A 52-year-old man with long-standing COPD is treated with sustained-release theophylline for symptom control. His respiratory symptoms have recently worsened despite good adherence. Which of the following patient factors or drugs is most likely to reduce the clinical effect of theophylline?
A. Clarithromycin
B. Recent binge alcohol intake
C. Long-term cigarette smoking
D. Cimetidine
E. Ciprofloxacin
Answer
C. Long-term cigarette smoking
Detailed discussion
Theophylline (and aminophylline, its ethylenediamine salt) is a methylxanthine bronchodilator with a narrow therapeutic index, making it highly susceptible to drug interactions. It is metabolised predominantly in the liver by the cytochrome P450 system, especially CYP1A2. Any factor that induces this enzyme will increase hepatic clearance of theophylline, resulting in lower plasma concentrations and reduced therapeutic efficacy.
Cigarette smoking is a classic and clinically important CYP1A2 inducer. Polycyclic aromatic hydrocarbons in tobacco smoke up-regulate CYP1A2 activity, leading to accelerated theophylline metabolism. As a result, smokers often require higher doses of theophylline to achieve therapeutic levels. Conversely, sudden smoking cessation can precipitate theophylline toxicity if the dose is not reduced—an exam-favourite clinical trap.
In contrast, drugs such as ciprofloxacin, erythromycin/clarithromycin, cimetidine, and omeprazole are enzyme inhibitors, which increase theophylline levels and raise the risk of toxicity (nausea, tremor, seizures, arrhythmias). Acute alcohol intake also inhibits hepatic metabolism and may increase theophylline levels, whereas chronic alcohol use can act as an inducer, similar to smoking.
For MRCP candidates, it is crucial to remember that theophylline clearance is influenced by age, liver disease, heart failure, fever, smoking status, and interacting drugs. Because of its narrow therapeutic window, plasma level monitoring is often required, particularly when starting or stopping interacting agents.
Cheat sheet (exam-oriented)
- Theophylline/aminophylline → metabolised mainly by CYP1A2
- Smoking → CYP1A2 induction → ↓ theophylline levels → ↓ effect
- Smokers need higher doses
- Stopping smoking suddenly → ↑ levels → toxicity risk
- Narrow therapeutic index → frequent drug interactions
- Toxicity: nausea, vomiting, tremor, seizures, atrial/ventricular arrhythmias
Flash cards
Q: Which CYP enzyme primarily metabolises theophylline?
A: CYP1A2
Q: Does smoking increase or decrease theophylline levels?
A: Decreases levels (enzyme induction)
Q: Name a common antibiotic that raises theophylline levels.
A: Ciprofloxacin
Q: Effect of stopping smoking on theophylline dose?
A: Dose should be reduced
Q: Acute alcohol effect on theophylline?
A: Increases levels (enzyme inhibition)
Q: Chronic alcohol effect on theophylline?
A: Enzyme induction → ↓ levels
Q: Why is theophylline dangerous in practice?
A: Narrow therapeutic index
Q: Two life-threatening toxic effects of theophylline?
A: Seizures and arrhythmias
Q: Which patient group often needs higher doses?
A: Smokers
Q: Name a H2 blocker that increases theophylline levels.
A: Cimetidine
MCQs
- A smoker on theophylline requires increasing doses. The mechanism is:
A. Reduced absorption
B. Increased renal excretion
C. CYP1A2 induction
D. Protein binding displacement
Answer: C - Which drug is most likely to cause theophylline toxicity?
A. Rifampicin
B. Ciprofloxacin
C. Carbamazepine
D. St John’s wort
Answer: B - Sudden smoking cessation in a patient on theophylline increases risk of:
A. Loss of efficacy
B. Bradycardia
C. Seizures
D. Hypotension
Answer: C - Theophylline is best described as having:
A. Wide therapeutic index
B. Minimal drug interactions
C. Narrow therapeutic index
D. Renal elimination only
Answer: C - Which condition reduces theophylline clearance?
A. Smoking
B. Fever
C. Chronic alcohol intake
D. Rifampicin use
Answer: B - Which enzyme inhibitor raises theophylline levels?
A. Phenytoin
B. Carbamazepine
C. Omeprazole
D. Smoking
Answer: C - The major clinical use of aminophylline is:
A. Hypertension
B. Asthma/COPD
C. Pulmonary embolism
D. Heart failure
Answer: B - Which symptom suggests theophylline toxicity?
A. Constipation
B. Tremor
C. Bradycardia
D. Hypothermia
Answer: B - Which factor lowers theophylline plasma concentration?
A. Ciprofloxacin
B. Erythromycin
C. Smoking
D. Cimetidine
Answer: C - Best monitoring strategy for long-term theophylline therapy:
A. ECG only
B. Spirometry
C. Plasma drug levels
D. Chest X-ray
Answer: C
Summary for quick exam revision
Theophylline and aminophylline are methylxanthine bronchodilators with a narrow therapeutic index and extensive hepatic metabolism, mainly via CYP1A2. Cigarette smoking induces CYP1A2, increasing theophylline clearance and reducing its clinical efficacy, meaning smokers often require higher doses. In contrast, enzyme inhibitors such as ciprofloxacin, macrolides, cimetidine, and omeprazole raise theophylline levels and increase toxicity risk. Acute alcohol intake inhibits metabolism, while chronic alcohol use induces enzymes and lowers levels. Sudden smoking cessation can precipitate toxicity if the dose is not adjusted. Key toxic effects include nausea, tremor, seizures, and potentially fatal arrhythmias. Understanding enzyme induction and inhibition is essential for MRCP success, as theophylline interactions are classic exam favourites and clinically high-risk scenarios.