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Theophylline

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Theophylline toxicity

Xanthine

Mode of action

  • relaxes bronchial smooth muscle but mode of action not completely understood
  • probably involves competitive inhibition of adenosine receptors
  • inhibition of phosphodiesterase is negligible at therapeutic concentrations
  • improves diaphragmatic contraction
  • increases rate and force of cardiac contraction
  • increases rate of urine production

Pharmacokinetics

Administration: PO: absorption usually rapid and complete. Theophylline is relatively insoluble. Aminophylline is a mixture of theophylline with ethylenediamine which is sufficiently soluble for IV use. Give by slow IV injection/infusion
Distribution: widely distributed
Elimination: 90% metabolized by liver. There is evidence that the process is saturable at therapeutic doses. T1/2 8h but there is marked variation. Prolonged in patients with severe cardiovascular disease and in cirrhosis. Obesity and prematurity associated with reduced rates of elimination whereas tobacco or cannabis enhances clearance by inducing hepatic enzymes

Clinical uses

  • chronic and acute asthma
  • COPD

Adverse effects

  • nausea and diarrhoea at high therapeutic levels
  • cardiac arrhythmias and fits when plasma concentration exceeds recommended range

Drug interactions

  • Reduced rates of theophylline elimination due to enzyme inhibition by: erythromycin, ciprofloxacin, allopurinol, oral contraceptives
  • Enhanced elimination due to enzyme induction by: carbamazepine, phenobarbitone, phenytoin

Poisoning

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© Charles Gomersall December 1999

 

©Charles Gomersall, April, 2014 unless otherwise stated. The author, editor and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from the use of this webpage.
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