Inclusion Criteria:

* Subjects meeting all of the following criteria will be considered for admission to the study:

1. Male or female 18 to 65 years of age, inclusive;
2. Self-reported history of physician-diagnosed and treated asthma for ≥ 3 months prior to randomization;
3. A diagnosis of an acute exacerbation of asthma upon presentation at the ED as defined by dyspnea and evidence of bronchospasm;
4. Received the following Standardized Treatment within a 2-hour time window and prior to obtaining the Qualifying Spirometry value(FEV1):

* Supplemental oxygen given to maintain oxygen saturation as measured by pulse oximetry of ≥ 90% as needed;
* Albuterol 5-15mg of albuterol via nebulizer prior to the qualifying spirometry evaluation; simultaneously with
* Ipratropium 0.5-1.5 mg of ipratropium via nebulizer prior to the qualifying spirometry evaluation;
* One dose of corticosteroid of at least 50 mg given orally (prednisone) or intravenously (methylprednisolone) or the equivalent dose of another corticosteroid.
5. FEV1 of ≤ 50% of predicted; NOTE: Spirometry to measure the subject's FEV1 expressed as % of predicted within 30 minutes of completing administration of 5 mg (but not more than 15 mg) albuterol and 0.5 mg (but not more than 1.5 mg) of ipratropium..
6. Negative urine pregnancy test for all females of child-bearing potential;
7. ECG with no dysrhythmias (except sinus tachycardia);
8. No clinical or electrocardiographic signs of ischemic heart disease as determined by the Investigator; and
9. Legally effective written informed consent obtained prior to starting any mandated study procedures

Exclusion Criteria:

Subjects will be excluded if they meet any of the following criteria:

1. Administration of a parenteral (intravenous or subcutaneous) beta agonist (e. g., albuterol, terbutaline, epinephrine) within 6 hours prior to randomization;
2. A current or prior diagnosis or suspected diagnosis of COPD or other chronic lung disease other than asthma;
3. Presence of pneumonia;
4. Presence of significant other respiratory dysfunction such as pneumothorax, pneumomediastinum, or pulmonary edema;
5. Known or suspected vocal cord dysfunction syndrome;
6. Presence of aspirated foreign body (known or suspected);
7. History or any current clinical evidence suggesting cardiomyopathy or congestive heart failure;
8. History or presence of tachyarrhythmias, with the exception of sinus tachycardia;
9. Heart rate ≥ 140 bpm;
10. Hypokalemia, defined as subjects with serum potassium level of \<2.8 mEq/L (≤2.8 mmol/L) obtained at Screening (local stat lab, blood gas analysis, or other point of care device) with the following exception:

For the subjects using non-potassium-sparing diuretics (i.e. loop-diuretic or thiazide diuretic) without "potassium-sparing diuretics" (e.g., Triamterene or Spironolactone) OR without potassium supplementation of at least KCl 20 mEq/day whose potassium level \<3.5 mEq/L (\<3.5 mmol/L) at Screening.
11. Significant cardiac, renal, hepatic, endocrine, metabolic, neurologic or other systemic disease. A significant disease will be defined as one which, in the opinion of the Investigator, may either put the subject at risk because of participation in the study, or may influence the results of the study or the subject's ability to participate in the trial;
12. Self-reported history of greater than 20 pack-yr smoking history;
13. Fever ≥ 102.0 ºF (38.9 ºC);
14. Uncontrolled hypertension defined as a blood pressure ≥ 170/100 mm Hg (22.7/13.3 kPa);
15. Need for immediate intubation, mechanical ventilation, or non-invasive positive pressure ventilation as determined by the Investigator;
16. Pregnant or lactating females;
17. Participated in another clinical study with an investigational drug within 30 days of randomization;
18. Positive urine drug screen for cocaine, methamphetamine or PCP unless, in the Investigator's clinical judgment, a single positive result is explained by exposure to a non-illicit drug product (i.e., is a false positive). For example, phenylpropanolamine or methylphenidate may read positive in a methamphetamine screen; dextromethorphan in a PCP screen.
19. Any subject with a known allergy to components of the MN-221 drug product;
20. Any subject with a known allergy to other beta agonists;
21. Previous exposure to MN-221; or
22. Use of theophylline, beta blockers, digoxin, MAO inhibitors, or tricyclic antidepressants within 2 weeks prior to randomization.

Use of non-potassium-sparing diuretics (i.e. Thiazide or Loop-diuretic) without potassium-sparing diuretic OR without potassium supplementation \>20 mEq/day within 2 weeks prior to randomization and if serum potassium level at Screening \<3.5 mEq/L (\<3.5 mmol/L).