Phase 1 & 2 drug metabolism and toxicity

Question 1

What is the need for metabolism (3)

Answer 1

1. Generally, drug molecules are lipophilic allowing them to pass through biological membranes
2. However, in order to be excreted the kidney requires chemicals to be hydrophilic
3. Metabolism (primarily in the liver) is responsible for making lipophilic chemicals more hydrophilic

Question 2

What are phase 1 metabolism functionalisation reactions (4)

Answer 2

1. Generally aiming to introduce a more polar functional group e.g. –OH, -NH2, -SH, -COOH
2. CYP450 family of enzymes can carry out a wide range of transformations, many of them are oxidation reactions, reductions are possible too
3. All tissues have some metabolic capability; the most important being the liver, kidney, GI tract, skin and lungs
4. However, reactive metabolites can be produced

Question 3

What is phase 2 metabolism (4)

Answer 3

1. Conjugation reactions
2. Addition of an endogenous substrate to the functionalised drug e.g. glucuronic acid, sulfate, acetate or an amino acid
3. Makes the resulting chemical extremely hydrophilic
4. Phase 2 reactions can occur without phase 1 reactions (for example, if the drug already has a hydroxyl group)

Question 4

How do pro-drugs work (2)

Answer 4

1. Pro-drugs work by taking advantage of Phase 1 metabolism to convert an inactive chemical into a biologically active one
2. Note: N-phenyl acetamide is not a good pro-drug for paracetamol!

Question 5

What is CYP450 hydroxylation and epoxidation (3)

Answer 5

1. hydroxylation = addition of OH group
2. Epoxidation = addition of a =O group
3. Phase 1 metabolism

Question 6

What is phenol phase 2 metabolism (2)

Answer 6

1. Sulfation
2. Glucuronidation

Question 7

Is sulphate an acid or base

Answer 7

Sulphate is an acid - there are three resonance forms

Question 8

What are the three resonance forms of sulphate

Answer 8

Question 9

How is paracetamol toxic (5)

Answer 9

1. Paracetamol is toxic to the liver (hepatotoxic)
2. Many drugs are metabolised by phase 1 CYP450 into reactive metabolites. These metabolites are electrophiles and can react with nucleophilic centres in proteins to form covalently bound adducts
3. This is so common that th body has a special protein called glutathione that can react with these electrophilic metabolites detoxifying them in the process.
4. Glutathione has a nucleophilic thiol group
5. However, the body’s supply of glutathione is limited and if it is consumed before all of the electrophile is detoxified then the electrophile will react with thiol groups in other proteins. This leads to the loss of normal protein function and ultimately cell death

Question 10

What is the common antidote to paracetamol toxicity (2)

Answer 10

1. acetylcysteine
2. The efficacy of the treatment decreases sharply if it is administered more than eight hours after an overdose

Question 11

Can you suggest a route of administration for N-acetylcysteine and why?

Answer 11

Oral

Contains =O which can form hydrogen bonds and polar SH and OH groups which makes it hydrophilic and suitable for oral administration

Question 12

The efficacy of the treatment of paracetamol toxicity with N-acetylcysteine decreases sharply if it is administered more than eight hours after an overdose, why might this be? (3)

Answer 12

1. NAC replenishes glutathione levels, allowing the liver to detoxify NAPQI.
2. If given within 8 hours, NAC: Effectively restores glutathione and prevents irreversible damage to liver cells.
3. NAC is still administered after 8 hours (up to 24-36 hours) because it may mitigate further damage, though the outcomes may not be as favorable.