Rifampicin is a potent inducer of cytochrome P450 enzymes. Which specific CYP isoforms are most significantly induced, explaining interactions with warfarin, oral contraceptives, and protease inhibitors?

• A CYP1A2 and CYP2D6
• B CYP2D6 and CYP3A4
• C CYP2E1 and CYP2B6
• D CYP2C9, CYP2C19, and CYP3A4 ✓

Explanation

Rifampicin activates the pregnane X receptor (PXR), inducing multiple CYP isoforms including CYP3A4, CYP2C9, and CYP2C19. CYP3A4 metabolises oral contraceptives, protease inhibitors, and calcineurin inhibitors. CYP2C9 metabolises warfarin. Rifampicin is the most potent CYP inducer among anti-TB drugs, reducing plasma levels of co-administered drugs by 50-90%.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

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