Development History of Retatrutide: From Discovery to Phase 3

Overview

The development of retatrutide (LY3437943) represents one of the most significant programs in metabolic drug development in recent years. From its origins in Eli Lilly’s peptide research laboratories to its current position as a Phase 3 investigational compound, retatrutide’s journey illustrates the evolution of incretin-based therapeutics from single-target to multi-target strategies.

Preclinical Discovery (2016-2019)

Concept and Rationale

The scientific foundation for retatrutide emerged from Eli Lilly’s deep expertise in incretin biology, cultivated over more than two decades of research that also produced tirzepatide (Mounjaro/Zepbound). Lilly researchers, led by Tamer Coskun and colleagues in the diabetes and obesity research division, recognized that the incretin-based therapy field was converging on a key insight: engaging multiple metabolic hormone receptors with a single molecule could produce synergistic effects exceeding those of single-receptor agonists.

The specific rationale for adding glucagon receptor agonism to the GIP/GLP-1 dual agonist framework was grounded in preclinical evidence that glucagon promotes energy expenditure and hepatic lipid oxidation. While this concept had been explored by academic groups and other pharmaceutical companies, Eli Lilly was uniquely positioned to execute it given their experience engineering multi-receptor peptides.

Peptide Engineering

The discovery phase involved extensive structure-activity relationship (SAR) studies to design a single peptide capable of activating GIP, GLP-1, and glucagon receptors with appropriate potency ratios. Starting from a GIP-based peptide backbone, Lilly chemists systematically modified amino acid residues to broaden receptor specificity while maintaining favorable pharmacokinetic properties. The addition of a C20 fatty diacid moiety via a gamma-glutamic acid linker provided albumin binding for half-life extension.

Multiple candidate molecules were synthesized and screened in cell-based receptor activation assays and animal models before LY3437943 was selected as the development candidate based on its optimal balance of receptor potencies, metabolic effects, and pharmacokinetic profile.

Preclinical Pharmacology

Comprehensive preclinical studies in diet-induced obese mice, diabetic mouse models, and non-human primates established the pharmacological proof of concept. Key preclinical findings included:

• Dose-dependent weight loss exceeding that of single- or dual-receptor agonists
• Improved glucose tolerance and insulin sensitivity
• Increased energy expenditure attributable to the glucagon receptor component
• Reduced hepatic steatosis
• Acceptable preclinical safety and toxicology profile

These data supported an Investigational New Drug (IND) application to the FDA, enabling the transition to human clinical testing.

Phase 1: First-in-Human (2019-2021)

Study Design

The first-in-human Phase 1 study was a randomized, double-blind, placebo-controlled trial conducted in healthy adult volunteers and patients with type 2 diabetes. The study employed single ascending dose (SAD) and multiple ascending dose (MAD) cohorts to characterize safety, tolerability, and pharmacokinetics across a range of dose levels.

Key Phase 1 Findings

• Safety: Retatrutide was generally well tolerated. Gastrointestinal adverse events (nausea, vomiting, diarrhea) were the most common side effects, consistent with the known pharmacology of GLP-1R agonism.
• Pharmacokinetics: The half-life was determined to be approximately six days, supporting once-weekly dosing. Dose-proportional increases in drug exposure were observed.
• Pharmacodynamic signals: Even at early dose levels, reductions in blood glucose and preliminary evidence of weight loss were observed, providing early biological proof of concept.
• Dose selection: Phase 1 data informed the dose range (0.5-12 mg) and escalation schedules used in the subsequent Phase 2 program.

The Phase 1 results were incorporated into the comprehensive pharmacology publication by Coskun et al. (2022) in Cell Metabolism, which presented both preclinical and Phase 1 clinical data in an integrated analysis.

Phase 2: Proof of Efficacy (2021-2023)

Trial Design and Execution

Two Phase 2 trials were initiated in 2021 and represented the most critical milestone in retatrutide’s development:

Phase 2 Obesity Trial (NCT04881706): A 48-week, randomized, double-blind, placebo-controlled, dose-ranging study in 338 adults with obesity or overweight. Participants were randomized to placebo or one of five retatrutide dose groups (1 mg, 4 mg escalating, 4 mg fixed, 8 mg escalating, 12 mg escalating).

Phase 2 Type 2 Diabetes Trial (NCT04867785): A 36-week, randomized, double-blind, placebo- and active-comparator-controlled study in 281 adults with type 2 diabetes. Participants were randomized to placebo, dulaglutide 1.5 mg (active comparator), or one of four retatrutide dose groups (0.5 mg, 4 mg, 8 mg, 12 mg).

Landmark Results

The Phase 2 results, presented at the American Diabetes Association (ADA) Scientific Sessions in June 2023 and simultaneously published in the New England Journal of Medicine and The Lancet, were transformative:

• Weight loss: Up to 24.2% mean weight reduction at 48 weeks in the obesity trial (12 mg group), the highest ever reported for a pharmaceutical agent
• Glycemic control: Up to 2.02% HbA1c reduction in the diabetes trial, with most participants at higher doses achieving non-diabetic HbA1c levels
• Liver fat: Up to ~82% relative reduction in liver fat content
• Tolerability: GI adverse event profile consistent with the incretin class; manageable with dose escalation

Scientific and Media Impact

The Phase 2 results generated substantial attention in both the scientific community and mainstream media. The magnitude of weight loss, in particular, was widely noted as approaching bariatric surgery outcomes, prompting discussion about the potential for pharmacotherapy to serve as an alternative or complement to surgical intervention for severe obesity.

Phase 3: The TRIUMPH Program (2023-Present)

Program Design

Following the compelling Phase 2 results, Eli Lilly advanced retatrutide into Phase 3 development under the TRIUMPH banner (Triple Receptor Agonist Intervention Utilizing Metabolic Pathways in Health). The TRIUMPH program comprises multiple pivotal trials designed to provide the definitive efficacy and safety data required for regulatory submissions.

TRIUMPH-1 (Obesity)

TRIUMPH-1 is the pivotal obesity trial, evaluating retatrutide in adults with obesity (BMI ≥30) or overweight (BMI ≥27) with weight-related comorbidities. The trial is expected to enroll thousands of participants across multiple countries, with a treatment duration sufficient to capture maximum weight loss and sustained safety data.

TRIUMPH-2 (Type 2 Diabetes)

TRIUMPH-2 is the pivotal type 2 diabetes trial, evaluating retatrutide in adults with type 2 diabetes who are inadequately controlled on existing therapies. Primary endpoints include HbA1c reduction and weight loss.

Additional Trials

The broader TRIUMPH program may include additional trials evaluating retatrutide in specific populations or for specific indications, such as MASLD/NASH, though detailed information about all planned studies has not been fully disclosed.

Key Milestones Timeline

Eli Lilly’s Strategic Context

Retatrutide’s development occurs within Eli Lilly’s broader metabolic disease portfolio, which also includes tirzepatide (approved as Mounjaro for type 2 diabetes and Zepbound for obesity) and orforglipron (an oral GLP-1R agonist in Phase 3 development). This portfolio reflects Lilly’s strategy of pursuing multiple complementary approaches to metabolic disease:

• Tirzepatide: Dual GIP/GLP-1 agonist (approved, injectable)
• Retatrutide: Triple GIP/GLP-1/glucagon agonist (Phase 3, injectable)
• Orforglipron: Oral GLP-1R agonist (Phase 3, oral)

If retatrutide is approved, it could be positioned as a more potent option for patients requiring greater weight loss or those with MASLD, while tirzepatide and orforglipron serve other patient segments. This tiered approach would provide physicians with a range of options matched to patient needs.

Challenges and Considerations

Manufacturing

Producing complex peptide therapeutics at scale is a significant manufacturing challenge. Eli Lilly has invested billions of dollars in manufacturing capacity expansion to support its metabolic portfolio, and retatrutide production capacity will be a factor in the drug’s commercial launch, if approved.

Market Competition

The metabolic therapeutics landscape is intensely competitive. By the time retatrutide potentially reaches the market, several other incretin-based agents will be available, including next-generation candidates from competitors like Novo Nordisk, Amgen, and others. Retatrutide will need to demonstrate differentiated value, whether through superior efficacy, unique hepatic effects, or other clinical advantages, to carve out a distinct market position.

Regulatory Considerations

The regulatory pathway for retatrutide involves demonstrating efficacy and safety in large Phase 3 trials for each proposed indication. The FDA’s requirements for obesity drugs include demonstration of clinically meaningful weight loss versus placebo, an acceptable safety profile, and sufficient long-term data. Additional regulatory pathways for potential MASLD indications would require separate clinical programs.

Summary

Retatrutide’s development history traces a clear arc from rational drug design through preclinical validation, early clinical proof of concept, and into large-scale pivotal trials. Each development phase has reinforced the hypothesis that triple receptor agonism produces metabolic benefits exceeding those of simpler receptor engagement strategies. The ongoing TRIUMPH program represents the critical next step in determining whether retatrutide’s promise will translate into regulatory approval and clinical practice.