Theophylline is one of the most commonly used medications for the treatment of the symptoms of chronic asthma. It relaxes and opens air passages in the lungs, making it easier to breathe. Its most important actions are to prevent the signs and symptoms of asthma, especially during the night, and to reduce the need for cortisone type medication. It also reduces or prevents symptoms from triggers of asthma such as exercise and allergens. It is a modest bronchodilator relaxing muscles around the airway, allowing air to travel more freely in and out of the lungs. Theophylline is 60% protein bound and has a distribution volume of 0.5 L/kg. Therapeutic serum levels range from 10-20 mcg/mL. Toxic levels are considered to be higher than 20 mcg/mL.
Theophylline affects the cardiovascular (CV), neurological, GI, and metabolic systems. The main mechanism of action of theophylline is that of adenosine receptor antagonism. Theophylline is a non specific adenosine antagonist, antagonizing A1, A2 and A3 receptors almost equally. Theophylline is available in long-acting and short-acting forms. Long-acting theophylline, used daily to prevent chronic asthma, is taken as an easy-to-remember, once-or-twice a day dose. The use of theophylline is complicated by the fact that it interacts with various drugs, chiefly cimetidine and phenytoin, and that it has a narrow therapeutic index, so its use must be monitored to avoid toxicity. It can also cause nausea, diarrhea, increase in heart rate, arrhythmias and CNS excitation.