Metabolism

Question 1

5 Major Isoforms of CYP

Answer 1

CYP1A2
CYP2C9
CYP2C19
CYP2D6**
CYP3A4*****

*****Most important: About 50% of drugs

**2nd most important

Question 2

Co-Factors of Cytochrome P450

Answer 2

NADPH*
flavoprotein
O2*
2H+

Question 3

Major chemical/functional group transformations by CYP (7 rxns)

Answer 3

1. aliphatic and aromatic hydroxylation
2. alkene and aromatic epoxidation
3. N-, O-, S-dealkylation
4. N-oxidation; N-hydroxylation
5. oxidative deamination and desulfuration
6. S-oxidation
7. oxidative dehalogenation

Question 4

Other Phase 1 enzymes (5)

Answer 4

1. Flavin adenine dinucleotide (FAD) monoxygenase
2. monoamine and diamine oxidase
3. alcohol dehydrogenase
4. acetaldehyde DH
5. xanthine oxidase

Question 5

5 Phase 2 Reactions

Answer 5

Time 2 say “Good Night SAM”

􀁺 Glucuronidation
􀁺 Sulfation
􀁺 Mercapturic acid formation
􀁺 N-, O-, S-methylation
􀁺 Acetylation

Question 6

Glucuronidation:

Answer 6

UGTs: uriidine diphosphate-glucuronyl transferase

􀁺 19 isoforms; mainly found in liver but also intestine, kidney, skin

􀁺 Adds glucuronic acid to acceptor molecules (hydroxyls,carboxylic acids, thiols, amines)

Endogenous substrates include bilirubin, bile acids, steroid hormones

Result: Increased polarity –>increased excretion

Question 7

Sulfation

Answer 7

sulfotransferases

􀁺 11 isoforms; found in liver, intestine, lung, adrenal glands

􀁺 Attaches sulfate groups to hydroxyl groups

􀁺 3’-phosphoadenosine-5’-phosphosulfate (PAPS) is donor

Result: Increased MW and polarity; now above receptor threshold and can be excreted

Question 8

Mercapturic acid formation

Answer 8

glutathione-S-transferase
(GST)

􀁺 Found in liver, kidney, lung, intestine

􀁺 Adds glutathione (Gly-Cys-Glu) to acceptor molecules

􀁺 Followed by hydrolysis of the glutamate and glycine, then acetylation of the cysteine

Result: chemistry has changed majorly and so
now it’s super unlikely to bind to its
receptor site

Question 9

N-, O-, S-methylation

Answer 9

methyl transferases

􀁺 Phenylethanolamine-N-methyltransferase, histamine-Nmethyltransferase,
indolethylamine-N-methyltransferase,
catechol-O-methyl transferase, thiol-S-methyltransferases

􀁺 Tissue distribution: liver, kidney, lung, brain

􀁺 Adds methyl group from S-adenosyl methionine to acceptor molecules

Result: ?

Question 10

Acetylation

Answer 10

Arylamine N-acetyltransferase

2 isoforms are important in xenobiotic metabolism

􀁺 Catalyzes the transfer of an acetyl group from acetyl CoA tovarious amine and hydrazine acceptor molecules

Question 11

CYP Induction

Also UGT and GST can be induced

Answer 11

Increases Phase I metabolism (functionalization)

Polycyclic aromatic hydrocarbons (found in combustion products: smoke, BBQ, etc.)
􀁺 Anticonvulsants (carbamazepine, phenobarbital, phenytoin)
􀁺 Glucocorticoids (dexamethasone, prednisone, triamcinolone)
􀁺 *****Peroxisome proliferator activated receptor α agonists (clofibrate, fenofibrate):lowers TGs w/known side effect of CYP induction

􀁺 Rifampin-antibiotic

􀁺 St. John’s Wort: potent CYP3A4 inducer

􀁺 HIV drugs (efavirenz, nevirapine, ritonavir)

Question 12

UGT Inducers

Answer 12

Phenobarbital
􀁺 3-methylcholanthrene (antiseizure)
􀁺 Carbamazepine (antiseizure)
􀁺 Nicotine

Question 13

GST Inducers

Answer 13

Phenobarbital
􀁺 3-methylcholanthrene
􀁺 Allyl isothiocyanate (wasabi, horseradish)
􀁺 Carvone (spices)

Question 14

Enzyme Inhibition : Types

Answer 14

Cofactor depletion

􀁺 Reversible competitive inhibition:

Covalent inhibition

􀁺 Pseudoirreversible inhibition:

Question 15

Enzyme Inhibition : Covalent Inhibition

Answer 15

reactive intermediates covalently
modify and inactivate enzymes
􀁺 Disulfiram: inhibits aldehyde dehydrogenase
􀁺 Polyhalogenated compounds
􀁺 Olefinic and acetylenic drugs

Question 16

Enzyme Inhibition : Reversible Competitive Inhibition

Answer 16

substrates for metabolic enzymes can compete for binding sites with other drugs and inhibit their metabolism
Examples:
􀁺 Antibiotics (clarithromycin, erythromycin)
􀁺 Gemfibrozil
􀁺 Azole-class antifungals (itraconazole, ketoconazole,
posaconazole, voriconazole)
􀁺 HIV protease inhibitors (indinavir, ritonavir, saquinavir)
􀁺 1st generation H2 antagonists (cimetidine)
􀁺 Grapefruit juice

Question 17

Enzyme Inhibition : Pseudoirreversible

Answer 17

metabolism results in

intermediates which slowly dissociate

Question 18

Therapeutic examples of enzyme inhibitors

Answer 18

MAO inhibitors
􀁺 Example: pargyline 􀁯 ↑ concentration of biogenic monoamines (serotonin, norepinephrine, dopamine)

􀁺 Aromatic L-amino acid decarboxylase inhibitors
􀁺 Example: carbidopa 􀁯 longer half-life of levodopa 􀁯 ↑ plasma concentration and CNS availability
􀁯 can decrease the dose of levodopa required to treat Parkinson’s disease

􀁺 Xanthine oxidase inhibitors
􀁺 Example: allopurinol 􀁯 ↓ uric acid biosynthesis 􀁯 ↓ gouty deposits

􀁺 Peptidase inhibitors
􀁺 Example: cilastatin 􀁯 inhibits renal dehydropeptidase-I, which is responsible for b-lactam inactivation
􀁯 ↑ therapeutic activity of blactamantibiotics (penicillin)

Question 19

Genetic Variations in CYP

Answer 19

CYP polymorphisms: CYP2C9, CYP2C19, CYP2D6
􀁺 Drugs affected: warfarin, omeprazole, codeine,
dextromethorphan, most SSRIs, many antipsychotic agents, beta-blockers

􀁺 CYP2D6 mutations are the most common CYP
polymorphism
􀁺 Ultra-rapid metabolizers vs. Poor Metabolizers

Question 20

Other Genetic issues affecting drug metabolism

Answer 20

1.. N-acetyl transferase activity leads to Isoniazid and procainamide inactivation
􀁺 Fast acetylators-lower serum [drug] and smaller T1/2
-slow acetylators

2. COMT deficiency
   􀁺 Decreased isoproterenol metabolism
3. Thiopurine-S-methyltransferase deficiency
   􀁺 Azathioprine, 6-mercaptopurine toxicity
4. Butyrylcholinesterase (pseudocholinesterase) activity
   􀁺 Succinylcholine sensitivity
   􀁺 Succinylcholine resistance