Swedish Cardiac And Renal Failure Study-1

Inclusion Criteria:

* The participant has given their written consent to participate
* A diagnosis of HFrEF according to current criteria, for at least three months before the screening visit
* Echocardiography within 24 months of the screening visit with ejection fraction ≤ 40%. The responsible investigator is allowed to order a new TTE at their own discretion if clinically indicated - e.g. following the initiation of markedly intensified HFrEF-treatment or in the event of significant clinical deterioration. If the new TTE shows an EF \> 40%, the participant will not be eligible for inclusion. However, a potential echocardiographic worsening should not, by itself, preclude enrollment
* New York Heart Association class II-III
* Optimally treated and stable HFrEF (according to the investigator) since at least four weeks before the screening visit. Treatment should include beta-blockers, sodium/glucose co-transporter 2 inhibitors, angiotensin-converting enzyme inhibitors, or angiotensin receptor blockers if eGFR ≥ 20 ml/min/1.73m2 according to the revised Lund-Malmö method. Participants should also have cardiac resynchronization therapy or an implantable cardioverter-defibrillator if the indication exists according to current guidelines
* eGFR \< 30 ml/min/1.73m2 according to the revised Lund-Malmö method at least once during the 12 months before the screening visit, and eGFR \< 45 ml/min/1.73m2 at the time of inclusion

Exclusion Criteria:

* P-K ≥ 5.6
* For the first ten study participants:

eGFR \< 20 ml/min/1.73m2 according to the revised Lund-Malmö method, or projected decline in eGFR to \< 10 ml/min/1.73m2 during the 36-week study period. The projected decline will be estimated using the three most recent eGFR values from the previous 6-12 months

\- For the remainder of the study participants: eGFR \< 10 ml/min/1.73m2 according to the revised Lund-Malmö method, or projected decline in eGFR to \< 10 ml/min/1.73m2 during the 36-week study period. The projected decline will be estimated using the three most recent eGFR values from the previous 6-12 months

* Ongoing/planned dialysis
* Systolic blood pressure \< 90 mmHg
* Uncontrolled hypertension as judged by the investigator
* Severe hepatic impairment (Child-Pugh C)
* History of, or planned, heart transplantation or left ventricular assist device
* Unwillingness to comply with highly effective contraceptive methods, or ongoing/planned pregnancy, or breastfeeding
* Previous allergic reaction to an MRA or a potassium binder
* Ongoing treatment with lithium, cyclosporine, tacrolimus, nonsteroidal anti-inflammatory drugs, trimethoprim, or strong CYP3A inhibitors (ketoconazole, itraconazole, ritonavir, nelfinavir, clarithromycin, telithromycin, and nefazodone) or inducers (rifampicin, carbamazepine, phenytoin, phenobarbital, and St. John's Wort)
* QTc(f) ≥ 550 msec, history of QT prolongation associated with any medication requiring medication discontinuation, or congenital long QT syndrome
* Uncontrolled arrhythmia as judged by the investigator
* Acute cardiac hospitalization or procedure within four weeks before inclusion
* Not suitable as judged by the investigator (presumed inability to participate, severe or terminal co-morbidity, and expected survival \< 12 months)
* Previously enrolled in this trial or participation in another trial not approved for co-enrollment