro).
- Development phase: ADME (in vitro + in vivo).
- Clinical phase: human PK/toxicity/safety.
- Post-marketing: side effects, pharmacovigilance, PMS.
ADR’s are a major clinical problem.
When a drug is put on the market, the safety data is limited.
Lareb: the central collection point for all side effects of pharmaceutical drugs in the Netherlands.
REACH: Registration, Evaluation and Authorization of CHemical substances. Data about safe use of
chemical substances. European community regulations.
3
, Lecture 2: ADME & Toxicokinetics (Olinga)
ADME determines the exposure to the final toxic compound.
Toxicity
- Local toxicity: harmful without being absorbed.
- Systemic toxicity: harmful after uptake in the systemic circulation.
Bioavailability: fraction of dose that reaches the systemic circulation. Decreased by the first pass
effect (metabolism in the intestine and liver).
- Bioavailability = AUCoral/AUCi.v.
Absorption takes place in the epithelial cells.
Membrane transport
Membranes are lipophilic hydrophobic compounds are taken up easily. Hydrophilic compounds
are taken up via transporters.
- Passive transport: due to concentration difference. Via simple diffusion, a channel, or a
carrier. Lipophilic drugs can diffuse, hydrophilic drugs need to be carrier mediated.
- Active transport: against electrochemical gradient energy required. Transport through a
channel.
Absorption in GI
Depends on:
- pH (varies from 2-7).
- Blood flow.
- Metabolism in intestinal epithelial.
- Transporters in intestinal epithelium.
- Nutritional status.
- Intestinal contractions.
- Bacteria (metabolism, bioactivation, detoxification).
Surface small intestine = 250 m2.
Compound properties that affect uptake
- Lipophilicity
- Pka
- Affinity for uptake carriers
- Size
- Solubility
- Affinity for metabolic enzymes.
- Affinity for excretion carriers (P-gp).
Factors influencing intestinal toxicity (exposure)
- High concentrations in intestinal lumen after oral administration.
- Degree of uptake/excretion in intestinal epithelium.
- Metabolism (bacteria, through wall epithelial cells, pH)
Enterohepatic circulation: compound is metabolised in the liver excreted back into GI tract
taken up by the liver again.
4
