Exam 1 Ch 4 Drug Biotransformation

Question 1

What happens to lipophilic xenobiotics during drug biotransformation, or metabolism?

Answer 1

termination or alteration of biological activity; they are transformed to more polar and hence more readily excreted products

Question 2

What is the mechanism for Phase I reactions of drug biotransformation?

Answer 2

Conversion of parent drug to more polar metabolite by introducing or masking a functional group (i.e. -OH, -NH2, -SH)

Question 3

Which types of enzymes are involved in Phase I reactions?

Answer 3

1. Cytochrome P450s (metabolize almost 75% of drugs, ~90% oxidation reactions
2. Alcohol/aldehyde dehydrogenase
3. Monoamine oxidase
4. Hydrolases (esterases, amidases)
   2-4 involve non-CYP oxidation reactions

Question 4

Explain a scenario in which a Phase II reaction precedes Phase I and describe the example discussed in lecture.

Answer 4

Some parent drugs may already possess a functional group that may directly form a conjugate.
e.g. Isoniazid. The hydrazide moiety forms the N-acetyl conjugate, a hepatotoxic metabolite, in Phase II reaction. This conjugate is the substrate for Phase I type reaction (in this example, hydrolysis to acetylhydrazine)

Question 5

Explain the nomeclature for cytochrome P450’s, and explain their main main location, structure and function in drug metabolism.

Answer 5

Designated as CYP, number, capital letter, then another number to indicate isozyme (e.g. CYP3A4). The cytochrome P450’s are composed of many families of heme-containing isozymes, located primarily in liver and GI tract. They are responsible for more than 95% of oxidative reactions of endogenous compounds (e.g. steroids, lipids) and biotransformation of exogenous substances (xenobiotics).

Question 6

What is the net reaction of cytochrome P450 oxidation?

Answer 6

NADPH + H+ + O2 + RH -> NADP+ + H2O + ROH

Question 7

What is a possible effect of cytochrome P450 enzyme induction in the case of a drug being transformed to reactive metabolites?`

Answer 7

Enzyme induction may exacerbate metabolite-mediated toxicity.

Question 8

By what mechanism are environmental chemicals and pollutants capable of inducing P450 enzymes?

Answer 8

Exposure to benzo[a]pyrene & other polycyclic aromatic hydrocarbons, present in tobacco smoke, charcoal-broiled meat, and other organic pyrolysis products is known to induce CYP1A enzymes and to alter rates of drug metabolism.

Question 9

Which six isoforms/isozymes are responsible for the vast majority of P450-catalyzed reactions?

Answer 9

CYP3A4, CYP2D6, CYP2C9/10, CYP2C19, CYP2E1, CYP1A2

Question 10

Can drugs inhibit cytochrome P450 activity? If so, how and what type of drug(s)?

Answer 10

Imidazole-containing drugs (cimetidine, ketoconazole) bind tightly to P450 heme iron, effectively reducing the metabolism of endogenous substrates (e.g. testosterone) or other co-administered drugs through competitive inhibition.
Erythromycin metabolites form a complex with cytochrome P450 heme iron and render it catalytically inactive.
Chloramphenicol is metabolized by CYP2B1 to a species that modifies P450 protein, thus also inactivating the enzyme.
Suicide inhibitors (steroids, grapefruit furanocouarins, clopidogrel, ritonavir, propylthiouracil).

Question 11

What happens during a Phase II reaction?

Answer 11

Parent drug or their phase I metabolites that contain suitable chemical groups undergo coupling or conjugations reactions with an endogenous substance to yield drug conjugates. Conjugate formation involves high-energy intermediates and transferases located in microsomes and the cytosol.

Question 12

How does morphine-6-glucuronid, the Phase II reaction conjugate of morphine, compare to morphine?

Answer 12

It is more potent.

Question 13

What does UGT stand for, what type of reaction is it? List key facts about it.

Answer 13

UDP-glucuronosyl transferase.
Phase II reaction.
Most dominant enzymes.
Type of conjugation: Glucuronidation.
Transferase: UDP-glucuronosyl transferases, microsomal enzymes.
Endogenous reactant: UDP glucuronic acid
Examples: drug, endogenous substrates (sulfonamides, morphine, acetaminophen, digoxin)

Question 14

What does SULT stand for, what type of reaction is it? List key facts about it.

Answer 14

Type of conjugation: sulfation
Transferases: sulfotransferases
Endogenous reactant: 3’-phosphoadenosine 5’-phosphosulfate (PAPS)
Examples: Drug, endogenous substrates (methyldopa, acetaminophen)

Question 15

What does GST stand for, what type of reaction is it? List key facts about it.

Answer 15

Type of conjugation: Glutathione
Transferase: Glutathione (GSH) transferases (cytosol, microsomes)
Endogenous reactant: Glutathione (GSH)
Examples: Drugs, endogenous substrates (acetaminophen, leukotrienes, prostaglandins)

Question 16

What does NAT stand for, what type of reaction is it? List key facts about it.

Answer 16

Type of conjugation: Acetylation
Transferase: N-acetyltransferase (cytosol)
Endogenous reactant: Acetyl-CoA
Examples: Drugs, endogenous substrates (isoniazid (INH), sulfonamides)

Question 17

What does MT stand for, what type of reaction is it? List key facts about it.

Answer 17

Type of conjugation: Methylation
Transferase: Transmethylases (cytosol)
Endogenous reactant: S-adenosylmethionine
Examples: drugs, endogenous substrates (catecholamines such as dopamine and epinephrine)

Question 18

What does EH stand for, what type of reaction is it? List key facts about it.

Answer 18

Type of conjugation: H2O
Transferase: Epoxide hydrolases (microsomal or cytosolic)
Endogenous reactant: H2O
Examples: Drugs. endogenous substrates (endobiotic, drug [carbamazepine epoxide], and xenobiotic epoxides generated via P450-catalyzed oxidations)
Conjugation of activated drugs such as S-CoA derivative of benzoic acid, with endogenous substrate, such as glycine, also occurs

Question 19

How does the speed of Phase II reactions compare to that of Phase I reaction, and what do the conjugations of Phase II reactions represent in the processing of drugs? Are there any exceptions?

Answer 19

Phase II reactions are relatively faster than P450-catalyzed reactions, thus effectively accelerating drug biotransformation.
Drug conjugations represent terminal inactivation events (“true detoxification” reactions)
Exceptions:
-Certain conjugation reactions may lead to formation of reactive species responsible for toxicity of drugs (acyl glucuronidation of NSAIDs, O-sulfation of N-hydroxyacetylaminofluorene, N-acetylation of isoniazid)
-sulfation activates orally active prodrug Minoxidil into very efficacious vasodilator
-morphine-6-glucuronide is more potent than morphine

Question 20

What would be the mechanistic reason for acetaminophen toxicity in an overdose?

Answer 20

When acetaminophen intake far exceeds therapeutic doses, this could lead to acetaminophen-induced hepatotoxicity.
Glucuronidation and sulfation pathways are saturated, so P450-dependent GSH conjugation pathway becomes increasingly important. In the absence of intracellular nucleophile such as GSH, reactive metabolite (N-acetylbenzoiminoquinone) reacts with nucleophilic groups of cellular proteins and participates in redox cycling, producing ROS