Once the subject has received the standardized initial treatment regimen and has been assessed for response to that treatment (signs and symptoms of acute asthma exacerbation), an informed consent to participate in the study will be obtained, study entry criteria will be reviewed, a 12-lead ECG will be performed, a dyspnea index scale assessment will be conducted, and spirometry will be performed. If the subject's FEV1 is ≤ 55% of predicted and the subject meets all other study entry criteria the subject will be randomized to receive either MN-221 or placebo. Throughout the screening process the subject will continue to receive the appropriate medical care consistent with the NAEPP guidelines for the treatment of acute exacerbations of asthma.