Retatrutide Phase 3 Trials: What Researchers Are Watching in the TRIUMPH Program

The Phase 2 retatrutide data published in 2023 set a high bar — approximately 24.2% mean body weight reduction at the highest dose over 48 weeks. That result was compelling enough to accelerate the conversation about what Phase 3 would need to prove and what outstanding questions remained. This article summarizes what is publicly known about the TRIUMPH Phase 3 program as of 2024, explains why Phase 3 design matters in ways that Phase 2 cannot address, and outlines the key signals researchers are watching as the program advances.

From Proof-of-Concept to Pivotal: Why Phase 3 Is Different

Phase 2 trials are designed to establish proof of concept, find the right dose range, and generate an initial safety signal in a controlled population. They typically enroll hundreds of participants and run for months. Phase 3 trials — the pivotal studies required for regulatory approval — are designed to answer entirely different questions in a much larger, longer, and more diverse setting.

For obesity pharmacology specifically, Phase 3 trials in this class have generally followed several design features:

• Duration: 72 weeks or longer (compared to the 48-week Phase 2)
• Sample size: Thousands of participants (compared to hundreds in Phase 2)
• Endpoints: Primary weight loss endpoints plus secondary cardiovascular, metabolic, and functional outcomes
• Populations: Broader enrollment criteria including special populations (older adults, those with type 2 diabetes, cardiovascular disease, etc.)

The tirzepatide SURMOUNT-1 trial, for example, enrolled approximately 2,539 participants over 72 weeks (Jastreboff et al., 2022). For retatrutide, Phase 3 would need to match or exceed that level of rigor to support a regulatory filing.

The TRIUMPH Program: What Is Publicly Known

Eli Lilly has initiated the TRIUMPH Phase 3 program for retatrutide. As of 2024, the program encompasses multiple trials targeting different populations, consistent with how other major obesity pharmacology programs have been structured. The trials are expected to include participants with obesity without type 2 diabetes, participants with obesity and type 2 diabetes, and potentially those with obesity-related cardiovascular risk factors.

Key design elements being evaluated and monitored by the research community include:

• 72-week primary efficacy endpoint: This longer duration will test whether the weight loss trajectory seen at 48 weeks in Phase 2 continues, plateaus, or diverges from the Phase 2 projection
• Larger sample size: Thousands of enrolled participants will allow detection of adverse events that were too rare to appear in the Phase 2 cohort, and will provide adequate statistical power for secondary endpoint analysis
• Cardiovascular outcomes as secondary endpoints: Following the cardiovascular outcomes trial (CVOT) precedent established in the GLP-1 class, retatrutide's Phase 3 program is expected to include CV safety data

Why Cardiovascular Safety Is a Priority Signal

The GLP-1 receptor agonist class established an important cardiovascular safety precedent with trials demonstrating not just neutral but potentially beneficial CV outcomes. Semaglutide's CVOT data helped drive adoption across both diabetes and obesity indications. For retatrutide, researchers are watching closely for two related questions: first, whether the cardiovascular safety profile is at minimum non-inferior to placebo; and second, whether triple agonism with GCGR engagement produces any CV signals — positive or negative — that differ from the dual agonist class.

Glucagon receptor agonism has historically raised theoretical concerns about cardiovascular effects, including heart rate variability and effects on cardiac output. In the Phase 2 trial, retatrutide showed heart rate increases consistent with the class, but larger, longer, more powered trials are needed before any firm conclusions can be drawn.

What Researchers Are Watching: Durability

One of the outstanding questions from Phase 2 is durability. The 48-week trial cannot answer whether weight loss is maintained at 72 weeks, 2 years, or beyond. Obesity pharmacology has a well-documented history of weight regain following drug discontinuation. One critical Phase 3 question is whether retatrutide-associated weight loss is stable at 72 weeks, and what the off-treatment trajectory looks like.

If weight loss continues to increase between weeks 48 and 72 — as the no-plateau Phase 2 dose-response might suggest — that would be a positive durability signal. If weight loss plateaus or reverses during the same period, it would revise the Phase 2 interpretation substantially.

Head-to-Head Comparisons and the Competitive Landscape

Phase 3 trials typically compare active drug against placebo rather than against other active agents. As a result, the TRIUMPH program will not directly answer whether retatrutide outperforms tirzepatide in a controlled head-to-head setting. Researchers will continue to draw inferential comparisons across trials, with all the cross-trial limitations that entails.

However, the overall competitive context matters: if retatrutide achieves superior weight reduction with a comparable or better safety profile relative to what tirzepatide demonstrated in its Phase 3 program, that distinction will shape clinical research priorities, future combination therapy exploration, and regulatory strategy.

Special Populations and Phase 3 Broadening

Phase 2 enrollment was relatively tight — adults with obesity without type 2 diabetes. Phase 3 will likely include:

• Participants with obesity and type 2 diabetes
• Participants with obesity and established cardiovascular disease
• Participants with obesity-related liver disease (MASH/NAFLD)
• Potentially adolescent populations, depending on regulatory strategy and post-approval study plans

Each of these subgroups introduces new questions about efficacy and safety that Phase 2 was not designed to address.

Expected Timelines

As of 2024, the TRIUMPH trials were ongoing. Given that Phase 3 obesity trials in this class typically require 72–96 weeks of active treatment plus follow-up and regulatory preparation time, regulatory filing would not be anticipated before the 2026–2027 timeframe at the earliest, depending on enrollment pace and regulatory discussions. These timelines are subject to change based on Eli Lilly's development decisions and emerging data.

Key Takeaways

• The TRIUMPH Phase 3 program represents the pivotal development stage for retatrutide, scaling from hundreds to thousands of participants over 72+ weeks
• Phase 3 will test durability (does weight loss hold past 48 weeks?), cardiovascular safety, and efficacy in broader and more diverse populations
• Cardiovascular secondary endpoints are expected and will be watched closely, particularly given GCGR agonism's theoretical CV implications
• No head-to-head comparison against tirzepatide is expected within the Phase 3 program; cross-trial comparisons remain methodologically limited
• Special populations (T2D, CV disease, MASH) will likely be included in Phase 3 design, expanding on Phase 2's narrower enrollment