Series: Landmark Trials in Endocrinology & Metabolism | Study #9
Category: GLP-1 Receptor Agonists · Cardiovascular Outcomes | Design: Multicentre, double-blind, placebo-controlled RCT | n: 3,297 | Follow-up: 2.1 years (median)
📋 Summary
Authors: Marso SP et al., for the SUSTAIN-6 Investigators
Journal: N Engl J Med 2016;375:1834–1844 | DOI: 10.1056/NEJMoa1607141
SUSTAIN-6 was designed as a preapproval cardiovascular safety study for once-weekly subcutaneous semaglutide, enrolling 3,297 patients with type 2 diabetes and high cardiovascular risk, either from established atherosclerotic cardiovascular disease or chronic kidney disease or being aged 60 years or older with cardiovascular risk factors. Patients were randomised to semaglutide 0.5 mg or 1.0 mg once weekly or corresponding volume-matched placebo for a median of 2.1 years. The primary outcome was 3-point MACE. The trial met its primary endpoint of noninferiority and additionally demonstrated superiority: 6.6% in the semaglutide group versus 8.9% in the placebo group (HR 0.74; 95% CI 0.58 to 0.95; p<0.001 noninferiority, p=0.02 superiority). Among the components, nonfatal stroke was significantly reduced (HR 0.61; 95% CI 0.38 to 0.99; p=0.04), nonfatal MI showed a non-significant reduction (HR 0.74; 95% CI 0.51 to 1.08), and cardiovascular death was unchanged (HR 0.98; 95% CI 0.65 to 1.48). A prespecified safety endpoint of retinopathy complications was significantly higher in the semaglutide group (HR 1.76; 95% CI 1.29 to 2.40; p<0.001), a finding attributed to rapid glucose lowering in patients with pre-existing retinopathy and subsequently characterised as a metabolic phenomenon rather than a direct drug-related toxicity.
📊 Key Findings
| Outcome | Semaglutide | Placebo | Effect Size |
|---|---|---|---|
| 3-point MACE (CV death, MI, stroke) | 6.6% | 8.9% | HR 0.74 (0.58–0.95) · p=0.02 (superiority) |
| Nonfatal stroke | — | — | HR 0.61 (0.38–0.99) · p=0.04 |
| Nonfatal MI | — | — | HR 0.74 (0.51–1.08) · NS |
| Cardiovascular death | — | — | HR 0.98 (0.65–1.48) · NS |
| New or worsening nephropathy | — | — | HR 0.64 (0.46–0.88) |
| Retinopathy complications (safety) | — | — | HR 1.76 (1.29–2.40) · Safety signal |
| Absolute risk reduction (MACE) | ~2.3% over 2.1 years | NNT approximately 44 | |
💬 Expert Commentary
SUSTAIN-6 represents an important early evidence point for semaglutide as a cardiovascular drug, but its interpretation requires careful contextualisation. The trial was not powered as a definitive superiority cardiovascular outcomes trial. It was a regulatory safety study with a planned sample size and event target selected to rule out a 30% excess cardiovascular risk. The positive superiority result was therefore a signal rather than a definitive quantification of benefit, and the wide confidence interval (HR 0.58 to 0.95) reflects this. The stroke reduction was the most prominent component finding and aligned with what would later be seen in REWIND (dulaglutide) and echoed across GLP-1 receptor agonist class meta-analyses. The stroke benefit of this class remains one of the most consistent and clinically relevant signals in all of the cardiometabolic trial literature, and SUSTAIN-6 provided the first focused evidence for once-weekly semaglutide as a contributor to that signal.
The retinopathy finding was widely discussed at the time of publication and generated regulatory attention. Subsequent analyses established that the signal was concentrated in patients with pre-existing diabetic retinopathy who experienced rapid HbA1c reduction, a phenomenon well-recognised from studies of intensive insulin therapy in the 1990s. Subsequent large-scale SUSTAIN data and observational analyses have not identified a persistent or dose-dependent retinopathy signal with semaglutide, and the current consensus is that this was a consequence of acute metabolic normalisation rather than a direct drug toxicity. Nevertheless, clinical vigilance for retinopathy exacerbation in patients with pre-existing retinopathy starting semaglutide, particularly those with poor prior glycaemic control, remains warranted. SUSTAIN-6 was followed by the FLOW trial (published 2024), which demonstrated that semaglutide reduces renal endpoints, and the SELECT trial (Study #13 in this series), which extended the cardiovascular benefit to non-diabetic populations with obesity.
Limitations: SUSTAIN-6 was a safety trial not powered for superiority; the cardiovascular benefit should be interpreted as a hypothesis-generating signal confirmed subsequently by the SELECT and FLOW trials. Cardiovascular death was not reduced individually. The retinopathy signal requires monitoring in patients with established retinopathy. The study was industry-sponsored by Novo Nordisk.
🔑 BOTTOM LINE
SUSTAIN-6 demonstrated that once-weekly subcutaneous semaglutide reduced 3-point MACE by 26% in type 2 diabetes patients at high cardiovascular risk, driven primarily by a significant 39% reduction in nonfatal stroke, while also identifying a retinopathy complication signal in patients with pre-existing retinopathy undergoing rapid glucose lowering.
⭐ Clinical Impact Rating: ●●●●○ Practice-changing (confirmatory FLOW and SELECT trials elevate class)
Next in the series: Study #10 REWIND: Dulaglutide and Cardiovascular Outcomes in Type 2 Diabetes
