SUSTAIN-6 Trial: A Plain-English Overview of What It Established

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At a glance

| Detail | Value | |---|---| | Trial name | SUSTAIN-6 (Trial Comparing Cardiovascular Safety of Insulin Degludec vs Insulin Glargine in Subjects with Type 2 Diabetes at High Risk of Cardiovascular Events) | | N | 3,297 | | Intervention | Semaglutide 0.5 mg or 1.0 mg subcutaneous once weekly | | Comparator | Matching placebo (volume-matched, once weekly) | | Duration | Median 2.1 years | | Primary endpoint | First occurrence of 3-point MACE (cardiovascular death, nonfatal myocardial infarction, nonfatal stroke) | | Key result | HR 0.74 to 95% CI 0.58, 0.95, p <0.001 for noninferiority; p = 0.02 for superiority | | Registration | NCT01720446 |

The Question SUSTAIN-6 Was Designed to Answer

By 2012, the FDA required every new diabetes drug to prove it did not increase cardiovascular risk. That bar was set after the rosiglitazone scare, when a diabetes medication turned out to raise the odds of heart attack. The formal requirement: run a large trial, enroll patients at high cardiovascular risk, and demonstrate that the upper bound of the 95% confidence interval for major adverse cardiovascular events (MACE) stays below 1.8 compared to placebo.

SUSTAIN-6 was built to clear that regulatory hurdle for semaglutide. Novo Nordisk designed it as a noninferiority trial, meaning the primary statistical goal was to show semaglutide was "not worse" than placebo for heart outcomes. The trial was not initially powered to prove superiority, which makes the positive result more striking.

Who Was Enrolled

Participants had type 2 diabetes (HbA1c ≥ 7.0%) and were 50 years or older with established cardiovascular disease, chronic kidney disease, or heart failure (83% of the cohort), or 60 years or older with at least one cardiovascular risk factor (17%). The population was sick: roughly 60% had prior MI, stroke, or peripheral artery disease. About 73% were on metformin, 58% on insulin, and 43% on sulfonylureas at baseline.

The trial excluded anyone with type 1 diabetes, those on dialysis, and patients with a recent acute coronary or cerebrovascular event within 14 days of screening. Baseline HbA1c averaged 8.7%, mean age was 65, and 39% were female.

What Patients Actually Received

Randomization was 1:1:1:1 across four arms: semaglutide 0.5 mg, semaglutide 1.0 mg, placebo matching 0.5 mg, and placebo matching 1.0 mg. For analysis, the two semaglutide arms were pooled and the two placebo arms were pooled.

Semaglutide was started at 0.25 mg weekly for the first four weeks, then escalated to the target dose. Investigators could adjust other diabetes medications throughout the trial, which reflects real-world care but also means background therapy differed between groups over time.

One methodological detail worth noting: the trial used a treat-to-target design where clinicians adjusted background medications to hit glycemic goals. Because semaglutide lowered blood sugar more effectively, patients in the placebo group received more add-on therapy (particularly insulin). This partly narrowed the HbA1c gap between groups but did not eliminate it. By the end, semaglutide-treated patients had HbA1c roughly 1 percentage point lower than placebo.

The Primary Outcome: 26% Reduction in MACE

The composite 3-point MACE endpoint occurred in 6.6% of semaglutide patients versus 8.9% of placebo patients (HR 0.74 to 95% CI 0.58, 0.95). This crossed both the noninferiority margin (upper CI below 1.8) and the superiority threshold (p = 0.02).

Breaking the composite apart reveals where the benefit concentrated:

| Component | Semaglutide (n = 1,648) | Placebo (n = 1,649) | Hazard Ratio (95% CI) | |---|---|---|---| | 3-point MACE (composite) | 6.6% | 8.9% | 0.74 (0.58, 0.95) | | Cardiovascular death | 2.7% | 2.8% | 0.98 (0.65, 1.48) | | Nonfatal MI | 2.9% | 3.9% | 0.74 (0.51, 1.08) | | Nonfatal stroke | 1.6% | 2.7% | 0.61 (0.38, 0.99) | | All-cause death | 3.8% | 3.6% | 1.05 (0.74, 1.50) |

The stroke reduction was the strongest single driver, with a 39% relative risk reduction that just reached statistical significance (p = 0.04). Nonfatal MI trended in the same direction but did not reach significance on its own. Cardiovascular death and all-cause death showed no meaningful difference.

Secondary and Safety Findings

Semaglutide produced expected metabolic improvements. Mean body weight dropped 3.6 kg (0.5 mg) and 4.9 kg (1.0 mg) more than placebo. Systolic blood pressure fell by 1.3 mmHg (0.5 mg) and 2.6 mmHg (1.0 mg) versus placebo. LDL cholesterol showed modest reductions.

New or worsening diabetic nephropathy occurred in 3.8% of the semaglutide group versus 6.1% with placebo (HR 0.64 to 95% CI 0.46, 0.88), suggesting a renal benefit. This finding influenced later dedicated kidney outcome trials.

On the safety side, two signals stood out. Retinopathy complications (vitreous hemorrhage, blindness, need for photocoagulation or intravitreal treatment) occurred more often with semaglutide (3.0% vs. 1.8%, HR 1.76). This was unexpected and prompted concern, though subsequent analysis suggested it was related to the speed of HbA1c reduction rather than a direct drug effect. Patients with pre-existing retinopathy and poor baseline glycemic control were most affected. Gastrointestinal events (nausea, vomiting, diarrhea) were the most common adverse events, occurring in roughly 40 to 45% of semaglutide patients versus 25 to 30% on placebo. Most were mild to moderate.

Pancreatitis occurred in 9 semaglutide patients and 12 placebo patients, and pancreatic cancer in 1 versus 4. Neither signal raised alarm.

Real Limitations the Authors Acknowledged

The trial was relatively short. At a median of 2.1 years, SUSTAIN-6 captured early cardiovascular events but could not assess whether benefits persist, grow, or plateau over longer treatment. The lack of a cardiovascular death benefit is consistent with this short exposure.

The trial was designed for noninferiority with a generous 1.8 margin. Though it achieved superiority, the event count was modest (254 primary events total), and the confidence intervals for individual components were wide. The stroke finding, while exciting, rested on a small number of events (32 vs. 44).

The retinopathy signal was the most discussed limitation. While likely related to rapid glucose lowering (a phenomenon documented with insulin decades earlier), it raised the question of whether semaglutide should be started cautiously in patients with proliferative retinopathy.

Drop-in of other cardiovascular medications was not controlled. Because clinicians could add or remove background drugs, some of the observed benefit could theoretically be attributed to differences in co-prescribing, though sensitivity analyses adjusting for this did not materially change the result.

What Happened After SUSTAIN-6

The FDA approved semaglutide for type 2 diabetes (as Ozempic) in December 2017. The cardiovascular data from SUSTAIN-6 were central to the approval but were not initially included as a labeled indication. In January 2020, the FDA granted an expanded indication for semaglutide to reduce the risk of major adverse cardiovascular events in adults with type 2 diabetes and established cardiovascular disease.

The SELECT trial (published 2023, NEJM) later extended the cardiovascular story to patients with obesity but without diabetes, using higher-dose semaglutide (2.4 mg weekly). SELECT showed a 20% reduction in MACE in that broader population, confirming that the cardiovascular benefit was not simply a downstream effect of better glucose control.

The American Diabetes Association Standards of Care now recommend GLP-1 receptor agonists with proven cardiovascular benefit (semaglutide, liraglutide, dulaglutide) for patients with type 2 diabetes and established atherosclerotic cardiovascular disease, independent of baseline HbA1c or need for additional glucose lowering.

How to Read This Trial Honestly

SUSTAIN-6 did not prove that semaglutide prevents heart attacks or strokes in all people with diabetes. It showed that, in a high-risk population already receiving standard care, adding semaglutide reduced a composite cardiovascular endpoint over roughly two years. The benefit was real but modest in absolute terms (2.3 percentage point difference). The number needed to treat (NNT) to prevent one MACE event over 2.1 years was approximately 45.

The trial tells us nothing about cardiovascular outcomes in low-risk diabetes patients, in prediabetes, or in people without diabetes who use semaglutide for weight loss (though SELECT addressed some of that gap later). It also cannot tell us whether the benefit comes from weight loss, blood pressure reduction, glucose lowering, direct vascular effects, or some combination. Mechanistic work continues.

For clinicians, the practical takeaway is straightforward: in a patient with type 2 diabetes and known cardiovascular disease, semaglutide is among the first-line add-on therapies regardless of where HbA1c sits. The cardiovascular protection is an independent reason to prescribe it.

Frequently asked questions

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References

  • Marso SP, Bain SC, Consoli A, et al. Semaglutide and cardiovascular outcomes in patients with type 2 diabetes. N Engl J Med. 2016;375(19):1834-1844. PubMed
  • Ozempic (semaglutide) prescribing information. Novo Nordisk. FDA Label
  • Lincoff AM, Brown-Frandsen K, Colhoun HM, et al. Semaglutide and cardiovascular outcomes in obesity without diabetes. N Engl J Med. 2023;389(24):2221-2232. PubMed
  • American Diabetes Association Professional Practice Committee. Standards of Care in Diabetes, 2024. Diabetes Care. 2024;47(Suppl 1). ADA Standards
  • Marso SP, Daniels GH, Tanaka K, et al. Liraglutide and cardiovascular outcomes in type 2 diabetes (LEADER). N Engl J Med. 2016;375(4):311-322. PubMed