Potassium Reduction Initiative to Optimize RAAS Inhibition Therapy With Sodium Zirconium Cyclosilicate in Heart Failure

Inclusion Criteria: 1. Capable of giving signed informed consent which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in this protocol. 2. Provision of signed and dated, written informed consent form prior to any mandatory study specific procedures, sampling, and analyses. 3. Provision of signed and dated written genetic informed consent prior to collection of sample for genetic analysis. Individuals refusing to provide informed consent for genetic testing may still be included in the study, but will not have to provide samples for genetic analysis. 4. Subject must be ≥18 years of age inclusive, at the time of signing the informed consent form. 5. Individuals with established documented diagnosis of symptomatic Heart Failure with Reduced Ejection Fraction (HFrEF, NYHA functional class II-IV), which has been present for at least 3 months. 6. Individuals with left ventricular ejection fraction ≤ 40% (any measurement made within the past 12 months using echocardiography, multiple gate acquisition scan, computer tomography scanning, magnetic resonance imaging or ventricular angiography is acceptable, provided no subsequent measurement above 40%). 7. Individuals receiving background standard of care for HFrEF and treated according to locally recognized guidelines with both drugs and devices, as appropriate. Therapy with ACEi/ARB/ARNI, MRA and beta blocker should have been stable for ≥4 weeks. Subjects who are not being treated with beta blockers because of a contraindication are eligible. Subjects should be taking no MRA or a low dose of MRA (spironolactone, eplerenone, or canrenone) defined as less than or equal to 12.5 mg once a day (QD) or 25 mg every other day (QOD). If patients are taking a low dose MRA, the rationale for the low dose must be the patient could not tolerate a higher dose due to documented hyperkalemia observed at higher doses. 8. Individuals with mild hyperkalaemia or at risk of developing hyperkalaemia during the study, as defined by meeting all of the criteria in any one of the 3 categories listed below: 1. eGFR 20-44 ml/min/1.73m2 by CKD-EPI and local lab-K 4.0-5.5 mmol/L inclusive, or 2. eGFR 45-59 ml/min/1.73m2 by CKD-EPI and local lab-K 5.1-5.5 mmol/L inclusive, or 3. eGFR 45-59 ml/min/1.73m2 by CKD-EPI and local lab-K 4.0-5.0 mmol/L inclusive and a documented history of S-K \> 5.0 mmol/L due to RAASi. 9. Women of childbearing potential must have a negative pregnancy test during screening (before first dose of IP) performed locally. Exclusion Criteria: 1. HF due to restrictive cardiomyopathy, active myocarditis, constrictive pericarditis, hypertrophic (obstructive) cardiomyopathy or uncorrected primary valvular disease. 2. Current acute decompensated HF, hospitalization due to decompensated HF within 4 weeks prior to enrolment, or Myocardial infarction (MI), unstable angina, stroke or transient ischemic attack (TIA) within 12 weeks prior to enrolment. 3. Coronary revascularization (percutaneous coronary intervention \[PCI\] or coronary artery bypass grafting \[CABG\]) or valvular repair/replacement within 12 weeks prior to enrolment or planned to undergo any of these operations after randomization. 4. Implantation of a Cardiac Resynchronization Therapy (CRT) device or Implantable Cardioverter Defibrillator (ICD) within 12 weeks prior to enrolment or intent to perform atrial fibrillation ablation or to implant a CRT device. 5. Previous cardiac transplantation or implantation of a ventricular assistance device (VAD) or similar device, or transplantation or implantation expected after randomization. 6. Symptomatic bradycardia or second (Mobitz type 2) or third-degree heart block without a pacemaker. 7. Symptomatic hypotension or systolic blood pressure (BP) \<95 mmHg on 2 consecutive measurements. 8. Receiving dialysis or anticipated by the investigator to require dialysis therapy within 3 months. 9. Prior history of hypersensitivity to a RAASi drug, including but not limited to development of angioedema, icterus, hepatitis, or neutropenia or thrombocytopenia requiring treatment modification. 10. Addison's disease or other causes of hypoaldosteronism. 11. Known hypersensitivity to ZS. 12. Any condition outside the cardiovascular (CV) and renal disease area, such as but not limited to malignancy, with a life expectancy of less than 2 years based on investigator´s clinical judgement. 13. Active malignancy requiring treatment. 14. MRA therapies that are mainly investigational and/or are not widely available as an oral dosing formulation (eg, canrenoate and finerenone) are excluded. 15. Treated with potassium binding resins such as sodium polystyrene sulfonate (SPS;e.g. Kayexalate®) or calcium polystyrene sulfonate (CPS; e.g. Resonium®) or the cation exchange polymer, patiromer sorbitex calcium (Veltassa®) within 7 days prior to the first dose of study drug. 16. Treated with potassium supplements within 7 days prior to randomization. 17. Participation in another clinical study with ZS at any time or treatment with any investigational product (IP) during the last 3 months. 18. Involvement in the planning and/or conduct of the study (applies to both AstraZeneca staff, AstraZeneca representatives, and/or staff at the study site). 19. Judgment by the investigator that the subject should not participate in the study if the subject is unlikely to comply with study procedures, restrictions and requirements. 20. Previous randomisation in the present study. 21. Subjects with a family history of long QT syndrome, presence of cardiac arrhythmias or conduction defects that require immediate treatment, or a QTc (corrected QT interval) of ≥550 msec.