Tirzepatide – dual GIP/GLP-1 agonist in metabolism studies

Regulatory framing — a critical distinction

Tirzepatide exists under two separate legal statuses, which this article keeps apart from the first sentence to the last:

• as an authorised medicinal product — Mounjaro (type 2 diabetes, FDA and EMA authorisation in 2022) and Zepbound (obesity, FDA authorisation 2023, EMA 2024). Under this status, tirzepatide has dosing protocols in clinical settings, prescription availability and full medical oversight.
• as a research reagent (Research Use Only) — a chemical substance for laboratory research on the incretin axis. It is not a medicinal product, it is not a dietary supplement and it is not intended for human use.

This article describes the clinical literature and mechanism of tirzepatide in an educational context. Products labelled “tirzepatide” in the research-peptide catalogue operate within the RUO framework and are described solely through the lens of analytical data (sequence, HPLC purity, COA), not usage protocols.

Tirzepatide is a synthetic 39-amino-acid peptide, a dual agonist of the GIP and GLP-1 receptors, developed by Eli Lilly under the code LY3298176. The molecule combines the activity of two incretin pathways in a single peptide — this sets it apart from the older GLP-1 monoagonists (semaglutide, liraglutide) and from the GIP/GLP-1/glucagon tri-agonist (retatrutide). In the metabolic literature, tirzepatide occupies the position of the missing evolutionary link between single-receptor and triple-receptor activation of the incretin axis.

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

In the RUO reagent catalog, the triple-receptor variant is represented by Retatrutide (Triple G) 5 mg; the product page concerns laboratory research material, not a medicinal product.

Table of contents

• What tirzepatide is — structure and status
• Mechanism of action — why two receptors
• The GIP debate — agonism or antagonism
• State of the clinical evidence — the SURPASS and SURMOUNT programmes
• Ongoing research directions — cardiovascular system, kidneys, liver
• Tirzepatide against other incretins
• Safety and limitations from clinical trials
• Frequently asked questions

What tirzepatide is — structure and status

Tirzepatide is a 39-amino-acid peptide with structural modifications that extend the half-life to about 5 days, which in clinical protocols corresponds to a weekly regimen. The modification includes acylation (attachment of a C20 fatty-acid chain to the lysine residue at position 20), which enables binding to serum albumin and much slower excretion than for the unmodified peptide. The amino acids at positions relevant to receptor recognition were selected to ensure high affinity for both GLP-1R and GIPR — as confirmed by preclinical data published by the Eli Lilly team (Coskun et al. 2018¹).

Regulatory status — two frames in one peptide

In international trade, tirzepatide operates within two separate frames:

• Medicinal product — Mounjaro (FDA and EMA 2022, indication: type 2 diabetes), Zepbound (FDA 2023, EMA 2024, indication: obesity). Prescription sale, dosing protocol in clinical settings (once weekly subcutaneously, doses titrated from 2.5 mg to 15 mg).
• Research reagent (RUO) — in the category of peptides for laboratory research. Without any intended human use and without approved usage recommendations for the end purchaser. Batch documentation includes an HPLC certificate ≥98% and identity confirmation by mass spectrometry.

This distinction is fundamental for the researcher assessing tirzepatide’s place in the peptide catalogue. Mounjaro / Zepbound and the RUO reagent are two separate regulatory entities of the same chemical compound — with different documentation, descriptive languages and intended purposes.

Molecular parameters

Mechanism of action — why two receptors

Tirzepatide is the first clinically advanced unimolecular dual agonist of the incretin axis. Simultaneous activation of two receptors produces a pharmacological profile that could not previously be achieved with GLP-1 monoagonists alone or with a combination of two separate molecules.

Activity at the GLP-1 receptor (GLP-1R)

The GLP-1 component of tirzepatide accounts for the known effects of glucagon-like peptide-1 receptor agonists:

• Increased glucose-dependent insulin secretion
• Inhibition of glucagon secretion
• Slowing of gastric emptying
• Reduction of appetite and food intake (central action)

This is the same profile that drives the efficacy of semaglutide and liraglutide — the difference being that in tirzepatide the activation of GLP-1R is coupled with a second pathway.

Activity at the GIP receptor (GIPR)

Activation of the glucose-dependent insulinotropic polypeptide receptor contributes:

• A synergistic increase in insulin secretion (GIP is the physiological partner of GLP-1 in the incretin effect — together they account for more than 60% of the insulin response to a meal)
• Modulation of adipose-tissue metabolism — in a pharmacological context, GIPR activation favours body weight reduction, although the mechanism differs from the classical understanding of GIP as “pro-obesogenic”
• Potential protection of pancreatic beta cells

Classically, GIP was regarded as a pathway associated with fat deposition — the pharmacological paradox is that GIPR activation combined with GLP-1R under conditions of therapeutic intervention produces the opposite effect to classical physiology. The mechanism is not fully explained and remains an area of active research.

Why dual > single — incretin synergy

Two receptors activated simultaneously produce an effect that mathematically does not reduce to “GLP-1 plus GIP”. A single molecule reaching both pathways in one binding event avoids the opposing pharmacokinetic effects that would arise from administering two separate peptides. Simultaneous activation leads to:

• Greater glycaemic reduction than with a GLP-1 monoagonist
• Greater body weight reduction than with a GLP-1 monoagonist (an effect documented in the head-to-head SURPASS-2)
• A tolerability profile close to that of GLP-1 agonists (gastrointestinal adverse effects predominate)

This is the mechanistic rationale for tirzepatide’s position as a bridge between a monoagonist (semaglutide) and a tri-agonist (retatrutide) in the classification of incretin peptides.

Unimolecular design — one molecule, two receptors

The pharmacological elegance of tirzepatide lies in the fact that both pathways are triggered by a single, deliberately designed molecule — not a mixture of two peptides. The tirzepatide backbone derives from the GIP sequence, modified so that the molecule also recognises the GLP-1 receptor. This solution from the Eli Lilly team (Coskun et al. 2018¹) made it possible to obtain a single peptide with defined, reproducible activity at both receptors — in place of a hard-to-control combination of two separate substances.

The long duration of action is due to acylation with a C20 fatty acid attached to the lysine residue. This chain enables reversible binding to serum albumin, which slows elimination and extends the half-life to about 5 days — the basis of the weekly regimen in clinical protocols. It is the same mechanism of prolonged action used in semaglutide, transferred to a dual-receptor molecule.

An important nuance is biased agonism: tirzepatide does not activate both receptors in an identical manner. In pharmacological studies it was described as a molecule with a profile close to native GIP at the GIP receptor, yet with a different, “biased” mode of GLP-1R activation (including weaker β-arrestin recruitment). This asymmetry is sometimes cited as one of the factors explaining a profile different from a simple summation of two monoagonists.

The GIP debate — agonism or antagonism

The most scientifically interesting — and most frequently oversimplified in popular communication — aspect of tirzepatide is the role of the GIP component. It conceals a genuine pharmacological paradox.

The classical picture of GIP. For decades, glucose-dependent insulinotropic polypeptide (GIP) was described as an incretin with a “pro-obesogenic” action — favouring energy storage and fat deposition. From this perspective, adding GIP agonism to a molecule analysed in the context of body weight reduction seems counterintuitive.

The paradox. Yet tirzepatide, a GIP receptor agonist, was associated in clinical trials with greater body weight reduction than a GLP-1 monoagonist. Moreover — and this is the crux of the debate — in preclinical models antagonism of the GIP receptor (its blockade, not its stimulation) also showed a favourable effect on body weight. Both opposing directions of action at the same receptor appear to lead to a similar metabolic effect.

Proposed explanations. Several non-mutually-exclusive hypotheses are considered in the literature:

• Receptor desensitisation — chronic, strong agonism of the GIP receptor may lead to its functional desensitisation, making the net effect resemble antagonism.
• Central versus peripheral action — GIP in the central nervous system may affect appetite differently than in adipose tissue; the central component may dominate under pharmacological conditions.
• Dependence on GLP-1 background — the GIP effect is sometimes context-dependent and may manifest differently under concurrent strong activation of GLP-1R.

The debate remains unresolved and is the subject of active research. For the reader, the important methodological conclusion is this: GIP biology is ambiguous, and simple statements such as “GIP makes you fat” do not capture the state of knowledge. This is one of the reasons why tirzepatide remains an interesting object of research on incretin pharmacology — and a good example of how a mechanism can be more complex than popular communication suggests.

State of the clinical evidence — the SURPASS and SURMOUNT programmes

Tirzepatide has the broadest clinical base among the next-generation incretin analogues. Eli Lilly is conducting two extensive research programmes:

SURPASS — type 2 diabetes

The SURPASS programme comprises a series of phase III trials in patients with type 2 diabetes across various therapeutic configurations — monotherapy, add-on to metformin, head-to-head comparisons with other drugs.

SURPASS-2 (Frías et al. 2021²) is the most significant trial from the perspective of the “dual vs single agonist” classification. It is a head-to-head comparison of tirzepatide (5, 10, 15 mg/week) with semaglutide (1 mg/week) in 1,879 patients with type 2 diabetes over 40 weeks. Results:

• HbA1c reduction of 2.01–2.30% in the tirzepatide groups vs 1.86% in the semaglutide group
• Body weight reduction of 7.6–11.2 kg in the tirzepatide groups vs 5.7 kg in the semaglutide group
• Safety profile consistent with the incretin agonist class

SURPASS-2 provided the first head-to-head data confirming that adding GIP activity to a GLP-1 agonist translates into better clinical outcomes — not merely a mechanistic hypothesis.

The full SURPASS programme comprises five main phase III trials, testing tirzepatide in various therapeutic configurations. The table below sets them out in the order in which they are described in the literature. The mg values appearing in the descriptions (5/10/15 mg weekly arms, unless otherwise indicated) are clinical trial parameters — not usage recommendations for the reagent.

The common conclusion from the whole series: across different populations and different therapeutic backgrounds, tirzepatide shifted the outcomes of glycaemic control and body weight above the level observed for earlier classes — with a tolerability profile typical of incretin agonists.

SURMOUNT — obesity

The SURMOUNT programme comprises phase III trials in people with obesity without diabetes.

SURMOUNT-1 (Jastreboff et al. 2022³) is a landmark trial of tirzepatide in obesity pharmacology. 2,539 adults with obesity (BMI ≥30) or overweight with comorbidities (BMI ≥27), randomised to tirzepatide 5/10/15 mg/week or placebo for 72 weeks. Results:

• Body weight reduction of 15.0% / 19.5% / 20.9% in the tirzepatide groups (5/10/15 mg) vs 3.1% on placebo
• The highest dose approached the 22.5% reduction threshold in secondary analyses
• Tolerability profile: nausea (28–31%) and diarrhoea (19–23%) predominate, usually mild or moderate

This is an outcome not previously seen in obesity pharmacology in non-surgical trials. SURMOUNT-1 provided the basis for the authorisation of Zepbound by the FDA (2023) and EMA (2024) in the obesity indication.

The SURMOUNT programme comprises four main phase III trials, testing tirzepatide in various obesity-related scenarios — including in people with diabetes, after a lifestyle intervention, and in the context of maintaining the effect.

SURMOUNT-4 is particularly interesting scientifically, because it illustrates the regulated nature of body weight described in set-point research: after withdrawal of the intervention, partial weight regain was observed — a phenomenon consistent with the physiological defence of body weight, not with “ineffectiveness” of the molecule. This is an observation from a clinical trial, not a recommendation regarding any course of action.

The most significant trials in the programme

The remaining trials in the programme (SURPASS-1, 3, 4, 5; SURMOUNT-2, 3, 4) describe tirzepatide in more specific populations — with varying baseline glycaemic control, on different therapeutic backgrounds. The consensus across the whole series: tirzepatide shifts the outcomes of metabolic pharmacology to a level previously reserved for surgical interventions.

Ongoing research directions — cardiovascular system, kidneys, liver

Beyond glycaemia and body weight, tirzepatide is the subject of research on so-called hard endpoints in several areas. The overview below deliberately keeps a cautious tone — many of these trials are still ongoing or are early in nature, and the results should not be over-interpreted.

Cardiovascular system. A dedicated CV endpoint trial (of the CVOT type) for tirzepatide in a high-risk population is ongoing; until it concludes, cardiovascular data come mainly from safety analyses within the SURPASS programme (including SURPASS-4 in an elevated-risk population). This is an area awaiting definitive data, not an established effect.

Kidneys. Analyses of renal endpoints within existing trials, as well as dedicated trials, are the subject of ongoing work. This direction fits into a broader line of research on the effect of incretin agonists on renal function — but it remains a subject of research, not a clinical conclusion.

Liver (MASH/MASLD). The most advanced of these directions is metabolic dysfunction-associated steatotic liver disease. In a phase II trial (Loomba et al. 2024¹², SYNERGY-NASH), tirzepatide was analysed in people with steatohepatitis (MASH) with fibrosis; MASH resolution rates higher than in the placebo group were reported. These are phase II data requiring confirmation in phase III — a promising direction, not an established indication.

The common denominator of these directions: research is ongoing, and tirzepatide remains under scientific analysis within them. In the One Peptides catalogue it functions solely as a research reagent (RUO) — the clinical data above concern the medicine (Mounjaro/Zepbound) in controlled trials under medical supervision, not the reagent, and do not constitute a suggestion of any use.

Tirzepatide against other incretins

Tirzepatide’s position within the class of incretin peptides becomes clear in comparison with a monoagonist (semaglutide) and a tri-agonist (retatrutide).

Tirzepatide vs semaglutide. Tirzepatide adds GIP activity to what semaglutide does on its own at GLP-1. The head-to-head SURPASS-2 showed that adding GIP translates into greater reduction of HbA1c and body weight with a comparable safety profile. A detailed comparison of the two peptides requires a separate article — tirzepatide’s position relative to semaglutide is the “single → dual evolution” of the incretin axis.

Tirzepatide vs retatrutide. Retatrutide adds a third receptor to tirzepatide — the glucagon receptor (GCGR). GCGR activation increases energy expenditure through thermogenesis and hepatic lipolysis, adding to the metabolic effect a mechanism that neither the dual nor the monoagonist possesses. Retatrutide is currently in phase III trials (the TRIUMPH programme), without authorisation as a medicine in any major jurisdiction. The details of the mechanism and the phase II data are described in a separate article on retatrutide.

In practice: tirzepatide is the current state of clinical incretin pharmacology (an authorised medicine, extensive phase III data), retatrutide is the state of the coming years (triple activation, authorisation expected after TRIUMPH concludes).

Safety and limitations from clinical trials

The safety profile of tirzepatide in the SURPASS and SURMOUNT trials corresponds to the profile of the incretin agonist class, with a few nuances.

The most common adverse effects reported in clinical trials:

• Nausea (25–31% in the tirzepatide groups vs 8–10% on placebo)
• Diarrhoea (15–23%)
• Vomiting (8–13%)
• Constipation (6–12%)
• Decreased appetite (8–17%)

Most reported adverse effects are mild or moderate in severity and resolve spontaneously during dose titration in the clinical protocol. The rate of treatment discontinuation due to adverse effects in SURMOUNT-1 was 4.3–7.1% in the tirzepatide groups vs 2.6% on placebo.

What the trials have not yet resolved:

• Long-term safety data over a 5+ year horizon (current data cover up to ~2 years of follow-up)
• The effect on cardiovascular risk in populations without baseline cardiovascular disease
• The full profile in special populations (pregnant women, breastfeeding women, patients with severe renal impairment)
• The withdrawal profile — maintenance of body weight reduction after cessation of pharmacotherapy

These are genuine limitations of the evidence worth flagging in a description of tirzepatide — in contrast to the “tirzepatide works miraculously” narrative, an honest picture takes into account that the observation horizon remains short compared with decades of data for older metabolic drugs.

The adverse effects and evidence limitations described above come from clinical trials of tirzepatide as an authorised medicine (Mounjaro, Zepbound). In the context of the RUO research reagent, the peptide is not intended for human use — the dosing protocols from clinical trials do not translate into how the reagent is used in laboratory research. For clinical questions regarding the medicine Mounjaro/Zepbound, consult your treating physician, not a supplier of research peptides.

Frequently asked questions

How does tirzepatide differ from semaglutide?

Tirzepatide activates two incretin receptors (GLP-1R + GIPR), semaglutide only one (GLP-1R). Adding GIP activity translates into greater reduction of HbA1c and body weight — the head-to-head SURPASS-2 demonstrated a statistically and clinically significant advantage of tirzepatide over semaglutide 1 mg in type 2 diabetes. The safety profile is comparable across both classes (nausea and diarrhoea predominate).

What are the SURPASS and SURMOUNT trials?

SURPASS is a series of phase III clinical trials of tirzepatide in type 2 diabetes (various configurations — monotherapy, add-on to metformin, head-to-head with semaglutide). SURMOUNT is a parallel phase III series in obesity without diabetes. Both series are conducted by Eli Lilly; their results became the basis for the authorisation of Mounjaro (type 2 diabetes, 2022) and Zepbound (obesity, 2023).

Tirzepatide and retatrutide — the difference?

Retatrutide adds a third receptor to tirzepatide — the glucagon receptor (GCGR). GCGR activation increases energy expenditure and hepatic lipolysis, adding a mechanism absent in the dual agonist. Tirzepatide is an authorised medicine (Mounjaro, Zepbound); retatrutide remains in phase III (TRIUMPH), without authorisation as a medicine in any major jurisdiction.

Is tirzepatide the same as Mounjaro?

Tirzepatide is the active substance (the peptide LY3298176). Mounjaro is the trade name of the medicine containing tirzepatide in the type 2 diabetes indication (Eli Lilly). Zepbound is the trade name of the same peptide in the obesity indication. All three names refer to the same chemical molecule in different legal and regulatory contexts. Tirzepatide in the trade of research reagents (Research Use Only) is a fourth context — a chemical substance for laboratory research, without the status of a medicine.

What is the legal status of tirzepatide in the European Union?

Tirzepatide has two separate statuses in the EU. As a medicine (Mounjaro in type 2 diabetes — EMA authorisation 2022; Zepbound in obesity — EMA authorisation 2024) it is available on prescription, under a clinical protocol, with pharmacovigilance conducted by the European Medicines Agency. As a research reagent it operates within the trade of Research Use Only peptides — as a chemical substance for laboratory research, subject to general legislation on chemical substances (REACH) and RUO labelling requirements. The legal status of both instances is clearly defined — different regulatory frameworks, different documentation, different intended purposes.

Summary

Tirzepatide is a synthetic 39-amino-acid peptide, the first clinically advanced dual agonist of the GIP and GLP-1 receptors, developed by Eli Lilly. Its authorisation as a medicine (Mounjaro in type 2 diabetes, Zepbound in obesity) is based on two extensive research programmes — SURPASS and SURMOUNT — with phase III data covering thousands of patients. Mechanistically, tirzepatide occupies an intermediate position between a GLP-1 monoagonist (semaglutide) and a GIP/GLP-1/glucagon tri-agonist (retatrutide). In the trade of research reagents (RUO) it functions as a chemical substance for laboratory research on the incretin axis — a separate regulatory status from the medicine Mounjaro/Zepbound.

Bibliography

1. Coskun T, Sloop KW, Loghin C, et al. (2018). 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
2. Frías JP, Davies MJ, Rosenstock J, et al. (2021). Tirzepatide versus Semaglutide Once Weekly in Patients with Type 2 Diabetes
3. Jastreboff AM, Aronne LJ, Ahmad NN, et al. (2022). Tirzepatide Once Weekly for the Treatment of Obesity
4. Rosenstock J, Wysham C, Frías JP, et al. (2021). Efficacy and safety of a novel dual GIP and GLP-1 receptor agonist tirzepatide in patients with type 2 diabetes (SURPASS-1): a double-blind, randomised, phase 3 trial
5. Min T, Bain SC (2021). The Role of Tirzepatide, Dual GIP and GLP-1 Receptor Agonist, in the Management of Type 2 Diabetes: The SURPASS Clinical Trials
6. Ludvik B, Giorgino F, Jódar E, et al. (2021). Once-weekly tirzepatide versus once-daily insulin degludec as add-on to metformin with or without SGLT2 inhibitors in patients with type 2 diabetes (SURPASS-3): a randomised, open-label, parallel-group, phase 3 trial
7. Del Prato S, Kahn SE, Pavo I, et al. (2021). Tirzepatide versus insulin glargine in type 2 diabetes and increased cardiovascular risk (SURPASS-4): a randomised, open-label, parallel-group, multicentre, phase 3 trial
8. Dahl D, Onishi Y, Norwood P, et al. (2022). Effect of Subcutaneous Tirzepatide vs Placebo Added to Titrated Insulin Glargine on Glycemic Control in Patients With Type 2 Diabetes: The SURPASS-5 Randomized Clinical Trial
9. Garvey WT, Frias JP, Jastreboff AM, et al. (2023). Tirzepatide once weekly for the treatment of obesity in people with type 2 diabetes (SURMOUNT-2): a double-blind, randomised, multicentre, placebo-controlled, phase 3 trial
10. Wadden TA, Chao AM, Machineni S, et al. (2023). Tirzepatide after intensive lifestyle intervention in adults with overweight or obesity (SURMOUNT-3): a randomized controlled trial
11. Aronne LJ, Sattar N, Horn DB, et al. (2024). Continued Treatment With Tirzepatide for Maintenance of Weight Reduction in Adults With Obesity: The SURMOUNT-4 Randomized Clinical Trial
12. Loomba R, Hartman ML, Lawitz EJ, et al. (2024). Tirzepatide for Metabolic Dysfunction-Associated Steatohepatitis with Liver Fibrosis

Tirzepatide in the One Peptides catalogue functions solely as a chemical reagent intended for laboratory research (Research Use Only). It is not a medicinal product, a dietary supplement or a foodstuff. The information in this article is educational in nature and describes the clinical literature concerning tirzepatide as an authorised medicine (Mounjaro in type 2 diabetes, Zepbound in obesity) — it does not constitute medical advice, does not encourage use of the peptide in any way other than for research, and does not contain dosing protocols for the end purchaser. For clinical questions regarding the medicine Mounjaro or Zepbound, consult your treating physician. The clinical trial data concern the pharmaceutical product; the RUO reagent serves laboratory research only and is not intended for human consumption.

Read also

• A full overview of GLP-1 peptides and metabolism
• Retatrutide — a triple GLP-1/GIP/glucagon agonist in clinical trials
• Semaglutide — clinical trials of the GLP-1 analogue
• GLP-1, GIP and glucagon — how incretins work
• Semaglutide vs Retatrutide — a comparison of incretin peptides
• Quality testing and certificates at One Peptides

Tirzepatide in the One Peptides catalogue is available as a lyophilisate, with an HPLC certificate of analysis ≥98% and identity confirmation by mass spectrometry — analytical data for the current batch in the COA available from the product page. Cyclical use, dosing and clinical protocols concern the medicine Mounjaro/Zepbound and do not transfer to the research reagent.

For mechanistic comparisons, see semaglutide (comparison), retatrutide / triple agonist, cagrilintide / CagriSema and the GLP-1 / incretin guide.

Bartosz BartczakPhysiotherapist, Personal Trainer

Physiotherapist with 20 years of clinical experience, graduate of Poznan University of Medical Sciences. Founder of One Peptides and lead author of clinical-perspective content.

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