Study in HFpEF Patients to Compare the Effect of AZD9977 and Spironolactone on Serum Potassium

Inclusion Criteria: * Symptoms and signs consistent with a diagnosis of heart failure (HF) * Left ventricular ejection fraction (LVEF) ≥40% by transthoracic echocardiography within the past 12 months * Plasma NT-proBNP level ≥125 pg/ml (≥ 600pg/mL if patient has persistent or permanent atrial fibrillation, atrial flutter, or atrial tachycardia) * Estimated glomerular filtration rate (eGFR) ≥40 and ≤70ml/min/1.73m2 (by CKD-EPI formula). * Baseline serum potassium (sK+) level ≥3.5 and ≤4.8mmol/l. * Stable doses of the following medications at baseline for at least 4 weeks prior to randomisation: Angiotensin receptor blocker (ARB) or Angiotensin converting enzyme inhibitor (ACEi) Loop diuretic (up to a 120mg furosemide dose or equivalent Exclusion Criteria: * Patients with documented LVEF \<40% at any time (i.e. patients with previously impaired LVEF that has now improved are not permitted). * Patients experiencing acute decompensation of heart failure requiring hospital admission or escalation in therapy. * Primary cardiomyopathy (e.g. constrictive, restrictive, infiltrative, toxic, hypertrophic, congenital or any primary cardiomyopathy in judgment of Investigator). * Hyponatraemia, defined as serum Na+ \<135 mmol/L at the time of enrollment. * Persistent resting sinus tachycardia \>110 bpm or sinus bradycardia \<45bpm. * Systolic blood pressure (BP) \<110mmHg or \>180mmHg. * Diastolic BP \<60 mmHg or \>100 mmHg. * Patients previously intolerant to MRA (e.g. spironolactone or eplerenone) for any reason. * Medical conditions associated with development of hyperkalaemia (e.g. Addison's disease). * Alanine aminotransferase (ALT) or aspartate aminotransferase (AST) level ≥2.0 times the upper limit of normal (ULN). * Patients who have been taking an MR antagonist within 1 month prior to randomization.