Erythromycin is a potent inhibitor of the cytochrome P450 isoenzyme CYP3A4, leading to clinically significant drug interactions. Azithromycin does NOT share this property because:

• A Azithromycin is not metabolised at all and is excreted unchanged in bile
• B Azithromycin lacks the desosamine sugar that forms a nitrosoalkane complex with the ferrihaem iron of CYP3A4
• C Azithromycin has a methyl-substituted nitrogen in its lactone ring, preventing nitrosoalkane formation ✓
• D Azithromycin is renally excreted and bypasses hepatic metabolism entirely

Explanation

Erythromycin and clarithromycin have an unsubstituted dimethylamino group on the desosamine sugar. During CYP3A4 metabolism, this forms a reactive nitrosoalkane that binds irreversibly to the ferrihaem of CYP3A4, inactivating it. Azithromycin has a methyl-substituted nitrogen in the 15-membered lactone ring, preventing this complex formation and thus avoiding significant CYP3A4 inhibition. This is a well-established structure-activity relationship.

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

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Written and medically reviewed by the StethoPrep medical team.