methylxanthine
Summary
Methylxanthines (theophylline, theobromine, caffeine) are bronchodilators that nonselectively inhibit phosphodiesterase, increasing cAMP, and block adenosine receptors. Theophylline is rarely used today because of its narrow therapeutic index and cardiac/CNS toxicity.
Detail
Mechanism: nonselective PDE inhibition raises intracellular cAMP in airway smooth muscle, producing bronchodilation; methylxanthines also block adenosine A1/A2 receptors, contributing to CNS stimulation and cardiac effects. Theophylline is a second-line bronchodilator in asthma and COPD. Toxicity is the high-yield point: narrow therapeutic index with cardiotoxicity (tachyarrhythmias) and neurotoxicity (tremor, seizures); metabolized by CYP1A2, so levels rise with cimetidine, ciprofloxacin, and erythromycin and fall with smoking and rifampin. Caffeine is used for apnea of prematurity in neonates. Theobromine in chocolate is the reason chocolate is toxic to dogs.
Sources
- First Aid for USMLE Step 1 2024
- Sketchy Pharm
- Katzung Basic & Clinical Pharmacology
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