Principles of Toxicology III

Question 1

A bodies natural method for detoxifying and/or removing foreign substances (xenobiotics) involves…

Answer 1

Biotransformation/metabolism

Excretion

Question 2

What is a xenobiotic?

Most are..

Answer 2

A foreign substance.
Most are..
-weak acids or bases
-Ionization depends on pH
-lipid soluble when unionized

Question 3

****What are the organs MOST commonly involved in metabolism/biotransformation..

Answer 3

LIVER and GI

Question 4

What are the advantages of the LIVER and GI for metabolism and biotransformation?

Answer 4

• Receives the greatest fraction of total cardiac output after the lungs.
• Contains the widest breadth of metabolic enzymes.
• Liver has significant endothelial surface for filtration of blood.
• Liver can distribute products to blood or bile for excretion via the kidneys or GI tract.

Question 5

What organ in the body receives 100% of cardiac output?

Answer 5

LUNGS
Upper and lower respiratory tracts have also a large amount of surface area for absorption. Act to metabolize aerosolized xenobiotics. Thick nasal conchae but lungs as well- receives 100% of cardiac output.

Question 6

What are phase I reactions?

Answer 6

3 types: oxidation, reduction and hydrolysis.
act to..
-increase the polarity of metabolites
-provide reactive sites for phase II enzymes

Question 7

What are phase II reactions?

Answer 7

Conjugation reactions that couple xenobiotics to other molecules
act to..
-increase molecular weight (glutathione, methyl groups are added)
-increases polarity of metabolites

Question 8

Both phase I and phase II reaction..

Answer 8

Modify the biological activity of parent compounds.

Question 9

What are the ways that drugs can be changed in the body?

Answer 9

• Once it is absorbed by an animal.. Can pass straight through to the urine or feces.
• Can have a drug in-effected by phase I.. Conjugated by phase 2.
• Most commonly* phase 1 changes the molecule, the products conjugated and eliminated.
• Lipophilic molecule + enzymes => gets changed to hydrophilic and is excreted.

Question 10

Chemical inactivation of a parent compound generally means…

Answer 10

loss of parent activity.

not the same as loss of ANY activity!

Question 11

Biotransformation can..

Answer 11

REDUCE
ENHANCE or
COMPELETLY CHANGE biological activity.

Question 12

Phase II conjugation reactions cause..

Answer 12

large changes (MW and polarity) usually resulting in the COMPLETE LOSS of activity.

Question 13

Phase I reactions can…

Answer 13

create molecules with modified activity..

• don’t increase the polarity as much as in phase II reactions.
• modify reactive groups in a molecule instead of large MW changes.
• metabolites are still capable of crossing membranes.

Question 14

Cellular distribution of phase I and II enzymes can be… (2)

Answer 14

microsomal or non-microsomal

Question 15

Cellular distribution of phase I and II enzymes

MICROSOMAL

Answer 15

Derived from the endoplasmic reticulum.

*Means the way the cells are processed. These detoxifying enzymes.

Question 16

Cellular distribution of phase I and II enzymes

NON-MICROSOMAL

Answer 16

Derived from the cytoplasm

Question 17

Phase I

Microsomal/ER

Answer 17

Esterase's
Epoxide hydrolase (inducible)
Flavin monooxygenase (inducible)
Cytochrome P450s (inducible)

Question 18

Phase I

Microsomal/ER

Answer 18

Glucuroniation (inducible)
Glutathione conjugation (inducible)
Glycine conjugation
Methylation

Question 19

Phase I

Cytoplasmic

Answer 19

Esterases
Epoxide hydrolase (inducible)
Peptidase
Alcohol dehydrogenase
aldehyde dehydrogenase

Question 20

Phase II

Cytoplasmic

Answer 20

Sulfation
Glutathione Conjugation (inducible)
Acylation
Methylation

Question 21

Phase I

Mitochondrial

Answer 21

Monamine oxidase

aldehyde dehydrogenase

Question 22

Phase II

Mitochondrial

Answer 22

Glycine conjugation

acylation

Question 23

What is the physiological relevance of inducible enzymes/cytochrome p450?

Answer 23

Largest family of biotransformation enzymes.

• Both with regard to number and family members
• Diversity of enzymatic reactions.
• *90% of phase I metabolism
• *Phase I enzymatic activity, small changes of molecular weight,
• *Phase I microsomal/ER

Question 24

Inducible enzymes/cytochrome p450 can be…

Answer 24

INDUCED by xenobiotic agents.

• Xenobiotic agents inducing p450s may increase other inducible phase I enzymes because they act together in sequential reactions. Can lead to other phase one reactions…
• Increased transcription
• Stabilization of mRNA
• Stabilization of protein (prevents destabilization)
• Long-term changes that occur slowly.

Question 25

*****What drugs are p450/microsomal inducers?

Answer 25

- Rifampin - Phenobarbital - Glucocorticoids - Tobacco smoke - Chronic ethanol - Fipronil * *These drugs ten to be given for long periods of time because long term changes occur slowly.

Question 26

*****What are p450/microsomal inhibitors?

Answer 26

Ketoconazole Macrolide antibiotics a-naphthoflavone aminotriazole

Question 27

Inducible enzymes/cytochrome p-450s can be inhibited by...

Answer 27

xenobiotic agents - xenobiotic agents inhibiting p450s do not necessarily inhibit other biotransformation enzymes - mechanisms include... - -competitive inhibition and noncompetitive inhibition * **Long and short term changes!***

Question 28

Both inhibiton and induction of inducible enzymes/cytochrome p450s..

Answer 28

can be caused by either parent xenobiotics or their metabolites do not affect all p450s uniformly (each inhibitor/inducer acts on a subset of p450s.)

Question 29

Toxins may alter the effect of treatment and...

Answer 29

treatments may alter the effect of toxins!

Question 30

Biotransformation of paracetamol/acetaminophen; | In dogs phase II metabolism inactivates acetaminophen by..

Answer 30

GLUCURONIDATION (adding glucuronic acid) | SULFATION (adding a sulfate group)

Question 31

Biotransformation of paracetamol/acetaminophen; | In CATS phase II metabolism inactivates acetaminophen by..

Answer 31

Glucuronidation is DEFICENT | Sulfate donors are RAPIDLY DEPLETED

Question 32

When glucuroniation or sulfation pathways are missing or ineffective..

Answer 32

CYP3A4 activity is elevated | *This can be due to prolonged fipronil administration.

Question 33

What is NAPQI?

Answer 33

Metabolized glutathione conjugation. (GSH, Phase II) GSH gets depleted. NAPQI is a product of CYP3A4 and acetaminophen. It forms a free radical that reacts with proteins, membranes and DNA. Causes centriolobular hepatonecrosis. Causes damage to the kidney medulla. Converts Fe3+ to Fe2+ in RBC heme (Fe3+ does not bind to O2)

Question 34

What does NAPQI do inside the body?

Answer 34

NAPQI is a product of CYP3A4 and acetaminophen. - It forms a free radical that reacts with proteins, membranes and DNA. - Causes centriolobular hepatonecrosis. - Causes damage to the kidney medulla. - Converts Fe3+ to Fe2+ in RBC heme (Fe3+ does not bind to O2)

Question 35

Why does NAPQI cause oxidative stress in the body?

Answer 35

You can conjugate NAPQI to get rid of it with GSH, then you have a molecule that can be eliminated. But you run low on GSH, increases oxidative stress because GSH is needed to control oxidative stress which leads to increased Fe2+ and Fe3+ and decreased O2 transport by RBCS. That’s why this causes red blood cell damage in cats. *METHEMOGLOBINEMIA

Question 36

Theoretically inducible enzymes/cytochrome p450...

Answer 36

can alter the effect of treatments treatments may alter the effect of toxins documented real word examples are sparse.

Question 37

Fipronil indications?

Answer 37

``` Anti-flea medication Used in dogs and cats Administered topically every month -INDUCES CYP3A4 and CYP1A1 (less so) --CYP3A4 biotransforms acetaminophen ```

Question 38

What is the difference between dogs and cats when it comes to the glucuroniation pathway?

Answer 38

Dogs CAN Cats CANT

Question 39

Cats are at greater risk of xenobiotics due to..

Answer 39

Prolonged presence of toxic xenobiotics resulting from slower biotransformation in the absence of glucuroniation. Increased generation of toxic metabolites results from shifting in the biotranformative pathways used to compensate fro the absence of glucuroniation.

Question 40

GI excretion pathway

Answer 40

- Direct route of excretion from the liver via the bile duct. - Xenobiotics undergoing phase II metabolism and with a molecular weight of more then 350 Da are eliminated this route (smaller urinary) - Xenobiotics can be reabsorbed following bacterial metabolism in the GI lumen.

Question 41

Kidneys excretion pathway..

Answer 41

The most vascular tissue in the body | Receives the second greatest portion of the cardiac output.

Question 42

Kidney | Filtration, diffusion and active transport..

Answer 42

Passive filtration of xenobiotics less then 40 kDA (except due to plasma protein binding) - In the PCT... * Charged xenobiotics can be actively transported into and out of the filtrate. * Neutral/lipid soluble xenobiotics will be reabsorbed via passive diffusion.

Question 43

Absorption, distribution and elimination are..

Answer 43

OVERLAPPING PROCESSES. | -Starts when the first molecule of a xenobiotic contacts the animal.

Question 44

Excretion of a drug implies..

Answer 44

physical removal of the drug from the body

Question 45

Elimination of a drug implies...

Answer 45

removal of a drug by ANY mechanism; excretion or metabolism. | -can be calculated using a 2-compartment model.

Question 46

What is the two compartment model?

Answer 46

The body has two compartments.. -Central = plasma -Peripheral = tissues Volume of distribution = sum of compartment volumes Xenobiotics equilibrate between the plasma and periphery.

Question 47

******What is first order elimination?

Answer 47

Applies to many xenobiotics Elimination follows exponential kinetics the same PROPROTION of the agent is eliminated at a CONSTANT rate -one half-life is when the amount of xenobiotic decreases by 50% -half-life is constant only when elimination is first order! *rate of elimination graph curves, proportional to the plasma drug concentration.

Question 48

******What is ZERO order elimination?

Answer 48

the SAME amount of agent is eliminated at a CONSTANT rate occurs due to saturation of elimination processes examples (for a 70kg animal) -environmental toxicants -alcohol -aspirin *CONSTANT amount is eliminated per unit of time (straight line)

Question 49

Aspirin toxicosis | TOXICITY:

Answer 49

pKa ~3.5 (an organic acid) sheds protons into the blood large doses inhibit the respiratory center causes respiratory and metabolic acidosis

Question 50

Aspirin toxicosis | METABOLISM

Answer 50

hydrolyzed to salicylate (pka ~3) | salicylate can be glucoronidated

Question 51

Aspirin toxicosis | excreted by the..

Answer 51

KIDNEYS - filtered by the glomerulus - transported by the organic acid transporter (per urate)

Question 52

Aspirin toxicosis | rate of elimination:

Answer 52

rates of filtration + active transport - rate of diffusion + active reabsorption

Question 53

Aspirin toxicosis | Pharmacological management?

Answer 53

NaHCO3 pKa = 10.3 Na+ + HCO3- is freely filtered by the kidney In the blood and urine HCO3- ions absorb H+ from ASA keeping it in the A- state Traps ASA in urine by reducing diffusion of HA *Put it into the blood sucks out protons, will reverse the acidosis… will also freely filter out of the glomerulus. Urine bicarb looks like blood bicarb. Freely dumping base, radical shift in pH. Aspirin can become more and more charged, increase the elimination rate of the drug.