RCT: Sodium Zirconium Cyclosilicate Improves Spironolactone Uptitration in HFrEF but Raises Concern for Increased HF Events
28 Jan, 2025 | 10:00h | UTCBackground: Hyperkalemia remains a major barrier to optimal use of mineralocorticoid receptor antagonists (MRAs) in patients with heart failure and reduced ejection fraction (HFrEF). While MRAs significantly reduce morbidity and mortality, real-world data show that fears surrounding hyperkalemia often lead to MRA down-titration or discontinuation. Sodium zirconium cyclosilicate (SZC), a newer oral potassium binder, has shown promise in managing hyperkalemia. However, its impact on MRA optimization and clinical outcomes in HFrEF has not been fully established.
Objective: To determine whether SZC can enable optimal dosing of spironolactone (≥25 mg/day) by preventing hyperkalemia in patients with symptomatic HFrEF, and to assess potential effects on heart failure outcomes.
Methods: This prospective, double-blind, randomized-withdrawal trial (REALIZE-K) enrolled adult patients with left ventricular ejection fraction ≤40% (NYHA class II-IV), on guideline-directed medical therapy but not on a full-dose MRA due to either prevalent hyperkalemia (serum potassium [K⁺] 5.1-5.9 mEq/L) or high hyperkalemia risk. During an open-label run-in, spironolactone was up-titrated (target 50 mg/day), and participants with hyperkalemia received SZC. Only those who achieved normokalemia (3.5-5.0 mEq/L) on spironolactone ≥25 mg/day continued into a 6-month randomized-withdrawal phase (SZC vs placebo). The primary endpoint was maintenance of normokalemia while on ≥25 mg/day spironolactone without rescue therapy for hyperkalemia. Key secondary outcomes included the time to first hyperkalemia event, time to spironolactone dose reduction/discontinuation due to hyperkalemia, and changes in health-related quality of life (Kansas City Cardiomyopathy Questionnaire–Clinical Summary Score).
Results: A total of 203 participants were randomized (SZC: n=102; placebo: n=101). SZC significantly increased the proportion of participants who maintained normokalemia on ≥25 mg/day spironolactone (71% vs 36%; OR: 4.45; 95% CI: 2.89-6.86; p<0.001). Rates of hyperkalemia were also lower with SZC (HR: 0.51; 95% CI: 0.37-0.71; p<0.001), and fewer patients required spironolactone down-titration or discontinuation (HR: 0.37; 95% CI: 0.17-0.73; p=0.006). There was no difference in KCCQ-Clinical Summary Score between groups (p=0.72). Notably, more participants in the SZC arm had adjudicated heart failure events (10 vs 2 in placebo) over the 6-month period, although the trial was not powered to detect differences in clinical outcomes.
Conclusions: In patients with symptomatic HFrEF and prevalent or incident hyperkalemia, sodium zirconium cyclosilicate (SZC) successfully enabled higher spironolactone doses by maintaining potassium levels in the normal range. However, the increased rate of heart failure (HF) events in the SZC group raises concern, particularly given the study’s limited power for clinical outcomes and baseline imbalances favoring higher HF risk in the SZC group (older age, lower eGFR, higher NT-proBNP). Although these findings do not establish a definitive causal relationship, they underscore a possible risk of fluid retention and the importance of cautious use in higher-risk populations. Larger, event-driven trials are crucial to confirm whether SZC-related sodium exchange may exacerbate HF decompensation despite the benefits of more effective MRA therapy.
Implications for Practice: SZC offers clinicians a means to sustain or escalate spironolactone dosing in hyperkalemia-prone HFrEF patients who would otherwise face MRA discontinuation. Still, the unexpected signal of increased HF events highlights the need for vigilant patient selection, fluid status monitoring, and regular follow-up. Clinicians should balance the advantages of maintaining guideline-recommended MRA doses against potential costs, the patient’s baseline HF severity, and possible sodium retention. Individualized care, particularly in older patients or those with high NT-proBNP levels, remains critical until more robust outcome data are available.
Study Strengths and Limitations: Strengths include strict definitions of hyperkalemia, a rigorous protocol for titrating spironolactone and SZC, and high rates of concurrent guideline-directed therapies. Limitations encompass the modest sample size, short follow-up period, and imbalance in baseline risk factors favoring the placebo group. Also, the study was not powered for major clinical events, limiting interpretability of the heart failure outcome signals.
Future Research: Further large-scale, long-term investigations are needed to clarify whether the higher incidence of HF events seen with SZC reflects underlying population differences or a genuine treatment-related risk. Such studies should prioritize hard clinical endpoints (eg, HF hospitalization, cardiovascular mortality), capture real-world tolerability data, and incorporate cost-effectiveness analyses. Research into the mechanisms of sodium exchange in compromised HF patients may also help identify subgroups that stand to gain the most from potassium-binding strategies or, conversely, experience undue risk.
Reference: Kosiborod MN, Cherney DZI, Desai AS, et al. “Sodium Zirconium Cyclosilicate for Management of Hyperkalemia During Spironolactone Optimization in Patients With Heart Failure.” Journal of the American College of Cardiology. 2025; DOI: https://doi.org/10.1016/j.jacc.2024.11.014