A 70-year-old man presents with progressive exertional dyspnoea and orthopnoea. Echocardiography reveals an ejection fraction of 30% and global hypokinesis. He is already on maximally tolerated ACE inhibitor, beta-blocker, and mineralocorticoid receptor antagonist. His eGFR is 65 mL/min/1.73 m², and potassium 4.5 mmol/L. According to CCS heart failure guidance, which additional class of medication should be added to improve prognosis?