Bosentan for Poorly Controlled Asthma

Hypothesis: The endothelin-1 receptor antagonist, bosentan when added to the treatment of asthma patients who are symptomatic despite the use of controller therapy will improve asthma symptoms and physiology. Twenty patients with a diagnosis of asthma, between the ages of 21 and 70 who are symptomatic despite the use of at least one controller medication will be randomized to either placebo or active medication for an 8 week period (initial 4 weeks is at 1/2 of final dose as per package insert and FDA approval). Measures of lung function and symptoms will be recorded. Patients will then cross over, so that patients initially on placebo will receive active drug for 8 weeks and those initially on active drug will receive placebo. The same endpoints will be measured. The acute bronchodilator effects of the drug will also be tested on the first day of therapy at the full therapeutic dose.