Toxicology

Steps in development of toxicity

*1. Delivery from site of exposure to target* a. Absorption b. Activation (metabolic) *2. Reaction of toxicant with target* a. Covalent/Non-covalent binding b. Generation of free radicals c. Electron transfer *3. Cellular dysfunction* a. Altered gene expression b. Altered signal transduction/production c. Altered enzymatic/protein activity *4. Failure to (or overwhelmed) repair can lead to damage of:* a. Proteins b. DNA (RNA) c. Lipids

**KNOW** *Lead's effects on vitamins*

*Lead interferes with Vitamin D synthesis/calcium, Heme synthesis/iron*

Some chelation agents also cannot completely block metal from biological targets Stable complex vs basket complex

*in stable complex - you get complete shielding of the mercury/lead/other metal inside the chelator and thus bio-molecules which like to come in and cause issues/react with the metal CANNOT GET IN AT ALL* *in basket complex - you get incomplete shielding of the mercury/lead/other metal inside the chelator and thus bio-molecules which like to come in and cause issues CAN STILL GET IN*

**KNOW** *Nervous system effect of lead toxicity*

*lead affects the CNS, though individuals may be asymptomatic* - impaired concentration, headache, encephalopathy, ↓intelligence, altered neurobehavioral development, cognitive decline (children) • PNS: Neuropathy (motor weakness, upper extremities)

What is oxidative stress?

- an alteration of REDOX balance - a disturbance in the pro-oxidant vs antioxidant balance in favor of pro-oxidants, leading to potential damage. IMPLICATED IN MANY TOXICITIES AND DISEASES

*Toxin: Cyanide* What does it interfere with? Where does cyanide come from? What does it do (mechanism)? What is the antidote?

-*Cyanide interferes with ATP generation* -Cyanide comes from chemical synthesis, metal plating, *apricot pits, cassava (edible root)*, and other plants/seeds, nitroprusside, amygdalin, burning plastics releases HCN (hydrogen cyanide), and lastly nitriles can be converted to CN as well Mechanism: Cyanide is a chemical asphyxiant. It binds to and inhibits Cytochrome oxidase --> preventing aerobic respiration **Know: specifically cyanide binds to the Fe3+ in cytochrome a3) The antidote includes cyanogenic glycoside(s) - which will prevent the immediate onset (minutes to hours) of cyanide effects that would occur without the antidote HCN gas (>50 ppm) or >200mg cyanide salts will cause death

*Toxin: Parathione (and other organophosphate/carbamate insecticides)* What is it? What does it do (mechanism)? What is the crisis that occurs with parathione called? What are the antidotes (2)?

-*Parathione* is an organophosphate insecticide -Mechanism: parathione is metabolized to *paraoxon* --> paraoxon then phosphorylates *acetylcholinesterase* --> acetylcholinesterase gets inhibited! -Crisis that occurs = cholinergic crisis: bronchospasms, altered HR, muscle weakness (respiratory muscles), seizures and peripheral neuropathy Antidotes: 1. *Atropine sulfate - is anti muscarinic, blocks Ach from binding to its receptors* 2. Pralidoxime: ACHase (AchE?) activator

Other effects of lead -probably not on test

-CV effects: increased BP, increased change of CV related mortality -hematologic effects: anemia -renal effects: tubular dysfunction -reproductive effects: abnormal sperm production, miscarriage, impaired neuro development, etc

Antioxidants include

-GSH -Vit C -tocopherol SOD (superoxide dismutase) -catalase -glutathione peroxidase -GSH reductase

Risk factors for drug interactions: review

-age (elderly or infants) -multiple prescribers -inadequate monitoring -number of drugs prescribed = *polypharmacy* -pharmacogenetics -impaired drug elimination - due to renal and liver failure -concomitant disease state -drugs with narrow therapeutic index

Mechanisms of anecdotal action

-complexation with poison -acceleration of bioconversion to nontoxic metabolite -block formation of a more toxic metabolite -accelerate excretion -block or compete for essential receptors -bypass effect of the poison

General affects of chronic toxicity --> NOT ON THE TEST

-cumulative dysfunction -chronic inflammation or wounding -cell proliferation --> potentially cancer -dysfunctional repair enzymes -fibrosis (scarring) as a result of inflammation

Why is chronic exposure/toxicity more difficult to pinpoint or explain?

-for one, we cannot predict chronic toxicity for a drug thats just been placed on the market -need to account for confounding variables when trying to determine the effect of chronic exposure -*we have no idea or way to see how much of the compound a person is truly exposed to*

Toxicity of lead: -increased absorption occurs when: -acute effects appear how long after exposure? What are some of the symptoms of acute effects?

-increased absorption of lead happens when a person is fasting or when someone has a calcium/iron deficiency acute effects take hours/days to appear. Acute effects symptoms include both GI and constitutional things: -abdominal pain -constipation -fatigue -nausea -anemia -insomnia -weight loss -myalgia

What are the problems with chelation therapy?

-many chelators cannot penetrate cell membranes, so they can only remove metals in extracellular compartments -they can redistribute metals from one place but then just drop them off in another (causing damage to the new area) --> ex: removal from non neuronal tissues, enters the brain instead -they can form complexes with essential metals that you need, like copper -they can cause liver and kidney toxicity -they have limited effectiveness

What is a toxin?

-many definitions -but basically EVERYTHING can be a toxin -its the level of EXPOSURE that determines toxicity

*Mechanism of mercury toxicity*

-mercury causes toxicity by *reacting with sulfhydryl (SH) groups (cysteines) on proteins, inhibiting these proteins.* Mercury *also interacts with the vital mineral/antiooxidant selenium*

Benzene exposure

-occupation (solvent, used in synthesis of many chemicals) -tobacco smoke (1-2mg daily) -crude oil/gas with high octane value of 1% -acute toxicity: CNS depression, myocardial insensitivity to catecholamines, chemical pneumonia -chronic toxicity: hematoxicity (*bioactivation of benzene by myeloperoxidase*) --> leukemia

General affects of acute toxicity --> NOT ON THE TEST

-severe cell or organ dysfunction -receptor interactions (ex: *neurotoxins*) -cell death -interference with body's vital processes/functions -inflammation, as a consequence of nonspecific irritation or cell death

Exposure to mercury usually occurs with....

-typically occupational; electrical equipment, dental amalgams, "Mad hatters"

*Toxin: Paraquat* What is it? How are people typically exposed? What does it cause (symptoms wise)?

1. Paraquat is an herbicide 2. exposure typically occurs via preparation/application of herbicides, ingestion 3. symptoms wise, if ingested, can cause mouth/throat pain, swelling, severe GI problems, *pulmonary fibrosis (fatal)* Paraquat causes *lung toxicity* via: -it is selectively taken up by pulmonary alveolar cells -ROS are generated

Exposure can be characterized by duration/frequency: the two ways to define duration are: NOTE: toxicity can be altered by how acutely it occurs (acuity) ... example of benzene toxicity... what happens?

1. acute (single exposure leads to toxicity) 2. chronic (repeated exposure) leads to toxicity With *benzene toxicity*: 1. an acute exposure can lead to *CNS depression* 2. a chronic exposure can lead to *leukemia*

Situations in which toxic events may happen

1. adverse drug reaction (ex: due to neutropenia and cytotoxic cancer therapy --> when chemo drugs kill white blood cells; due to hepatic or renal failure) 2. drug-drug or drug-gene interactions 3. accidental or intended drug/chemical overdoses 4. exposure to certain specific poisons or toxins (ex: cholera or botulinum) *all associated with increased exposure!

Describe the key points about the 3 main routes of exposure: 1. inhalation 2. ingestion 3. dermal absorption

1. inhalation - inhalation may include breathing in gases/vapors/fumes or solutes; can also inhale particulates; *it is the most common route of occupational exposure* 2. ingestion - first pass in the liver may have an effect (ex: making a toxic metabolite). pH variation along the GI tract will affect absorption. *It is the most common route of exposure in poisonings*; frequently seen in the ER 3. dermal absorption - the *second most common route of exposure...* People do not wear gloves and then handle solvents **other routes can include 4. wounds/inflammation and 5. ocular

There are many mechanisms for lead toxicity - know these 2

1. reactivity with sulfhydryl groups (SH), or phosphate carboxyl groups on proteins (COOH) 2. mimics calcium (zinc and iron), and interferes with processes that depend on these metals... pretends that its a good metal like calcium and then interferes with the good things calcium does!

Factors that may impact exposure/toxicity

1. route/dose/time drug is taken 2. co-morbidities 3. loss of protective mechanisms: ex) if someone is malnourished 4. co-exposures 5. genetics

What things can introduce oxidative stress? (the bad things; and the good things/defenses getting depleted)

Bad things - p450 metabolism of drugs and xenobiotics (foreign substances) can lead to production of ROS -oxidative phosphorylation - produces free radicals if it leaks out of the mitochondria (mitochondrial toxicity) -xenobiotics in general -*ischemia and reperfusion...restoring blood flow to an organ or tissue introduces a whole bolus of air that can cause oxidative stress* Good things getting depleted/overwhelmed -GSH depletion -Genetic factors -Malnutrition *DOES NOT necessarily mean that taking tons of antioxidants prevents cancer

Antidotes for mercury toxicity

DMSA/Succimer!!! also unithiol (DMPS), and N-acetylcysteine

Antidote for lead

DMSA/succimer!! Also EDTA, BAL

Mercury - what are its forms and whats the most important/most toxic form (and why is it considered the most toxic form)?

Elemental mercury (Hg0): >10mg/m3 immediately dangerous to life and health - Inorganic salts (HgCl2): >1g lethal - Organic mercury (methylmercury): >10mg lethal, with lower exposure you get neuro/repro effects Organic mercury (methylmercury) is the most life-threatening. *it is the most life threatening because the inorganic forms do not cross the BBB well, but methylmercury is able to cross right past and into the brain*

Toxicological Dose-Response Curves: Hormetic model ...describe and draw it

Hormetic curve: J shape. Hormetic model: lets say this is an environmental model. Having a little bit of exposure to the toxin is GOOD. Maybe it stimulates the immune system in a good way. However at higher dose - bad. ex: we need some iron in our blood to transport O2, but too much could be toxic and lead to ROS.

Why does benzene damage the bone marrow?

Hydroquinone, a metabolite of benzene, is converted by human myeloperoxidase to 1,4-benzoquinone, a highly toxic species. Myeloperoxidase is only found in bone marrow --> this is why it damages the bone marrow. One of only a few examples where a chemical has a tissue specific toxicity (only acts on an enzyme specific to a certain tissue) *note: MPO = myeloperoxidase The protective enzyme that opposes this reaction is called NQO1

Mercury - bioaccumulation

Japan and Minomata disease: Mercury containing contaminants were dumped into the river --> small fish ingested the mercury --> large fish ingested the small fish --> and then we eat the large fish...all while the mercury is bioaccumulating *causes minomata disease*: a *neurological syndrome* caused by severe mercury poisoning. Symptoms include ataxia, numbness in the hands and feet, general muscle weakness, narrowing of the field of vision and damage to hearing and speech

Toxicological Dose-Response Curves: Linear model ...describe and draw it

Linear model: The more you take/the more you're exposed, the more bad effects you have. THERE IS NO SAFE DOSE HERE! (For a drug that follows this curve). Any chemical that causes DNA damage can be placed on this curve - there is no safe dose because a single DNA base change can lead to cancer

Toxicity: Hg0 (elemental mercury) and HgCl2 (mercury salts) are *CNS toxicants* <--KNOW What happens with methylmercury in low exposure? What happens when dimethylmercury is exposed to the skin? What do Hg0 vapors do? What do mercury inorganic salts (HgCl2) do?

Low exposure to methylmercury causes neurodevelopment disorders, reproduction effects --> ex: minamata disease Skin: dimethylmercury is very easily absorbed topically, and this can be LETHAL... a long latent period may occur, with progressive neurodegeneration Hg0 vapors: are pulmonary irritants and can cause edema Inorganic salts (HgCl2): are extremely corrosive and can cause GI hemorrhage/gastroenteritis and are nephrotoxic

Mechanism of paraquat --> draw it out

Mechanism: -Paraquat undergoes oxidation by NADPH oxidase to become a free radical -Radical interacts with molecular oxygen -KNOW *hydroxyl radical forms* -Lipid peroxidation and cell death occur -*pulmonary fibrosis develops directly on the lung tissue*

Metals - we will focus on mercury and lead ....also talked about arsenic a bit

Mercury in general: -the speciation of mercury you are exposed to is important -bioaccumulation of mercury in small fish --> large fish --> humans occurs Lead in general: -causes renal damage -exposure in childhood results in learning/emotional disabilities *side note: Arsenic -causes decreased circulation in the vascular periphery -leads to skin cancer

*Toxin: Methemoglobinemia Inducers* What is methemoglobin? What does it result from? What does it do (mechanism)? What is the antidote?

Methemoglobin is hemoglobin containing *Fe3+*... Hb-Fe3+ instead of Fe2+. This messes with iron's ability to hold oxygen. Methemoglobin is made as a result of exposure to *nitrates,* aromatic amines, nitro compounds, antibiotics and local anesthetics. Mechanism of methemoglobinemia: The Hb-Fe2+ complex reacts with *NO2* to form the Hb-Fe3+ (metHb). Met Hb then interacts with oxygen transport systems and causes oxidative stress. -*Methemoglobin reductase* can convert the Hb-Fe3+ (MetHb) back to normal hemoglobin Antidote: *methylene blue*: acts as an electron donor.... reduces Fe3+ (ferric) to Fe2+ (ferrous) to get rid of MetHb (reconvert it to Hb) and prevent further oxidative stress generation **symptoms: not covered, but would be blueish tint to the skin, fatigue, shortness of breath

Some chemicals interact with macromolecules to cause: -protein damage -lipid damage -DNA damage **repair mechanisms listed -not emphasized in class

Protein damage: - oxidized thiols (i.e. cysteines) reduced by thioredoxin and glutaredoxin Lipid damage: - peroxidation repaired by GSH/GSH reductase and NADPH DNA damage: -DNA repair

What does the phenotype for slow metabolism look like? What does the phenotype for intermediate metabolism look like? What does the phenotype for fast metabolism look like? What would slow vs fast metabolizer look like on a dose response curve?

Slow: variation on both copies of the gene Intermediate: variation on one copy of the gene Fast: gene duplication Note: we are more worried about the toxicity of a drug that is metabolized slowly!

Onto Lead: What are the sources of lead? How are people exposed to lead?

Sources: smelting and refining; used in batteries, pipes, cables, paints, glass, solder, ammunition Exposure occurs through: -paint (in older homes) ...children are at risk for effects; also industrial paint -lead-glazed ceramics or containers

Treatments for Metal Poisoning ... what do all treatments involve?

They all involve chelation of the metal!! Some are given by IV, others are oral. The chelator to know is: -*Dimercaptosuccinic acid (DMSA, also called Succimer for oral delivery)* - CHELATES MERCURY, LEAD AND ARSENIC!

Toxicological Dose-Response Curves: Threshold model ...describe and draw it

Threshold model: there is a level at which it is safe...at one point though, the drug begins to become toxic....example: drug starts producing more oxidants and you get ROS (protective mechanisms get overwhelmed). Most drugs fit into this category.

More information about cyanide antidotes... two step antidote (compounds to give)

Two step antidote process: 1. Give *amyl/sodium nitrates* - these generate MetHb (HbFe3+), which has a very high affinity for CN and will bind to it, forming an inactive complex 2. At the same time, give *hydroxycobalamin* which binds to free CN and forms cyanocobalamin 3. Or, instead of hydroxycobalamin, you can give *sodium thiosulfate,* which will bind to the cyanide, producing thiocyanate (SCN-) which is then excreted in the urine

Detection of lead

Typically done via blood -venous lead level -free erythrocyte or zinc protoporhyrin (due to heme inhibition) -can also be detected via xray fluorescence of lead in bone -by urinary excretion tests

Mechanisms of toxicity (how xenobiotics exert their effects) - overview (6 mechanisms) --> will not be tested on the mechanisms

Xenobiotics causing toxicity can do so by affecting/altering: 1. receptor-ligand interaction (bisphenols/BPA --> found to be a carcinogen, interacts with the estrogen receptor --- found in plastic water bottles) 2. membrane function or permeability 3. ATP generation 4. interaction with macromolecules 5. calcium homeostasis 6. generation of ROS

Antioxidants can donate an ______________ to radicals.

electron. GSH = the chemical "mop".... glutathione's -SH group donates an electron to the OH radical and makes it water ...glutathione can then form a cysteine bond until it gets reduced again

Many toxic exposure scenarios involve inappropriate _____________ in blood concentrations

elevation. Something happens that INCREASES THE BIOAVAILABILITY OF THE DRUG

True or false: you cannot generate carbon radicals

false

The U Shaped Dose Response Curve

low dose may be best - high dose can cause toxicity, and no dose at all can be deadly as well. EX: VITAMINS! You need some but too much can be harmful.

One of the biggest mechanisms of toxicity is generation of ______________

oxidative radicals and oxidative stress

Acetaminophen overwhelms ____________ mechanisms, leading to liver toxicity. Probably do not need to know for exam

protective mechanisms

Toxicity depends on the ______________ of a drug

therapeutic index (TI) tells you how much wiggle room you have. low dose = pharmacological benefits high dose = toxicity!

True or false: in occupational and ER medicine, you typically see a lot of toxin cases.

true

True or false: acute lethality does not take into account carcinogenicity or teratogenicity of certain compounds with low acute toxicity,

true! Note: ethyl alcohol has a high LD50 ..... botulinum toxin has a very low LD50. there is a dose dependence to toxicity!