Therapeutic consequences of drug interactions with theophylline pharmacokinetics.
Elimination of theophylline from the body occurs mainly (approximately 90%) by biotransformation, followed by excretion of the metabolites. Consequently, drugs affecting microsomal enzyme systems in the liver may alter the elimination of theophylline. Since theophylline has a rather narrow therapeutic window, dose adjustment might be necessary. Of the sympathomimetics, isoproterenol reduces theophylline clearance by 25%, but metaproterenol and terbutaline do not. Of the antibiotics, erythromycin and troleandomycin decrease theophylline clearance by 25% and 50%, respectively. No effect was observed with tetracycline, doxycycline, amoxicillin, cefaclor, and co-trimoxazole. On the other hand, rifampin (antituberculotic agent) increases theophylline clearance by 30%. Corticosteroids (hydrocortisone, methylprednisolone, and prednisone) do not affect theophylline kinetics. Influenza vaccination also has no influence. The antiulcerative agent (H2 antagonist) cimetidine decreases theophylline clearance by 30%, but the agent ranitidine does not have any effect. Oral contraceptives may decrease theophylline elimination by 30%. Barbiturates and phenytoin may enhance theophylline clearance substantially (up to 75%). If an interaction is expected, careful monitoring of theophylline plasma concentrations is required to optimize the dose.
D000305:Adrenal Cortex Hormones / Q000494:pharmacology; D000863:Antacids / Q000494:pharmacology; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D004347:Drug Interactions; D006635:Histamine H2 Antagonists / Q000494:pharmacology; D006801:Humans; D007252:Influenza Vaccines / Q000494:pharmacology; D007700:Kinetics; D013566:Sympathomimetics / Q000494:pharmacology; D013806:Theophylline / Q000378:metabolism*
D000305:Adrenal Cortex Hormones; D000863:Antacids; D000900:Anti-Bacterial Agents; D006635:Histamine H2 Antagonists; D007252:Influenza Vaccines; D013566:Sympathomimetics; D013806:Theophylline