Honest Criticisms and Limitations of the SUSTAIN-6 Trial

At a glance

Parameter	Detail
N	3,297
Intervention	Semaglutide 0.5 mg or 1.0 mg subcutaneous, once weekly
Comparator	Placebo (added to standard of care)
Duration	104 weeks (median follow-up ~2.1 years)
Primary endpoint	First occurrence of 3-point MACE (CV death, nonfatal MI, nonfatal stroke)
Key result	HR 0.74, 95% CI 0.58, 0.95; p <0.001 for noninferiority

Why a Limitations Page Exists

The SUSTAIN-6 trial changed the trajectory of GLP-1 receptor agonist prescribing. Published in the New England Journal of Medicine in 2016, it was the second major CVOT (after LEADER for liraglutide) to show a cardiovascular signal favoring a GLP-1 RA over placebo in patients with type 2 diabetes and established or high-risk cardiovascular disease. That signal, a 26% relative reduction in three-point MACE, helped reshape FDA labeling and ADA treatment guidelines.

But every trial has design constraints that bound the confidence we can place in its conclusions. The limitations below are not reasons to dismiss SUSTAIN-6. They are reasons to read it carefully.

1. Designed for Noninferiority, Not Superiority

SUSTAIN-6 was powered as a noninferiority trial. The prespecified margin was an upper bound of the 95% confidence interval for the hazard ratio of 1.8, the same threshold the FDA's 2008 cardiovascular guidance for diabetes drugs required. The trial was not designed or powered to demonstrate superiority for individual MACE components.

The HealthRX.com Noninferiority-vs-Superiority Interpretation Framework:

The distinction matters here. SUSTAIN-6 accumulated 254 primary endpoint events across 3,297 patients. By comparison, LEADER enrolled 9,340 patients and accumulated 1,302 MACE events to power a superiority finding for liraglutide. The SUSTAIN-6 hazard ratio of 0.74 crossed the superiority boundary in a hierarchical testing procedure, but the trial's smaller event count means the confidence interval (0.58, 0.95) is wide. The point estimate could plausibly sit anywhere from a 42% reduction to a 5% reduction.

This is not a flaw in execution. It is a feature of the design. Novo Nordisk structured SUSTAIN-6 to satisfy the FDA's pre-approval cardiovascular safety requirement efficiently, not to generate the definitive superiority dataset.

2. Enriched Population Limits Generalizability

Enrollment criteria required either established cardiovascular disease (83% of participants) or chronic kidney disease, or both. Participants were older (mean age 64.6 years), had long-standing diabetes (mean duration 13.9 years), and had a mean baseline HbA1c of 8.7%.

This enrichment achieved two things: it increased the event rate (enabling the trial to finish faster with fewer patients) and it tested the drug in the population where cardiovascular harm would be most dangerous. Both are reasonable from a regulatory standpoint.

The trade-off is that the SUSTAIN-6 results tell us relatively little about:

• Younger patients with newly diagnosed T2D who lack established CVD
• Primary prevention populations where MACE event rates are far lower
• Patients without renal impairment, who represented a minority of the cohort
• Non-White populations, given that 83% of enrolled participants were White

The 2018 ADA/EASD consensus report acknowledged this by recommending GLP-1 RAs with proven CV benefit specifically for patients with established atherosclerotic cardiovascular disease, not as a blanket recommendation across all T2D phenotypes.

3. Two-Year Follow-Up Is Short for Cardiovascular Conclusions

The median follow-up was approximately 2.1 years. Cardiovascular outcome trials in diabetes have trended toward longer observation periods. LEADER ran for 3.8 years. The UKPDS legacy studies demonstrated that glycemic intervention effects on macrovascular outcomes sometimes take a decade to separate clearly from control.

Two years is long enough to detect a safety signal (the FDA's primary concern) and to identify large treatment effects. It is not long enough to answer several clinically important questions:

• Does the MACE benefit persist, grow, or attenuate after year two?
• Are there late-emerging adverse effects (thyroid C-cell concerns, pancreatic events) that require longer observation?
• Does the retinopathy signal (HR 1.76, 95% CI 1.11, 2.78) stabilize, worsen, or reverse with continued treatment?

The retinopathy finding is particularly relevant. The SUSTAIN-6 publication noted a statistically significant increase in diabetic retinopathy complications with semaglutide. The authors attributed this primarily to rapid HbA1c reduction in patients with pre-existing retinopathy, a phenomenon observed with insulin intensification as well. But without extended follow-up, the natural history of this signal remains uncertain.

4. Composite Endpoint Masking

Three-point MACE bundles cardiovascular death, nonfatal myocardial infarction, and nonfatal stroke into a single outcome. This is standard for diabetes CVOTs, but composite endpoints can obscure which components are driving the overall result.

In SUSTAIN-6, the component breakdown was:

The composite was driven primarily by nonfatal stroke and nonfatal MI reductions. Cardiovascular death showed no meaningful separation (HR 0.98). This pattern is worth noting: patients prescribed semaglutide based on the headline 26% MACE reduction might reasonably assume their risk of dying from cardiovascular causes is lower, but the trial data do not support that specific interpretation with any statistical confidence.

The nonfatal stroke reduction (HR 0.61) reached nominal significance, but the event count was small (27 vs. 44). Small absolute numbers in post-hoc component analyses are susceptible to random variation, and the trial was not powered for these individual comparisons.

5. Sponsor Design and Conduct

Novo Nordisk designed the trial, funded it, and was involved in data collection, analysis, and manuscript preparation. The trial publication discloses this clearly: "The sponsor (Novo Nordisk) designed the trial, with input from the steering committee."

This is standard practice for pharmaceutical CVOTs and does not inherently invalidate the findings. But it introduces structural considerations:

• Protocol design choices (noninferiority margin, enrichment criteria, follow-up duration) were optimized for regulatory efficiency, not necessarily for clinical informativeness
• Data access was managed through the sponsor, with academic investigators analyzing data under contractual arrangements
• Publication timing aligned with regulatory submissions, creating incentive pressure to present results favorably
• The steering committee had "final responsibility for the decision to submit for publication," per the manuscript, though the degree of truly independent analytic access has been debated in post-publication commentary

A 2017 editorial in The Lancet Diabetes & Endocrinology by Cefalu et al. discussed the broader challenge of interpreting sponsor-run CVOTs, noting that while results are valid, clinicians should weigh the distinction between regulatory-grade evidence (is the drug safe enough?) and practice-changing evidence (should this drug be preferred?).

6. Background Therapy Was Not Standardized

Participants continued their existing diabetes medications, and investigators could adjust background therapy (including insulin) at their discretion throughout the trial. This pragmatic design mirrors real-world practice but introduces confounding.

If semaglutide's glucose-lowering effect led to reductions in insulin dose or discontinuation of sulfonylureas, the MACE benefit could be partially attributable to reduced hypoglycemia or changes in concomitant medication rather than a direct vascular effect of semaglutide. The trial did not adjudicate or systematically report background therapy changes in a way that fully addresses this possibility.

7. Withdrawal and Adherence Patterns

Approximately 20% of participants in the semaglutide groups discontinued treatment prematurely, primarily due to gastrointestinal side effects. The analysis used a modified intention-to-treat approach, counting events regardless of treatment discontinuation, which is conservative and appropriate. But a 20% discontinuation rate means the observed treatment effect reflects a mixture of full exposure, partial exposure, and no exposure to semaglutide.

In a superiority framework, high discontinuation dilutes the treatment effect and biases toward the null, meaning the true on-treatment benefit may be larger. In clinical practice, however, the relevant question is whether patients who stop the drug due to nausea at month three still carry any residual cardiovascular protection. SUSTAIN-6 does not answer that question.

8. What Post-Publication Commentary Added

Several letters to the editor and subsequent review articles raised points that did not appear in the primary publication:

• The retinopathy signal generated substantial discussion, with multiple correspondents noting the need for prospective ophthalmologic monitoring in future semaglutide trials
• Questions about the interaction between rapid A1c lowering and microvascular outcomes were compared to historical data from the DCCT/EDIC studies
• The SELECT trial (2023, NEJM), which tested semaglutide 2.4 mg in patients with obesity and CVD but without diabetes, later confirmed a 20% MACE reduction in a dedicated superiority design with 17,604 participants, partially addressing the power limitations of SUSTAIN-6

Putting It Together

SUSTAIN-6 accomplished what it was designed to do: it demonstrated cardiovascular safety and generated a credible signal of benefit for subcutaneous semaglutide in high-risk type 2 diabetes. The 26% MACE reduction is real in the statistical sense. The question that the limitations above collectively raise is not "was semaglutide harmful?" but "how confident should we be in the magnitude and breadth of the benefit?"

For patients resembling the trial population (established CVD, long-duration T2D, older age, predominantly White), the evidence is relatively strong. For everyone else, the evidence is borrowed, extrapolated, or pending from other trial programs.

Frequently asked questions

›Was SUSTAIN-6 designed to prove semaglutide reduces heart attacks and strokes?

No. SUSTAIN-6 was designed as a noninferiority trial to satisfy FDA cardiovascular safety requirements. The superiority finding was a secondary result from hierarchical testing, not the primary design goal. The trial enrolled 3,297 patients, far fewer than the 9 to 340 in LEADER, which was powered for superiority from the outset.

›Why does the 26% MACE reduction come with a wide confidence interval?

The 95% confidence interval (0.58, 0.95) spans a range from a 42% benefit to a 5% benefit because the trial accumulated only 254 primary endpoint events. Larger event counts produce narrower intervals and more precise estimates. The relatively small sample size, appropriate for a noninferiority trial, limits the precision of the point estimate.

›Did semaglutide reduce cardiovascular death in SUSTAIN-6?

No. The hazard ratio for cardiovascular death was 0.98 (95% CI 0.65, 1.48), showing no separation between semaglutide and placebo. The overall MACE reduction was driven by reductions in nonfatal MI and nonfatal stroke.

›What was the retinopathy concern in SUSTAIN-6?

Semaglutide was associated with a 76% increase in diabetic retinopathy complications (HR 1.76, 95% CI 1.11, 2.78). The authors attributed this to rapid HbA1c lowering in patients with pre-existing retinopathy, a known risk with insulin intensification. The finding prompted calls for ophthalmologic monitoring in subsequent semaglutide studies.

›How many participants dropped out of SUSTAIN-6?

Approximately 20% of semaglutide-treated participants discontinued the drug prematurely, most commonly due to gastrointestinal adverse effects. The modified intention-to-treat analysis counted events regardless of discontinuation, which is methodologically conservative but means the observed effect reflects mixed exposure levels.

›Can SUSTAIN-6 results be applied to patients without established cardiovascular disease?

The trial enrolled 83% of participants with established CVD. Results in primary prevention populations (patients without existing heart disease) cannot be directly inferred from this data. The ADA/EASD 2018 consensus specifically recommended GLP-1 RAs with proven CV benefit for patients with established atherosclerotic CVD.

›Did the SELECT trial address SUSTAIN-6's limitations?

Partially. SELECT (2023) tested semaglutide 2.4 mg in 17,604 patients with obesity and CVD but without diabetes, using a superiority design. It confirmed a 20% MACE reduction with much greater statistical power and longer follow-up, addressing the sample size and design concerns of SUSTAIN-6 while extending the evidence to a non-diabetic population.

›Does Novo Nordisk's sponsorship of SUSTAIN-6 compromise the results?

Sponsorship does not invalidate results, but it shaped protocol decisions (noninferiority design, enriched population, two-year duration) that favored regulatory efficiency over clinical informativeness. The steering committee retained publication authority, and the results have been replicated directionally by subsequent independent analyses and the larger SELECT trial.

›Why was the follow-up period only two years?

Two years was sufficient to meet the FDA's cardiovascular safety requirement and to accumulate enough events for the noninferiority analysis. Longer follow-up would have answered additional questions (durability of benefit, late adverse effects, retinopathy trajectory) but would have delayed regulatory approval and increased trial costs.

›How does SUSTAIN-6 compare to LEADER in trial design quality?

LEADER enrolled nearly three times as many patients (9,340 vs. 3,297), followed them for 3.8 years (vs. 2.1), accumulated five times the MACE events (1,302 vs. 254), and was powered for superiority from the start. LEADER provides a more statistically precise estimate of GLP-1 RA cardiovascular benefit, though both trials point in the same direction.

References

1. 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
2. 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
3. Davies MJ, D'Alessio DA, Fradkin J, et al. Management of hyperglycemia in type 2 diabetes, 2018: a consensus report by ADA and EASD. Diabetes Care. 2018;41(12):2669-2701. PubMed
4. Lincoff AM, Brown-Frandsen K, Colhoun HM, et al. Semaglutide and cardiovascular outcomes in obesity without diabetes (SELECT). N Engl J Med. 2023;389(24):2221-2232. PubMed
5. FDA. Ozempic (semaglutide) prescribing information. Revised 2020. FDA Label
6. Cefalu WT, Kaul S, Gerstein HC, et al. Cardiovascular outcomes trials in type 2 diabetes: where do we go from here? Lancet Diabetes Endocrinol. 2017;5(5):325-328. PubMed