Compounds Flashcards
Drugs of abuse
MK-801, Ketamine and phencyclidine inhibit NMDA receptors with hypnotic and anaesthetic properties. Disruption of glutamate homeostasis can cause a schizophrenia-like state.
Ketamine is unusual as it seems to be a respiratory stimulant, unlike most anaesthetics that cause respiratory depression. It activates breathing and abolishes the coupling between loss of consciousness and upper airway dilator muscle dysfunction. In some ways it can be considered a safe anaesthetic due to this. However, there have been cases where anaesthesia has been very difficult to reverse, leading to a prolonged coma.
Gamma-hydroxyl-butyric acid (GHB) is a drug of abuse that acts as a GABA agonist, with risk of death through respiratory depression.
METH increases brain ammonia levels, leading to increases in glutamate and excitotoxicity through disruption of EAATs.
MDMA causes similar excitotoxicity. MDMA-induced microglia activation causes the release of pro-inflammatory cytokines, which can exacerbate glutamate release. Astrocytes, responsible for clearing excess glutamate from the synapse, become dysfunctional when activated by MDMA, further increasing extracellular glutamate levels and causing excitotoxicity. MDMA metabolism can also generate reactive quinolones, which cause DNA damage, affecting neurons.
Methapyrilene
Methapyrilene is an antihistaminergic with some anticholinergic properties and strong sedative action, used to treat insomnia.
It was withdrawn after reports of hepatic carcinogenicity in rats following chronic treatment → toxicogenomics showed that genes associated with the cell cycle were upregulated.
Carcinogenicity is mediated by metabolic activation via the rat CYP2C11, possibly via a thiophene-s-oxide.
This reaction depletes hepatocyte glutathione and causes hepatotoxicity, with evidence of mitochondrial toxicity including reduced fatty acid oxidation (steatosis) and elevated ammonia.
This is an unusual toxin as the damage caused is predominantly periportal necrosis.
Methapyrilene is therefore a non-genotoxic carcinogen via modified histone methylation, thereby influencing DNA replication indirectly.
Aminoglycosides - Gentamicin
Aminoglycosides include antibiotics, such as gentamicin, used against gram-negative bacteria.
These antibiotics have limited clinical use due to their nephrotoxicity, as they cause acute proximal tubule necrosis and renal failure.
Gentamicin enters proximal tubule cells. As it is a basic drug, it can fuse with acidic lysosomes, destabilising them and causing them to rupture and release their hydrolytic enzymes. The cell starts to digest itself.
Gentamicin can also directly damage mitochondria.
There is a complete failure of urine production
When prescribing aminoglycoside antibiotics, clinicians carefully monitor renal function.
Polyaspartic acid helps reduce toxicity by preventing the binding of aminoglycosides to negatively charged phospholipids and so cell entry.
Thioacetamide
Thioacetamide is used in industrial processes.
It is oxidised to thioacetamide-s-oxide and a very reactive thioacetamide s,s-dioxide in the liver.
It preferentially reacts with amino groups, especially lysine residues and phosphatidylethanolamine (PE), a membrane phospholipid.
Lysine residues are associated with histone proteins, so it affects DNA coiling and therefore replication, having non-genotoxic carcinogenic effects in this way.
This leads to hepatotoxicity including centrilobular necrosis, hyper-bilirubinaemia and cholestasis.
Phytohaemagglutinin (PHA)
Phytohaemagglutinin (PHA) is a lectin found in many pulses, especially red kidney beans. PHA inhibits glucose transport in the gut, so toxicity results from bacterial overgrowth.
Rats exposed to crude red kidney beans for 24 h showed weight loss and increased intestinal permeability, as well as increased bacterial load and translocation to the liver.
Pulses need to be soaked and thoroughly boiled to avoid adverse gut reactions.
Tubocurarine
Tubocurarine is a nondepolarizing competitive antagonist at nicotinic AChRs on the motor end plate of the NMJ, leading to relaxation of skeletal muscle.
Tubocurarine is produced from the bark of a South American climbing vine.
It was used as an arrow poison as the flesh of animals killed by curare was ‘safe’ to eat.
Curare does not readily cross from the gut to the blood, so absorption from the GIT is limited. This is due to positive charges on the molecule.
This is why consumption of animals killed with curare was considered safe
Acute toxicity LD50 150 mg/kg in mice when administered orally and 420 ug/kg (0.42 mg/kg) when administered intraperitoneally.
300 times less toxic orally BUT still considered toxic orally
Historically used as a muscle relaxant as an aid to surgery during anaesthesia.
Mechanical ventilation may be necessary due to paralysis of the diaphragm and intercostal muscles.
Relatively little effect on heart function.
Replaced in recent years by suxamethonium and rocuronium, which have a shorter half life and rapid onset of action.
Rye and Ergot fungi
Rye infected with the Ergot fungus Claviceps Purpura can cause ergotism.
These occur in damp conditions, particularly in the late autumn before harvest.
Ergotism can cause gangrene and limb loss.
Ergot alkaloids are classified as tryptophan-derived alkaloids. Many derivatives are heat tolerant and so not destroyed by cooking.
Ergotism is characterised by pain in limbs, paresthesia (pins and needles) and burning sensations. Later clinical syndromes include vasoconstriction leading to gangrene of the extremities, with loss of limbs.
Vasoconstriction likely occurs through ergot acting as an alpha-1 receptor agonist.
Other symptoms include hallucinations and convulsions.
Ergot alkaloids include ergometrine and ergotamine. These show structural similarities to lysergic acid diethylamide (LSD).
Treatment typically involves the use of vasodilators and heparin.
In humans, ergot is used pharmacologically to inhibit lactation, treat postpartum haemorrhage, and induce labour.
Ergot derivatives are used as vasoconstrictors to treat serious migraines.
Historically, ergot was used to treat Parkinsonism.
Plants in accidental poisoning
The plant Deadly nightshade (Atropa Belladonna) produces the toxin atropine.
Atropine binds to and inhibits muscarinic ACh receptors.
Atropine poisoning signs: red as beetroot, dry as a bone, blind as a bat, hot as a hare, mad as a hatter.
One or two berries are enough to be lethal to a child
Atropine may cause ‘pseudo allergy’ through histamine release from mast cells.
Hyoscine (Scopolamine) is another mAChR inhibitor that comes from the closely related Henbane.
It is used in travel sickness pills as it inhibits muscarinic receptors in the vestibular nuclei and inhibits gastric secretions.
Henbane has been confused with Fat Hen Weed, which is harmless - misidentification could be lethal.
Woody nightshade contains solanine, a cholinesterase inhibitor, resulting in excess ACh signalling.
Solanine is also found in green potatoes and tomatoes, but large quantities need to be consumed for toxicity to occur.
Solanine can cause gastrointestinal and neurological disorders, with symptoms including nausea, diarrhoea, vomiting, hallucinations, paralysis, and even death in severe cases.
Atropine is actually used as an antidote for solanine poisoning.
Calabar beans contain the active substance physostigmine (eserine), a cholinesterase inhibitor.
Physostigmine causes an increase in the size of electrically invoked reactions.
Calabar beans were historically used in ritual torture of criminals as they cause vomiting, diarrhoea, convulsions, breathing difficulties, and hallucinations.
However, physostigmine is also used pharmacologically to treat glaucoma, constipation, and myasthenia gravis, and to reverse neuromuscular blockade
Aconitase can be inhibited when fluoroacetate is converted to fluorocitrate, leading to convulsions and death.
Fluoroacetate is found in the ‘poison pea’ plant
Aconitase is the enzyme that catalyses the isomerization of citrate to isocitrate, one of the first biochemical reactions in the TCA cycle.
It effectively inhibits ATP synthesis.
Ethanol
The first step of ethanol metabolism is fast, but the second is slow.
Acetaldehyde is the main toxic agent.
Some populations, such as east asian populations, have a deficiency in ALDH, making them more susceptible to alcohol toxicity.
Durian fruit also inhibit this enzyme, increasing the risk of toxicity in these populations
The ALDH inhibitor diethyldithiocarbamate was used as an alcohol deterrent (anti-buse) → makes people ill quickly and is therefore also a high-risk agent.
Acute alcohol toxicity involves respiratory depression, potentially via GABA activation and inhibition of glutamate transmission.
Chronic toxicity involves inhibition of fatty acid beta-oxidation in the mitochondria, causing early steatosis through steatohepatitis, eventually causing cirrhosis.
Acetaldehyde induces mitochondria damage and impairs glutathione function, leading to oxidative stress and apoptosis.
This occurs as acetaldehyde binds proteins, including mitochondrial proteins, causing formation of autoantigens.
It can also bind DNA, which can lead to hepatocellular carcinoma (HCC).
Ethanol inhibits PPAR-α signalling, further suppressing beta oxidation.
Toxicity markers include raised plasma ammonia and bilirubin, and later on suppressed albumin and reduced clotting factors.
Ethanol intoxication leads to an increase in gut-derived LPS in the hepatic portal vein being presented to the liver due to a leaky gut. This leads to Kupffer cell activation, induction of iNOS and increased levels of peroxynitrite. This inflammatory response further contributes to steatohepatitis and cirrhosis.
Iron overload is considered a factor in liver pathology, so ferroptosis may be involved.
Haemolysis and bone marrow suppression also occur as a result of iron accumulation
The hormone hepcidin normally inhibits excess iron absorption, but this is suppressed in alcoholic liver disease.
Iron accumulation causes the production of hydroxyl radicals through the Fenton reaction.
Alcohol induces the liver enzyme CYP2E1, which can lead to increased generation of ROS. This stimulates local synthesis of inflammatory mediators.
Autoimmune reactions involving CYP2E1 are detectable in about one third of the patients with advanced alcoholic liver disease.
Alcohol can also cause hemolysis.
Mustard gas
Mustard gas is a liquid delivered as a vapour mist, with the odour of mustard/horseradish.
Highly lipophilic and easily absorbed on contact with the skin or mucous membranes.
Symptoms may be slow to appear but include initial itching, followed by blistering and eventually leading to chemical burns equivalent to 3rd degree burns. Death can occur from opportunistic infections.
Mustard gas is highly reactive and it alkylates DNA.
Diethylenetriamine is a useful agent to deactivate mustard gas.
N-Acetyl cysteine may prove to be a useful additional treatment.
Nitrogen mustards were originally used as warfare gases, but they were then the prototype early chemotherapy agents when it was observed that persons exposed to mustard gas had lower lymphocyte numbers.
Mustine was one of the earliest chemotherapy agents.
Furanocoumarins
UV light can activate furanocoumarins, like 5-methoxypsoralen, found in celery and parsnip, causing phototoxicity.
The photo-activated psoralen seems to bind thymine and possibly guanine in DNA, causing cross-linking and death by apoptosis.
Phototoxicity is used clinically in PUVA therapy (Psoralen and UVA light) in treatment of psoriasis, vitiligo and cutaneous T-cell lymphoma.
Photoallergy is a T-cell mediated allergy that develops following light activation of some chemicals.
6-Methyl coumarin, which is used in sunscreens and has a coconut scent, is one of the most notorious photoallergens.
Issue as it will be continuously exposed to sunlight since it is found in sunscreen
This may be exacerbated by relatively low level sun exposure
For treatment, keep out of the sun, and use topical steroids
UV-A actually induces IL-10, an anti-inflammatory cytokine, in keratinocytes
Benoxaprofen
Benoxaprofen was a NSAID developed as an anti-rheumatic remedy, suppressing inflammation and preventing bone destruction.
Produced by Eli Lilly, aggressively marketed as “major advance in the treatment of arthritis”.
It was an unusual NSAID, acting as an inhibitor of lipoxygenase but having milder inhibitory effects on cyclooxygenase. This mechanism was thought to reduce side effects.
Phase 3 trials included 2000 subjects, including some elderly patients.
It was withdrawn following severe adverse effects and deaths, with 12 occuring in the UK.
It caused liver and kidney failure and bleeding peptic ulcers.
There was also evidence of phototoxicity through photochemical decarboxylation with singlet oxygen production and single strand breaks in DNA. The drug has similar structural features to psoralen, which could have indicated this effect.
Long half-life of 40 hrs and extended half-life in elderly patients of 100-140 hr was the principal reason for drug toxicity.
Domoic acid and Amnesic shellfish poisoning
Domoic acid (DOM) is produced by different species of marine plankton and other organisms such as the red algae.
DOM can potentially enter the food chain by contaminating shellfish, most commonly blue mussels, and it can also be present in crustaceans.
Toxicity causes memory impairment which led to the name amnesic shellfish poisoning (ASP).
Gastrointestinal symptoms occur within 24 hours and neurological symptoms within 48 hours.
Symptoms included nausea, vomiting, abdominal cramps, diarrhoea, headache, unstable blood pressure, cardiac arrhythmias and neurological dysfunction, including memory loss, coma, and seizures.
DOM is structurally similar to another known toxin, kainic acid (KA). Both are excitatory amino acids (EAA) and analogues of glutamate.
Therapeutics and peripheral neuropathy
Paclitaxel is another chemotherapeutic that causes peripheral neuropathy, similar to cisplatin. It is derived from western pacific yew.
Thalidomide, a treatment of ankylosing spondylitis and rheumatoid arthritis, mostly affects sensory nerves, reversible after drug suspension.
Isoniazid, used to treat tuberculosis, interferes with Vitamin B6 synthesis, which causes neuropathy, principally affecting motor nerves. Effects are reversed by pyridoxine, a 4-methanol form of vitamin B6.
TCDD
Dioxins like TCDD are primarily byproducts of human activities.
The main industrial sources are waste incinerators, ferrous and non-ferrous metal production, power generation and heating.
They accumulate in the food chain through a process called biomagnification. Meat, fish, and eggs are major sources of dioxin in humans. Once in the body, dioxins are partly metabolised and eliminated, with the rest stored in body fat. Their half-life can range from 5 to 10+ years.
Seveso 1976 - chemical plant was trying to make 2,4,5-Trichrlorophenol as a precursor to hexachlorophene, a general antibacterial often found in mouthwash, but the reaction got too hot and they made TCDD, which exploded over northern Italy.
Agent orange, a mixture of chlorinated herbicides, was used as a defoliant in the Vietnam war. It was contaminated with TCDD.
TCDD activates the nuclear aryl hydrocarbon receptor (AhR), which is linked with excess induction of the liver enzyme CYP1A1. This metabolises aromatic hydrocarbons to carcinogenic products, promoting tumour development, particularly in the form of lymphoma and stomach cancer.
Teratogenicity may also occur, with birth defects as a result of the incident still occurring today.
Pathology observed includes peripheral neuropathy, thymic atrophy and general immunosuppression, autoimmunity, progressive weight loss, and increased incidence of cardiovascular disease.
TCDD is an endocrine disruptor, leading to reduced levels of testosterone and thyroxine. It can also cause atherosclerosis, hypertension and diabetes.
Skin toxicity also occurs in the form of chloracne, an acneiform eruption of blackheads, cysts and pustules.
Terfenadine and Astemizole
Terfenadine and Astemizole are non-sedating anti-histaminergic drugs, both withdrawn from the market due to potent potassium channel blocking effect leading to QT-prolongation and Torsades de Pointes.
The QT prolongation from terfenadine was relatively small (< 10ms), yet caused an excess of 125 deaths in USA and 14 deaths in UK.
Cadmium (Cd2+)
Cadmium (Cd2+) is an industrial agent and environmental pollutant that’s a major cause of kidney disease via apoptotic and necrotic pathways.
Cadmium can end up in drinking water or it can be taken up by plants grown in contaminated soil.
Cd2+ has a high affinity for sulfhydryl (SH) groups including glutathione and metallothionein (MT), and it can bind these in the liver.
Once internalised, cadmium enters hepatocytes and forms Cd-MT complexes.
These complexes are filtered by the kidneys and taken up by proximal epithelial cells, leading to polyuria and appearance of the protein albumin in urine.
Cd2+ is effectively delivered from the liver to the kidney.
Central to cadmium’s toxicity is that Cd-MT complexes can dissociate, so Cd2+ can then be released in the kidney and the process can start again as cadmium is delivered back to the liver in a cyclical process. This accounts for the long half-life of cadmium, which is 6-38 years.
Traditional chelators used for other metal poisonings can worsen kidney damage by increasing cadmium delivery to the kidneys.
In the kidney, low levels of cadmium cause apoptosis and high levels cause necrosis.
These occur due to disruption of cadherin-mediated cell-cell adhesion, oxidative stress through glutathione depletion and impaired cell signalling.
Kim-1 is highly expressed in injured proximal tubule cells and is a promising tool for early detection of cadmium nephrotoxicity.
Cadmium can also cause immunosuppression as it inhibits antibody synthesis.
Cd2+ ions can affect signalling mediated by other ions, as they can act as Na+ channel inhibitors, having TTX-like effects on the cardiovascular system.
Mercury (Hg2+) is another heavy metal that can cause proximal tubule injury and inhibits antibody synthesis.
Mercury may however also cause autoimmunity
Penicillin allergy
Penicillin can cause a type 1 hypersensitivity reaction in allergic individuals.
IgE-mediated, involves degranulation of mast cells and release of histamine and tryptase.
Characterised by vasodilation and bronchoconstriction
May result in life threatening anaphylactic reaction
1% of the population have an allergy, with some reports suggesting that this is an overestimate
0.01% experience anaphylaxis
Genetic predisposition due to high N-acetyltransferase activity is associated with penicillin allergy.
Penicillin may also cause autoimmune haemolytic anaemia
Cisplatin
Cisplatin is an alkylating-like cancer treatment.
Renal toxicity seen in 28-36% of patients.
Renal excretion is the primary way cisplatin is removed from the body, so the kidneys accumulate a greater amount of drug, which can result in toxicity manifesting as injury in the loop of Henle and collecting ducts. This is primarily a result of uptake by the proximal tubule cells of the nephron and it can be largely controlled by diuretics and pre-hydration.
Cisplatin can also cause peripheral neuropathy. Sensory nerves are the most affected but deficits in motor/autonomic function are also seen, including microtubule disruption, mitochondrial damage and myelin sheath damage. Damage to sensory nerves causes shooting pain in the extremities.
Alpha-lipoic acid and Amifostine are used to treat cisplatin-induced neuropathy. There is a concern, particularly with amifostine, that it is protecting the cancer cells from toxicity too, undoing the effects of cisplatin.
Acetyl carnitine raises ATP levels, protecting against mitochondrial damage.
Paraquat
Paraquat was a widely available herbicide, but it was banned in the EU in 2007.
Paraquat contains two N+ ions.
It is highly toxic via the skin and mucous membrane routes, as well as orally.
Linked to suicide attempts.
Paraquat exposure causes severe skin injury involving ulceration and pulmonary toxicity, including interstitial pneumonitis and pulmonary oedema.
It causes acute respiratory distress syndrome (ARDS) followed by pulmonary fibrosis.
It is eliminated through the kidneys, so these are also a target organ for toxicity, particularly due to their concentrating effect.
The brain is also a target organ for toxicity, causing cerebral oedema with acute exposure and neurodegeneration of dopaminergic neurons and Parkinson’s with chronic exposure.
Paraquat is structurally similar to MPTP, which may account for the similarities in CNS toxicity.
20-30 mg/kg is an approximate human lethal dose
High mortality rate (50-90%)
If plasma levels are <2 ug/ml, patients may survive, but delayed death can still occur due to fibrosis.
Paraquat is selectively toxic to the lungs because lung tissue has a high concentration of polyamines, and paraquat can hijack the polyamine uptake system since the distance between the positive N+ charges is similar in paraquat and polyamines.
Paraquat undergoes redox cycling. It is reduced to a free radical that reacts with molecular oxygen to generate superoxide (O2-) radicals and regenerate the oxidised version of paraquat.
The process continues with endless generation of oxygen radicals leading to glutathione depletion and lipid peroxidation causing ferroptosis.
This causes oxidative stress.
Antioxidants, including N-Acetylcysteine, vitamin E and C, and iron chelators such as desferrioxamine, reduce oxidative stress, glutathione depletion and lipid peroxidation.
Iron chelators are useful as a lot of free radical reactions are dependent on the availability of free iron.
Paraquat causes ferroptosis, which is dependent on lipid peroxidation → iron chelators inhibit this
Steroids can be used as paraquat toxicity is exacerbated by an inflammatory response. Immunosuppressive cytotoxic drugs can also be used.
Extracorporeal toxin removal may be used in the form of dialysis, which can spare the kidneys of toxicity.
Diquat is another herbicide that undergoes similar redox cycling to paraquat and menadione, but toxicity is primarily in the liver.
Doxorubicin
Doxorubicin is used in a variety of cancers, including breast, ovarian, AIDS-related Karposi’s sarcoma, multiple myeloma, leukaemia and Hodgkin’s lymphoma.
Doxorubicin intercalates with DNA to block the action of topoisomerase II, leading to double-strand DNA breaks and cell death.
Cardiotoxicity is a major complication. It is seen as dilated cardiomyopathy, which is enlarged heart chambers and reduced contractile ability.
It involves oxidative stress, downregulation of contractile genes and p53-mediated apoptosis, or more specifically ferroptosis.
Mechanisms of cardiomyocyte toxicity include generation of free radicals through interactions with iron and disruption of calcium homeostasis.
Cardiac damage leads to heart failure with up to 50% mortality rate, which limits the usefulness of doxorubicin.
Dexrazoxane may reduce the cardiotoxicity of doxorubicin in some cases by chelating iron, thus reducing doxorubicin-iron complex formation.
Antioxidants may also be beneficial.
The cardio-selective toxicity of Doxorubicin is not fully understood.
Carbon tetrachloride
Carbon tetrachloride (CCl4) is a dry cleaning fluid that was used pre-1970. It is a model liver toxin, causing lipid peroxidation, steatosis and centrilobular hepatic necrosis.
Carbon tetrachloride is metabolised by CYP450 to a free radical. This reacts with free oxygen and then causes lipid peroxidation and steatosis. It can also covalently bind to proteins.
Metabolism also produces phosgene and chloroform.
Chloroform stimulates GABA receptors, and inhibits Na+ channels on glutamatergic neurons, reducing neuronal firing. Chloroform itself can also produce phosgene.
Phosgene reacts with NH2, OH and SH groups. It causes pulmonary oedema, nephrotoxicity, hepatotoxicity and it is a possible carcinogen.
Alpha Tocopherol (Vitamin E) protects against CCl4 necrosis by acting as an antioxidant and inhibiting lipid peroxidation. It is a radical scavenger, delivering H to the free radical.
Cyclophosphamide
The antitumor drug cyclophosphamide can be activated by CYP450s to the active anticancer compound phosphoramide mustard and the toxic metabolite acrolein.
Acrolein exposure can lead to decreased pulmonary function, pulmonary oedema and chronic respiratory disease.
There are acute cardiotoxic effects like endothelial and myocyte damage, leading to diastolic contractile dysfunction.
Congestive heart failure, haemorrhagic myo-pericarditis and sudden death have also been reported.
Bladder toxicity also occurs as the metabolites are relatively stable.
Prostaglandin H synthase also activates cyclophosphamide to the same reactive metabolite. Indomethacin is an inhibitor of this enzyme and reduces cyclophosphamide toxicity.
Organs where immune cells are found in high numbers (i.e. lung), or organs where inflammation occurs, will express high levels of prostaglandin-H synthase.
Cyclophosphamide metabolites are toxic to immune cells, causing alopecia and hemorrhagic cystitis in the bladder. Cyclophosphamide, methotrexate and many other anticancer agents cause general bone marrow suppression, having anti-proliferative effects with a higher selectivity for T-cells.
Organophosphates
Organophosphates were originally developed as chemical warfare agents, but are now commonly used as household and agricultural insecticides.
Organophosphates used as insecticides include Parathion, Malathion (used to treat head lice), Diazinon and Chlorpyrifos.
Organophosphates do also include the nerve agents sarin, malathion, VX nerve agent, and novichok 5 and 7.
Organophosphate insecticides inhibit acetylcholinesterase, causing accumulation of ACh at the synapse.
This leads to cholinergic syndrome, which has widespread manifestations, including GIT disturbances, bronchoconstriction, CV effects and convulsions.
Initial supportive measures in acute organophosphate poisoning include:
Decontamination
Airway stabilisation
Activated charcoal for body decontamination
Anyone trying to treat someone for organophosphate poisoning should take precautionary measures to protect themselves from contamination first.
Organophosphates also reduce T and B-cell numbers and the respiratory burst in macrophages, having immunosuppressive effects.
Some organophosphates pose a risk of QT prolongation.
Antidotes include:
Atropine, a muscarinic antagonist
Pralidoxime, a chemical antagonist used to regenerate active cholinesterase → only useful if you get it in quickly
NSAIDs
NSAIDs inhibit COX enzymes and so prostaglandin synthesis, which is responsible for their therapeutic effect.
However, inhibition of COX-1causes peptic ulceration and gastric bleeding by reducing PG production in the GIT and therefore decreasing mucus production.
NSAIDs, such as aspirin, ibuprofen, naproxen and indomethacin, can cause renal injury with high-dose or long-term use.
COX inhibition also inhibits the production of the vasodilators PGE2 and PGI2 (prostacyclin), which causes hypertension and pre-renal toxicity in the form of reduced renal blood flow and GFR with oliguria.
Toxic effects are often reversed if the drug is withdrawn, but long-term chronic NSAID use may lead to irreversible kidney damage.
Kidney damage is characterised by papillary necrosis and interstitial nephritis, both secondary to renal vasoconstriction and reduced glomerular filtration rate.
Nephritis is a condition in which the tissues in the kidney become inflamed and have problems filtering waste from the blood. Nephrotic syndrome may occur, characterised by proteinuria, low blood protein, increased cholesterol and swelling. This occurs when NSAIDs directly injure podocytes, leading to the development of autoantibodies and an autoimmune response
Raised creatinine and BUN are seen.
Renal toxicity can cause electrolyte imbalances which can have systemic effects, and it can also progress to chronic kidney disease (CKD) in patients with pre-existing hypertension or diabetes.
Some individuals can be allergic to NSAIDs, so there is also an immune component to renal toxicity.
NSAIDs may also induce phototoxicity.
4-Ipomeanol
4-Ipomeanol is a pulmonary toxin generated in mouldy sweet potatoes via the action of the fungus Fusarium Solani.
Toxicity is also reported in the liver and kidneys.
Toxicity is primarily in bronchiolar Clara cells, characterised by pulmonary oedema, congestion and haemorrhage.
The furan group (oxygen heterocycle) is activated by P450s to generate a reactive metabolite, an unsaturated di-aldehyde that depletes glutathione and binds to macromolecules.
This can be detoxified by forming a glucuronide.
4-Ipomeanol tends to accumulate in the lungs and has been considered a drug candidate for treatment of lung cancer. Trials were however stopped after phase 2 due to hepatotoxicity.
Endogenous ammonia
Ammonia is generated in the liver through transamination reactions, where it is consumed by the urea cycle and converted to urea.
Normal adult plasma ammonia concentrations are less than 50 uM but may rise to up to 1 mM in liver failure patients.
Ammonia levels of >200 uM may be associated with neurotoxicity and convulsions.
Ammonia crosses the BBB and enters astrocytes, where it reacts with glutamate to generate glutamine via glutamine synthase.
Glutamine is osmotically active, leading to cerebral oedema.
Glutamine can be converted back to ammonia and glutamate in the mitochondria.
Ammonia also activates NMDA receptors to cause excitotoxicity and neuronal death.
Both forms of ammonia, NH3 and NH4+, can disrupt the pH balance within cells.
Excessive ammonia may lead to neuropsychiatric disorders, hepatic encephalopathy and cerebral oedema.
Children are more susceptible than adults. If they go undiagnosed and untreated, this may lead to mental retardation and learning difficulties.
Excess ammonia is linked with Alzheimer’s disease as elevated brain ammonia have been observed in patients.
Ammonia can trigger neuroinflammation by activating microglia and astrocytes, which leads to the release of pro-inflammatory cytokines and upregulation of iNOS and COX-2.
Ammonia can cause oxidative and nitrosative stress.
Treatments for ammonia toxicity include:
- Glutamine synthetase inhibitors including methionine sulfoximine (MSO)
- Lactulose to acidify the gut such that more ammonia is retained as the non-transportable NH4+ ion, reducing exposure of the CNS to ammonia.
- NMDA antagonists, such as MK-801, phencyclidine and ketamine.
Amanitin as a therapeutic
The toxic effects of amanitin from death cap are beginning to be exploited for therapeutic purposes in oncology.
Mechanism of toxicity involves inhibition of RNA polymerase II and so blockade of protein synthesis.
Achievement of the total synthesis of amanitin in recent years has allowed for the development of new amanitin-conjugated antibodies targeting certain tumor characteristics. Since amanitin is no longer a substrate for OATP1B3 when coupled to antibodies, the particularities of the conjugated antibody allow targeting a precise cell population while limiting non-selective toxicity.
Allergic contact dermatitis
Caused by sensitisers through hapten formation.
Can be caused by:
- Balsam of Peru from the trunk of Myroxylon balsamum, a widely used ingredient in the food industry and in perfumes, as well as other household items.
- Latex, detergents, and plants, such as poison ivy and mango which both contain urushiol
- Phenylenediamine (Hair dyes, cosmetics). In 1993, Phenylenediamine was found in Lash Lure mascara, causing blisters, abscesses, blindness and even death following infection in one case.
Tear gas (CS gas)
Used in civil control by riot police.
When deployed it causes coughing and pulmonary damage.
Allegedly it is safe, but being a known sensitizer the word “safe” needs applying with a degree of scepticism. Deaths have occurred following CS-gas use.
Known to activate TRPA-1 receptors causing a pain sensation.
Bleomycin
Bleomycins are a family of compounds of bacterial origin from gram-positive bacteria.
They are used in chemotherapy, but around 50% of treated individuals experience pneumotoxicity, which limits therapeutic use.
The lungs and skin lack bleomycin hydrolase, making them particularly vulnerable to toxic effects.
Bleomycins partially intercalate into DNA and bind Fe2+, creating a more reactive oxygen species and leading to DNA strand breaks.
Toxicity results from necrosis of endothelial and alveolar type 1 cells leading to oedema and haemorrhage, and later apoptosis of alveolar type 2 cells.
Bleomycins activate alveolar macrophages, causing cytokine release and inflammation.
Fibroblasts are activated, causing production of collagen leading to pulmonary fibrosis.
Oxalic acid
Oxalic acid can be found in rhubarb.
Oxalic acid can also be produced as a metabolite of ethylene glycol, which is the main component of antifreeze but can also be found in some cheap wines and historically in children’s medicine, which led to some deaths.
Oxalic acid can form calcium oxalate, which crystallises in renal tubules, leading to nephritis, fibrosis and tubular atrophy.
Symptoms of toxicity include metabolic acidosis, elevated BUN, and increased blood creatinine.
Sodium thiosulphate, which reduces the formation of kidney stones, may be a beneficial treatment, potentially acting as a vasodilator to help dissolve and remove the crystals.
4-methyl pyrazole, an inhibitor of alcohol dehydrogenase, can prevent the toxicity of ethylene glycol, if administered in time.
Coxibs
Gastrointestinal bleeding is a common side effect of NSAIDs, leading to 100k hospitalizations and 16.5k deaths per year in the USA.
One hypothesis claims that constitutive COX-1 is associated with integrity of the gastrointestinal mucosa and platelet aggregation, whilst inducible COX-2 with inflammation and pain.
Therefore selective inhibition of COX-2 should reduce gastrointestinal bleeding and produce better NSAIDs.
COXIBs do in fact reduce gastrointestinal bleeding, but the hypothesis is simplistic.
COX-2 has an important cardio-protective role, so inhibiting it removes the cardioprotective properties of the vasodilators prostaglandin E2 and prostacyclin (PGI2). Toxicity shifts from the gut to the cardiovascular system.
This causes pre-renal toxicity in the form of reduced GFR.
Rofecoxib is a COX-2 selective inhibitor marketed by Merck for osteoarthritis.
Merck withdrew the drug after disclosures that it withheld information about rofecoxib’s risks for 5 years, resulting in 88-140k cases of serious heart disease and strokes.
There is also evidence of hERG potassium channel blockade by rofecoxib metabolites.
The cardiovascular risk probably extends to all COX-2 inhibitors, though celecoxib is still available in Europe.
Copper
Copper can cause oxidative stress, protein aggregation and enzyme inhibition, leading to widespread toxicity.
Nickel
Nickel is a contact sensitiser in the skin through hapten formation via ionic bonds.
This is a type IV delayed hypersensitivity reaction mediated by T-cells.
The naive T-cell repertoire can respond to various drugs and chemicals. This means that within the pool of naive T cells, which have not yet encountered their specific antigen, there exist T cells capable of recognising and being activated by drugs and chemicals.
Treatment options for nickel allergy include topical corticosteroids, avoidance of nickel-containing products, and nickel desensitisation therapy.
However, systemic effects are contradictory, as nickel can also cause immunosuppression, inhibiting the development of immune organs and immune surveillance and causing lymphocyte apoptosis.
NK cell suppression is the most pronounced effect.
Nickel is a type 1 carcinogen.
Oxidative stress is considered an important factor in the damage nickel causes to the organs of the immune system. Nickel chloride can suppress the activities of antioxidant enzymes and reduce the content of glutathione, which leads to ROS accumulation and apoptosis.
Botulinum toxin
Botulinum toxin (BOTOX) comes from Clostridium botulinum spore-forming anaerobic bacteria found in the soil, river water and the intestines of animals.
The toxin enters the general circulation and binds to the presynaptic membranes of motor neuron nerve terminals.
It can then enter the neuronal cytosol and block the release of acetylcholine, causing a neuromuscular blockade.
This causes flaccid paralysis.
Canning processes, if not followed correctly, can lead to C. botulinum growing
In 1978, 4 people became ill after consuming a tin of John West Salmon
2 died, 2 survived with artificial ventilation
Acidity of the fish means that the bacteria is more susceptible to spore survival
Canning or bottling make conditions more favourable for the growth of C. botulinum as this is an anaerobic bacteria, and particular care must be taken when preserving foods by these methods.
Extremely toxic, LD50 of 10 ng/kg
Botulism is a food borne illness resulting from ingestion of food that has become contaminated with spores that germinate and grow in an anaerobic environment, releasing botulinum toxin.
Infection of wounds can occur from contamination with spores that subsequently germinate, resulting in production of toxin and symptoms of botulism.
Spores can be found in soil
Signs and symptoms include:
- Inhibition of skeletal muscle innervation
- Flaccid muscular paralysis
- Blurred vision
- Difficulty swallowing
- Respiratory paralysis (cause of death)
An antitoxin is available
Medical uses of the toxin were introduced in 1983.
Very small amounts of this toxin are used both as a cosmetic treatment and to treat painful muscle spasms, such as in the oesophagus.
It is sold under the brand names Botox and Dyspot, respectively.
Tetrodotoxin (TTX)
TTX is a Na+ channel inhibitor.
TTX is found in the liver, skin and ovaries of puffer fish, but this is considered a delicacy in Japan.
Several ubiquitous varieties of bacteria also produce TTX.
TTX is water soluble, heat stable and about 1,000 times more toxic than cyanide
Initial symptoms include tingling (paresthesia) of the tongue and lips, followed by or concurrent with headache and vomiting, which may progress to muscle weakness and ataxia. In severe cases death may occur due to respiratory and/or heart failure
TTX is a sodium channel blocker selective for nerves, not affecting muscle channels, so it is a useful pharmacological agent as it helps us identify if a response is neurological or a muscle effect.
Zomepirac
Zomepirac was a NSAID withdrawn because of anaphylactic reactions and renal injury.
Renal injury was mediated by a reactive electrophilic acyl glucuronide → example of phase 2 metabolism leading to reactive metabolites.
Renal injury is exacerbated by monocyte and neutrophil infiltration with release of inflammatory cytokines, including IL-1, IL-6 and IL-8.
Reactive acyl glucuronides are also probably associated with anaphylactic reactions.
Relatively short half-life, so the concentrating effect of the kidneys is a relevant factor to renal injury.
This is idiosyncratic toxicity.
Formation of reactive glucuronides may be widespread amongst the NSAIDs, so this is possibly a drug-class effect. Ibuprofen may suffer of this but not as badly.
Methyl isocyanate (MIC)
In December 1984, at an insecticide manufacturing plant in Bhopal (India), water was inadvertently added to a tank of methyl isocyanate, leading to an exothermic reaction and the explosive release of methyl isocyanate.
500,000 persons were exposed to the gas cloud, and around 4,000 deaths have been attributed to the incident. Many people have life-changing injuries and the legal case continues.
Effects are initially coughing, sneezing, choking and pulmonary oedema, sometimes with lethal consequences.
Methyl isocyanate is highly reactive, depleting GSH, and failure to inform hospitals of the nature of the toxin released lead to many more deaths.
In retrospect, diethylenetriamine, N-Acetyl-cysteine and 2-methoxyethanol would have been helpful treatments if there wasn’t a delay.
Sulphadiazine
Sulphadiazine is an antibiotic that may cause systemic lupus erythematosus. This is widespread inflammation.
Cyanide
Cyanide can be generated in industrial processes, it can be an environmental toxin and it can also be found in some food sources.
Hydrogen cyanide (HCN) gas is a rapidly acting poison.
Cyanides are also known as nitriles. Organic nitriles are much less toxic.
It is the cyanide ion that is responsible for toxicity, so more complex forms are not as toxic.
Complex ferricyanide and ferrocyanide are less toxic.
Cyanates and thiocyanates (SCN) are less toxic metabolites.
Cyanide is an industrial chemical used in gold extraction.
Cyanides bound to sugars, known as cyanogenic glycosides, are widely found in nature from cassava yams, apple pips, bitter almonds, and other stones from prunus species, such as cherries, plums and apricot kernels.
Cyanide exposure is a major risk factor in smoke inhalation.
Sodium nitroprusside, an emergency anti-hypertensive agent, may release cyanide following metabolism or decomposition.
LD50 of around 4 mg/kg, and toxicity occurs rapidly.
Cyanide inhibits the cytochrome oxidase enzyme, the terminal oxidase of the mitochondrial respiratory chain. This causes NADH to accumulate and mitochondrial ATP generation to cease.
The blood is fully oxygenated, but the oxygen cannot be utilised for ATP production.
It therefore has the ability to induce extreme chemical suffocation of cells and disrupt enzymatic processes.
Toxic effects of cyanide include: acidosis, vomiting, seizures, coma and death.
Cyanide has rapid physiological effects, with initiation within 15 seconds and cardiac arrest within 6-8 minutes when large enough doses are consumed.
Some antidotes exist, including:
- Cobalt edetate - chelates cyanide to form a relatively non-toxic complex. May however cause convulsions and angioneurotic oedema if mistakenly given when CN is not present, so it has been discarded as a treatment option.
- Amyl nitrite or sodium nitrate - convert haemoglobin to methemoglobin, which binds CN more avidly
- Sodium thiosulphate, which acts as a sulphur donor for the mitochondrial enzyme rhodanese, which converts CN- to thiocyanate, detoxifying it. Needs to be injected, alternative sulphur donors may be more effective. Rhodanese is also associated with longevity in kidney failure patients, leanness and good health. It therefore probably functions to protect the mitochondrial respiratory chain as it shuffles sulphurs.
- Hydroxocobalamin (hydroxyl-vitamin B12)
- Hyperbaric oxygen and first aid support, but no mouth to mouth resuscitation
Cyanide is generated endogenously by the action of myeloperoxidase (produced by neutrophils) on glycine and other amino acids, such as serine.
Cyanide may be another gaseous neurotransmitter alongside NO, CO, and possibly H2S.
Cyanide may be generated by cyanogenic bacteria, so it is a toxin associated with some serious bacterial infections, such as Pseudomonas in cystic fibrosis.
Hydroxyurea is a mild chemotherapy agent. However, it is also produced endogenously - sheep studies found plasma concentration of 30 uM.
Hydroxyurea is unstable and may release cyanide, which may be a source of endogenous CN- production.
Cisapride
Cisapride was used to treat GERD. It acted as a 5-HT4 agonist to increase ACh release in the enteric nervous system and increase gastric motility and emptying.
Withdrawn due to cardiovascular side effects, linked to children’s deaths, LQTS, ventricular arrhythmias and ventricular fibrillation due to interactions with hERG/Kv11.1 channels.
Mosapride is a newer 5HT-4 agonist with similar mode of action without QT prolongation.
Snakes and spiders
The bite of the Brazilian jumping (wandering) spider causes calcium channel blockade, inhibiting glutamate release.
Bite causes tachycardia, paralysis, arrhythmia and convulsions
Can also cause priapism.
α-Latrotoxin is found in the venom of the black widow spider. It causes massive presynaptic release of acetylcholine by synaptic vesicle exocytosis when applied to nerve terminals.
After a person has been bitten, cramps and muscle spasms usually occur within an hour.
Only in the most serious cases do black widow spider bites cause latrodectism, a syndrome consisting of generalised muscle pain, abdominal cramps, profuse sweating, a rise in blood pressure and tachycardia, with death encountered only on very rare occasions, although symptoms can persist for days.
The Banded Krait snake produces Bungarotoxins, such as α-BuTx, which binds to and inhibits postsynaptic nicotinic receptors at the neuromuscular junction.
Useful toxin for isolating nicotinic receptors, as it binds strongly.
α-BuTx targets the peripheral nerves and synapses, causing paralysis of skeletal muscles.
At a molecular level, α-BuTx binds to the α1 nAChR subunit, a mechanism that qualifies it as a curare-mimetic
Similar to tubocurarine, a non-depolarizing competitive antagonist at nAChRs on the motor end plate of the NMJ → musculoskeletal toxicity.
Also similar to coniine from hemlock, which has biphasic nicotine-like effects
In addition to peripheral synapses,α-BuTx is also capable of blocking neuronal cholinergic receptors in the central nervous system, but to a lesser extent.
Lead (Pb)
Lead is a heavy metal and environmental neurotoxin, widely found near mining operations.
Lead is soft and malleable, so it was used for roofing material and pipes, and tetra-ethyl lead was used as an anti-knocking agent in petrol. It also formed part of an alloy in children’s toys.
It has major effects on neuronal migration, differentiation and development.
Lead is able to substitute calcium in a number of biochemical reactions, which is the primary toxic mechanism.
As it is able to replace calcium, lead can accumulate in bones.
Interferes with mitochondrial calcium release, promoting mitochondrial pore transition and apoptotic neuronal cell death.
It has multiple effects on receptor systems, including NMDA antagonism.
Capable of binding to sulfhydryl, amine, phosphate, and carboxyl groups.
Generalised effect occurs through inhibition of membrane Na+/K+-ATPase.
A blood lead concentration of 10 ug/dl leads to reduced cognitive functions and necrosis of the cerebral cortex.
High lead concentrations cause lead-encephalopathy. Symptoms include irritability, headache, mental dullness, memory loss and hallucinations.
Lead can induce autoimmunity. It can also promote macrophage maturation toward the M2 anti-inflammatory phenotype, having immunosuppressive effects.
Lead poisoning may be treated by chelation therapy
- Ca-diNa-EDTA
- Dimercaprol (BAL = British anti-Lewisite)
- Penicillamine - originally used for gold toxicity but useful in a range of heavy metal toxicities
Rimonabant
Rimonabant is an inverse agonist at cannabinoid CB1 receptors, and there have also been reports of antagonism at MORs.
Approved as an anti-obesity drug in many countries (2006), but not USA
It inhibits the cannabinoid system involved in the reward pathway and has anorexigenic effects, causing deceased motivation to eat palatable food. It also stimulates satiating signals in the GIT and inhibits lipogenesis in adipose tissue and the liver.
Withdrawn after 2 years due to depression and suicidal tendencies → high drop-out rate of 50%
Clinical trials data submitted to regulatory authorities showed that rimonabant caused depressive disorders or mood alterations in up to 10% of subjects and suicidal ideation in around 1%
This side effect should have been expected as the cannabinoid system is involved in promoting positive mood → exaggerated pharmacology.
DDT
DDT can accumulate in adipose tissue. It was an early insecticide but it was not pest selective. It acts by delaying the closing of Na+ channels and inhibits K+ channels, therefore causing spontaneous neuronal firing. DDT causes CNS toxicity, including hyperexcitability, tremors, convulsions, respiratory paralysis, heart failure and liver toxicity.
Polonium-210
radioactive metal that acts as an alpha particle emitter, causing DNA damage and carcinogenicity
Lewisite
Warfare gas with blistering properties, smells of geraniums.
Effects include stinging, lacrimation and burning sensations followed by blistering. When inhaled, lewisite causes respiratory distress.
Long-term effects include liver necrosis.
An antidote was developed specifically to counter effects of lewisite. This is British Anti-Lewisite (BAL), also known as dimercaprol, which acts to bind up the arsenic atom.
BAL is also useful in treating a range of heavy metal intoxications, including lead, mercury, and cadmium.
Halothane
Halothane was a volatile anaesthetic introduced in the 50s: non-flammable, well tolerated, pleasant smell, less nausea and vomiting.
Halothane undergoes reductive metabolism by CYP2E1 and is converted to a reactive trifluoroacetyl chloride.
This binds liver microsomal proteins to form trifluoroacetate (TFA) adducts, modifying hepatic microsomal proteins.
Altered liver microsomal proteins can be recognised as a foreign antigen and cause antibody production.
Severe auto-immune hepatitis may occur on second or subsequent exposure to halothane. This leads to immune-mediated hepatic injury, known as halothane hepatitis.
Azide
Azide, from sodium azide, NaN3, is an explosive gas that is the propellant in airbags.
Azide is a respiratory poison and inhibitor of cytochrome oxidase, acting as a metabolic poison.
It also acts as a strong stimulator of cGMP generation, leading to a dramatic fall of blood pressure on exposure. This causes collapse and death.
Cidofovir
Cidofovir is an antiviral drug that can accumulate in proximal tubules of the nephron via OAT-1 transporters and cause apoptosis.
Probenecid can be used to reduce renal toxicity by inhibiting this transporter.
Monocrotaline
Monocrotaline is a pyrrolizidine alkaloid from ragworth.
It enters hepatocytes via the OATP1 transporter.
It is then activated to monocrotaline pyrrole by CYP3A4.
Monocrotaline pyrrole is an alkylating agent. It is a hard electrophile that binds to glutathione, lysine residues, and DNA.
It causes intrahepatic haemorrhage, centrilobular tissue necrosis, cirrhosis and liver failure up to 2 years after exposure.
May cause liver cancer.
The lung is a secondary target for monocrotaline toxicity. The reactive metabolite may be transported from the liver or metabolic activation may also occur in the lung.
Major concern for the health of livestock.
MPTP
MPTP produces Parkinson’s-like toxicity, including tremor and bradykinesia. Affected patients had an immediate response to L-Dopa administration.
Produced as a contaminant of synthetic heroin, MPPP.
MPTP is highly lipophilic and can cross the BBB. Once in the brain, it enters astrocytes and is converted to MPP+ by MAO-B.
MPP+ is a substrate for for dopamine uptake transporters (DAT), so it selectively taken up into dopaminergic neurons in the substantia nigra.
MPP+ then concentrates in the mitochondria and inhibits complex I of the respiratory chain, leading to a fall in ATP and free radical generation.
Vesicular dopamine is displaced into the cytoplasm, leading to further ROS generation.
MAO-B inhibitors, like selegiline, reduce MPTP toxicity if administered early on.
Allylamine and acrolein
Allylamine is an atherogenic amine characterised as a model of chemically induced atherosclerosis.
A single dose of allylamine can cause a lifetime of atherosclerosis
Allylamine may be a causative factor in toxic oil syndrome from adulterated rapeseed oil, which occurred in Spain in the 80s.
Acute allylamine exposure causes subendocardial necrosis, vasospasm and vascular smooth muscle proliferation.
Allylamine is metabolically activated to acrolein and hydrogen peroxide by semicarbazide-sensitive amine oxidase (SSAO), a vascular-specific enzyme found in smooth muscle. Relatively high SSAO levels are also found in human coronary arteries.
Acrolein, a reactive metabolite, causes vasospasm in coronary arteries, which looks like a heart attack. It is associated with hypercontraction.
Acrolein can also cause decreased pulmonary function, pulmonary oedema and chronic respiratory disease.
Bladder toxicity also occurs.
Acrolein can also be produced as a reactive metabolite of the chemotherapeutic drug cyclophosphamide, which is activated by CYPs to acrolein and the active chemotherapeutic agent, phosphoramide mustard.
Recent studies have shown that acrolein is formed endogenously in biological systems by myeloperoxidase (MPO) mediated oxidation of threonine.
Acrolein may also be produced via oxidation of polyamines, which are found in high concentrations in the lungs.
Nicotine
Nicotine is found in cigarette smoke and in E-cigarette liquid and presents a significant toxic hazard.
Highly toxic: oral LD50 dose of 30–60 mg (0.8–1 mg/kg) is considered to be lethal for an adult.
Similar toxicity to potassium cyanide
Nicotine is an agonist of nAChRs which are present in the central and autonomic nervous systems, and the neuromuscular junction.
At low doses nicotine causes stimulatory effects on these receptors, predominantly seen as cardiac stimulation.
However, at high doses nicotine inhibits the CNS inducing hypotension and bradycardia followed by convulsions, coma and neuromuscular blockade with respiratory muscle paralysis, leading to death by asphyxia.
At higher doses, nicotine’s overactivation of the neuromuscular junction can cause a curare-like effect, leading to paralysis and respiratory failure. Death, typically occurring within an hour of ingestion (and sometimes within minutes), is usually attributed to paralysis of the respiratory muscles or circulatory collapse.
Cerivastatin
Statins inhibit HMG-CoA reductase and have a cholesterol lowering effect, particularly on LDL cholesterol.
Some statins are associated with muscle weakness (myalgia) and rhabdomyolysis.
Rhabdomyolysis leads to the release of creatine kinase into the bloodstream and can cause acute kidney injury. Renal failure may then manifest as cardiovascular complications, such as QT prolongation and sudden death.
Cerivaststatin, developed by Bayer, was withdrawn after links to rhabdomyolysis and kidney failure. Bayer was sued after unpublished documents that identified cerivastatin-induced myopathy.
Simvastatin may also cause rhabdomyolysis, but not as badly so it is still available.
Statin toxicity is linked to mitochondrial complex 1 inhibition and skeletal muscle ryanodine receptor (RyR) activation.
Cerivastatin was particularly potent at activating ryanodine channels in the 100-250 nM range, effects even demonstrated at 10 nM.
CYP3A4 metabolises statins but it is inhibited by the calcium-channel blocker amlodipine, increasing the plasma concentration of simvastatin. This causes a risk of toxicity.
Flosequinan
Flosequinan was approved in 1992 for the treatment of heart failure, marketed as a peripheral venous and arterial dilator that was paralleled by a reduction in “cardiac wall stress”.
Later studies reported higher levels of plasma noradrenaline, a non-specific stress biomarker, which should have been predictive of poor clinical outcomes.
It improved well-being and exercise tolerance and it was the first oral agent to augment effects of digitalis (Na+/K+ ATPase inhibitor for arrhythmia), diuretics and ACE inhibitors.
It however had a tendency to increase heart rate and enhance cardiac contractility → cardiotoxicity.
The PROFILE trial was conducted 24 years later in 2017 to evaluate the effectiveness of Flosequinan in heart failure. It found that mortality increased compared to placebo, despite patients feeling like it improved their quality of life.
Other halogenated hydrocarbons
Other halogenated solvents are also nephrotoxic.
Trichloroethylene causes nephrotoxicity via toxic cysteine conjugates.
Initially conjugated with glutathione, which normally detoxifies compounds, but this is then chopped up by CS-lyase in the kidney to a cysteine residue, which is subsequently converted to a reactive metabolite.
1,2-dichloroethane produces a sulphur half mustard in a similar reaction
This is an example of glutathione conjugation actually leading to enhanced toxicity via unusual metabolic routes.
Autoimmune reactions involving CYP2E1 play an important role in idiosyncratic liver injury induced by halogenated hydrocarbons.
Aconite (Wolfsbane)
Aconite, also known as Wolfsbane and Monkshood, is a widespread garden plant from the buttercup family.
It looks like horseradish or parsley when flowers are not present.
It is used in Chinese herbal medicine.
Aconite can be absorbed into the systemic circulation through the skin, so toxicity can also occur following gardening without gloves.
Aconitum plants have shown their pharmacological properties such as anti-inflammatory, analgesia, and anti-rheumatism properties.
Aconite is highly toxic to the heart and nervous system, as it produces aconitine.
Aconitine blocks the inactivation of VGSCs at the resting membrane potential, causing sustained Na+ influx. This leads to cardiac arrhythmia due to sustained neuronal firing.
Blockade of hERG potassium channel alongside nicotinic receptor blockade further contributes to toxicity.
Following ingestion, gastrointestinal disturbances occur (NVD), alongside neurologic effects, including numbness and paraesthesia.
This is followed by CNS depression, respiratory depression, seizures and cardiovascular symptoms, including hypotension, palpitations, ventricular tachyarrhythmias and fibrillation, and life threatening arrhythmias. Asphyxia may also occur.
No specific antidote, but local anaesthetics like lidocaine (VGSC blocker), and magnesium add-on have proved useful.
Chloroform
Chloroform (CHCl3) was used as an anaesthetic. It was later withdrawn as it caused too many deaths due to respiratory depression.
It is also used in industrial processes as a chemical intermediate and as a solvent.
Acute exposure causes unconsciousness through GABA receptor stimulation. This leads to overall inhibition of neuronal firing.
Chloroform also inhibits sodium channels on glutamatergic neurons, inhibiting glutamate release and excitatory neurotransmission.
Other receptors are affected, such as glycine and nicotinic acetylcholine receptors.
Death can occur through respiratory depression.
The primary cellular target of chronic chloroform exposure is the proximal tubule of the nephron, but it is also hepatotoxic.
CYP metabolism causes the production of the reactive metabolite phosgene, which can covalently bind nucleophilic groups on cellular macromolecules.
Proteinuria, glucose in urine and increased BUN levels are seen
Classified as a possible human carcinogen.
Strychnine
Strychnine is a highly toxic alkaloid derived from Strychnos nux vomica (Turkish rhubarb), allegedly used in small quantities as an athletic performance enhancer and also used as an arrow poison and a pesticide.
It inhibits glycine receptors in the spinal cord, inhibiting the influx of Cl- and so causing increased neuronal activity and muscle spasms. This eventually causes respiratory failure and convulsions which can lead to death.
Treatment is achieved by anticonvulsants, such as phenobarbitone and diazepam, or other hypnotics/anaesthetics, as these enhance inhibitory GABA neurotransmission.
Additional toxins from food
Patulin from mouldy apples, produced by Penicillium moulds, can cause liver toxicity.
Castor oil beans produce ricin, which can inactivate the 6S ribosomal subunit, inhibiting protein synthesis. This can lead to organ dysfunction, nausea, vomiting, and death.
Urushiol, found in poison ivy and in small quantities in mangoes, can cause irritant dermatitis.
Menadione (Vitamin K3)
Vitamin K is an essential cofactor for blood clotting, so deficiency can lead to hemorrhagic disorders and fatal bleeds in infants, particularly in the brain.
This was previously treated with an intramuscular injection of menadione (vitamin K3). However, this led to several infant deaths.
Menadione mediates superoxide (oxygen radical) generation in redox cycling.
Reductase enzymes form free radicals, which react with free oxygen species to form superoxide, or with glutathione to deplete stores.
Superoxide radicals are converted to hydrogen peroxide by superoxide dismutases (SODs), which can cause thiol oxidation, affecting cellular signalling and causing glutathione depletion.
H2O2 can also react with Fe2+ to produce hydroxyl radicals, which cause cellular damage.
H2O2 can be removed by glutathione peroxidase (GPX), peroxiredoxin (PRX) and catalase.
Vitamin K1 is given now.
Ranitidine
Ranitidine, a histamine H2 receptor antagonist used to reduce gastric acid secretion, was produced to counter problems with the precursor cimetidine, which acted as a P450 inhibitor.
However, it was withdrawn after being found to decompose to a nitrosamine (N-Nitrosodimethylamine), which is carcinogenic, associated with liver carcinogenicity.
Following P450-mediated activation to a reactive ethanolamine, particularly through ethanol-induced CYP2E1, DNA adducts are formed. It is therefore genotoxic.
N-Nitrosodimethylamine is also generated in food treated with nitrite, such as cured meat products including bacon, and found in cigarettes and some condoms.
Hemlock
Hemlock has white flowers and stems with purple spots.
It is easily mistaken for other edible plants of the carrot family and the flowers may be mistaken for elderflowers.
Conium alkaloids produce a biphasic nicotine-like effect, including salivation, mydriasis and tachycardia, followed by bradycardia.
Coniine, found in hemlock, can also produce rhabdomyolysis by either a direct toxic effect on skeletal muscle or a strychnine-like proconvulsant action on the central nervous system.
Chronic toxicity may result from eating hemlock poisoned birds, usually seen as renal failure from rhabdomyolysis.
Muscle wasting and rhabdomyolysis may also lead to the release of mitochondrial DNA from muscles, which is circular and can therefore be mistaken by the immune system for foreign DNA, potentially causing an immune reaction.
Metal salts
Barium carbonate, used in error to barium sulphate as a radiological contrast media, is a potassium channel blocker leading to GI toxicity and convulsions
In barium sulphate, barium is locked up as an insoluble precipitate.
A hospital used barium carbonate in error in the 50s, which releases barium as a free ion and allows it to block potassium channels
Lithium and gold salts can cause renal toxicity
Thalidomide
Thalidomide developed in 1957 as a sedative and anti-emetic particularly for morning sickness in the early stages of pregnancy.
Early reports of birth defects affecting 10-20k cases with 40% deaths around time of birth led to withdrawal of thalidomide in 1961.
Thalidomide inhibits the production of tumour necrosis factor (TNF) and other cytokines.
TNF promotes angiogenesis. Thalidomide therefore inhibits angiogenesis, an action linked to its teratogenic potential.
Thalidomide is also used as a treatment of arthritis, but it can cause peripheral neuropathy, mostly affecting sensory nerves, reversible after drug suspension.
Thalidomide was reintroduced for treatment of leprosy, and as an antitumor agent, where anti-angiogenic action is a useful therapeutic approach. Sedative and antiemetic effects provide additional benefit, avoiding the classic side effects of chemotherapeutic agents.
Teratogenicity might be picked up today by screenings
- In vitro models that use human pluripotent stem cells (hPSCs) to create 3D structures that mimic the early embryo.
- Animal-based models using rats.
Paracetamol
The paracetamol intermediate metabolite NAPQI is responsible for hepatotoxicity.
After low level doses (< 4 g/day), paracetamol is converted to glucuronide and sulphate conjugates (95%), with minor levels of NAPQI produced, or excreted unchanged.
At supra-therapeutic doses (> 4 g/day), the sulfation pathway is saturated, glucuronidation and oxidation increase, and a smaller amount is excreted unchanged.
After toxic doses (> 7-10 g), glucuronidation saturates as well and higher proportions oxidise to NAPQI via CYP450 metabolism.
Excess NAPQI eventually depletes GSH stores and starts to form protein adducts by binding cysteine groups on cellular proteins.
The patient is decontaminated with active charcoal. If required, there is further treatment with N-acetylcysteine, which works by replenishing hepatic glutathione, aiding in the metabolism of NAPQI.
Metacetamol, a meta-substituted version of paracetamol, is converted to reactive metabolites, but does not cause liver necrosis as glutathione depletion does not occur, so it is non-toxic.
Some formulations of paracetamol include the antidote, N-acetyl cysteine.
Phenacetin was an early analog of paracetamol, withdrawn for causing renal toxicity and cancer.
Carbon monoxide
Carbon monoxide (CO) is a colourless, odourless gas that is produced as a product of incomplete combustion of carbon or hydrocarbons.
CO binds ~220x as tightly to haemoglobin compared to oxygen, forming carboxyhaemoglobin (COHb). The effect is more pronounced at low oxygen levels.
The blood may be saturated with oxygen but we cannot utilise it.
CO binds to Fe2+ in myoglobin and components of the mitochondrial respiratory chain, inhibiting cytochrome oxidase.
This leads to oxygen radical production, oxidative stress, lipid peroxidation, and an inflammatory cascade, causing neurological effects.
Blood parameters: high lactate, but oxygen saturation is normal
Best treatment option is hyperbaric oxygen.
CO is generated endogenously as a byproduct of the action of haeme oxygenase on haeme during its breakdown into bilirubin.
At low concentrations, CO has a cytoprotective role through stimulation of mitochondrial biogenesis.
Symptoms of carbon monoxide poisoning vary with blood CO%, starting with headache and progressing to impaired judgement, shortness of breath, drowsiness, confusion and unconsciousness eventually causing death.
CO poisoning can be recognised by the cherry-red skin colouration it produces, alongside unconsciousness.
Survivors of CO poisoning can experience a range of long-term consequences, from severe brain damage (uncommon) to more prevalent but less severe persistent problems. The most common neurological issues include depressed mood, short-term memory impairment and difficulty concentrating. Loss of consciousness during CO exposure is a risk factor for long-term neurological effects.
Death Cap (Amanita Phalloides)
Amanitin and Phalloidin are both found in death cap mushrooms.
These can enter the liver via OATP1 bile transporters.
Phalloidin enters hepatocytes via the OATP1B1 transporter and causes cholestasis and haemorrhagic necrosis. Toxicity is limited by slow absorption from the GIT.
Amanitin is water soluble and rapidly absorbed from the GIT, stable to heat/cooking and not extensively metabolised. It enters the liver through OATP1B3.
Amanitin causes centri-lobular necrosis and intrahepatic haemorrhage. Renal toxicity is also reported.
Mechanism of toxicity involves inhibition of RNA polymerase II and so blockade of protein synthesis.
Causes necrosis rather than apoptosis as apoptosis is an active process dependent on protein synthesis.
Other features are apoptosis following loss of mitochondrial membrane potential, oxidative stress and lipid peroxidation.
Estimated human LD50 of 0.1 mg/kg (about 1 mushroom)
Poisoning can be treated with:
- diuretics and dialysis to flush the compounds out
- antioxidants like N-acetylcysteine
- silibinin - OATP1B3 inhibitor to stop liver permeation
- Rifampicin - OATP1 inhibitor
OATP inhibitors may cause cholestasis but this is not as big of an issue as it is not accompanied by necrosis and haemorrhage.
The toxic effects of amanitin are beginning to be exploited for therapeutic purposes in oncology.
Achievement of the total synthesis of amanitin in recent years has allowed for the development of new amanitin-conjugated antibodies targeting certain tumor characteristics. Since amanitin is no longer a substrate for OATP1B3 when coupled to antibodies, the conjugated antibodies allow targeting of a precise cell population while limiting non-selective toxicity.
Glyphosate
Glyphosate is a popular herbicide that can act as a renal toxin when ingested or inhaled. This includes acute kidney injury, chronic kidney disease and possibly renal fibrosis.
Saxitoxin and paralytic shellfish poisoning
Saxitoxin (STX) is a potent neurotoxin.
Ingestion of saxitoxin by humans, usually by consumption of shellfish contaminated by toxic dinoflagellate algal blooms, is responsible for the illness known as paralytic shellfish poisoning (PSP).
Algal blooms are red → cause ‘red tides’
STX inhibits Na+ channels to produce respiratory and general paralysis by blocking nerve conduction.
Effects include: tingling sensation to lips, gastrointestinal problems (nausea, vomiting, diarrhoea) breathing difficulties, leading to complete paralysis, respiratory arrest and death.
Radon
Radon has been identified as the second leading cause of lung cancer after cigarette smoking.
Radon is a highly radioactive inert gas, colourless and odourless, dense, with no obvious signs of exposure.
It arises from the radioactive decay of the radioactive heavy metals Uranium-238 and Radium-226.
Radon-222 is the principal isotope of concern, with a half life of 3.8 days.
Radon may accumulate in basements in areas like Cornwall, where background levels of radioactive elements are high. It is a risk in mining areas with poor ventilation, like Cornish tin mines.
Radon is an alpha particle emitter, so when inhaled the alpha particles are exceptionally destructive, leading to a widespread oxidative stress that is accompanied by DNA strand breaks and carcinogenesis.
Radon decomposition can also release the metal Polonium-210, another alpha particle emitter.
Diethylenetriamine is a useful treatment option.
Aluminium
Aluminium is an example of light metal toxicity.
In Cornwall, 20 tons of aluminium sulphate were inadvertently added to the water supply, raising the concentration to 3,000 times the admissible level.
Stomach cramps, skin rashes, diarrhoea, mouth ulcers and aching joints were reported with one death
Aluminium interacts with other metals, so this could be due to a combination of multiple heavy metal toxicities. For example, copper could have been stripped out of pipes.
Human intake of up to 30 mg day can occur from cooking utensils, saucepans, drinking water, and cosmetics - up to 0.1% is absorbed
Aluminium disrupts the actions of other metals, like Ca, Mg and Fe, leading to neurodegeneration, iron accumulation and oxidative stress. Aluminium stimulates iron-initiated lipid peroxidation.
Aluminium accumulates in the hippocampus and frontal cortex and is a contributing factor in pathogenesis of neurodegenerative disease, encephalopathy and seizures.
Aluminium has a biphasic effect on cholinesterase, causing an increase early on and a decrease later on.
Aluminium increases glutamate, causing excitotoxicity.
There is a suggested link between aluminium and Alzheimer’s disease, but this is controversial.
Unknown if there are clear effects caused in the brain.
Aluminium has been shown to promote the aggregation of amyloid-β (Aβ)
TGN1412
TGN1412 is a T-cell CD28 superagonist, so it stimulates T-cell activation and proliferation without requiring T-cell receptor activation.
This mechanism was thought to hold therapeutic potential for autoimmune diseases and cancers.
It was tested in phase I clinical trials in 6 volunteers. A first infusion dose 500 times smaller than that found safe in animal studies was used, given to all volunteers at the same time.
All volunteers faced life-threatening conditions involving a cytokine storm, a massive inflammatory response leading to multi-organ failure for which they were moved to the intensive care unit.
Pre-clinical testing of TGN1412 was carried out in cynomolgus macaques (primates), at doses of up to 50 mg/kg for 1 week. This was well tolerated with no signs of toxicity or systemic immune stimulation.
- Major species difference in immune response
- The assumption of 100% CD28 receptor homology between the primate species used in preclinical studies and humans was not supported by citations in the trial documentation, and later reviews revealed sequence differences of up to 4%.
In in vitro tests, no immuno-stimulatory response was seen when TGN1412 was simply mixed in solution with PBMCs. TGN1412 only stimulated a pro-inflammatory cytokine response in vitro if appropriately presented to PBMCs, e.g. by immobilisation onto a plastic surface.
An appropriate in vitro test could have actually alerted the trial coordinator of a possible immunotoxicity problem, before animal testing was even carried out.
Tienilic acid
Tienilic acid (1979) was a diuretic containing a thiophene group (sulphur ring) used for the treatment of hypertension with a uric acid lowering effect.
- Thiophene groups seem to be an indicator of toxicity potential → Methapyrilene
Withdrawn due to immuno-allergic hepatitis and acute renal failure
- Short half-life of 6h, concentrating effect of the kidney contributes to toxicity.
Activated by CYP2C9 and CYP2C19 to thiophene-s-oxide and thiophene epoxide leading to suicide inactivation of CYP2C9. Antibodies directed against CYP2C9 are formed, leading to allergic hepatitis.
There is also additional evidence of tienilic acid-induced cholestatic liver damage.
Arsenic
Arsenic is a metalloid, but its toxicity is similar to heavy metal poisoning.
Arsenic is found in drinking water, especially in mining areas as arsenate (AsO43-) and arsenite (AsH3O3).
Part of Lewisite.
Historically used as a skin lotion.
Early organic arsenicals were used as antibiotics → Salvarsan for Syphilis.
Arsenate replaces phosphate groups in ATP-mediated reactions.
The metabolite arsenite has a high affinity for SH groups, so it depletes glutathione and promotes oxidative stress.
Arsenic causes red or swollen skin, hyperpigmentation and hyperkeratosis, later leading to squamous cell carcinoma.
Arsenic can also cause pneumotoxicity if inhaled, manifesting as bronchitis.
Arsenic may be a useful chemotherapeutic agent for leukemia as it was found to lower WBC count, suppressing T and B cell proliferation and macrophage maturation. Arsenic trioxide (ATO) in an oral formulation is used to treat acute promyelocytic leukemia (APL). Immunosuppression can however also lead to characteristic arsenic induced keratosis and squamous cell carcinoma.
Tolerance can develop to arsenic as antioxidant defences are upregulated.
Aflatoxins
Aflatoxins are fungal toxins (Mycotoxins) produced by Aspergillus Flavus moulds that grow on nuts, corn, wheat, peanuts and rice.
These are potent hepatic carcinogens.
A minimal level of exposure is considered unavoidable - the EU limit is set at 8-10 ug/kg in food.
Aflatoxins include aflatoxin B1 (AFB1)
Aflatoxin is metabolised by CYPs to the reactive metabolite aflatoxin epoxide, which binds to guanine residues.
Glutathione can offer some protection by detoxifying the epoxide.
Binding to guanines on DNA disrupts DNA structure and function, leading to mutations and genomic instability. This causes cancer through genotoxic effects.
A characteristic mutation caused by epoxide guanine adducts is a G→T transversion in the p53 tumour suppressor gene.
AFB1 exposure can induce oxidative stress by generating reactive oxygen species, further contributing to DNA damage and cellular dysfunction.
Other aflatoxin metabolites, including aflatoxin di-hydrodiol and aflatoxin dialdehyde, may form lysine adducts, so there is also non-genotoxic carcinogenicity.
Symptoms of toxicity include abdominal pain, emesis, steatosis, liver necrosis and carcinoma, convulsions, teratogenicity.
Aflatoxins also cause immunosuppression by inhibiting proliferation of T and B lymphocytes and NK cells. This contributes to their carcinogenic potential.
Ciguatera fish poisoning
Ciguatoxins are produced by dinoflagellate algae species.
Dinoflagellates are a group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered protists.
Ciguatoxins are potent activators of sodium and/or calcium fluxes, acting as voltage-dependent Na+ channel openers. This leads to excess nerve activation.
Extremely toxic, LD50 of 0.45 μg/kg intraperitoneal
The closely related Maitotoxin primarily targets VGCCs instead, and it is even more toxic with an LD50 of 0.15 μg/kg
Brevetoxins from Karenia brevis also activate VGSCs but cause milder symptoms symptoms with loss of motor control and muscle pain.
Symptoms involve nausea, vomiting, diarrhoea, convulsions, breathing difficulties, asphyxia and death
Calcium channel blockers, such as gabapentin, are used as treatment
Asbestos
Asbestos belongs to a large group of fibrous minerals closely related to serpentine and talc.
The fibrous nature of these minerals dictates their toxicity. The length of the fibres is a critical component in asbestos toxicity.
Asbestos is a generic term for a range of hydrated silicate fibrous minerals.
Asbestos was widely used from the 19th century as a fire retardant material. As a result many historical buildings contain substantial amounts of asbestos.
Asbestos was extensively mined in the town of Asbestos (Canada). Today the residents have an extraordinarily high rate of mesothelioma, a type of cancer that occurs in the thin layer of tissue that covers the majority of your internal organs, the mesothelium. It often affects the lung mesothelium. Disease has a long latency period after initial exposure (30-50yr).
Asbestos causes pneumotoxicity including pulmonary fibrosis (asbestosis), malignancies, bronchogenic carcinoma and mesothelioma.
The iron content of asbestos minerals contributes to toxicity by facilitating the Fenton reaction, which produces hydroxyl radicals, causing oxidative stress including lipid peroxidation, antioxidant depletion and DNA damage.
The asbestos fibre can be successfully ingested by lung macrophages, where the fibre is cleared from the lung into the lymph system.
Fibres longer than the macrophage (20-25 µm) cannot be engulfed, leading to frustrated phagocytosis resulting in macrophages secreting lysozymes, acid hydrolases and reactive oxygen metabolites, leading to fibrosis and chronic inflammation.
Macrophages are long-lived, so they will continuously churn out reactive oxygen species and inflammatory mediators.
Some macrophages may be removed by the mucociliary escalator in phlegm.
Others, where the fibre is not properly ingested, continue to generate oxygen radicals and inflammatory mediators.
Half-life of asbestos can be 30-50 years due to the long life of macrophages
Generation of oxygen radical and inflammation can lead to autoimmunity.
The persistent inflammation causes a mutagenic environment.
ROS can cause DNA damage. Studies suggest that mutations in the BAP1 gene, a tumour suppressor gene, play a crucial role in asbestos-induced mesothelioma.
Asbestos fibres can physically interact with chromosomes during cell division, causing chromosomal breaks, fragments (micronuclei), and aneuploidy.
The reactive surface of asbestos fibres can adsorb carcinogenic molecules from the environment.
Asbestos related mechanisms may apply to lung toxicity of other fibrous particles, such as soot, nanoparticles (e.g. cerium oxide) and micro/nano-plastics.
Microcystin
Microcystin, toxins from blooms of blue-green algae, also referred to as cyanobacteria (Microcystis Aeruiginosa), comprise a cyclic penta-peptide, of which microcystin-leucine arginine (MC-LR) is the most studied.
These accumulate in the liver via the OATP-1 bile transporter
Cause toxicity to fish, humans and domestic animals and are a major risk in water with agricultural runoff.
They are potent inhibitors of protein phosphatases PP1 and PP2A, leading to cytoskeletal effects, apoptosis, liver failure and sometimes hepato-carcinoma.
There is unregulated phosphorylation of substrates, such as p53 and MAPK, disrupting cellular signalling. ROS are also created.
Mycocystins also cause renal, cardiac, and reproductive toxicity.
Rifampicin (treats mycobacterial infections) blocks OATP-1 so this is useful in the treatment of Microcystin poisoning
This may cause cholestasis, another form of toxicity, but this is better tolerated
Cyanobacteria also produce BMAA, a CNS toxin that causes excitotoxicity.
β-Methylamino-L-alanine (BMAA)
Cycad seeds and most cyanobacteria (blue-green algae) may produce β-methyl-amino alanine (BMAA), which can cross the BBB and act as an agonist at glutamate receptors, causing excitotoxicity.
Cycads found in pacific regions are seeds used for flour in cooking.
Cycad seeds cause bioaccumulation in bats
Cyanobacteria responsible for bioaccumulation in shellfish and other marine animals higher in the food chain, such as sharks → shark fin soup consumption causes toxicity in humans
BMAA may also be incorporated into proteins in place of serine
This causes protein misfolding, showing a possible link to Alzheimer’s
BMAA is linked with amyotrophic lateral sclerosis (ALS), a neurodegenerative disease.
Consequences of BMAA toxicity include:
- Neurodegeneration
- Limb muscle atrophy
- Behavioural dysfunction
BMAA excitotoxicity can be antagonised by the NMDA antagonist MK-801
Poisonous plants with therapeutic use
Foxglove (Digitalis) inhibits the Na+/K+ ATPase pump. It is used to treat congestive heart failure (CHF) and atrial arrhythmias as it increases the force of heart contractions.
The opium poppy contains morphine and papaverine
Morphine is a μ-opioid receptor agonist used as an analgesic
Papaverine is a phosphodiesterase (PDE) inhibitor acting as a vasodilator, used in the treatment of vasospasms and occasionally erectile dysfunction and acute mesenteric ischemia.
Calabar beans contain the active substance physostigmine (eserine), a cholinesterase inhibitor. Physostigmine can be used to treat glaucoma, constipation, and myasthenia gravis, and to reverse neuromuscular blockade.
Aconitine blocks the inactivation of VGSC and inhibits hERG potassium channels, increasing neuronal firing. It also blocks nicotinic receptors. Aconite plants have shown their pharmacological properties such as anti-inflammatory, analgesia, and anti-rheumatism properties.
While primarily an eye irritant, capsaicin has shown neuroprotective effects on retinal ganglion cells (RGCs). TRPV1 activation protects RGCs from damage caused by NMDA-induced excitotoxicity, potentially offering a therapeutic avenue for retinal diseases like glaucoma.
Opioids
Opioids are MOR agonists used as analgesics but they pose a risk of respiratory depression.
Opioid-induced respiratory depression can occur through the suppression of respiratory drive by interacting with neurons in the pons and medulla.
The risk of respiratory depression is greater with classic full mu-opioid receptor agonists, such as morphine, hydrocodone, and oxycodone. There is a major problem with newer fentanyl derivatives that act as ultra-potent opioid agonists.
Evidence suggests that the respiratory depression caused by opioids may also depend partly on antagonism at NMDA receptors in the pons and medulla.
Opioid effects can be reversed with Naloxone, an antagonist.
Morphine derivatives can also cause pseudo-allergy by causing the release of histamine from mast cells, without initiating an immune response.
Phosgene
Phosgene is a dense gas that tends to hang around at ground level and smells of cut grass.
It initially causes choking. Pulmonary irritant, leading to pulmonary oedema.
Highly reactive with NH2, OH and SH groups.
Phosgene is also nephrotoxic and implicated in the nephrotoxicity of chloroform, binding macromolecules and causing proximal tubule injury. It is also hepatotoxic and a possible carcinogen.
Used as a chemical intermediate for manufacture of polyurethane, polycarbonates and in insecticide manufacture, such as carbaryl insecticides. Insecticide manufacture also involves the production of methyl isocyanate.
It is a metabolite of chloroform and carbon tetrachloride, which are CNS neurotoxins.
Poisonous plants with therapeutic use - microtubule poisons
Western Pacific Yew contains taxol (paclitaxel). It is used as a chemotherapeutic agent. It induces the formation of stable bundles of microtubules, thereby interfering with cellular division. Peripheral neuropathy is however a side effect.
Rosy Periwinkle contains vincristine and vinblastine, which are also microtubule poisons that bind to tubulin and inhibit cell division, used as chemotherapeutics.
These also cause peripheral neuropathy, which can manifest as paraesthesia, neuropathic pain, motor nerve damage and orthostatic hypotension due to autonomic dysfunction.
Gastrointestinal side effects also occur, including constipation, abdominal cramps and nausea and vomiting.
Hydrogen sulphide (H2S)
H2S is a highly toxic gas with a rotten egg smell.
Toxicity is of a similar magnitude to hydrogen cyanide and the mechanism of toxic action is also similar, occurring through inhibition of cytochrome oxidase. It can however unbind from this, unlike cyanide, so it is slightly less toxic.
H2S also acts as a K+ channel opener and an inhibitor of phosphodiesterase-5 (PDE5).
H2S effects go beyond mitochondria, directly harming neurons and disrupting cardiac ion channels, potentially causing dysrhythmia.
H2S is produced endogenously by the action of cystathionine-ꞵ-synthase (CBS) and cystathionine-𝛾-lyase (CSE).
Antidotes:
- Amyl nitrite (inhalation) or sodium nitrite (IV). Nitrites convert haemoglobin to methaemoglobin (Fe3+), which binds H2S to form sulfhaemoglobin, a green pigment. Nitrite may also displace H2S from cytochrome oxidase.
- Hyperbaric oxygen
- Cobinamide, a precursor of hydroxocobalamin (vitamin B12 analogue)
- Vasopressor agents, like noradrenaline or phenylephrine, can also be used as H2S decreases blood pressure through potassium channel opening.
A little hydrogen sulphide is good for you. Dietary sources include durian fruit, garlic and broccoli. These act as sulphide donors, associated with health benefits.
Hydrogen sulphide has also been found to cause erections, so it could lead to the development of a new impotence drug. This is caused through PDE5 inhibition, leading to an increase in cAMP production. Sildenafil (Viagra) is also a PDE5 inhibitor.
3-Methyl indole
3-Methyl indole, also known as 3MI or Skatole, has a faecal odour at high concentrations.
3MI is generated endogenously as a fermentation product of tryptophan by Lactobacillus in ruminants following abrupt change in feeding conditions from poor to lush pasture.
Toxin in the diet of livestock, so more of a concern for the agricultural industry than for humans.
LD50 of 40 mg/kg in goats and 800 mg/kg in pigs, showing species differences.
It is also found as a pyrolysis product of tryptophan in cigarettes, and food and water sources can also occasionally become contaminated.
3MI causes acute pulmonary oedema and emphysema, primarily targeting Clara cells.
3MI is also activated to reactive metabolites by CYPs and prostaglandin-H-synthase.
Large immune cell involvement
Aspirin administration can reduce 3MI toxicity in goats
High activity of Prostaglandin-H-Synthase in the lung may arise from the large population of immune cells in the airways.
It is a possible human pulmonary carcinogen through generation of 3-methylindolenine, which may form nucleoside and DNA adducts → genotoxic.
Can undergo nucleophilic addition to guanine, adenine and cytosine nucleosides
Could also form adducts with DNA resulting in the single-strand breaks in primary human lung cells. The DNA damage could be observed with as low as 100 nM, implying that the skatole in cigarettes may be a potential lung carcinogen.
Can up-regulated lung CYP1A1 and CYP2F1 expression through aryl hydrocarbon receptor (AhR) activation.
Skatole was also demonstrated to exhibit AhR agonist activity in primary human hepatocytes.
3MI is used industrially as a fixative in perfumery, with notes of jasmine.
Compounds often smell very different in a concentrated vs very dilute forms → jasmine smell when very dilute
