GLP-1 peptides in metabolic research

📖 Contextual disclaimer
The following article is of an educational nature and is a review of published scientific literature on peptides from the class of GLP-1 analogues and dual and triple incretin agonists. The described research includes an animal model, in vitro and clinical trials on humans – each time with an indication of the phase and model. The text does not constitute medical advice or a protocol for use in humans. Peptides from the One Peptides catalog are offered exclusively as a Research Use Only reference reagent for laboratory research; not for human consumption, therapeutic, or diagnostic use.

In 1964, two independent teams noticed that glucose given orally caused a greater release of insulin than the same dose given intravenously. This effect – today called the “incretin effect” – had a molecular basis that was unknown at that time. It was only several decades later, when the intestinal hormones GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide) were identified, that it became clear that the body had a dedicated signaling system linking food intake with the pancreatic response.
This system turned out to be one of the greatest surprises of modern metabolic pharmacology. Fifty years passed between the discovery of the incretin effect and the introduction of the first long-acting GLP-1 analogue — as long as it took to understand why endogenous GLP-1 has a half-life measured in minutes. Then everything accelerated: first liraglutide, then semaglutide, and now retatrutide (LY3437943) – a triple agonist that targets the GLP-1, GIP and glucagon receptors (GLP-1R/GIPR/GCGR) simultaneously. Throughout this guide, every incretin compound is discussed solely as a research reagent: each One Peptides peptide is supplied as a Research Use Only reference reagent characterised by HPLC purity and a batch COA, not as a medicinal product.
The article collects the current state of knowledge about incretins in a research context. If you’re looking for a detailed overview of one particular sequence – I’ve linked to the individual articles within each section.
Offered exclusively as a Research Use Only reference reagent for laboratory research; not for human consumption, therapeutic, or diagnostic use.

Contents

1. What are incretins – GLP-1, GIP, glucagon
2. Molecular mechanism of GLP-1 — receptor, signaling pathways, effectors
3. Semaglutide – the first long-acting analogue in research
4. Retatrutide – GLP-1/GIP/glucagon triple agonist
5. GLP-1/GIP dual agonists – mechanism of synergy
6. Limitations and directions for further research
7. Analytical specification of incretin peptides
8. FAQ
9. Bibliography

1. What are incretins – GLP-1, GIP, glucagon

Incretins are peptide hormones secreted by enteroendocrine cells of the small intestine in response to food. Their task is to modulate the response of the pancreas, stomach and central nervous system to incoming energy. The two best described incretins are GLP-1 and GIP – together they are responsible for approximately 70% of postprandial insulin release in healthy people (Baggio and Drucker, 2007).

1.1 GLP-1 (glucagon-like peptide-1)

GLP-1 is a fragment of preproglucagon, the same precursor protein from which glucagon is made. Biological activity mainly concerns two forms: GLP-1 (7-36) amide and GLP-1 (7-37). The peptide is produced by L cells in the distal small intestine and colon.
The key feature of endogenous GLP-1 is its extremely short half-life – approximately 2 minutes. The enzyme dipeptidyl peptidase-4 (DPP-4) cleaves the N-terminal amino acids of the peptide, creating an inactive fragment of GLP-1 (9-36). This property has been a major challenge in the development of GLP-1-based compounds for years – the need to modify the sequence so that the peptide is resistant to DPP-4 while maintaining affinity for the GLP-1R receptor.

1.2 GIP (glucose-dependent insulinotropic polypeptide)

GIP is secreted by K cells in the proximal small intestine (duodenum and upper jejunum). In terms of amino acids, it has 42 residues. Unlike GLP-1, GIP has a stronger lipogenic effect – it stimulates adipocytes to store energy. GIP has long been characterised in the literature as having a predominantly lipogenic/anabolic profile; more recent data with retatrutide and tirzepatide indicate that the picture is more complex.

1.3 Glucagon – an antagonistic role in classical physiology

Glucagon is produced by the alpha cells of the pancreas in response to a decrease in glycemia. Its classic role is to mobilize glucose from the liver via glycogenolysis and gluconeogenesis. In the context of incretin pharmacology, glucagon-receptor activation represents a third pharmacological axis; preclinical work associates it with increased thermogenesis and energy expenditure as a studied mechanism.

2. Molecular mechanism of GLP-1 – receptor, signaling pathways, effectors

The GLP-1 receptor (GLP-1R) is a class B1 G-protein-coupled receptor. It is found in many tissues – pancreatic beta cells, stomach, intestines, brain stem nuclei, hypothalamus, heart and kidneys. The breadth of receptor distribution explains why GLP-1 agonists have the effects observed in such diverse organ systems.

2.1 Signaling pathways after receptor activation

Upon binding of the agonist, the GLP-1R receptor activates:

• cAMP/PKA pathway – the classic response by the Gs protein leads to an increase in cyclic AMP and activation of protein kinase A. In the pancreatic beta cell, this translates into increased glucose-dependent insulin secretion.
• PI3K/Akt pathway – important for beta cell survival and potential anti-apoptotic effect.
• MAPK/ERK pathway – involved in the proliferative processes of pancreatic beta cells (Drucker, 2018).

2.2 Organ effects

Tissue	Effect of GLP-1R activation
Pancreatic beta cells	Increased glucose-dependent insulin secretion
Pancreatic alpha cells	Inhibition of glucagon secretion
Stomach	Slowing down gastric emptying
Hypothalamus	Modulation of satiety-center signaling (appetite-regulating circuits)
Brain stem (NTS)	Induction of the feeling of satiety, anti-reward effect
Heart	Potential cardioprotective effect (limited data)
Bowel	Modulation of motility and mucosal barrier integrity

2.3 Glucose dependence – an important safety feature

A key feature of GLP-1 agonists observed in clinical trials is the glucose-dependent insulinotropic effect. This means that GLP-1 stimulates insulin secretion only when glucose levels are elevated – which in studies in animal models and clinical trials in healthy people translated into a low risk of hypoglycemia as a side effect in monotherapy (Nauck and Meier, 2018).

3. Semaglutide – the first long-acting analogue in research

Semaglutide is a GLP-1 analogue developed by Novo Nordisk, registered in clinical trials under the designation NN9535. The structure of the peptide is based on the human GLP-1 (7-37) backbone with three significant modifications:

1. Amino acid substitution at position 8 – replacement of alanine with α-aminoisobutyric acid (Aib). This amino acid does not occur in natural proteins and blocks cleavage by DPP-4.
2. Side chain modification at position 26 – fatty acid chain (C18 diacid) coupled by γ-glutamyl spacer. Increases affinity for serum albumin.
3. Substitution at position 34 – arginine instead of lysine, so that the fatty-acid side chain is attached only at position 26.

Together, the modifications give a peptide with a half-life of approximately 165 hours (7 days) – compared to 2 minutes for native GLP-1. That’s a difference of five thousand times.

3.1 Status of clinical trials

Semaglutide was evaluated in a series of Phase III clinical trials known as SUSTAIN (type 2 diabetes) and STEP. In the STEP 1 trial (n=1961, 68 weeks), Wilding et al. (2021) reported a mean body-weight change of −14.9% in the semaglutide group versus −2.4% in the placebo group. The SUSTAIN 6 trial reported a reduction in the risk of major cardiovascular events in people with type 2 diabetes and cardiovascular disease (Marso et al., 2016). These figures were reported in clinical trials of the medicine studied in human participants and are not characteristics of any research reagent.
Offered exclusively as a Research Use Only reference reagent for laboratory research; not for human consumption, therapeutic, or diagnostic use.

3.2 Adverse event profile in studies

The most common side effects reported in clinical trials included nausea, vomiting, diarrhea and constipation. Most of them were mild or moderate in severity and resolved spontaneously. Acute pancreatitis, cholelithiasis and retinopathy (the latter mainly in people with existing type 2 diabetes) occurred less frequently. These adverse events were reported in clinical trials of the medicine, not characteristics of the research reagent.

3.3 Semaglutide in the RUO research context

In the One Peptides catalog semaglutide 2 mg is available as a chemical reagent for research applications. Specification: sequence identical to NN9535, HPLC purity ≥98%, MS identity confirmation, lyophilized form. The peptide is not a medicinal product – applications include research on the mechanisms of action of GLP-1R agonists in animal models and comparative analyses of pharmacokinetics in the context of the development of new analogues.
→ Full overview of clinical trials: semaglutide – what we know from clinical trials

4. Retatrutide – GLP-1/GIP/glucagon triple agonist

Retatrutide (LY3437943) is a peptide developed by Eli Lilly that is a first-in-class triple agonist of three receptors simultaneously: GLP-1R, GIPR and the glucagon receptor (GCGR). The compound concept is based on the hypothesis that simultaneous activation of three incretin and catabolic-glucagon pathways produces a metabolic effect greater than the sum of individual agonists.

The informal shorthand used for retatrutide in online searches is explained in a separate entry: what the informal term “reta” means.

4.1 Structure and pharmacokinetics

Retatrutide is a 39-amino acid peptide with three important structural elements:

• A framework based on glucagon as a common precursor of three receptor activities
• Modifications at key positions for DPP-4 – protection against proteolysis
• Conjugation with a fatty chain (C20 diacid) – binding to albumin, extended half-life approx. 6 days

Affinity profile: strongest activity on GIPR, comparable and lower activity on GLP-1R and GCGR (Coskun et al., 2022).

4.2 Status of clinical trials

Retatrutide was evaluated in a Phase II trial published in 2023. In an obese cohort (n=338, 48 weeks), Jastreboff et al. (2023) reported a mean body-weight change of −24.2% at the highest dose tested. Retatrutide also achieved significant reductions in HbA1c and improvements in the lipid profile in the phase II trial reported. These outcomes were reported in a clinical trial of the investigational molecule studied in human participants and are not characteristics of any research reagent.
At the time of writing, the investigational molecule is in phase III clinical trials – in the TRIUMPH (type 2 diabetes) and TRIUMPH-MASH (metabolic dysfunction-associated steatohepatitis) programs. Full Phase III results are expected in the coming years; MASLD/MASH and cardiometabolic endpoints are reported in the literature as active research directions rather than established indications.
Offered exclusively as a Research Use Only reference reagent for laboratory research; not for human consumption, therapeutic, or diagnostic use.

4.3 Mechanistic hypothesis for the triple effect

Why does adding glucagon activity to GLP-1/GIP agonism amplify the metabolic effect studied in research? Glucagon stimulates lipolysis in adipose tissue and increases basal metabolic rate. The simultaneous modulation of satiety-center signaling by GLP-1 and optimization of insulin secretion by GIP creates a theoretically coherent picture: lower energy intake + higher energy expenditure as a studied mechanism. Verification of this hypothesis in full phase III is the direction of current research.
→ Detailed discussion of phase II: retatrutide – a triple agonist in clinical trials · available in the catalog: retatrutide 5 mg

In the One Peptides catalogue retatrutide is listed under the name Triple G — the Triple G peptide category groups its lyophilised and pen formats.

5. GLP-1/GIP dual agonists – mechanism of synergy

Between semaglutide (a GLP-1R monoagonist) and retatrutide (a triple agonist) lies an intermediate class – dual GLP-1/GIP agonists. The most important representative of this class is tirzepatide (Mounjaro/Zepbound). In a research context, reagents with similar structural architecture are available.

5.1 Synergy or addition

The question that defines this class: does simultaneous activation of GLP-1R and GIPR result in a synergistic or additive effect? Data from preclinical and clinical studies suggest a mixed picture:

• The insulinotropic effect appears to be additive or slightly synergistic (Frias et al., 2018)
• The effect on satiety-center signaling and metabolic endpoints appears to be synergistic – both activities act on different populations of hypothalamic neurons
• The effect on the lipid profile and insulin sensitivity was reported as more pronounced than in GLP-1 monoagonism in the cited studies (Frias et al., 2018).

Concomitant activation of GIPR in the presence of GLP-1R agonism may also modulate adverse effects, with some observations suggesting lower nausea intensity compared to an equivalent dose of pure GLP-1 agonist.

5.2 GLP-1/GIP Research Reagent

The GLP-1/GIP 5 mg peptide in the One Peptides catalog represents this class for research applications. The specification includes HPLC ≥98%, MS confirmation and batch COA. Research applications focus on comparative analysis of dual agonist pharmacokinetics, in vitro receptor studies, and animal models of metabolism.

5.3 Comparison: single, dual and triple agonist (clinical-trial data)

With all three classes introduced — single (semaglutide), dual (tirzepatide) and triple (retatrutide) — the table below places them side by side using their landmark obesity-trial figures. All values are reported from human clinical trials of approved or investigational medicines.

Class	Compound	Receptors	Landmark obesity trial	Mean body-weight change reported (top dose)	Regulatory status
Single agonist	Semaglutide	GLP-1	STEP 1 (Wilding 2021)	~14.9% at 68 wk (2.4 mg)	Approved (Wegovy / Ozempic)
Dual agonist	Tirzepatide	GIP + GLP-1	SURMOUNT-1 (Jastreboff 2022)	~20.9% at 72 wk (15 mg)	Approved (Mounjaro / Zepbound)
Triple agonist	Retatrutide	GIP + GLP-1 + glucagon	Phase 2 obesity (Jastreboff 2023)	~24.2% at 48 wk (12 mg)	Investigational — phase 3 TRIUMPH

⚠️ These figures come from clinical trials of approved or investigational medicines. They describe the molecules studied in human research — not outcomes of any research reagent, which is sold for laboratory use only. Sources: Wilding JPH et al., N Engl J Med 2021 (10.1056/NEJMoa2032183); Jastreboff AM et al., N Engl J Med 2022 (10.1056/NEJMoa2206038); Jastreboff AM et al., N Engl J Med 2023 (10.1056/NEJMoa2301972).

A pre-dissolved 10 mg pen format of this class is available as the GLP-1+GIP pen 10 mg (Research Use Only).

Ready-to-use pen formats of these incretin reagents are grouped in the research pens category.

6. Limitations and directions for further research

The development of incretin-based compounds is one of the fastest-growing areas of metabolic pharmacology. Despite the substantial clinical-trial results reported in the literature, the field has significant open questions:

6.1 Long-term stability of effect

Data from clinical trials cover a 1-4 year horizon. There are no observations from a full decade of use of GLP-1 analogues in a broad population yet. Questions about the stability of the metabolic effect after 10+ years, the impact on body composition (muscle vs. fat mass) under long-term treatment in the trials reported, and the risk of rebound after discontinuation all remain partially open.

6.2 Effect on muscle mass

Some observations from clinical trials indicate that the body-composition change reported under GLP-1 agonists affects not only fat tissue, but also muscle mass. The contribution of FFM (fat-free mass) to the total body-weight change reported is estimated at 20–40%, depending on the study and population. For the sports and geriatric context, this is an important direction for further analysis.

6.3 Central effects beyond appetite-regulating circuits

The GLP-1R receptor in the central nervous system is present not only in satiety centers. There are hypotheses about the potential impact of GLP-1 agonists on the reward system (anti-reward effect), addictions (alcoholism, behavioral addictions) and cognitive functions (research on Alzheimer’s disease). The data are limited and require further clinical trials dedicated to these research directions.

6.4 New directions – beyond the classic three

After retatrutide, the first publications on peptides with additional mechanistic elements appear – amylin agonists in combination with GLP-1, incretin peptides with the FGF21 (fibroblast growth factor 21) component, as well as analogues with modifications enabling oral administration (oral semaglutide is already registered, more are under development). The decade 2020–2030 will likely define a new map of metabolic pharmacology.

The broader context of this class of compounds is covered in metabolic modulators – research reagent category, from incretins to non-incretin compounds, exclusively as RUO reagents.

7. Analytical specification of incretin peptides

Incretin peptides are relatively large molecules (semaglutide: 31 amino acids + lipid modification, retatrutide: 39 amino acids). Their synthesis using the SPPS (solid-phase peptide synthesis) method requires precise quality control because the risk of sequence-related impurities is higher than for shorter peptides.

7.1 Quality Standards for One Peptides

All peptides from the incretin class in the One Peptides catalog have:

• HPLC purity ≥98% – measured on each production batch
• MS confirmation – confirmation of identity by mass spectrometry with a mass accuracy of less than 1 Da
• COA per batch – Certificate of Analysis available for each batch, linked to the batch number on the vial
• Cold chain 2–8°C – cold chain from synthesis to delivery
• Batch traceability – full batch traceability from production to the end customer

The guide explains how to read these parameters and what to expect from a research peptide supplier: peptide analytical standards.

7.2 Stability of incretin peptides

Peptides with a fatty chain (acylated) are in practice slightly more stable in aqueous solution than analogues without lipid modification – the fatty chain partially protects the peptide against final hydrolysis. When reconstituted in bacteriostatic water and stored in a refrigerator at 2–8°C, typical stability of the working solution reaches 28 days for semaglutide and retatrutide.
For full context on laboratory procedures with incretin peptides, it is worth reading the guide to laboratory practice – it discusses reconstitution, storage and batch documentation in detail.

7.3 Specification verification

Each vial supplied by One Peptides is marked with a batch number associated with the COA. Analytical documents can be downloaded from the quality testing and certificates page. For research teams preparing scientific publications, full QC documentation (HPLC chromatogram, MS spectrum, Karl Fischer report) is available on request.

FAQ

Do semaglutide and retatrutide have the same mechanism of action?

NO. Semaglutide is a GLP-1 receptor monoagonist. Retatrutide is a triple agonist of GLP-1R, GIPR and the glucagon receptor. Pharmacologically, they differ in their receptor-affinity profile and in the metabolic endpoints reported in clinical trials.

Why is endogenous GLP-1 not used as a drug?

Endogenous GLP-1 has a half-life of approximately 2 minutes due to rapid cleavage by DPP-4. Application in clinical practice would require continuous intravenous infusion. All GLP-1 analogues used in pharmacology (liraglutide, semaglutide) have structural modifications that protect against DPP-4 and extend the half-life to hours or days.

Is retatrutide available as a medicine?

Retatrutide (LY3437943) is an investigational molecule in phase 3 trials and is not approved as a medicine in any jurisdiction. One Peptides supplies it solely as a Research Use Only reference reagent for laboratory work; it is not for human consumption, therapeutic, or diagnostic use.

What does “glucose-dependent” insulinotropic effect mean?

This means that the GLP-1 agonist stimulates insulin secretion from pancreatic beta cells only when blood glucose levels are elevated. In normoglycemia or hypoglycemia, the insulinotropic effect is reduced. In clinical trials, this translated into a low risk of hypoglycemia in monotherapy with GLP-1 agonists – a feature that distinguishes this class from sulfonylureas or exogenous insulin.

How should reconstituted incretin peptides be stored?

The working solution in bacteriostatic water is stored in a refrigerator at 2-8°C for approximately 28 days. Do not freeze the reconstituted solution – freeze-thaw cycles degrade the peptide structure. Before reconstitution, the lyophilisate can be stored in a -20°C freezer for 18–24 months. For a complete guide to peptide storage, see the Laboratory Practice section.

Are GLP-1/GIP dual agonists more potent than GLP-1 monoagonists?

In available clinical trials (tirzepatide vs. semaglutide in SURPASS-2, for type 2 diabetes), dual agonism showed significantly stronger reductions in HbA1c and in body-weight change compared to semaglutide at equivalent doses. The mechanism of synergy is not fully elucidated – it probably involves both enhancement of insulinotropic pathways and modulation of satiety centers beyond the classic GLP-1 receptor sites.
Offered exclusively as a Research Use Only reference reagent for laboratory research; not for human consumption, therapeutic, or diagnostic use.

Related articles in the knowledge base

• Semaglutide – what we know from clinical trials
• Retatrutide – a triple GLP-1/GIP/glucagon agonist in clinical trials
• Semaglutide vs Retatrutide – comparison of mechanisms
• GLP-1, GIP, glucagon – how do incretins work
• How to prepare and store research peptides
• Peptid-Rechner
• What to pay attention to when buying research peptides

Available reagents from the incretin class in the catalog: semaglutide 2 mg, retatrutide 5 mg, GLP-1/GIP 5 mg. Full overview of the peptide class: research peptides.

Bibliography

1. Baggio, L. L., Drucker, D. J. (2007). Biology of incretins: GLP-1 and GIP
2. Drucker D. J. (2018). Mechanisms of action and therapeutic application of glucagon-like peptide-1
3. Nauck M.A., Meier J.J. (2018). Incretin hormones: Their role in health and disease
4. Wilding JPH, Batterham RL, Calanna S, et al. (2021). Once-weekly semaglutide in adults with overweight or obesity
5. Marso SP, Bain SC, Consoli A, et al. (2016). Semaglutide and cardiovascular outcomes in patients with type 2 diabetes
6. Jastreboff AM, Kaplan LM, Frías JP, et al. (2023). Triple-hormone-receptor agonist retatrutide for obesity — a phase 2 trial
7. Coskun T, Urva S, Roell WC, et al. (2022). LY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist for glycemic control and weight loss
8. Frías JP, Nauck MA, Van J, et al. (2018). Efficacy and safety of LY3298176, a novel dual GIP and GLP-1 receptor agonist, in patients with type 2 diabetes
9. Knudsen LB, Lau J (2019). The discovery and development of liraglutide and semaglutide

More articles from this cluster: all metabolic peptide articles.