Pharm 101: Theophylline

Class

Methylxanthine

Pharmacodynamics
  • Theophylline is a methylxanthine. It’s exact mechanism of action is not established but there are several propositions:
    • Inhibition of PDE enzyme family, thereby increasing intracellular cAMP and cGMP. This causes stimulation of cardiac function and relaxation of smooth muscle
    • Inhibition of cell surface receptors for adenosine. Adenosine causes airway smooth muscle contraction and release of histamine from airway mast cells
  • Multiple organ system effects, including;
  • Central nervous system:
    • Mild cortical arousal with increased alertness and deferral of fatigue
    • Tremor
    • Anxiety
  • Cardiovascular:
    • Positive chronotropic and inotropic effects
    • Decreases blood viscosity
  • Gastrointestinal:
    • Secretion of gastric acid and digestive enzymes
  • Renal:
    • Weak diuretic effect
    • Increased glomerular filtration rate, reduced tubular Na absorption
  • Smooth muscle:
    • Bronchodilation
    • Tolerance does not develop
  • Skeletal muscle:
    • Improved contractility
Pharmacokinetics
  • Bioavailability 96%
  • Volume of distribution 0.5L/kg
  • Narrow therapeutic window. Therapeutic and toxic effects are related to its blood level. Improvement in pulmonary function is seen at blood levels of 5-20mg/L
Clinical uses
  • Previously used in the treatment of asthma, the toxicity and need for blood concentration monitoring has made theophylline almost obsolete in asthma treatment
    • Acute asthma: bronchodilator effect relieves airflow obstruction
    • Chronic asthma: reduces severity of symptoms
Adverse effects
  • Adverse effects include sinus tachycardia, tremor and vomiting
  • Toxic effects are related to its blood level:
    • > 20mg/L: vomiting, abdominal discomfort, headache
    • > 40mg/L: seizures or arrhythmias
    • Hypotension, hypokalaemia and hyperglycaemia can occur due to beta-2 adrenergic activation (cause unknown)
  • Theophylline is an adenosine receptor blocker and reduces the effectiveness of adenosine used in the treatment of SVT
Further Reading
Pharm 101 700

Pharmacology 101

Top 200 drugs

MBBS CCPU (RCE, Biliary, DVT, E-FAST, AAA) Rob is an Emergency Medicine Advanced Trainee based in Melbourne, Australia. He has special interests in medical education, ECG interpretation, and the use of diagnostic and procedural ultrasound in the undifferentiated and unwell patient.

Follow him on twitter: @rob_buttner | ECG Library |

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.