Overview

Retatrutide (LY3437943) is a first-in-class triple agonist of the GLP-1, GIP, and glucagon receptors, developed by Eli Lilly. By activating three incretin and metabolic receptors simultaneously, retatrutide targets complementary pathways for appetite regulation, energy expenditure, and glucose homeostasis. Phase 2 results have demonstrated unprecedented weight loss efficacy, positioning retatrutide as a potential best-in-class anti-obesity agent.

Mechanism of Action

Retatrutide is a single peptide molecule that acts as a co-agonist at three receptors:

GLP-1 Receptor

• Reduces appetite via hypothalamic signaling
• Delays gastric emptying
• Enhances glucose-dependent insulin secretion
• Reduces glucagon secretion

GIP Receptor

• Enhances insulin secretion (incretin effect)
• Promotes脂肪 tissue lipid storage and browning
• Synergizes with GLP-1R for appetite suppression
• May improve insulin sensitivity

Glucagon Receptor

• Increases energy expenditure via hepatic lipid oxidation
• Promotes棕色脂肪 thermogenesis
• Enhances lipolysis and fatty acid oxidation
• May improve hepatic steatosis (MASH)

The triple agonism is hypothesized to produce additive or synergistic effects on weight loss compared to single or dual incretin agonists.

Phase 2 Trial Results

Study Design

The phase 2 trial (NCT04881706) was a randomized, double-blind, placebo-controlled study in adults with obesity (BMI ≥30) or overweight (BMI ≥27) with at least one weight-related comorbidity, without diabetes.

Dose Arms

• Retatrutide: 1 mg, 3 mg, 6 mg, 9 mg (subcutaneous, once weekly)
• Placebo
• Duration: 48 weeks
• Primary endpoint: Percent change in body weight from baseline

Key Results

Dose	n	Mean Weight Loss (48 wk)	≥5% Loss	≥10% Loss	≥20% Loss
1 mg	33	−7.2%	52%	26%	6%
3 mg	34	−14.5%	78%	54%	21%
6 mg	33	−17.1%	84%	65%	30%
9 mg	34	−24.2%	91%	82%	50%
Placebo	34	−2.1%	18%	3%	0%

Dose-Response Relationship

Weight loss showed a clear dose-response relationship with no plateau observed at 48 weeks at the highest dose. Modeling suggests that continued weight loss may occur beyond 48 weeks, with potential for >25% weight loss at 9 mg with extended treatment.

Weight Loss Trajectory

• Weight loss began within 4 weeks of treatment initiation
• Mean rate of weight loss: 0.5–0.8 kg/week at higher doses
• No evidence of weight plateau at 48 weeks at 9 mg dose
• Weight regain after discontinuation followed a similar trajectory to other incretin agents

Phase 3 Trials

TRIUMPH Program

Eli Lilly initiated the phase 3 TRIUMPH program in 2024, enrolling multiple pivotal trials:

Trial	Population	Primary Endpoint	Expected Completion
TRIUMPH-1	Obesity (no diabetes)	Weight change at 72 weeks	2026
TRIUMPH-2	T2D with obesity	HbA1c + weight change	2026
TRIUMPH-3	Obesity + CV risk	MACE	2027
TRIUMPH-4	Obesity + OSA	AHI change	2026
TRIUMPH-5	Obesity + MASH	Hepatic steatosis	2027

Enrollment

• Total enrollment target: >18,000 participants across all trials
• Study sites: North America, Europe, Asia, Australia
• Inclusion: BMI ≥30 or ≥27 with comorbidity
• Key exclusion: Type 1 diabetes, prior bariatric surgery

Comparative Efficacy

Retatrutide vs Semaglutide 2.4 mg

Parameter	Retatrutide 9 mg	Semaglutide 2.4 mg
Weight loss (48 wk)	−24.2%	−14.9%
≥20% weight loss	50%	~15%
Plateau at 48 wk?	No	Yes
GI tolerability	Moderate	Moderate

Retatrutide vs Tirzepatide 15 mg

Parameter	Retatrutide 9 mg	Tirzepatide 15 mg
Weight loss (48–72 wk)	−24.2%	−20.9%
Receptor targets	3 (GLP-1, GIP, GCGR)	2 (GLP-1, GIP)
Glucagon component	Yes	No
Energy expenditure	Increased	Minimal

Safety and Tolerability

Adverse Events (Phase 2)

Adverse Event	Retatrutide 9 mg	Placebo
Nausea	35%	10%
Diarrhea	25%	8%
Decreased appetite	18%	5%
Vomiting	15%	3%
Constipation	12%	5%
Injection site reaction	8%	3%

Serious Adverse Events

• No treatment-related serious adverse events in phase 2
• No cases of pancreatitis
• No medullary thyroid carcinoma signals
• Liver enzyme elevations: 2 patients (resolved spontaneously)

Glucagon Receptor-Specific Concerns

• Hepatic safety is monitored given glucagon receptor activation
• No significant liver enzyme elevations observed in phase 2
• Glucagon receptor agonism may actually improve hepatic steatosis
• Long-term hepatic safety data pending from phase 3

Pharmacokinetics

Parameter	Value
Half-life	~5 days
Tmax	24–48 hours
Bioavailability	~80% (SC)
Steady state	~3 weeks
Dosing	Once weekly
Albumin binding	>99%

The long half-life supports once-weekly dosing and contributes to sustained receptor activation across the dosing interval.

Biomarker Effects

Metabolic Markers

• Fasting glucose: −12% at 9 mg
• HbA1c: −0.8% at 9 mg (in non-diabetic subjects)
• Triglycerides: −25% at 9 mg
• HDL cholesterol: +8% at 9 mg
• hsCRP: −30% at 9 mg

Hepatic Markers

• ALT: −15% at 9 mg
• AST: −12% at 9 mg
• Hepatic fat fraction (MRI-PDFF): −55% at 9 mg (exploratory subgroup)

Current Status (2025)

• Phase 2: Completed, published in New England Journal of Medicine
• Phase 3 (TRIUMPH): Enrolled, ongoing
• FDA Breakthrough Therapy designation: Granted 2024
• Anticipated NDA filing: 2027 (pending phase 3 results)
• Potential approval: 2027–2028

Significance

Retatrutide represents the first triple agonist in the incretin class, with potential to surpass the efficacy of both semaglutide and tirzepatide. The addition of glucagon receptor agonism introduces a distinct mechanism for energy expenditure and hepatic fat reduction, which may be particularly relevant for patients with obesity-associated MASH or metabolic syndrome.

Deep dive: Read about Semaglutide vs Tirzepatide for comparison with dual agonists, or explore Clinical Trial Design for phase 3 methodology.

Test yourself: Take the Incretin Pharmacology Quiz or study with GLP-1 Receptor Flashcards.