HFpEF is the most common form of heart failure, accounting for roughly half of all heart failure cases. It disproportionately affects patients with obesity, hypertension, and diabetes — the same population using GLP-1 agonists for weight management. SUMMIT tested whether tirzepatide could improve cardiovascular outcomes in this overlap population.
Trial design and population
SUMMIT enrolled adults with HFpEF (ejection fraction ≥50%) and BMI ≥30. The primary endpoint was a composite of cardiovascular death and worsening heart failure events (hospitalization or urgent visit). Secondary endpoints included Kansas City Cardiomyopathy Questionnaire (KCCQ) score (a measure of heart failure symptoms and quality of life), 6-minute walk distance, and body weight change.
The trial population was older (mean age ~65), predominantly female (reflecting HFpEF demographics), and had multiple comorbidities beyond obesity and heart failure — hypertension, Type 2 diabetes, and atrial fibrillation were common.
Primary results
Tirzepatide reduced the composite primary endpoint by approximately 38% vs placebo. The reduction was driven primarily by fewer worsening heart failure events rather than cardiovascular death, consistent with the trial's power limitations for mortality as an individual endpoint.
| Endpoint | Tirzepatide | Placebo | Hazard Ratio |
|---|---|---|---|
| CV death + worsening HF (composite) | ~9% | ~15% | ~0.62 |
| Worsening HF events alone | ~7% | ~12% | ~0.56 |
| CV death alone | ~3% | ~4% | NS trend |
| KCCQ score improvement | +12-15 points | +3-5 points | Significant |
| 6-min walk distance | +25-35m | +5-10m | Significant |
Why this matters beyond weight loss
HFpEF has historically been treatment-resistant. Unlike heart failure with reduced ejection fraction (HFrEF), where multiple drug classes have proven mortality benefits, HFpEF has had limited pharmacological options. SGLT2 inhibitors (empagliflozin, dapagliflozin) were the first class to show meaningful benefit in HFpEF. Tirzepatide now joins that short list.
The mechanism may extend beyond weight loss. While the ~15-17% weight reduction in SUMMIT certainly contributed to improved cardiac function (reduced cardiac preload, improved diastolic filling, reduced systemic inflammation), the magnitude of heart failure event reduction suggests effects on cardiac metabolism and myocardial function that go beyond hemodynamic changes from weight loss alone.
SUMMIT is a landmark result that repositions tirzepatide from a weight loss medication to a cardiovascular therapy for HFpEF patients with obesity. Combined with SELECT (semaglutide for MACE reduction) and FLOW (semaglutide for CKD), the GLP-1 class is accumulating cardiovascular and organ-protective evidence that will reshape prescribing patterns, insurance coverage, and clinical guidelines.
Regulatory and clinical implications
Lilly is expected to file a supplemental indication for tirzepatide in HFpEF. If approved, Zepbound (the obesity brand) or Mounjaro (the diabetes brand) would gain a cardiovascular indication — potentially improving insurance coverage and reducing prior authorization barriers for the overlap population.
For clinical practice, SUMMIT data supports prioritizing tirzepatide over semaglutide in patients with both obesity and HFpEF. This is one of the few clinical scenarios where drug selection within the GLP-1 class has evidence-based guidance rather than relying on individual response and tolerability.