A 70-year-old man with COPD (GOLD 3, FEV₁ 45%) and a history of two exacerbations in the past year reports worsening dyspnea, eosinophiles count is 90 cells/L. He smokes one pack per day. Current medications include tiotropium and salmeterol. Which of the following is the best long-term therapy to reduce future exacerbations?