Week 8 part 1 - Toxicology & teratogenesis Flashcards
Adverse reaction vs side effect vs toxic effect?
Adverse drug reaction: a harmful effect of a drug (try to avoid).
Described by the dose, time course and patient susceptibility
Side effect: a secondary unwanted effect of a drug (eg. constipation after taking iron tablets)
Toxic effect: an adverse drug effect caused by an exaggeration of the therapeutic effect of a drug (e.g. cancer chemotherapeutics)
Harmful or toxic drug reactions
- Before a drug can enter human clinical trials, it needs to be thoroughly screened for toxicity in vitro (cells) and in vivo (in animal models, eg. rodents) to identify its likely toxic effects in humans.
- Some programs are available free of charge online which can predict someadverse drug effects. Eg. pkCSM.
- Many very effective drug candidates fail in pre-clinical trials as a result of excessive toxic or adverse reactions.
Therapeutic index (TI):
the ratio of a drug dose required to produce a lethal effect (LD50)
divided by the dose required to produce a therapeutic effect (ED50)
Therapeutic window:
The range of drug concentrations (in plasma) where the therapeutic effect is obtained without significant toxicity
LD50
The dose of a compound at which 50% of subjects die
ED50
The dose of a compound at which 50% of subjects experience a therapeutic effect
Maximum tolerated dose (MTD):
Maximum dose that can be given without leading to death/lethal effect
No observable effect limit (NOEL):
The highest level of compound exposure at which no effect is observed
Acute toxicity:
Immediate toxic response following a single or short term exposure to a compound
Chronic toxicity:
A toxic response to long term exposure to a compound
Toxicant:
A man made substance that causes disease or injury (an artificial toxin)
Carcinogen:
A compound or other substance that causes cancer
Mutagen:
A compound that causes physical changes in chromosomes or biochemical changes in genes
Teratogen:
A compound that changes ova, sperm or embryos to increase the risk of birth defects
Epigenetic:
Pertaining to non-genetic mechanisms by which compounds cause disease (e.g. environmental factors)
Organ toxicity targets
Liver and kidney are common targets!
Major drug elimination organs!
Organ toxicity in the liver
Drugs are commonly taken up into
hepatocytes where they are metabolised to metabolites by cytochrome P450s.
Eg. paracetamol.
* Some drugs which are cleared by the liver are also specifically hepatotoxic (eg. methotrexate and paracetamol)
* Intrinsic hetatotoxicity (e.g. methotrexate)
* Cholestasis (impaired bile flow jaundice; e.g. chlorpromazine)
* Immunological (e.g. halothane)
* Most forms of hepatotoxicity are manifested only as increases in the levels of liver enzymes in plasma (stopping drug treatment is not necessary). Others cause severe liver damage and need to be stopped.
Organ toxicity in the Kidneys
- Some drugs or their reactive metabolites are predominantly cleared via the urine after concentration in the kidney renal tubules
- Concentration dependent toxicity!
- (NSAIDs) are toxic to the kidneys by causing vasoconstriction in the kidneys and slowing glomerular filtration rate -> kidney cells are exposed to high concentrations of drug or toxic/reactive metabolites over a longer time
- Anything that effects kidney function and glomerular filtration rate will enhance the toxic effects of drugs in the kidneys
Organ toxicity in the brain
Neurotoxicity
E.g. MPTP (1-methyl-4-phenyl-1,2,3,6-
tetrahydropyridine) – biproduct of heroin synthesis
* Crosses the blood brain barrier and activated to a toxic metabolite (MPP+) by MAO-B
* Causes irreversible unusual motor defects resembling Parkinson’s disease
Organ toxicity in the blood
Haematotoxicity
E.g. Benzene. (used in the chemical industry)
* Chronic exposure leads to leukemias and anemia
* Caused by increased autophagy (cell degradation
and reuse) and decreased acetylation in bone
marrow mononuclear cells
Mechanisms of cell damage/death
Cell damage/death occurs via…
* Necrosis (uncontrolled cell damage and death)
* Apoptosis (controlled/programmed cell death mediated by the cell)
Non-covalent drug interactions
Covalent drug interactions
e.g.
Lipid peroxidation (& reactive oxygen species)
Glutathione (GSH) depletion
Modification of protein sulfhydryl (SH) groups
Types of ADRs
Type A: Dose related toxicity
* Related to the dose and main pharmacological effect of a drug and patient susceptibility
* Can be minimised by simply reducing the dose
* Generally predictable effects based on the known pharmacological and
pharmacokinetic effects of the drug and patient characteristics, eg. kidney damage
* Eg. Giving the anticoagulant drug warfarin – high doses can cause internal bleeding
Type B: Idiosyncratic effects
* Unpredictable adverse drug reactions (often immunological) that are unrelated to the pharmacological actions of the drug
* Often initiated by chemically reactive metabolites rather than active drug (e.g. paracetamol)
* Dose related, but only seen in SOME patients. Eg. hypersensitivity reactions
Type C: Carcinogenic/teratogenic effects
* Dose dependent and predictable. Usually a low incidence of these effects when using a ‘safe’ drug, but with eg. cancer chemotherapy drugs, a notable problem.
Other adverse effects (resulting from overdose, variable pharmacokinetics)
* OVERDOSE – adverse effects are unrelated to the pharmacological effects of the drug (goes back to, everything is toxic if you give enough). Eg. paracetamol hepatotoxicity.
* VARIABLE PK – result from polymorphisms in drug metabolism. Eg P450 2D6.
Non-covalent drug interactions
- Do not involve formation of physical bonds between drug and a target
- Lipid peroxidation (lipid-OO)
- Reactive oxygen species (O2, HOO, HO)
- Depletion of glutathione (GSH)
- Modification of sulfhydryl groups (-SH)
Covalent drug interactions
- Involve formation of physical bonds between drugs and DNA/proteins etc.
- Common mechanism involved in mutagenesis (with chemotherapy drugs)
Lipid peroxidation (& reactive oxygen species)
Reactive drug metabolites or oxygen species react with lipids in cell membranes and damage the membrane
- Reactive drug metabolites (or reactive oxygen species) can react with unsaturated lipids (like phospholipids) on the cell membrane
- Once one reactive drug metabolite attacks a membrane lipid, the process propagates to damage many lipids
- Cell death results from damage to the cell membrane or from the product of reactions between lipid radicals and cell proteins
Glutathione (GSH) depletion
GSH is an antioxidant that protects cells. When it becomes depleted to ~30%, cells cannot protect themselves against reactive metabolites
- GSH redox cycle protects cells from oxidative stress and reactive drug metabolites
- Excessive amounts of reactive drug metabolites can use up cellular GSH too rapidly for it to be sufficiently regenerated –> depleted GSH
- When cellular GSH falls to 20-30% of normal levels, cells cannot protect themselves against ROS and reactive drug metabolites, so the cell dies as a result of ROS overload.
Modification of protein sulfhydryl (SH) groups
SH groups are important for maintaining the functional structure of proteins. Reactive drug metabolites can bind to these and damage the protein structure
- Free –SH groups have an important role in the catalytic activity of many
enzymes (targets include cytoskeletal protein actin, GSH reductase and Ca+2 transporting ATPases – maintain intracellular Ca levels) - Cysteine is a highly reactive amino acid that contains thiols
- Reactive drug metabolites can react with these –SH groups (for instance, to form –S-S- crosslinks) by creating an oxidising environment and inactivate the protein
Reactive oxygen species (ROS)
- Drug metabolism reactions that require O2 involve redox actions that cause oxygen radicals to be produced
- These ROS react with nucleic acids, proteins, structural carbohydrates and lipids and are highly cytotoxic (kill cells)
- Superoxide anion (O2-˙)
- Hydroxyl radical (˙OH)
- Hydrogen peroxide (H2O2)
- Hydroperoxy radical (HOO˙)
- Singlet oxygen (O˙)
- Cause neurodegeneration and excitotoxicity
Adverse drug interactions
Pharmacodynamic
* Additive, synergistic, potentiation, antagonistic, functional antagonism
Chemical
* Drug-drug complexation or local chemical changes
Pharmacokinetic
* Competition for similar absorption, distribution or excretion pathways
Metabolic
* Changes in drug metabolising enzymes
Pharmacodynamic interactions
Additive
* If 2 or more drugs have the same pharmacodynamic effect, the effect of both drugs given at the same time is added together (ie. 1 + 1 = 2)
* Eg. magnitude of cyclosporine nephrotoxicity is increased ‘additively’ by aminoglycoside nephrotoxicity
Synergistic
* If 2 or more drugs have the same pharmacodynamic effect, the effect of both drugs given at the same time is greater than the added effect of both drugs (ie. 1 + 1 = 3)
* Eg. Blood anticoagulant effect of warfarin is dramatically increased in magnitude if given together with aspirin or NSAIDS (warning label on warfarin tablets)
Potentiation
* If one drug with no toxicological effect at the dose given is given with another drug that DOES exhibit toxicity, the magnitude of the toxic drug is increased (ie. 0 + 1 = 2)
* Isopropanol potentiates the hepatotoxicity of carbon tetrachloride
Antagonistic
* If the pharmacodynamic effect of 1 drug COMPROMISES the pharmacodynamic effect of another drug which should have the same ultimate clinical outcome (ie. 1 + 1 = 0.5)
* Bacteriocidal effects of penicillin on bacteria are inhibited by the bacteriostatic effect of other antibiotics (eg. bacteriostatic = stop growth, bacteriocidal = rely on cell growth to kill cells)
Functional antagonism
* Two or more substances produce the opposite effect to each other, counterbalancing each other (identical to physiological antagonism).
Chemical interactions
Altered drug absorption or receptor binding
- Drug binds to another substance
- Chemical environment around a drug is altered
Clinical examples:
Tetracycline chelates metals in
antacids and multivitamins
–>
Reduced tetracycline absorption
Cimetidine increases gastric pH
–>
Ketoconazole is only soluble at
low gastric pH
–>
Coadministration of ketoconazole with cimetidine decreases ketoconazole absorption
Pharmacokinetic interactions
Essentially, one drug/substance alters the absorption, distribution or elimination of another drug. (ADME)
Eg. Grapefruit and many drugs
Metabolic interactions
- Involves drug metabolising enzymes. Eg. cytochrome P450
- P450’s metabolise a large number of drugs
- Different P450’s metabolise different drugs (whether to activate or inactivate the drug)
- Some drugs can change the metabolic profile of another drug in the following ways:
- Competitive inhibition of P450’s
- Potent inhibition of P450’s
- Induction of P450’s
Competitive P450 inhibition
- When 2 drugs that have been administered competitively inhibit each others metabolism –> increase plasma concentrations of both drugs
- Ie. The enzyme can only work at a maximal capacity, so it becomes saturated by the drugs
Clinical example:
Co-administration of nifedipine and erythromycin
Both are metabolised by hepatic P450 3A4 and compete with each other for metabolism
Potent P450 inhibition
- One drug actively inhibits a P450 from working, preventing another drug from being metabolised by that P450
- Means that the second drug needs to be cleared via an alternate route
(different metabolic pathway, biliary excretion, urinary excretion etc)
Clinical example:
ketoconazole on erythromycin (??)
Induction of P450’s
One drug INCREASES the level of expression of a P450 that is required to metabolise another drug -> accelerated metabolism of the second drug –> reduced plasma concentrations
Clinical example:
Phenobarbital is used as an anticonvulsant…. But also upregulates the expression of lots of P450s (like 2B1 & 3A2)
But….lots of other drugs are metabolised by these P450s ….
Eg. oestrogen, doxycycline, corticosteroids, anticoagulants….
Other factors that cause ADRs
Anything that increases blood concentrations of a drug above ‘target’ levels can cause ADRs
‘Interindividual variability’
Ethnicity
* Differences in drug metabolising enzymes and transporters
* Eg. Asians poorly metabolise alcohol
Age
* Neonatal vs old age – differences in drug pharmacokinetics
* Eg. reduced renal clearance with increased age
Pregnancy
* Large changes in drug pharmacokinetics and immune function
* Eg. reduced drug metabolism in the ‘foetal compartment’
Disease
* Particularly diseases affecting the liver and kidneys
* Changes in plasma proteins, liver/kidney function or sensitivity to drugs
Genetic variations
* Changes in response to various drugs or pharmacokinetics
Mutagenesis
Drug (or metabolite/radiation/infectious agent/environmental agent) induced changes to DNA can cause carcinogenesis or teratogenesis
Often caused by covalent modification of DNA, but not always (e.g. methotrexate)
- DNA damage is distinct from mutagenesis:
- DNA damage = abnormal alteration in DNA structure that cannot be replicated.
- Mutation = replacement of a gene in DNA that can be replicated
- Mutations can be in the form of microlesions or macrolesions.
- DNA is most susceptible to change when in the process of replication
- Expose base pairs, particularly guanine
- Risk is related to how frequently cells are dividing
DNA Microlesions (gene mutations)
Change a single base nucleotide
to change the amino acid made
Addition or deletion of a nucleotide
to change the amino acid sequence
DNA macrolesions (chromosomal
mutations)
Include:
1. Change in the number of chromosomes
2. Structural changes in chromosomes
* Deletion: loss of chromosome segment
* Translocation: segment of one
chromosome becomes attached to a non homologous chromosome
* Inversion: A change in direction of the nucleotide code
* Duplication: repetition of a chromosome segment
3. Micronuclei formed (damaged
chromosome fragments or chromosomes not incorporated into the nucleus)
Carcinogenesis
Carcinogenesis, also called oncogenesis or tumorigenesis, is the formation of a cancer, whereby normal cells are transformed into cancer cells.
- Approx 90% of human cancers are caused by chemicals (ie drugs)
- All ‘primary’ chemical carcinogens are highly reactive electrophiles that react readily with nucleophilic sites in the cells (e.g. DNA)
- > 100 known human carcinogens
- Long latency between carcinogen exposure and cancer development
- Carcinogenesis involves more than a single gene mutation (need many targeted mutations). Include genetic alterations involved in:
- Sustaining proliferative signalling (mutation of proto-oncogenes)
- Evading growth suppressors (mutation of tumour suppressor genes)
- Resisting cell death
- Inducing angiogenesis
- Activating invasion and metastasis
- Reprogramming energy metabolism
- Evading immune destruction
Proto-oncogenes
Genes that helps cells grow or stay alive.
Oncogenes
Proto-oncogenes that have mutated, giving it the potential to cause cancer
Tumour suppressor genes
Genes that inhibit cell proliferation
Multistep model of carcinogenesis (3 steps of cancer causation)
Initiation: genetic alteration of a cell
* Normally repaired by cellular processes
* If NOT repaired before the cell replicates, it becomes a stable biological lesion
* Cells accumulate mutations from multiple sources over time (age related cancers)
* Repeated exposure is usually required to increase the number of mutations
Promotion: initiated cell clones itself
(proliferates) to form a mass of modified cells that can be either benign or preneoplastic (NOT cancerous)
Progression: cells undergo additional changes to make them malignant (cancerous). E.g. as cells divide they acquire addition mutations that make the cell more aggressive.
Types of cancer-causing drugs
Primary Carcinogen: Chemical that can directly modify DNA (e.g. alkylating drugs)
Secondary carcinogen: Requires metabolism/activation to a carcinogenic product
Co-carcinogen: Enhances the effect of a carcinogen (e.g. beta carotene)
Promoter: Enhances tumorigenicity(carcinogenicity) when given after a carcinogen (e.g. estrogen; affect the rate of cell division, terminal differentiation or death of tumour precursor cells)
Teratogenesis
the process where exposure to harmful agents during pregnancy causes structural or functional abnormalities in the developing fetus, resulting in congenital birth defects or malformations.
Known teratogens
an agent or factor which causes malformation of an embryo
e.g.
Thalidomide
- R enantiomer: sedative
- S enantiomer: teratogenic
Mechanisms of teratogenicity
Not clearly understood mechanisms!
Teratogens can bind to DNA, proteins or lipids
Bioactivation-dependent teratogens:
* requires metabolism to electrophilic metabolites
Non-bioactive dependent teratogens:
* do not require biotransformation
- disrupt cell differentiation directly (known mechanisms)
- E.g. Accutane (isotretinoin) causes apoptosis and cell cycle arrest (critical during neural development) after a single dose
Anticonvulsant agents (phenytoin):
* risk of birth defects in offspring of treated epileptic mums is 2x
* believed to involve a P450 generated epoxide metabolite
Cytotoxic chemotherapy drugs (cyclophosphamide):
* anticancer drugs target rapidly dividing cells
* direct DNA conjugation or alter DNA/nucleotide synthesis or replication
Thalidomide
Thalidomide
Introduced in 1957 as a hypnotic and sedative drug
* Recommended for use in pregnancy
* Prescribed to treat morning sickness
* Subject to toxicity testing in mice only (resistant to thalidomide teratogenicity)
* Evidence of teratogenicity first noticed in 1961 (withdrawn)
Fetal alcohol syndrome
- Most common non-genetic cause of mental retardation
- Increased risk of miscarriage, stillbirth, premature birth, low birth weight
- Characterised by:
- Impaired neurological development and function
- Social and behavioural problems
- Abnormal growth and physical deformities (e.g. limbs)
- Slow growth after birth
- Characteristic facial features
- Vision and hearing problems
- Kidney, heart and bone defects
