Has Tirzepatide Scaled the HFpEF/Obesity SUMMIT?

The results of the SUMMIT trial of the long-acting agonist of glucose-dependent insulinotropic polypeptide (GIP) and GLP-1 receptors, tirzepatide, in patients with heart failure with preserved ejection fraction (HFpEF) and obesity are positive. But the trial design leaves clinicians and regulators with big doses of uncertainty.
HFpEF has exceeded heart failure with reduced ejection fraction (HFrEF) as the most common form of heart failure. HFpEF differs from HFrEF in that patients with preserved ejection fraction often present later in life with more comorbidities.
Some of these comorbidities are on the causal pathway of heart failure. Obesity, for instance, both associates with HFpEF and surely causes the diastolic dysfunction central to the condition. This may be a direct effect via high excess adipose tissue or an indirect effect via pro-inflammatory pathways.
GLP-1 agonists and the dual-acting GIP/GLP1 agonist tirzepatide have proven efficacy for weight loss. Semaglutide has previously been shown to improve quality of life and physical functioning in two small trials of patients with HFpEF and obesity. Semaglutide also reduced hard clinical outcomes in patients with obesity and these other conditions: chronic kidney disease, diabetes, and established atherosclerotic vascular disease.
This class of drugs is costly. The combination of both high drug costs and highly prevalent conditions such as obesity and HFpEF forces clinicians to make both value and clinical judgements when translating evidence.
The SUMMIT trial aimed to evaluate tirzepatide’s effect on typical heart failure events, health status and functional capacity in patients with obesity and HFpEF. A total of 731 patients were randomly assigned to receive to tirzepatide or placebo.
Investigators chose two co-primary endpoints. The first was a composite of cardiovascular (CV) death and worsening heart failure events—the latter could be a hospitalization for heart failure, a visit for intravenous diuretics, or intensification of oral diuretics. The idea behind this rather unique composite was to comprehensively capture all heart failure events. The second co-primary endpoint was a change in baseline Kansas City Cardiomyopathy Questionnaire clinical summary score (KCCQ-CSS) at 1 year.
Characteristics of the patients included an average age of 65 years, 55% were female, the average body mass index was 38, and the mean left ventricular ejection fraction was 61% (the minimum for trial entry was 50%). Just under half had been hospitalized for heart failure in the year before trial entry.
The primary outcome of CV death and first heart failure event occurred in 36 patients (9.9%) in the tirzepatide group and 56 patients (15.3%) in the placebo group, for a hazard ratio of 0.62 (95% CI, 0.41-0.95; P =.026).
The 5.4% absolute risk reduction in the primary endpoint was completely driven by lower rates of heart failure events (8% vs 14.2%). CV death was actually higher in the tirzepatide arm, but the number of deaths was low in both arms (8 vs 5).
The rate of hospitalizations due to heart failure was lower with tirzepatide (3.3% vs 7.1%), as was intensification of oral diuretics (4.7% vs 5.7%).
The second co-primary endpoint of change from baseline in KCCQ-CSS favored tirzepatide.
Other secondary endpoints also favored tirzepatide: longer 6-minute walk distance, greater change in body weight (-11.6%), and lower high-sensitivity C-reactive protein levels and systolic blood pressure (-4.7 mm Hg).
At the American Heart Association Scientific Sessions, the primary investigator Milton Packer, MD, said SUMMIT was the first trial of patients with obesity and HFpEF that had major heart failure outcomes as the primary endpoint. And that tirzepatide changed the clinical trajectory of the disease.
Jennifer Ho, MD, associate professor of medicine at Harvard Medical School, Boston, Massachusetts, said, “This really is a practice-changing trial and cements this type of therapy as one of the cornerstones of obesity and HFpEF treatment.”
Other experts cited a recently published pooled analysis of semaglutide trials looking specifically at patients with HFpEF and found lower rates of HF events with the GLP-1 agonist.
The SUMMIT trial results were covered in 53 news outlets— nearly all with glowing headlines.
The trial delivered statistically robust findings. What’s more, patients lost weight, and a greater than 11% weight loss difference is meaningful. Patients with a baseline weight of more than 100 kg who lose this much weight are bound to feel and function better.
The first problem comes when we ask whether the results are disease-modifying. There was no difference in CV death. And the number of hospitalizations for heart failure — the more standard endpoint — was low, at only 12 and 26, respectively. Contrast this with the DELIVER trial of the sodium-glucose cotransporter 2 inhibitor dapagliflozin in HFpEF where there were nearly 750 hospitalizations for heart failure and PARAGON-HF of sacubitril-valsartan vs valsartan in HFpEF, where there were nearly 1500. SUMMIT simply had too few events to make conclusions — a point Packer has made regarding AF ablation trials in patients with heart failure.
I have previously called GLP-1 drugs disease-modifying in patients with obesity and atherosclerotic disease. This is because the SELECT trial of semaglutide randomized more than 17,000 patients and recorded a 20% reduction in hard outcomes. And there were more than 1200 primary outcome events. SUMMIT does not come close to this measure.
The second issue is short follow-up. These were 65-year-old patients and with only 2 years of follow-up, it is hard to make conclusions regarding whether or not these drugs can provide long-term benefit.
The third issue is that SUMMIT authors don’t tell us the number of all-cause hospitalizations. I was part of a recently published meta-analysis of more than 100 heart failure trials that raised questions regarding the value of hospitalizations for heart failure as a surrogate for heart failure outcomes.
For instance, we found that in large trials there was great variability in the ability of a reduction in HF hospitalizations to predict a reduction in all-cause hospitalization. In small trials, such as SUMMIT, it would likely be impossible to predict how the reduction in HF hospitalization would predict all-cause hospitalization. I believe all-cause hospitalization is a more inclusive endpoint because it is bias free; it captures benefits and potential harms of the therapy; and it is patient-centered, because patients probably do not care what type of hospitalization they avoid.
The fourth issue with SUMMIT is the difficulty in maintaining blinding, which reduces confidence in outcomes that require clinical decisions or patient judgements. Owing to gastrointestinal symptoms, decreased appetite, and weight loss, patients on this class of drugs are very likely to know their treatment assignment. This is a criticism of not only SUMMIT but all GLP-1 agonist trials. The fact that blinding is difficult to maintain argues for choosing endpoints less susceptible to bias, such as CV death or all-cause hospitalization.
Proponents of tirzepatide for this indication might argue that unblinding is less of an issue because of objective endpoints such as biomarkers. And they have a point, but nearly all other endpoints, especially the co-primary endpoint of KCCQ-CSS, are largely susceptible to bias.
The fifth and main problem comes in translating this evidence in the clinic. Should doctors give up on nondrug means of weight loss? All of the positive outcome trials in this class of drugs have also shown weight loss. I believe we should take these data and use them to re-invigorate our advocacy for weight loss without medication. I know the standard answer to this proposal is nihilism: It just will not work. And I cannot deny that we have failed previously in our efforts to help patients lose weight. But perhaps now, with the vast amount of data, we can be more persuasive. Imagine a world where key opinion leaders made weight loss the message rather than prescription of a drug.
Finally, if you approach SUMMIT from the view of a regulator, with its small numbers of outcome events and bias-susceptible endpoints, you cannot allow a disease-modifying claim. For that we would need a properly powered trial that shows that the drug reduces both CV death and all-cause hospitalization.
In the end, SUMMIT is not close to changing treatment norms in patients with HFpEF. As evidence-based clinicians, we should demand more from our partners in industry and academia.
John Mandrola practices cardiac electrophysiology in Louisville, Kentucky, and is a writer and podcaster for Medscape. He espouses a conservative approach to medical practice. He participates in clinical research and writes often about the state of medical evidence.