Mast Therapeutics announced that it has entered into a definitive agreement to acquire Aires Pharmaceuticals, a clinical stage pharmaceutical company developing therapies to treat pulmonary vascular disorders such as pulmonary arterial hypertension and pulmonary hypertension due to heart failure. Aires' lead product, AIR001, is an intermittently nebulized formulation of nitrite and has orphan drug status with the FDA and the EMA for the treatment of pulmonary arterial hypertension. Under the terms of the all-stock transaction, Aires would become a wholly-owned subsidiary of Mast Therapeutics. The acquisition is expected to close in February 2014, subject to customary closing conditions.