GLP-1 Class Peptides: Liver (MASLD/MASH) Research – Literature Review

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Abstract

This review summarises peer-reviewed literature on glp-1 class peptides, with a focus on liver (masld/mash) research. It describes receptor pharmacology, preclinical findings and outcomes reported in published randomised controlled trials, and notes limitations of the current evidence. It does not provide dosing, administration or preparation information.

Background

Tirzepatide (LY3298176) is a 39-amino-acid synthetic peptide with agonist activity at the GIP and GLP-1 receptors. It is an authorised prescription-only medicine in the UK for specific indications under medical supervision; material supplied for laboratory research is not a medicinal product and is not for human or veterinary use.^[1]

Semaglutide is a long-acting GLP-1 receptor agonist analogue, engineered with fatty-acid side-chain acylation and amino-acid substitution to enable albumin binding and resistance to DPP-4 degradation. It is an authorised prescription-only medicine in the UK for specific indications under medical supervision; research-grade material is not a medicinal product.^[2]

Retatrutide (LY3437943) is an investigational single-peptide agonist of the glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1) and glucagon receptors, developed by Eli Lilly and Company. It is not authorised as a medicine by the MHRA, EMA or FDA and remains under clinical investigation.^[3,4]

Mechanism of action

Tirzepatide is an imbalanced dual agonist: in vitro it shows affinity for the GIP receptor comparable to native GIP and weaker GLP-1 receptor affinity than native GLP-1. Preclinical work described glucose-dependent insulin secretion, effects on food intake and improvements in insulin sensitivity in rodent models.^[1,5]

Semaglutide activates the GLP-1 receptor, a class B G-protein-coupled receptor expressed on pancreatic beta cells, in the gastrointestinal tract and in brain regions involved in appetite regulation. GLP-1 receptor signalling enhances glucose-dependent insulin secretion, suppresses glucagon secretion and slows gastric emptying.^[2,5]

In preclinical characterisation, retatrutide showed agonism at human GIP and GLP-1 receptors with comparatively greater relative potency at GIPR, and additional glucagon-receptor (GCGR) agonism. Glucagon-receptor engagement is hypothesised to contribute increased energy expenditure and hepatic lipid oxidation, complementing incretin-mediated effects on insulin secretion and central appetite signalling observed in rodent models.^[3,5]

Summary of published research

Tirzepatide – Published research: MASH

In the phase 2 SYNERGY-NASH trial in biopsy-confirmed MASH with stage F2–F3 fibrosis, a higher proportion of participants receiving tirzepatide met the primary endpoint of MASH resolution without worsening of fibrosis at 52 weeks (44–62% vs 10% placebo).^[6]

Semaglutide – Published research: MASH

In the phase 3 ESSENCE trial (biopsy-defined MASH with F2–F3 fibrosis), resolution of steatohepatitis without worsening of fibrosis at week 72 occurred in 62.9% with semaglutide versus 34.3% with placebo, and fibrosis improvement in 36.8% versus 22.4%.^[7]

Retatrutide – Published research: hepatic fat

A substudy of the phase 2 obesity trial measured liver fat by MRI-PDFF in participants with metabolic dysfunction-associated steatotic liver disease (MASLD) and reported substantial, dose-dependent relative reductions in liver fat at 24 and 48 weeks, with many participants reaching normal liver-fat levels.^[8]

Limitations of the evidence

Most clinical data come from industry-sponsored trials of defined pharmaceutical formulations under medical supervision; results cannot be extrapolated to research-grade material, which is not manufactured to medicinal GMP standards. Trial populations, durations and comparators differ, so cross-trial comparisons should be interpreted cautiously. Longer-term safety data for newer agents remain limited.

Conclusion

The published literature characterises these peptides as potent incretin-pathway receptor agonists with substantial bodies of preclinical and clinical-trial data. For laboratory work, researchers should rely on primary publications and verified analytical documentation (HPLC/MS certificates of analysis) for any material used.

References

1. Coskun T et al. LY3298176, a novel dual GIP and GLP-1 receptor agonist for the treatment of type 2 diabetes mellitus: From discovery to clinical proof of concept. Mol Metab. 2018; 18:3-14. doi:10.1016/j.molmet.2018.09.009 PMID: 30473097.
2. Lau J et al. Discovery of the Once-Weekly Glucagon-Like Peptide-1 (GLP-1) Analogue Semaglutide. J Med Chem. 2015; 58(18):7370-80. doi:10.1021/acs.jmedchem.5b00726 PMID: 26308095.
3. Coskun T et al. LY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist for glycemic control and weight loss: From discovery to clinical proof of concept. Cell Metab. 2022; 34(9):1234-1247.e9. doi:10.1016/j.cmet.2022.07.013 PMID: 35985340.
4. Urva S et al. LY3437943, a novel triple GIP, GLP-1, and glucagon receptor agonist in people with type 2 diabetes: a phase 1b, multicentre, double-blind, placebo-controlled, randomised, multiple-ascending dose trial. Lancet. 2022; 400(10366):1869-1881. doi:10.1016/S0140-6736(22)02033-5 PMID: 36354040.
5. Drucker DJ et al. Mechanisms of Action and Therapeutic Application of Glucagon-like Peptide-1. Cell Metab. 2018; 27(4):740-756. doi:10.1016/j.cmet.2018.03.001 PMID: 29617641.
6. Loomba R et al. Tirzepatide for Metabolic Dysfunction-Associated Steatohepatitis with Liver Fibrosis. N Engl J Med. 2024; 391(4):299-310. doi:10.1056/NEJMoa2401943 PMID: 38856224.
7. Sanyal AJ et al. Phase 3 Trial of Semaglutide in Metabolic Dysfunction-Associated Steatohepatitis. N Engl J Med. 2025; 392(21):2089-2099. doi:10.1056/NEJMoa2413258 PMID: 40305708.
8. Sanyal AJ et al. Triple hormone receptor agonist retatrutide for metabolic dysfunction-associated steatotic liver disease: a randomized phase 2a trial. Nat Med. 2024; 30(7):2037-2048. doi:10.1038/s41591-024-03018-2 PMID: 38858523.

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