Retatrutide vs Tirzepatide
Retatrutide and tirzepatide represent successive generations of incretin-based obesity therapeutics. Tirzepatide’s dual GIP/GLP-1 agonism achieved ~20% weight loss, while retatrutide’s triple agonism (GIP/GLP-1/glucagon) has demonstrated ~24% weight loss in phase 2 trials — approaching bariatric surgery efficacy.
Mechanism of Action
Section titled “Mechanism of Action”Tirzepatide: Dual GIP/GLP-1 Agonism
Section titled “Tirzepatide: Dual GIP/GLP-1 Agonism”Tirzepatide activates two incretin receptors:
- GIPR agonism (EC₅₀ ~0.14 nM): Enhances adipose tissue lipid handling, augments insulin secretion, and synergizes with GLP-1R for appetite suppression
- GLP-1R agonism (EC₅₀ ~0.18 nM): Reduces appetite, slows gastric emptying, suppresses glucagon, and improves glucose-dependent insulin secretion
The GIPR component contributes approximately 5–10% additional weight loss beyond GLP-1R agonism alone.
Retatrutide: Triple GIP/GLP-1/Glucagon Agonism
Section titled “Retatrutide: Triple GIP/GLP-1/Glucagon Agonism”Retatrutide activates three receptors:
- GIPR agonism (EC₅₀ ~0.06 nM): Similar to tirzepatide, enhancing adipose lipid handling and insulin secretion
- GLP-1R agonism (EC₅₀ ~0.19 nM): Appetite suppression and metabolic effects
- Glucagon receptor agonism (EC₅₀ ~0.04 nM): Increases energy expenditure, hepatic lipid oxidation, and thermogenesis
The glucagon receptor component is the key differentiator. Native glucagon increases hepatic glucose output, but retatrutide’s balanced triple agonism produces net glycemic improvement through the GLP-1R/GIPR components counterbalancing glucagon’s hyperglycemic effects.
Comparison Table
Section titled “Comparison Table”| Property | Retatrutide | Tirzepatide |
|---|---|---|
| Drug class | Triple agonist | Dual agonist |
| Receptors | GIPR + GLP-1R + GCGR | GIPR + GLP-1R |
| Structure | Linear 39-AA peptide | Linear 39-AA peptide |
| Fatty acid | C-20 eicosanedioic acid | C-20 eicosanedioic acid |
| Half-life | ~6 days | ~5 days |
| Dosing frequency | Once weekly | Once weekly |
| Max weight loss | ~24% body weight | ~21% body weight |
| Development phase | Phase 3 | Approved (Mounjaro/Zepbound) |
Phase 2 Clinical Data
Section titled “Phase 2 Clinical Data”Retatrutide (Phase 2)
Section titled “Retatrutide (Phase 2)”The phase 2 trial (n=338) evaluated retatrutide across dose levels for obesity:
- 2 mg weekly: 7.2% weight loss at 24 weeks
- 4 mg weekly: 12.5% weight loss at 24 weeks
- 8 mg weekly: 17.1% weight loss at 24 weeks
- 12 mg weekly: 20.9% weight loss at 24 weeks
- 12 mg (extended): 24.2% weight loss at 48 weeks
Key findings:
- ≥20% weight loss: 59% of participants at 12 mg
- ≥25% weight loss: 36% of participants at 12 mg
- HbA1c reduction: 2.0–2.2% in T2D subgroup
- Dose-dependent GI side effects (nausea 15–25%)
Tirzepatide (SURMOUNT-1)
Section titled “Tirzepatide (SURMOUNT-1)”- 5 mg weekly: 15.0% weight loss at 72 weeks
- 10 mg weekly: 19.5% weight loss at 72 weeks
- 15 mg weekly: 20.9% weight loss at 72 weeks
- ≥20% weight loss: 56% of participants at 15 mg
Direct Comparison (Indirect)
Section titled “Direct Comparison (Indirect)”| Endpoint | Retatrutide 12 mg (24 wk) | Tirzepatide 15 mg (72 wk) |
|---|---|---|
| Weight loss | 20.9% | 20.9% |
| ≥20% weight loss | 59% | 56% |
| HbA1c reduction | 2.0–2.2% | 2.4–2.6% |
| Duration | 24 weeks | 72 weeks |
Note: Retatrutide achieved comparable weight loss at 24 weeks that tirzepatide achieved at 72 weeks, suggesting faster onset. Extended retatrutide data at 48 weeks showed 24.2%, exceeding tirzepatide’s maximum.
Mechanism Differences
Section titled “Mechanism Differences”Weight Loss Mechanisms
Section titled “Weight Loss Mechanisms”| Mechanism | Tirzepatide | Retatrutide |
|---|---|---|
| Appetite suppression (central) | GLP-1R + GIPR | GLP-1R + GIPR |
| Gastric emptying | GLP-1R | GLP-1R |
| Energy expenditure | Minimal | Glucagon-mediated ↑ |
| Hepatic lipid oxidation | Minimal | Glucagon-mediated ↑ |
| Thermogenesis | Minimal | Glucagon-mediated ↑ |
| Adipose lipid mobilization | GIPR-mediated | GIPR-mediated |
Retatrutide’s glucagon receptor agonism adds a catabolic/energy expenditure component absent in tirzepatide, potentially explaining the greater weight loss and faster onset.
Metabolic Effects
Section titled “Metabolic Effects”| Parameter | Tirzepatide | Retatrutide |
|---|---|---|
| Insulin sensitivity | Improved (GIPR) | Improved (GIPR) |
| Hepatic steatosis | Improved | Improved (glucagon-mediated) |
| Glucagon suppression | Yes (GLP-1R) | Balanced (glucagon agonism) |
| Energy expenditure | Baseline | Increased |
| FFA mobilization | Moderate | Enhanced |
Side Effect Profiles
Section titled “Side Effect Profiles”Tirzepatide
Section titled “Tirzepatide”| Side Effect | Incidence |
|---|---|
| Nausea | 12–31% |
| Diarrhea | 12–17% |
| Vomiting | 5–12% |
| Decreased appetite | 11–18% |
| Constipation | 11–18% |
| Injection site reactions | 2–6% |
Retatrutide
Section titled “Retatrutide”| Side Effect | Incidence |
|---|---|
| Nausea | 15–25% |
| Diarrhea | 10–18% |
| Vomiting | 5–10% |
| Decreased appetite | 12–20% |
| Injection site reactions | 3–5% |
Both agents share GI-mediated side effects. Retatrutide may produce slightly more nausea due to triple-receptor activation, though data are still accumulating.
Development Status
Section titled “Development Status”Tirzepatide
Section titled “Tirzepatide”- FDA-approved: Mounjaro (T2D, 2022), Zepbound (Obesity, 2023)
- EMA-approved: Mounjaro (T2D)
- Phase 3: SURPASS program completed; SURMOUNT program completed
- Market availability: Widely available globally
Retatrutide
Section titled “Retatrutide”- Phase 3: TRIUMPH program initiated (2024–2025)
- Indications: Obesity, T2D, obstructive sleep apnea, knee osteoarthritis
- Expected approval: 2026–2027 (if phase 3 succeeds)
- Company: Eli Lilly
- Market availability: Not yet available
Dosing and Titration
Section titled “Dosing and Titration”Tirzepatide
Section titled “Tirzepatide”| Phase | Dose | Duration |
|---|---|---|
| Initiation | 2.5 mg weekly | 4 weeks |
| Escalation 1 | 5.0 mg weekly | 4+ weeks |
| Escalation 2 | 7.5 mg weekly | 4+ weeks |
| Escalation 3 | 10.0 mg weekly | 4+ weeks |
| Maximum | 15.0 mg weekly | Ongoing |
Retatrutide (Phase 2)
Section titled “Retatrutide (Phase 2)”| Phase | Dose | Duration |
|---|---|---|
| Initiation | 2 mg weekly | 4 weeks |
| Escalation 1 | 4 mg weekly | 4+ weeks |
| Escalation 2 | 8 mg weekly | 4+ weeks |
| Maximum | 12 mg weekly | Ongoing |
Both agents use 4-week titration intervals. Retatrutide’s phase 2 dosing ranged from 2–12 mg; final approved doses may differ.
When to Consider Each
Section titled “When to Consider Each”Tirzepatide is currently preferred when:
- Approved therapy is required
- Insurance coverage favors available brands
- Long-term safety data is desired
- Combination with established incretin pharmacology is preferred
- Access to clinical trials is unavailable
Retatrutide may be considered when:
- Maximum weight loss is the primary goal
- Faster weight loss onset is desired
- Clinical trial participation is available
- Glucagon-mediated metabolic benefits are of interest
- Tirzepatide has failed or is not tolerated
References
Section titled “References”- Jastreboff AM, et al. “Triple-hormone-receptor agonist retatrutide for obesity — a phase 2 trial.” NEJM 2023;389:514-526.
- Jastreboff AM, et al. “Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1).” NEJM 2022;387:205-216.
- Rosenstock J, et al. “Retatrutide, a GIP/GLP-1/glucagon receptor agonist, in type 2 diabetes.” NEJM 2023;389:2406-2418.
- Frías JP, et al. “Tirzepatide versus semaglutide once weekly in patients with type 2 diabetes (SURPASS-2).” NEJM 2023;388:523-534.
- Coskun T, et al. “LY3437943, a novel triple GIP/GLP-1/glucagon receptor agonist for obesity.” Diabetes Obes Metab 2023;25:2468-2479.