Correct Answer: A. Erythromycin
Erythromycin is a macrolide antibiotic that is a potent inhibitor of cytochrome P450 enzymes, particularly CYP3A4 and CYP1A2. Theophylline is metabolized primarily by CYP1A2 in the liver. When erythromycin is co-administered with theophylline, it significantly reduces theophylline metabolism, leading to elevated serum theophylline levels and increased risk of toxicity. Theophylline has a narrow therapeutic index (10–20 µg/mL), and toxicity manifests as arrhythmias, seizures, and severe nausea. This is a classic drug–drug interaction that Indian clinicians must avoid. Macrolides like erythromycin are commonly prescribed in India for respiratory infections, and theophylline is still used in asthma and COPD management, making this interaction clinically relevant. The interaction is so significant that theophylline dose reduction of 25–50% is required if erythromycin must be used, or alternative antibiotics should be chosen.
Why the other options are wrong
B. Amoxicillin — Amoxicillin is a β-lactam antibiotic that does not inhibit cytochrome P450 enzymes. It is safe to use with theophylline and does not alter theophylline metabolism or serum levels. No dose adjustment of theophylline is required when amoxicillin is co-prescribed. This is a commonly used combination in Indian clinical practice for respiratory infections. C. Cotrimoxazole — Cotrimoxazole (trimethoprim–sulfamethoxazole) has minimal CYP450 enzyme inhibition compared to macrolides. While some older literature suggested a minor interaction, it is not clinically significant enough to contraindicate concurrent use with theophylline. The combination is used safely in India for urinary tract infections and other infections in asthmatic patients. D. Cefotaxime — Cefotaxime is a third-generation cephalosporin that does not inhibit cytochrome P450 enzymes. It is metabolized independently and does not interfere with theophylline metabolism. Cephalosporins are safe alternatives to macrolides when theophylline co-prescription is necessary, and no dose adjustment is required.
High-Yield Facts
- Erythromycin + theophylline → CYP1A2 inhibition → ↑ theophylline levels → toxicity (arrhythmias, seizures).
- Theophylline therapeutic index = 10–20 µg/mL; narrow margin makes drug interactions clinically critical.
- Macrolide interactions: erythromycin >> azithromycin (azithromycin is safer with theophylline).
- β-lactams (amoxicillin, cefotaxime) and cotrimoxazole do NOT inhibit CYP450 → safe with theophylline.
- Management: if erythromycin necessary, reduce theophylline dose by 25–50% or switch to non-interacting antibiotic.
Mnemonics
MACE (Macrolides Avoid with Cytochrome Enzyme substrates) Macrolides (erythromycin, clarithromycin) inhibit CYP450 → avoid with theophylline, warfarin, statins. Azithromycin is safer (weaker inhibitor). Cephalosporins and Ethers (β-lactams) are safe alternatives. Narrow Index = No Interactions Drugs with narrow therapeutic index (theophylline, digoxin, warfarin) are vulnerable to drug interactions. Always check CYP450 inhibitors before prescribing. Macrolides are the classic culprit in India.
NBE Trap
NBE pairs erythromycin with theophylline to test knowledge of CYP450-mediated drug interactions rather than direct antimicrobial efficacy. Students may incorrectly assume all antibiotics are safe with theophylline or confuse macrolide safety profiles (azithromycin is safer than erythromycin).
Clinical Pearl
In Indian practice, when a patient on theophylline for asthma/COPD develops a respiratory infection requiring macrolide therapy, erythromycin must be avoided—switch to azithromycin (weaker inhibitor) or use β-lactams. Theophylline toxicity presenting as palpitations or tremor in an asthmatic on erythromycin is a red flag for this interaction.
_Reference: KD Tripathi Pharmacology Ch. 48 (Macrolide Antibiotics & Drug Interactions); Harrison Ch. 157 (Antimicrobial Agents)_