L9: CYP450

Question 1

Where are CYP450 isoforms encoded?

Answer 1

Each CYP450 isoform is encoded by a different gene

Question 2

What is a key property of CYP450 isoforms?

Answer 2

They have distinct substrate specificities but show considerable overlap.

Question 3

Why is identifying the CYP450 that metabolises a drug important?

Answer 3

It’s essential for drug licensing, safety, and dosage prediction.

Question 4

Which three CYP450 isoforms are the most clinically significant?

Answer 4

CYP3A4, CYP2D6, and CYP2C9.

Question 5

Why are CYP3A4, CYP2D6, and CYP2C9 considered the main ones?

Answer 5

Because they metabolise approximately 90% of all drugs that are CYP450 substrates.

Question 6

What are the three main approaches to identifying drug-metabolising CYP isoforms?

Answer 6

Correlation analysis, chemical/antibody inhibition, and purified enzyme studies.

Question 7

What is correlation analysis in CYP450 identification?

Answer 7

Comparing drug metabolism with probe substrate metabolism in human liver microsomes to find statistical correlation.

Question 8

How is CYP450 involvement confirmed through correlation analysis?

Answer 8

A high R² value between the probe and test substrate suggests the involvement of that CYP isoform.

Question 9

How does chemical inhibition help identify CYP450 isoforms?

Answer 9

A specific inhibitor is added to microsomes with drug and NADPH; reduced metabolite formation points to the inhibited CYP’s involvement.

Question 10

How does antibody inhibition help identify CYP450 isoforms?

Answer 10

Antibodies block specific CYP enzymes, and a decrease in metabolite formation confirms the enzyme’s role.

Question 11

How are purified or expressed enzymes used to identify CYPs?

Answer 11

The CYP gene is transfected into a cell line, the drug is added, and metabolism is measured directly.

Question 12

What is the substrate for CYP1A1?

Answer 12

Benzo[a]pyrene, an environmental toxin and polycyclic aromatic hydrocarbon (PAH).

Question 13

What are substrates of CYP1A2?

Answer 13

Caffeine and heterocyclic arylamines.

Question 14

What is the substrate for CYP1B1?

Answer 14

Benzanthracene.

Question 15

What is the main substrate for CYP2A6?

Answer 15

Coumarin.

Question 16

What is the primary substrate of CYP2B6?

Answer 16

Cyclophosphamide.

Question 17

What are the substrates for CYP2C8?

Answer 17

Taxol (paclitaxel) and retinoic acid.

Question 18

What drugs are metabolised by CYP2C9?

Answer 18

Warfarin, ibuprofen, diclofenac. It is a polymorphic enzyme requiring dose titration.

Question 19

What are the substrates of CYP2C19?

Answer 19

Omeprazole and clopidogrel.

Question 20

What drugs are metabolised by CYP2D6?

Answer 20

Codeine, metoprolol, tamoxifen, tricyclic antidepressants, and more.

Question 21

What are the substrates of CYP2E1?

Answer 21

Ethanol and carbon tetrachloride.

Question 22

What are the substrates of CYP3A4?

Answer 22

Nifedipine, cyclosporine, and many others.

Question 23

What is a key structural feature of CYP2D6 substrates?

Answer 23

Basic nitrogen (ionised at physiological pH) and a hydrophobic region ~0.5–0.7 nm from N.

Question 24

What is the population frequency of poor metabolisers for CYP2D6?

Answer 24

5–10% due to gene mutations.

Question 25

Can CYP2D6 be induced by drugs or external factors?

Answer 25

No — CYP2D6 is not inducible.

Question 26

What reaction type is common for CYP2D6?

Answer 26

Hydroxylation.

Question 27

What features define CYP2C9 substrates?

Answer 27

Small, acidic, hydrophobic, capable of forming hydrogen bonds or ion pairs ~0.5–1 nm from metabolic site.

Question 28

What are CYP2C9's common drug substrates?

Answer 28

NSAIDs like warfarin, ibuprofen, diclofenac.

Question 29

Is CYP2C9 polymorphic and inducible?

Answer 29

Yes — polymorphic and inducible by barbiturates and rifampicin.

Question 30

What makes CYP3A4 structurally unique?

Answer 30

Its binding site can accommodate large, hydrophobic molecules due to structural diversity.

Question 31

Which reactions are commonly catalysed by CYP3A4?

Answer 31

N-dealkylation and aromatic hydroxylation.

Question 32

Is CYP3A4 inducible?

Answer 32

Yes — highly inducible by glucocorticoids, rifampicin, and St. John’s Wort.

Question 33

Does CYP3A4 exhibit strong genetic polymorphism?

Answer 33

No — genetic variation has limited impact on its function.

Question 34

What effect does grapefruit juice have on CYP3A4?

Answer 34

It causes irreversible inhibition, leading to increased drug plasma concentration.

Question 35

List five other CYP isoforms important in drug metabolism.

Answer 35

CYP2C19, CYP1A2, CYP2B6, CYP2C8, CYP2A6.

Question 36

What are the 4 steps of imipramine metabolism and involved CYPs?

Answer 36

1. 40%: Demethylation by CYP3A4, 2C19, 1A2 → hydroxylation → conjugation 2. 25%: Hydroxylation by CYP2D6 → conjugation 3. 15%: CYP3A4 dealkylation + CYP2D6 conjugation 4. 13%: Dealkylation by CYP3A4 alone.

Question 37

What three main factors determine CYP isoform-substrate specificity?

Answer 37

1. Topography of the enzyme’s active site 2. Steric hindrance to Fe=O complex 3. Ease of electron or hydrogen abstraction.

Question 38

How does the active site topography influence metabolism?

Answer 38

Shape/size of the binding pocket affects which substrates can access the catalytic centre.

Question 39

How does steric hindrance affect CYP metabolism?

Answer 39

Bulky groups near oxidation sites can block access to the reactive Fe=O centre.

Question 40

Why are some substrates more easily metabolised?

Answer 40

Substrates with easily abstractable H/e⁻, especially from heteroatoms, are more reactive.

Question 41

How can tolbutamide metabolism be modified?

Answer 41

Removing the methyl group eliminates the hydroxylation site for CYP2C9, reducing metabolism.

Question 42

Which of the following CYP450 enzymes is most abundant in the liver and responsible for metabolizing ~50% of drugs? A. CYP2D6 B. CYP2C9 C. CYP3A4 D. CYP1A2

Answer 42

C

Question 43

What feature is most important in guiding substrate oxidation by CYP2D6? A. Presence of hydroxyl group B. High polarity C. Basic nitrogen and hydrophobic region 5–7 Å from it D. Aromatic ring structure

Answer 43

C

Question 44

Which method involves using R² values to identify isoform-specific drug metabolism? A. Antibody inhibition B. Chemical inhibition C. Recombinant enzyme assays D. Correlation analysis with human microsomes

Answer 44

D

Question 45

What consequence may arise in patients who lack functional CYP2D6? A. Enhanced drug clearance B. Increased risk of side effects from poor metabolism C. Complete loss of metabolism for acidic drugs D. Inhibition of CYP3A4

Answer 45

B

Question 46

Which enzyme metabolises codeine, metoprolol, and tamoxifen? A. CYP2E1 B. CYP3A4 C. CYP2D6 D. CYP2C19

Answer 46

C

Question 47

Which isoform is inhibited by grapefruit juice and induced by St John’s Wort? A. CYP2C8 B. CYP2C19 C. CYP2E1 D. CYP3A4

Answer 47

D

Question 48

. Which CYP450 isoform is polymorphic and involved in metabolising warfarin and NSAIDs? A. CYP1A2 B. CYP2C9 C. CYP2B6 D. CYP2E1

Answer 48

B

Question 49

Which factor does NOT influence CYP isoform-specific metabolism? A. Substrate polarity B. Topography of active site C. Steric hindrance to Fe=O site D. Ease of hydrogen abstraction

Answer 49

A

Question 50

Which CYP450 isoform is most associated with ethanol metabolism? A. CYP2A6 B. CYP2C8 C. CYP2E1 D. CYP1B1

Answer 50

C

Question 51

Which pair of drugs illustrates modification of metabolism due to loss of a hydroxylation site? A. Diclofenac vs Fenclofenac B. Omeprazole vs Clopidogrel C. Tolbutamide vs Chlorpropamide D. Metoprolol vs Propranolol

Answer 51

C