The primary objective of this study is to evaluate the dose response, efficacy and safety of 4 different doses of fluticasone propionate (50, 100, 200, and 400mcg) delivered as Fluticasone Spiromax® Inhalation Powder (Fp Spiromax) when administered twice daily in subjects 12 years of age and older with severe persistent asthma who are uncontrolled on high dose ICS therapy.