A 55-year-old male former smoker presents to the clinic for evaluation of productive cough and recurrent respiratory infections for the past 2 years. He has been treated multiple times with antibiotics without complete resolution of symptoms. His symptoms seem to be worse in the morning and are accompanied by wheezing. A computed tomography (CT) scan of the chest reveals bronchial wall thickening. Pulmonary function testing demonstrates a post-bronchodilator FEV1/FVC ratio of < 70%, consistent with obstructive lung disease. He has not had a significant response to inhaled bronchodilators or corticosteroids. What drug is the best next step in management?