Drug toxicity Flashcards
Define teratogenesis.
the process by which congenital malformations are produced in an embryo or fetus.
Name a drug that causes teratogenesis.
Thalidomide
Outline the mechanism by which Thalidomide causes teratogenesis.
The teratogenesis caused by thalidomide manifests itself as severe limb reduction but other organ defects often accompany this.
Thalidomide is thought to bind to G-C rich sequences in DNA and it can therefore act as a competitive inhibitor of the SP1 transcription factor. Genes that don’t contain TATAA boxes would be more susceptible to inhibition by thalidomide. This is because TATAA boxes are also responsible for recruiting polymerases to the transcription start site. So transcription of these genes will continue.
In genes where there aren’t TATAA boxes, Sp1 plays a critical role in the recruitment of the transcription factor so transcription of these genes will be far more affected by inhibition of SP1 binding.
Some of the genes that lack TATAA boxes include those that code for factors such as integrins which are involved in cell motility and angiogenesis in a developing embryo. Reduction in integrin expression could account for the abnormal limb development seen in response to thalidomide.
- Cell movement is also key in tumour development, as they need to be able to migrate. Thalidomide is being reinvestigated as an anti-cancer agent.
- It exists in two isomers: only one can cross the placental barrier. This means that isolation of the isomers has resulted in a much safer form of thalidomide for therapeutic use.
Thalidomide is metabolised through Phase I into compounds that can be considered teratogenic. What are these compounds? Outline the mechanism of metabolism and toxicity.
There is also evidence to suggest that the metabolites could also be responsible for producing the toxic effects.
A number of metabolites arise by hydrolysis of the amide bond in the piperidine ring. Hydrolysis of this bond leads to opening of the piperidine ring and we get phthalyl derivatives of glutamic acid and glutamine.
- Phthalylglutamic acid has been shown to be teratogenic and this is possibly as a result of Phase I metabolism by a minor cytochrome P450 pathway which could generate an epoxide intermediate. From the structure it can be seen that there are several candidate places for epoxide formation. Epxoides, are reactive and can easily form adducts with DNA and proteins.
- In vitro studies have shown that inhibiting epoxide hydralase, which would further metabolise the epoxide increases the toxicity of thalidomide whereas adding the purified enzyme to cell cultures decreased the toxicity levels.
Define immunotoxicity.
Immunotoxicity is defined as adverse effects on the functioning of the immune system that result from exposure to xenobiotics.
Name a drug that causes immunotoxicity.
Penicillin.
Outline the mechanism by which penicillin causes immunotoxicity.
Up to 10% of people taking penicillin are likely to suffer from allergic reactions and all 4 types of hypersensitivity reaction have been seen in allergic patients.
The mechanism of toxicity relies on the formation of covalent conjugates with soluble or cellular proteins. Penicillin is a reactive molecule in vivo and in vitro and it undergoes biotransformation into a variety of products. Some of these are more reactive than the parent drug and can covalently bind to proteins as they react with the nucleophilic amino, hydroxyl, mercapto and histidine groups. Therefore a number of antigenic determinants may be generated from a single penicillin derivative and these will vary between individuals.
Penicillin can spontaneously convert between some of its derivatives such as penicillenic acid or penicilloic acid and all of these derivatives can from conjugates with cellular proteins
- The conjugate most commonly formed involves the formation of an α-amide derivative with the ε-amino group on the lysine of the protein via the β-lactam ring in the penicillin molecule. It is also possible that the thiazolidine ring may break open resulting in conjugates forming through the sulphur atoms.
Benzyl-penicillenic acid is 40 times more reactive towards cellular proteins than benzyl-penicillin is. Formation of penicilloic acid results in loss of immunogenicity of the resulting conjugate so we see a much reduced immune reaction to this molecule.
Name a xenobiotic that causes multi-organ toxicity.
- Lead
- Ethylene glycol
Outline ethylene glycol poisoning.
Ethylene glycol is a liquid used in paints, polishes, and cosmetics, but it’s better known for its role in anti-freeze. It has a sweet taste so has been used as an alcohol substitute which is where it hit the news 15 or so years ago.
100ml is the minimum lethal dose required and death can occur within 24 hours if the CNS is damaged, or more slowly over 8-12 days if kidney function is affected.
Clinically there are 3 recognisable stages:
- (30 min-12 hr) Permanent optic damage may occur in the first stage. Intoxication, nausea, vomiting, depressed reflexes (can be confused with alcohol intoxication).
- (12 hr- 24 hr) Tachypnoea (excessively rapid breathing), tachycardia, hypertension, pulmonary oedema.
- (24 hr- 72 hr) Kidney damage.
The clinical biochemical features reflect the physiological changes that take place. There is reduced plasma bicarbonate, low calcium and increased potassium. It is also possible to detect crystals, protein and blood in the urine.
The toxicity of ethylene glycol does require metabolism for its effects but it does not involve activation of the parent compound to a more reactive metabolite.
Outline the mechanism of ethylene glycol toxicity.
Ethylene glycol is metabolised via several oxidation steps that eventually yield oxalic acid.
- The first step is catalysed by alcohol dehydrogenase and successive steps by aldehyde dehydrogenase and lactate dehydrogenase.
- Each step in this series of reactions results in the production of NADH. This results in an imbalance of the oxidation states within the cell and needs to be corrected. The body does this by oxidising the NADH back to NAD+ which is a reaction that is coupled to the anaerobic conversion of pyruvate into lactate.
- Therefore there is an increase in the lactate concentration in the cell which results in lactic acidosis.
The intermediates formed by the oxidation of ethylene glycol also have effects on metabolism as they can inhibit: glucose metabolism, the Krebs cycle and oxidative phosphorylation resulting in a decrease in cellular ATP.
What are the pathological consequences of ethylene glycol poisoning?
The pathological damage that we see includes cerebral oedema, haemorrhaging, and deposition of crystals.
The lungs also show oedema, and sometimes crystal accumulation.
It is also possible to get degenerative myocardial changes as well as the muscle cells won’t be working as well if ATP production is impaired.
There is degeneration of the kidney tubules and fat droplets become detectable in some tubular epithelial cells.
How is ethylene glycol poisoning treated?
- Firstly, gastric lavage or stomach pumping is carried out to reduce the absorption of the ethylene glycol, followed by supportive therapy for shock or respiratory distress, saline drip/oxygen.
- Patients are then treated with
- Ethanol: competes with the ethylene glycol for alcohol dehydrogenase. Ethanol is a better substrate than ethylene glycol so essentially the first step in the metabolism of ethylene glycol becomes blocked. Studies have shown that this doubles the LD50 value.
- IV Sodium bicarbonate: to correct the acidosis., this increases the LD50 approx 4 fold.
- Calcium gluconate: corrects the low levels of calcium caused by oxalate crystal formation
- Dialysis to remove the ethylene glycol.
These treatments have been developed as a result of understanding the biochemical aspects of the toxicity.
Exposure, pathology, biochemical
Outline lead poisoning.
Lead has been known as a poisonous compound for 100s of years. Many workers over the years, especially those in lead mines or the manufacture of lead based paint have been exposed to lead, as part of their occupation.
- Exposure to lead can occur in a variety of ways, via food, water or inhalation and for most people it is inhalation that is the major route of exposure. Lead is found in the air as a result of petrol fumes from the combustion of leaded petrol. Levels of lead in food tend to be higher than in the air but absorption via the lungs is greater than the digestive tract so air contamination is the major source. Lead is also found in batteries, some water pipes especially in rural areas insecticides and paint in older houses. Paint flakes were a major cause of lead poisoning in children up until relatively recently, although modern paint is now lead free. Lead exposure may be to the metal, lead salts or organic lead.
Lead causes damage to a variety of organs and also causes significant biochemical effects. The kidneys, testes, bone, GI tract and nervous system are all damaged by lead.
- Acute exposure to inorganic lead causes renal damage and this is detectable as amino aciduria and glycosuria. Lead has a negative effect on both the male and female reproductive systems.
After lead is absorbed into the body it enters the blood stream and 97% of it gets taken up by red blood cells. In the blood, lead it has a half life of 2-3 weeks so there is then time for some re-distribution of lead into the liver and kidney where it is excreted into the bile, or it can become deposited in the bones. To detect lead levels and calculate lead exposure the level circulating in the blood plasma as well as in the red blood cell needs to be known.
The major biochemical effect is interference in the synthesis of Haem which gives rise to anaemia.
Outline the mechanism for lead toxicity.
Lead acts to inhibit the enzymes involved in haem synthesis. So as well as haemoglobin, the synthesis of myoglobin and the cytochrome p450s may also be affected because they have haem prosthetic groups.
The synthesis of haem occurs in several steps.
- Initially succinyl CoA and glycine condense to form δ-aminolaevulinate (ALA) which is a reaction catalysed by δ-aminolaevulinate synthase (ALA synthase).
- 2 molecules of ALA then condense to produce a porphobilinogen intermediate (catalysed by ALA dehydrase). For each haem molecule the reaction requirs 8 ALA molecules which produce 4 porphobilinogen molecules which condense to form uroporphyrinogen via a liner intermediate.
- Uroporphyrinogen is the porphyrin skeleton that is now oxidised to form coproprophyrinogen by decarboxylation of the side chains which is followed by desaturation of the prophyrin ring and conversion of the propionate side chains to vinyl groups (catalysed by coproprophyrinogen oxidase).
- Finally Haem is formed by chelation of the iron in the form of a ferrous group and this reaction is catalysed by ferrochelatase.
The enzymes ferrochelatase, ALA synthase and ALA dehydrase are especially sensitive to inhibition by lead.
- Inhibition of ferrochelatase means that protoporphyrin is inserted into the globin molecule rather than haem and a zinc molecule tends to replace the iron. As it is the iron that is responsbile for oxygen binding the resulting zinc containing molecule can no longer bind oxygen.
- A lack of haem usually results in a negative feedback reaction to stimulate the ALA synthase but as this is also inhibited, more haem can’t be made.
- Therefore the Succinyl CoA and glycine starting substrates accumulate and level of haemoglobin decreases.
Anaemia is the result of a lack of haemoglobin and the fact that the red blood cells have a slightly shorter life span as their membranes become more fragile.
Outline the pathologies associated with lead toxicity.
-
Acute lead exposure leads damage to the kidneys that is reversible, however chronic exposure results in permanent damage to the tubules which involves tubular cell atrophy and fibrosis (excess fibrous connective tissue in an organ).
- Lead protein complexes are seen as inclusion bodies in the tubular cells and the mitochondria have impaired oxidative phosphorylation which affects the cell function as ATP will not be being produced as efficiently. This is because the cytochrome enzymes involved in ox-phos are haem containing molecules.
- The effects on the CNS are probably the most profound and children are especially vulnerable. The neurotoxicity that is observed as encephalopathy which is accompanied by various pathological changes such as cerebral oedema, neurone degeneration and necrosis. Lead can have direct affects on neural transmission as it can inhibit the functions of neurotransmitters and interfere with calcium regulation.
- This results in amino acids and glucose being detected in the urine as they are excreted rather than re-absorbed.
- Anaemia is seen in high or prolonged exposure.
