Retatrutide, Tirzepatide and Semaglutide Compared in a New MC4R-Deficient Obesity Mouse Study
TL;DR: Mouse study: tirzepatide cut weight 31.6%, retatrutide 24.1%, semaglutide 19.7% in MC4R-deficient obesity (Int J Obes, Feb 2026).
What Did This New Comparative Study Investigate?
A study published in the International Journal of Obesity in February 2026 asked a question that the headline human TRIUMPH and SURMOUNT trials cannot answer on their own: do GLP-1-class agonists still drive weight loss when the melanocortin-4 receptor (MC4R) pathway — a completely separate hypothalamic circuit — is genetically knocked out?[1] The paper, titled "Efficacy of GLP-1 analog peptides, semaglutide, tirzepatide, and retatrutide on MC4R deficient obesity and their comparison," is credited to Hitaka, Sugawara, Matsumoto and colleagues and appears in Int J Obes volume 50, pages 928–937.[1]
MC4R sits downstream of leptin and POMC signalling in the hypothalamus and is one of the best-characterised monogenic causes of severe, early-onset human obesity. Because it is anatomically and mechanistically distinct from the GLP-1, GIP and glucagon receptors that incretin-class peptides target, MC4R-knockout (KO) rodents give researchers a way to test whether GLP-1-class agonism still works when the melanocortin arm of appetite regulation is broken — a question with direct relevance to genetically defined obesity subtypes. This is a preclinical, in vivo rodent study, entirely separate from Eli Lilly's Phase 3 TRIUMPH programme in human obesity, which is covered in our retatrutide research overview and retatrutide vs tirzepatide vs semaglutide comparison.
Velox Peptides supplies retatrutide as an HPLC-verified (≥99% purity) lyophilised reagent for in vitro research use only. View the product page →
How Was the Study Designed?
The researchers used MC4R-knockout mice, a genetically engineered strain that develops severe obesity from disrupted melanocortin signalling regardless of diet. Separate cohorts received once-daily dosing with semaglutide, tirzepatide or retatrutide for 21 days, with body weight, food intake and body composition tracked throughout the protocol.[1]
Body composition — fat mass and lean mass — was measured with an Echo-MRI system, a non-invasive quantitative NMR technique standard in rodent metabolic research. Alongside weight and composition, the team tracked plasma insulin, the HOMA-IR insulin-resistance index, cholesterol, and the liver-enzyme markers AST and ALT, plus liver hypertrophy.[1]
Hitaka K, Sugawara T, Matsumoto M, et al. Int J Obes 2026;50:928–937.
Source: nature.com, DOI 10.1038/s41366-026-02025-2 · PMID 41723268
Which Compound Reduced Weight the Most in MC4R-Knockout Mice?
Across the 21-day protocol, all three GLP-1-class peptides significantly reduced body weight relative to baseline in MC4R-KO mice, despite the disrupted melanocortin pathway. Tirzepatide, the dual GLP-1/GIP receptor agonist, produced the largest reduction; retatrutide, the triple GLP-1/GIP/glucagon receptor agonist, was second; semaglutide, the single-receptor GLP-1 agonist, produced the smallest reduction of the three.[1]
The authors' framing is that receptor count alone does not predict rank order in this model: retatrutide's added glucagon-receptor activity did not translate into the largest weight effect here, and tirzepatide's dual-receptor profile outperformed both the single-receptor and triple-receptor compounds. This is a single preclinical dataset in one knockout strain, and it should not be read across to relative efficacy in humans, where the compounds' receptor-binding ratios, half-lives and titration schedules differ from the doses used in this mouse protocol.
What Happened to Body Composition and Metabolic Markers?
Echo-MRI analysis found that all three peptides significantly suppressed both fat mass and lean mass in the MC4R-KO mice — a pattern consistent with the broader GLP-1-class literature, where lean-mass loss alongside fat-mass loss is a well-documented feature of this drug class in rodent and human studies alike.[1]
Insulin sensitivity and glycaemic markers
All three compounds improved plasma insulin levels and the HOMA-IR insulin-resistance index relative to untreated MC4R-KO controls, indicating better glycaemic handling despite the ongoing melanocortin-pathway disruption.
Cholesterol and liver markers
Cholesterol levels improved across all three treatment groups. The liver-damage markers AST and ALT were reduced, and liver hypertrophy — enlargement associated with obesity-driven hepatic fat accumulation — was lessened in treated animals compared with untreated MC4R-KO controls.
Taken together, the metabolic data suggest the anti-obesity and metabolic benefits of GLP-1-class agonism are not solely dependent on an intact melanocortin pathway in this rodent model — a mechanistic finding relevant to research into genetically defined obesity subtypes, independent of any human treatment claim.
Is There Any Human Data on GLP-1 Drugs in MC4R-Deficient Obesity?
Tirzepatide: a real-world genetic subgroup analysis exists
A retrospective analysis of the licensed SURMOUNT-1 trial (Bhatnagar et al., Nature Medicine, 26 August 2025) found 32 of 2,291 genotyped participants carried pathogenic MC4R mutations. Their 72-week weight loss on tirzepatide (18.3%) was comparable to non-carriers (19.9%), suggesting the human drug remains effective in MC4R-mutation carriers.[2] This is licensed human clinical-trial data for the pharmaceutical product tirzepatide — not research-reagent data, and not related to any Velox Peptides product.
Retatrutide and semaglutide: mouse data only, so far
No equivalent human MC4R-mutation subgroup analysis has yet been published for retatrutide or semaglutide. The February 2026 findings for those two compounds in this specific genetic-obesity context currently come only from the MC4R-knockout mouse study covered here — a gap the field has not yet closed with human genetic-subgroup data.
No change to retatrutide's regulatory status
Retatrutide remains an unlicensed, investigational compound. It is not approved by the MHRA or FDA for any indication, and neither this mouse study nor the tirzepatide subgroup analysis changes that. Eli Lilly's Phase 3 TRIUMPH programme, tracked in our FDA & MHRA approval timeline guide, is unrelated to either dataset.
What Are the Limitations Researchers Should Note?
Several caveats matter for anyone citing this dataset. First, this is a single peer-reviewed study in one genetically engineered mouse strain; replication in independent cohorts and other rodent models of monogenic obesity has not been reported. Second, the 21-day dosing window captures short-term weight and metabolic change only — it says nothing about durability, tolerance, or long-term safety in this model. Third, mouse dosing regimens in knockout-strain studies are species- and model-specific research protocols with no established translation to human dosing; nothing in this paper should be read as dosing guidance of any kind. Fourth, MC4R-knockout mice are a model of monogenic obesity and do not represent the polygenic, diet-driven obesity that dominates most human and rodent GLP-1 research, including Lilly's own TRIUMPH programme.
None of these findings constitute evidence of any weight-loss or metabolic effect of retatrutide, tirzepatide or semaglutide in humans, and they do not describe any property of Velox Peptides' research-grade retatrutide product, which is supplied strictly for in vitro laboratory use and carries no therapeutic or health-benefit claims.
Velox Peptides Supply Information
Retatrutide is supplied as a research reagent only. It is not a medicine and has not been evaluated by the MHRA or FDA. Not for human or veterinary use. Tirzepatide and semaglutide are licensed pharmaceutical products, not sold by Velox Peptides. See our Research Use Policy and MHRA Statement.
References
- Hitaka K, Sugawara T, Matsumoto M, et al. Efficacy of GLP-1 analog peptides, semaglutide, tirzepatide, and retatrutide on MC4R deficient obesity and their comparison. International Journal of Obesity 2026;50:928–937. nature.com · PMID 41723268.
- Bhatnagar P, Ahmad NN, Li X, Coghlan M, Kaplan LM, Farooqi IS. Tirzepatide leads to weight reduction in people with obesity due to MC4R deficiency. Nature Medicine, 26 August 2025. nature.com, DOI 10.1038/s41591-025-03913-2.
- Eli Lilly and Company. Retatrutide Phase 3 TRIUMPH programme topline data. See our TRIUMPH-1 guide for full sourcing.
- Velox Peptides. Retatrutide vs Tirzepatide vs Semaglutide: mechanism and human clinical-trial comparison. See our comparison guide for background on receptor pharmacology and human trial data.
Frequently Asked Questions
What did the new MC4R mouse study find about retatrutide, tirzepatide and semaglutide?
A study published in the International Journal of Obesity in February 2026 (Hitaka et al.) dosed MC4R-knockout mice, a genetic model of melanocortin-4-receptor-deficient obesity, with semaglutide, tirzepatide or retatrutide for 21 days. Body weight fell by 19.7% (semaglutide), 31.6% (tirzepatide) and 24.1% (retatrutide) versus baseline, with all three peptides also improving insulin, HOMA-IR, cholesterol and liver-enzyme markers. This is rodent data, not human data.
What is MC4R-deficient obesity and why does it matter for GLP-1 research?
The melanocortin-4 receptor (MC4R) sits downstream of leptin and POMC signalling in the hypothalamus and is one of the most common monogenic causes of severe early-onset obesity. Because this pathway is anatomically distinct from the GLP-1, GIP and glucagon receptors that incretin-class peptides target, researchers use MC4R-knockout rodents to test whether GLP-1-class agonists remain effective anti-obesity tools when the melanocortin pathway itself is broken.
Is there human data on GLP-1 drugs and MC4R deficiency?
Yes, but so far only for tirzepatide. A retrospective genetic-subgroup analysis of the SURMOUNT-1 trial (Bhatnagar et al., Nature Medicine, 26 August 2025) found 32 of 2,291 genotyped participants carried pathogenic MC4R mutations, and their 72-week weight loss (18.3%) was comparable to non-carriers (19.9%). No equivalent human MC4R-mutation subgroup data has been published yet for retatrutide or semaglutide; the February 2026 findings for those two compounds come only from the mouse study.
Does this mouse study tell researchers anything about human dosing?
No. The mouse study used species-specific dosing, a genetically engineered knockout strain, and a 21-day protocol with no established human equivalent. It provides no evidence about human dosing, safety or outcomes for any of the three compounds. Retatrutide is not approved by the MHRA or FDA for any indication, and Velox Peptides makes no therapeutic, weight-loss, or health-benefit claims for the retatrutide it supplies, which is sold strictly as an in vitro research reagent.
Where can I buy retatrutide for research in the UK?
Velox Peptides supplies retatrutide (LY3437943) for in vitro research use in the UK. It is HPLC-verified at ≥99% purity, supplied as lyophilised powder in 10mg and 20mg vials, and dispatched from the UK within 24 hours. See our Research Use Policy before purchasing. Order here →