Understanding GLP-1 Receptor Agonists in Research

Published: March 2026

GLP-1 Receptor Agonists: Research Overview of Semaglutide, Tirzepatide, and Retatrutide

Overview

Glucagon-like peptide-1 (GLP-1) receptor agonists represent one of the most extensively characterized classes of peptide compounds in contemporary metabolic research. GLP-1 is an incretin hormone secreted postprandially by intestinal L-cells, and its receptor (GLP-1R) is expressed across multiple tissues including pancreatic beta cells, the central nervous system, the gastrointestinal tract, and the cardiovascular system. Early clinical investigation of GLP-1R agonists such as exenatide and liraglutide demonstrated meaningful effects on glycemic regulation in individuals with type 2 diabetes, and subsequent research expanded to examine their role in body weight regulation, cardiovascular risk, and a growing range of metabolic endpoints. Semaglutide (SEMA), a long-acting GLP-1R agonist with an extended plasma half-life achieved through fatty acid conjugation and albumin binding, emerged as a second-generation compound and has since become one of the most widely studied peptides in the incretin class.

The recognition that glucose-dependent insulinotropic polypeptide (GIP) receptor signaling complements GLP-1R activation — particularly in adipose tissue, where GIPR is expressed but GLP-1R largely is not — motivated the development of dual agonist molecules. Tirzepatide (TIRZ), a synthetic 39-amino-acid peptide engineered on the native GIP backbone with balanced activity at both GIPR and GLP-1R, was the first dual GLP-1/GIP agonist to enter phase 3 clinical investigation. The SURMOUNT and SURPASS trial programs examined tirzepatide across populations with obesity and type 2 diabetes and reported weight reductions exceeding those observed with GLP-1R monoagonists in head-to-head comparisons. This progression from single to dual receptor engagement established a pharmacological rationale for further expansion of agonist targets.

The most recent frontier in this research area is triple receptor co-agonism. Retatrutide (RETA), developed by Eli Lilly, simultaneously activates GLP-1R, GIPR, and the glucagon receptor (GCGR). Glucagon receptor agonism is hypothesized to contribute additive effects on energy expenditure, hepatic lipid metabolism, and thermogenesis through brown adipose tissue — mechanisms distinct from, and potentially synergistic with, GLP-1R and GIPR signaling. A phase 2 clinical trial published in 2023 reported body weight reductions at 48 weeks that, at the highest dose studied, exceeded those reported for either monoagonist or dual agonist comparators in analogous timeframes.

Molecular Profiles

Semaglutide (SEMA) — GLP-1 Receptor Agonist

Tirzepatide (TIRZ) — GLP-1 / GIP Dual Receptor Agonist

Retatrutide (RETA) — GLP-1 / GIP / Glucagon Triple Receptor Agonist

Mechanism of Action

GLP-1 receptor signaling is initiated when GLP-1 or a GLP-1R agonist peptide binds the seven-transmembrane GLP-1R, a class B G protein-coupled receptor. Receptor activation drives coupling to G_αs proteins, which stimulate adenylyl cyclase, elevate intracellular cyclic adenosine monophosphate (cAMP), and activate protein kinase A (PKA) and exchange proteins directly activated by cAMP (Epac2). In pancreatic beta cells, this signaling cascade potentiates glucose-dependent insulin secretion — the canonical incretin effect — and suppresses inappropriately elevated glucagon release from alpha cells. Simultaneously, GLP-1R activation in the gastrointestinal tract slows gastric emptying, and signaling through hypothalamic and brainstem GLP-1R circuits reduces food intake by modulating satiety and appetite centers. As reviewed by Drucker (2018), these combined central and peripheral mechanisms account for the weight-regulatory properties observed in clinical research with GLP-1R agonist compounds.

GIP receptor (GIPR) co-agonism, as incorporated in tirzepatide (TIRZ), engages a parallel but mechanistically distinct axis. GIPR is expressed on adipocytes, osteoblasts, and cortical neurons, whereas GLP-1R is largely absent from mature adipose tissue. In preclinical studies, GIPR agonism in adipocytes has been shown to enhance insulin-stimulated glucose uptake, increase glycerol synthesis from glucose, and modulate fatty acid trafficking — effects that complement rather than duplicate GLP-1R signaling. Research has suggested that the superior efficacy of dual GLP-1/GIP agonism may partly reflect GIPR-mediated adipose tissue remodeling and an attenuation of GLP-1–associated gastrointestinal effects. The relative contribution of each receptor target to overall efficacy remains an area of active mechanistic investigation.

Glucagon receptor (GCGR) agonism, the third target incorporated in retatrutide (RETA), introduces a further mechanistic layer centered on energy expenditure rather than energy intake alone. GCGR is expressed in hepatocytes, brown adipose tissue, and the central nervous system. Preclinical data indicate that glucagon receptor activation stimulates thermogenesis in brown adipose tissue, promotes hepatic fatty acid oxidation, and increases whole-body energy expenditure through mechanisms involving uncoupling protein 1 (UCP1) induction. Within the context of a triple agonist, GCGR engagement is hypothesized to amplify the energy deficit created by GLP-1R– and GIPR-mediated food intake reduction. The precise interplay between all three receptor systems in human physiology is an active area of research.

Key Areas of Investigation

GLP-1 Receptor Agonism: Foundational Research

The research foundation for GLP-1R agonists in obesity was substantially advanced by the SCALE program, which examined liraglutide at 3.0 mg in adults with obesity (Pi-Sunyer et al., NEJM 2015, PMID: 26132939). The STEP program subsequently investigated subcutaneous semaglutide at 2.4 mg once weekly across diverse populations. The STEP 1 trial enrolled 1,961 adults without type 2 diabetes and reported a mean body weight reduction of 14.9% in the semaglutide group versus 2.4% with placebo over 68 weeks (Wilding et al., NEJM 2021, PMID: 33567185). The SUSTAIN-6 cardiovascular outcomes trial in 3,297 adults with type 2 diabetes at high cardiovascular risk reported a statistically significant reduction in the composite endpoint of cardiovascular death, nonfatal myocardial infarction, or nonfatal stroke with semaglutide versus placebo (Marso et al., NEJM 2016, PMID: 27633186). Separate research programs have investigated GLP-1R signaling in the central nervous system, including effects on dopaminergic reward pathways, neuroinflammation, and addictive behaviors in preclinical models.

Dual GLP-1/GIP Agonism: Tirzepatide (TIRZ) Research

Tirzepatide (TIRZ) was examined across the SURPASS phase 3 diabetes program and the SURMOUNT obesity program. SURPASS-2, a 40-week head-to-head trial comparing tirzepatide versus once-weekly semaglutide 1 mg in 1,879 adults with type 2 diabetes, reported that all three tirzepatide doses were statistically superior to semaglutide for glycated hemoglobin reduction (Frías et al., NEJM 2021, PMID: 34170647). SURMOUNT-1, a 72-week phase 3 trial in 2,539 adults with obesity without diabetes, reported mean body weight reductions of 15.0%, 19.5%, and 20.9% with tirzepatide 5, 10, and 15 mg, respectively, versus 3.1% with placebo (Jastreboff et al., NEJM 2022, PMID: 35658024). These data represent some of the largest placebo-corrected weight reductions reported in a randomized controlled trial of a pharmacological agent. Mechanistic research has focused on tirzepatide’s differential engagement of GIPR in adipose tissue as a potential explanation for its incremental efficacy over GLP-1R monoagonism.

Triple Receptor Agonism: Retatrutide (RETA) Research

Retatrutide (RETA; LY3437943) is an investigational triple agonist of GLP-1R, GIPR, and GCGR currently in clinical development. A phase 2, double-blind, randomized, placebo-controlled trial published by Jastreboff et al. in 2023 enrolled adults with a BMI ≥30 (or ≥27 with a weight-related condition) and administered once-weekly subcutaneous retatrutide at doses of 1, 4, 8, or 12 mg for 48 weeks (PMID: 37366315). At the 12 mg dose, the trial reported a mean body weight reduction of 24.2% at week 48 versus 2.1% with placebo. The 8 mg dose reported 22.8% reduction at week 48. Gastrointestinal adverse events consistent with the broader incretin class profile were the most common findings. Phase 3 investigations of retatrutide are ongoing.

Metabolic and Adipose Tissue Research

A substantial body of preclinical and translational research has examined how GLP-1R, GIPR, and GCGR agonism affect metabolic tissues beyond the pancreas. In adipose tissue, GIPR agonism has been shown in preclinical models to modulate triglyceride-derived fatty acid uptake, regulate adipokine secretion, and alter lipid storage dynamics. Glucagon receptor agonism in both rodent and human systems has been associated with increased resting energy expenditure, enhanced hepatic beta-oxidation, and induction of thermogenic programs in brown adipose tissue through UCP1-dependent mechanisms. Research interest has extended to metabolic-dysfunction-associated steatohepatitis (MASH), where preclinical data with dual and triple agonist compounds have shown reductions in hepatic steatosis and liver fat fraction. The relative contributions of direct receptor engagement versus indirect effects mediated by caloric restriction and weight loss remain subjects of active investigation.

CNS and Neuroprotective Research

GLP-1R is expressed in multiple brain regions including the hypothalamus, hippocampus, substantia nigra, and cortex. Preclinical research has investigated whether GLP-1R agonism exerts neuroprotective effects relevant to neurodegenerative conditions. In rodent and non-human primate models, GLP-1R agonists have been reported to reduce amyloid-beta deposition, attenuate tau phosphorylation, decrease neuroinflammatory markers, and preserve synaptic integrity in models relevant to Alzheimer’s disease. Separate lines of investigation in Parkinson’s disease models have documented reductions in dopaminergic neuron loss and improvements in motor function following GLP-1R agonist administration. GIPR is also expressed in cortical neurons and hippocampal tissue, and dual GLP-1/GIP receptor agonists have been reported to demonstrate superior neuroprotective effects compared to GLP-1R monoagonists in some preclinical paradigms. Whether these CNS effects translate meaningfully in human populations remains an open research question.

Key Published References

1. Drucker DJ. Mechanisms of Action and Therapeutic Application of Glucagon-like Peptide-1. Cell Metab. 2018;27(4):740–756. PMID: 29617641
2. Pi-Sunyer X, Astrup A, Fujioka K, et al. A Randomized, Controlled Trial of 3.0 mg of Liraglutide in Weight Management. N Engl J Med. 2015;373(1):11–22. PMID: 26132939
3. Marso SP, Bain SC, Consoli A, et al. Semaglutide and Cardiovascular Outcomes in Patients with Type 2 Diabetes (SUSTAIN-6). N Engl J Med. 2016;375(19):1834–1844. PMID: 27633186
4. Wilding JPH, Batterham RL, Calanna S, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). N Engl J Med. 2021;384(11):989–1002. PMID: 33567185
5. Frías JP, Davies MJ, Rosenstock J, et al. Tirzepatide versus Semaglutide Once Weekly in Patients with Type 2 Diabetes (SURPASS-2). N Engl J Med. 2021;385(6):503–515. PMID: 34170647
6. Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). N Engl J Med. 2022;387(3):205–216. PMID: 35658024
7. Jastreboff AM, Kaplan LM, Frías JP, et al. Triple–Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. N Engl J Med. 2023;389(6):514–526. PMID: 37366315

Product Availability

The following research-grade peptides are available for qualified investigators through White Market Peptides:

• Semaglutide (SEMA) — 10 mg — GLP-1 receptor agonist
• Tirzepatide (TIRZ) — 10 mg — GLP-1/GIP dual receptor agonist
• Retatrutide (RETA) — 10 mg — GLP-1/GIP/glucagon triple receptor agonist