Bosentan for Poorly Controlled Asthma

Inclusion Criteria: * Diagnosis of asthma, maintained on a minimum of 1 anti-inflammatory/controller and daily long acting B-agonist therapy with inadequate control of symptoms. (Defined as symptoms including wheezing, chest tightness or shortness of breath occurring at least 3 times a week or requiring use of "rescue" short-acting B-agonist at least 3 times a week). * FEV1 less than 80% of predicted and greater than 40% at screening visit. * A minimum of 12% reversibility of FEV1 after albuterol on screening visit or previously documented during the prior two years. * Women of childbearing potential must use 2 non-hormonal methods of birth control (2 methods between the subject and her partner) while on the study and for 1 month after the last dose of study medication. * Male subjects must use two non hormonal methods of birth control (2 methods between the subject and his partner) while on the study and for 1 month after the last dose of study medication. Exclusion Criteria: * History of liver disease, clinically significant cardiac disease, renal disease or pulmonary disease other than asthma. Patients with clinically significant laboratory abnormalities of LFTs/bilirubin (AST/ALT, TBili, alkaline phosphatase) and clinically significant anemia will be excluded. Clinically significant anemia will be defined as any anemia resulting in serum Hgb more than 1 gm/dl below the LLN, Patients with isolated minimal elevations of bilirubin (eg. as occurs in Gilbert's disease) or minimal elevations in transaminases (less than 1.2 x ULN) without a history of liver disease, risk factors for liver disease or symptoms of liver disease may still be included in the study at the investigator's discretion, but will have LFT testing at each study visit.) * Cigarette history of \>10 pack years. * Predicted inability to adhere to medication regimen or documentation requirements of the study (symptom and medication diaries). * Respiratory infection during 30 days preceding screening visit. * Requirement for change in scheduled asthma medication use, including oral steroid dose change, or acute medical care for asthma during the 30 days preceding screening visit. * Predicted inability to safely refrain from B-agonist use for the required amount of time on study visit days. * Use of tiotropium * Pregnancy, breast feeding or the use of hormonal methods of birth control as the only means of birth control during the study. * Use of potent CYP3A4 and CYP2C9 inhibitors, including, but not limited to azole antifungals, amiodarone, glyburide, warfarin, cyclosporine, ritonavir, other medications potentially toxic to the liver or bone marrow. * Use of any illegal drugs or alcohol abuse.