SUMMIT Trial: Tirzepatide Cut Heart Failure Risk 38%
GLP-1 Drugs and Heart Disease
38%
Reduction in the composite of cardiovascular death or worsening heart failure events with tirzepatide vs. placebo in patients with HFpEF and obesity.
Packer et al., NEJM, 2025
Packer et al., NEJM, 2025
View as imageHeart failure with preserved ejection fraction (HFpEF) has been one of cardiology's most treatment-resistant conditions. The heart pumps normally on imaging, but stiffened walls and impaired filling leave patients breathless and fatigued. When HFpEF occurs alongside obesity, the two conditions reinforce each other: excess adipose tissue increases blood volume, promotes inflammation, and deposits fat around the heart itself. Until recently, no drug had shown a clear benefit on hard clinical endpoints in this population. The SUMMIT trial changed that. Tirzepatide, a dual GIP/GLP-1 receptor agonist already approved for diabetes and obesity, reduced the composite of cardiovascular death or worsening heart failure by 38% in 731 patients with HFpEF and BMI 30 or above.[1] This makes SUMMIT the first randomized trial to demonstrate that a GLP-1-class drug improves heart failure outcomes directly, not just cardiovascular events in patients with atherosclerosis.
Key Takeaways
- SUMMIT randomized 731 patients with HFpEF (EF 50% or above) and BMI 30 or above to tirzepatide (up to 15 mg weekly) or placebo for at least 52 weeks across 14 countries (Packer et al., NEJM, 2025)
- The primary composite of CV death or worsening heart failure occurred in 9.9% of tirzepatide patients vs. 15.3% on placebo (HR 0.62, 95% CI 0.41-0.95, P=0.026)
- Worsening heart failure events alone were reduced by 46% (HR 0.54, 95% CI 0.34-0.85), driven primarily by fewer HF hospitalizations
- KCCQ clinical summary score improved by 6.9 points more with tirzepatide than placebo at 52 weeks (P less than 0.001), crossing the clinically meaningful threshold of 5 points
- Tirzepatide reduced left ventricular mass by 12.4 g and paracardiac fat volume by 21%, suggesting structural cardiac remodeling (Kramer et al., JACC, 2025)
- Benefits were consistent regardless of diabetes status, baseline kidney function, or BMI level
What was the SUMMIT trial?
SUMMIT (Study of Tirzepatide in Participants With Heart Failure With Preserved Ejection Fraction and Obesity) was a phase 3, randomized, double-blind, placebo-controlled trial conducted at sites across 14 countries. Enrollment required an ejection fraction of 50% or higher, a BMI of 30 or above, and documented evidence of heart failure including elevated natriuretic peptides or structural heart disease on imaging.[1]
The 731 patients were randomized 1:1 to tirzepatide (titrated up to 15 mg subcutaneously once weekly) or matching placebo for a minimum of 52 weeks. The coprimary endpoints were the composite of cardiovascular death or a worsening heart failure event, and the change from baseline in the Kansas City Cardiomyopathy Questionnaire clinical summary score (KCCQ-CSS), which measures symptoms, physical limitations, and quality of life.
The trial population had a mean age of 64 years, mean BMI of 38 kg/m2, and mean ejection fraction of 60%. About 60% had type 2 diabetes. Baseline NT-proBNP levels were elevated, confirming hemodynamic congestion. This was a population where obesity and heart failure overlapped to create a compound pathology that neither cardiologists nor obesity specialists had effective tools to address.[1]
Primary results: 38% reduction in the composite endpoint
The composite of cardiovascular death or worsening heart failure occurred in 36 patients (9.9%) in the tirzepatide group and 56 patients (15.3%) in the placebo group. The hazard ratio was 0.62 (95% CI 0.41-0.95, P=0.026). This represented a 38% relative risk reduction.[1]
The signal was driven primarily by worsening heart failure events, which occurred in 29 tirzepatide patients (8.0%) versus 52 placebo patients (14.2%), producing a hazard ratio of 0.54 (95% CI 0.34-0.85). Heart failure hospitalizations specifically were reduced by 56%.
Cardiovascular death alone was numerically lower with tirzepatide (7 events vs. 10) but the trial was not powered for this individual endpoint. The low event rate for CV death meant the treatment effect was captured primarily through heart failure event reduction.
The second coprimary endpoint also reached significance. The KCCQ-CSS improved by 19.5 points in the tirzepatide group versus 12.7 points with placebo, a between-group difference of 6.9 points (95% CI 3.3-10.6, P less than 0.001). A 5-point change in the KCCQ-CSS is considered clinically meaningful. The nearly 7-point difference means tirzepatide-treated patients experienced substantially less shortness of breath, less fatigue, and fewer limitations in daily activities than those on placebo.[1]
How tirzepatide changes the heart itself
The SUMMIT cardiac magnetic resonance (CMR) substudy, published by Kramer and colleagues in JACC, provided imaging evidence of structural cardiac remodeling. In 91 patients who underwent CMR, tirzepatide reduced left ventricular mass by 12.4 grams more than placebo and decreased paracardiac adipose tissue volume by 21%.[2]
This is significant because paracardiac fat (the fat depot surrounding and infiltrating the heart) is not inert tissue. It secretes pro-inflammatory cytokines, contributes to pericardial constraint, and may directly impair diastolic filling. Reducing this fat depot could explain part of the improvement in heart failure symptoms independent of systemic weight loss.
Left ventricular mass reduction reflects decreased cardiac workload. In HFpEF, the heart compensates for stiff, non-compliant walls by developing hypertrophy. Reversing this hypertrophy suggests tirzepatide addresses the structural pathology, not just symptoms. The CMR substudy confirmed what the clinical endpoints suggested: tirzepatide was not simply masking heart failure through volume depletion, but was remodeling the underlying cardiac pathology.[2]
Effects on circulatory overload and organ damage
Borlaug and colleagues published a secondary analysis in Nature Medicine examining tirzepatide's effects on markers of circulatory overload and end-organ damage. Tirzepatide reduced NT-proBNP (the standard biomarker for heart failure severity), reduced high-sensitivity C-reactive protein (a marker of systemic inflammation), and improved kidney function markers.[3]
The kidney findings are particularly relevant. HFpEF and obesity commonly coexist with chronic kidney disease, and many heart failure treatments worsen kidney function. Tirzepatide appeared to improve or maintain renal parameters, suggesting the hemodynamic benefits extended to the kidneys.
Packer and colleagues separately analyzed the interplay of chronic kidney disease with tirzepatide's effects, finding that the heart failure benefits were consistent regardless of baseline estimated glomerular filtration rate. Patients with reduced kidney function at baseline derived similar benefit to those with normal kidney function.[4]
Clinical trajectory and sustained benefit
Zile and colleagues published an analysis of the clinical trajectory of SUMMIT patients in Circulation. They found that the benefits of tirzepatide were not limited to preventing discrete heart failure events but also improved the overall clinical trajectory across multiple measures of disease severity. Patients on tirzepatide were more likely to improve across multiple domains simultaneously and less likely to deteriorate across any domain.[5]
Packer's analysis of diabetes status showed that the benefits were consistent in patients with and without type 2 diabetes at baseline. This is a critical finding because it demonstrates that tirzepatide's heart failure benefits are not mediated solely through glucose control. Like the SELECT trial showed for semaglutide and atherosclerotic events, the cardiovascular benefit appears to be independent of the metabolic indication.[6]
How SUMMIT compares to other GLP-1 cardiovascular trials
SUMMIT is distinct from previous GLP-1 receptor agonist cardiovascular outcomes trials in a fundamental way. The LEADER, SUSTAIN-6, and SELECT trials all tested whether GLP-1 drugs reduced major adverse cardiovascular events (MACE): heart attacks, strokes, and cardiovascular death in patients with atherosclerotic disease. SUMMIT tested whether tirzepatide could improve heart failure, a different condition with different pathophysiology.
The distinction matters because heart failure and atherosclerosis are different diseases. GLP-1 drugs reduce cardiovascular inflammation and slow atherosclerosis progression, which explains MACE reduction. Heart failure with preserved ejection fraction involves diastolic dysfunction, myocardial stiffness, and volume overload, where the mechanisms of benefit are different.
Kruger and colleagues compared semaglutide and tirzepatide in HFpEF in 2025, noting that both drugs showed benefits in this population but through partially overlapping mechanisms. Tirzepatide's dual GIP/GLP-1 mechanism may confer additional benefits through GIP receptor activation in cardiac tissue, though this remains under investigation.[7]
He and colleagues published a systematic review and meta-analysis in 2025 pooling tirzepatide heart failure data, confirming the signal from SUMMIT and suggesting the effect is robust across different analytic approaches.[8]
What SUMMIT does not prove
The trial was not designed to determine whether tirzepatide reduces all-cause mortality. Only 17 deaths occurred across both groups, too few to draw conclusions about survival.
The 52-week duration leaves open the question of long-term durability. Heart failure is a chronic condition managed over decades. Whether the structural remodeling seen on CMR persists after drug discontinuation is unknown. Weight regain after stopping GLP-1 class drugs is well documented, and whether heart failure benefits would reverse with weight regain is a legitimate concern.
The SUMMIT population was obese (mean BMI 38). Whether tirzepatide would benefit HFpEF patients who are merely overweight (BMI 25-30) or normal weight is not addressed. SUMMIT specifically tested the obesity-HFpEF phenotype, not HFpEF in general.
Gastrointestinal side effects led to treatment discontinuation in 6.3% of tirzepatide patients versus 1.4% on placebo. This dropout rate, while manageable, means that approximately 1 in 16 patients could not tolerate the treatment. The GI side effect profile mirrors what has been seen in the SURMOUNT weight loss trials, where nausea, diarrhea, and vomiting were the most common reasons for discontinuation. For heart failure patients who may already have reduced appetite and fluid balance challenges, these gastrointestinal effects require careful clinical monitoring.
Hellenkamp's editorial in Med assessed both the promise and risks, noting that while the efficacy data is compelling, the long-term safety profile of tirzepatide in heart failure patients specifically (who may have different pharmacokinetic profiles due to impaired cardiac output and organ perfusion) requires ongoing surveillance.[9]
A 2026 narrative review by Abdul-Hafez and colleagues placed SUMMIT in the broader context of tirzepatide's cardiovascular program, noting that the trial establishes a new therapeutic category: metabolic drugs that treat structural heart disease through weight-dependent and weight-independent mechanisms.[10]
The blood pressure implications are also worth noting. Tirzepatide reduced systolic blood pressure in SUMMIT, consistent with findings across GLP-1 agonists and blood pressure. For HFpEF patients, where afterload reduction can improve diastolic filling, even modest blood pressure reductions contribute to the hemodynamic benefit. The question of whether the cardiac remodeling seen in SUMMIT would persist after drug discontinuation remains among the most important unanswered questions. Weight regain after stopping GLP-1-class drugs is well established, and whether the reduced LV mass and paracardiac fat would return toward baseline is an active concern among cardiologists managing these patients long-term.
The Bottom Line
The SUMMIT trial demonstrated that tirzepatide reduces the composite of cardiovascular death or worsening heart failure by 38% in patients with HFpEF and obesity. The benefit was accompanied by structural cardiac remodeling (reduced LV mass and paracardiac fat), improved kidney markers, reduced inflammation, and a clinically meaningful 6.9-point improvement in heart failure symptoms. Benefits were consistent regardless of diabetes status or kidney function. SUMMIT establishes tirzepatide as the first incretin-based therapy to show direct heart failure benefit, moving GLP-1-class drugs beyond atherosclerotic event prevention into structural cardiac disease.