Which of the following mechanisms best explains the pharmacokinetic interaction between rifampicin and warfarin, resulting in decreased anticoagulant effect?

Question

Accepted Answer

C. Induction of hepatic CYP2C9 enzyme. ## Mechanism of Rifampicin-Warfarin Interaction

**Key Point:** Rifampicin is a potent inducer of the hepatic cytochrome P450 enzyme system, particularly CYP2C9, which is responsible for the metabolism of warfarin (S-enantiomer).

### How Enzyme Induction Works

Rifampicin increases the expression of CYP2C9 by acting as a ligand for the pregnane X receptor (PXR), leading to:
1. Increased hepatic metabolism of warfarin
2. Reduced plasma concentration of warfarin
3. Decreased anticoagulant effect (INR reduction)

**High-Yield:** Rifampicin is one of the most potent broad-spectrum enzyme inducers in clinical practice. It induces CYP3A4, CYP2C9, CYP2C19, and CYP2B6.

### Clinical Consequence

Patients on warfarin who start rifampicin require:
- Increased warfarin dosing (often 2–3 fold)
- More frequent INR monitoring
- Dose adjustment when rifampicin is discontinued (to avoid over-anticoagulation)

**Clinical Pearl:** The interaction becomes clinically apparent within 5–7 days of starting rifampicin, as enzyme induction reaches steady state.

### Other Notable CYP2C9 Inducers

| Inducer | Target Enzymes | Clinical Significance |
|---------|---|---|
| Rifampicin | CYP3A4, CYP2C9, CYP2C19 | Most potent; broad spectrum |
| Phenytoin | CYP3A4, CYP2C9, CYP2C19 | Moderate; long half-life |
| Carbamazepine | CYP3A4, CYP2C9 | Moderate; auto-induction |
| Phenobarbital | CYP3A4, CYP2C9, CYP2C19 | Moderate; long-acting |

**Mnemonic:** **PC BRAS** — Phenytoin, Carbamazepine, Barbiturates, Rifampicin, Alcohol (chronic), St. John's wort — all induce CYP450 enzymes.

Answer

A. Inhibition of renal tubular secretion

Answer

B. Displacement from plasma protein binding

Answer

D. Inhibition of hepatic CYP2C9 enzyme

Which of the following mechanisms best explains the pharmacokinetic interaction between rifampicin and warfarin, resulting in decreased anticoagulant effect?

Explanation

Mechanism of Rifampicin-Warfarin Interaction

How Enzyme Induction Works

Clinical Consequence

Other Notable CYP2C9 Inducers

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Inducer	Target Enzymes	Clinical Significance
Rifampicin	CYP3A4, CYP2C9, CYP2C19	Most potent; broad spectrum
Phenytoin	CYP3A4, CYP2C9, CYP2C19	Moderate; long half-life
Carbamazepine	CYP3A4, CYP2C9	Moderate; auto-induction
Phenobarbital	CYP3A4, CYP2C9, CYP2C19	Moderate; long-acting