L67: Drug Metabolism And Elimination

Question 1

2 phases of Metabolism

Answer 1

Phase I = Functionalization reactions (within ER)

• result in loss of pharmacological activity (sometimes enhancement)
• Oxidation (introduce functional group)/ Hydrolysis (expose functional group)

Phase II = conjugation reactions (within cytosol)

• covalent linkage of endogenously derived glucuronate, sulfate, glutathione, a.a., acetate to drug metabolite of phase I
• result in polar conjugate —> facilitate excretion

Question 2

Site of metabolism

Answer 2

1. Liver
2. GI tract
3. Kidneys
4. Lungs

Within cells:

1. ER (Phase I)
2. Cytoplasm (Phase II)

Question 3

Phase I metabolism

Answer 3

Oxidation:

1. CYP450 monooxygenase system: heme-thiolate protein
   - microsomal enzyme
   - terminal oxidase to introduce single atom of oxygen onto substrate
   - other oxygen atom incorporated into water
   - electrons from NADPH via P450 reductase (closely associated with P450 in lipid membrane of SER)
   - Aromatic hydroxylation, N-dealkylation, deamination etc.

Hydrolysis

1. Esterases and amidases (in ER): expose alcohol and amine group —> suitable for conjugation
2. Microsomal epoxide hydrolase (in ER) (detoxification enzyme)
3. Protease and peptidase

Question 4

Phase II metabolism

Answer 4

Conjugation reactions: by Transferase —> conjugated metabolite

1. Glucuronidation
   - UGT: transfer of glucuronate to various groups to form glucuronides
   - microsomal —> direct access of phase I metabolite
2. Cytosolic sulfation
   - Sulfotransferase: transfer of inorganic sulphur
3. Acetylation
   - N-acetyltransferase
   - less soluble metabolite —> crystalluria

Question 5

Factors affecting drug metabolism

Answer 5

1. Genetic variation
   - allelic variants with different catalytic activity
   - autosomal recessive
   - CYP2D6 —> 70 SNPs
2. Environmental determinants
   - drug-drug, drug-diet interaction, induction and inhibition
   - induction: enhanced transcription of enzyme —> reduced bioavailability
   - inhibition: competition of substrate for enzyme e.g. CYP3A inhibitors (ketoconazole, grapefruit juice), reduced efflux pump (P-glycoprotein function)
3. Disease factors
   - Dysfunction of liver (P450 enzymes), heart (lidocaine metabolism)
4. Age, Gender and Pregnancy
   - impairment of bilirubin glucuronidation at birth —> hyperbilirubinemia
   - Pregnancy: induce drug metabolising enzyme during 2nd and 3rd trimester

Question 6

Excretion

Answer 6

1. Renal excretion
   - only unbound drug is filtered
   - proximal tubule:
   —> active secretion of conjugated / lipophilic metabolite (P-glycoprotein, MRP-2)
   —> passive reabsorption of non-ionic form
   - distal tubule:
   —> active reabsorption of non-ionic form
   —> passive reabsorption
   - concentration gradient created by reabsorption of water and Na
   - pH dependent (acidification / alkalisation of urine)
2. Biliary and fecal excretion
   - Biliary: Canalicular membrane of hepatocyte into bile (P-glycoprotein: amphipathic anion, MRP-2: conjugated metabolite)
   - may be secreted into bile or by enterocyte directly into GI lumen (reabsorption possible via hydrolysis by normal flora)
3. Sweat, saliva, tears (pH dependent; lipid-soluble, non-ionised form through epithelial cells)
4. Breast milk
5. Hair and skin