A 67-year-old man with a history of reduced ejection fraction heart failure (LVEF 30 percent) presents for routine follow-up. He is euvolemic and on maximally tolerated doses of sacubitril-valsartan, metoprolol succinate, and spironolactone. His eGFR is 55 mL/min/1.73 m2 and potassium is 4.4 mEq/L. Which additional medication class should be added to optimize guideline-directed medical therapy?