Clinical Context
This landmark Phase 2 trial, published in The Lancet in 2018, introduced the world to LY3298176—a molecule that would later become tirzepatide (Mounjaro/Zepbound). It represented a novel therapeutic concept: simultaneously activating both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 receptors, the two major incretin hormones released after eating.
GLP-1 receptor agonists like semaglutide and liraglutide had already proven highly effective for diabetes and weight management. However, GIP had been largely overlooked—early research suggested GIP was less effective in diabetes due to receptor desensitization. Eli Lilly’s scientists hypothesized that a molecule activating both receptors might provide synergistic or additive benefits beyond GLP-1 agonism alone.
The rationale was compelling: GIP and GLP-1 have complementary mechanisms. Both enhance glucose-dependent insulin secretion, but they differ in effects on glucagon (GLP-1 suppresses; GIP’s effect is glucose-dependent), gastric emptying (primarily GLP-1), and adipose tissue (GIP may enhance fat storage in fed state but improve lipid metabolism overall). The combination was hypothesized to provide superior metabolic benefits.
This Phase 2 trial tested whether the dual agonist concept translated into clinical benefit—and the results were striking enough to launch one of the most successful drug development programs in diabetes history.
Study Summary (PICO Framework)
Summary: In patients with poorly controlled type 2 diabetes, LY3298176 (now tirzepatide), a dual GIP and GLP-1 receptor agonist, significantly improved glycemic control and reduced body weight beyond the GLP-1 agonist dulaglutide compared to placebo and dulaglutide, with gastrointestinal adverse events typical of incretin-based therapies.
| PICO | Description |
|---|---|
| Population | Adults with type 2 diabetes inadequately controlled on diet, exercise, and metformin. |
| Intervention | LY3298176 (tirzepatide) at varying doses (1, 5, 10, or 15 mg weekly). |
| Comparison | Placebo and dulaglutide 1.5 mg weekly (GLP-1 agonist active comparator). |
| Outcome | Superior HbA1c reduction and weight loss with LY3298176 vs both placebo and dulaglutide. GI side effects (nausea, vomiting) were common but manageable. |
Clinical Pearls
1. This trial validated the dual incretin agonist hypothesis. By demonstrating that LY3298176 outperformed dulaglutide (a selective GLP-1 agonist), the study proved that adding GIP agonism to GLP-1 agonism provides incremental benefit. This was not a given—previous GIP research had been discouraging, and combining two mechanisms doesn’t always improve outcomes.
2. Weight loss exceeded what was expected from GLP-1 agonism alone. The dual agonist produced more weight loss than dulaglutide, suggesting GIP contributes to weight effects. Interestingly, GIP alone tends to promote fat storage in the fed state, but in combination with GLP-1 (and in the context of reduced caloric intake), the effect was enhanced weight loss.
3. The GI side effect profile was similar to GLP-1 agonists. Nausea and vomiting were common, as expected with incretin-based therapies, but rates were not dramatically higher than dulaglutide. This was reassuring—there was concern that dual agonism might cause intolerable GI effects.
4. This study launched a transformative drug development program. Based on these Phase 2 results, Eli Lilly proceeded with the SURPASS Phase 3 program, which confirmed tirzepatide’s superiority over semaglutide. The drug was subsequently approved as Mounjaro (for diabetes) and Zepbound (for obesity).
Practical Application
Current relevance: While this is a historical Phase 2 trial, it provides the scientific foundation for understanding tirzepatide’s mechanism. Clinicians prescribing tirzepatide today should understand that its enhanced efficacy comes from dual receptor agonism, not simply higher GLP-1 activity.
How tirzepatide compares to GLP-1 agonists: Based on subsequent Phase 3 data (SURPASS-2), tirzepatide produces greater HbA1c reduction (~0.5% more) and weight loss (~5-6 kg more) compared to semaglutide 1 mg. This positions tirzepatide as the most potent incretin-based therapy currently available.
Appropriate prescribing: Tirzepatide is indicated for type 2 diabetes (Mounjaro) and obesity without diabetes (Zepbound). Start at 2.5 mg weekly and titrate monthly to 5 mg, 10 mg, and 15 mg as tolerated. Slow titration minimizes GI side effects.
Broader Evidence Context
This Phase 2 trial was followed by the comprehensive SURPASS Phase 3 program: SURPASS-1 through SURPASS-5 established tirzepatide’s efficacy across different patient populations. In all trials, tirzepatide demonstrated superior or non-inferior efficacy with a favorable safety profile.
The obesity program (SURMOUNT) subsequently demonstrated even greater weight loss in non-diabetic patients (~20-22% at 72 weeks), leading to FDA approval for obesity management. The success of tirzepatide has spawned interest in other multi-agonist approaches, including triple agonists like retatrutide.
Study Limitations
As a Phase 2 trial, the study was relatively small and short (26 weeks). The comparator was dulaglutide, not semaglutide (the most potent GLP-1 agonist at the time of Phase 3 development). Long-term safety and cardiovascular outcomes were not assessed—these have since been addressed in the ongoing SURPASS-CVOT trial.
Bottom Line
This pivotal Phase 2 trial established that dual GIP and GLP-1 receptor agonism provides superior glycemic and weight benefits compared to GLP-1 agonism alone. LY3298176, now known as tirzepatide, became the first-in-class dual incretin agonist and has since been proven the most effective incretin-based therapy available.
Source: Frias JP, et al. “Efficacy and safety of LY3298176, a novel dual GIP and GLP-1 receptor agonist, in patients with type 2 diabetes: a randomised, placebo-controlled and active comparator-controlled phase 2 trial.” The Lancet. Read article.
