A research-level overview of the incretin system, GLP-1, GIP and glucagon receptor biology, amylin, and the multi-receptor agonists that now dominate metabolic peptide research, with the published findings behind each.
Few areas of peptide science have moved as quickly as metabolic research. Over roughly two decades, work on a handful of gut and pancreatic hormones has produced single-receptor agonists, then dual agonists, and now triple agonists that act on three receptors with one molecule. This guide walks through the receptor biology that makes that progression make sense, summarizes what the key published studies actually found, and closes with the design questions that matter when these compounds are used in a laboratory model.
Incretins are gut peptides released after a meal that amplify insulin secretion when blood glucose is elevated. The two known incretins are glucose-dependent insulinotropic polypeptide (GIP), secreted by K cells of the upper small intestine, and glucagon-like peptide-1 (GLP-1), secreted by L cells of the lower gut. In human studies, together they account for the incretin effect: a two- to three-fold larger insulin response to oral glucose than to the same glucose given intravenously [1].
In people with type 2 diabetes, the incretin effect is diminished or absent. A review of the human physiology attributes this mainly to a sharply reduced response of the diabetic pancreas to GIP, while the insulin-stimulating and glucagon-suppressing actions of GLP-1 are largely preserved [1]. That asymmetry is why the GLP-1 receptor, rather than the GIP receptor, became the first target for incretin-based drugs.
GLP-1 is cut from the proglucagon precursor. The same gene is expressed in pancreatic alpha cells, in intestinal L cells concentrated in the distal ileum and colon, and in a population of neurons in the nucleus tractus solitarii of the brainstem, and tissue-specific processing determines which peptides each cell releases [2]. The two bioactive circulating forms, GLP-1(7-36)amide and GLP-1(7-37), are reported to be equally potent at stimulating insulin secretion [2]. Both act on the GLP-1 receptor, a class B G protein-coupled receptor that signals largely through cAMP.
Across rodent and human studies, the reported actions of GLP-1 include glucose-dependent insulin secretion, slower gastric emptying, reduced food intake, increased sodium excretion, and, in rodents, modulation of beta-cell proliferation [2]. The glucose dependence is the defining feature: insulin release is amplified when glucose is high and fades as glucose falls, which is part of why the pathway attracted so much interest.
Native GLP-1 survives only about 1 to 2 minutes in circulation, depending on species, because the enzyme dipeptidyl peptidase-4 (DPP-4) removes its first two amino acids and the kidney clears what remains [2]. Much of the medicinal chemistry in this field is a response to that constraint. Common strategies described in the literature include substitutions near the N-terminus that resist DPP-4 and attachment of a fatty acid chain that binds reversibly to serum albumin and slows clearance [2]. Tirzepatide, for example, is described by its developers as a fatty acid modified peptide [3]. For researchers, the practical consequence is that native GLP-1 and an engineered analog behave very differently in time-course experiments, and assays for active GLP-1 generally require that DPP-4 activity be stopped at the moment a sample is collected.
Because GIP loses much of its insulin-stimulating effect in type 2 diabetes, it was long regarded as the less useful incretin. GIP and GLP-1 also act outside the pancreas, including on fat tissue, bone, and the cardiovascular system [1], and the question became whether adding GIP receptor activity to GLP-1 receptor activity would do more than GLP-1 alone.
The discovery paper for LY3298176, the compound later named tirzepatide, set out to test exactly that. In cell lines expressing incretin receptors it activated both GIP and GLP-1 receptor signaling. In mice it produced glucose-dependent insulin secretion and improved glucose tolerance by acting on both receptors, and with chronic administration it reduced body weight and food intake significantly more than a selective GLP-1 receptor agonist [3]. The accompanying phase 1 and 1b studies enrolled 142 human subjects, and the most frequently reported side effects were gastrointestinal: nausea, vomiting, decreased appetite, diarrhea, and abdominal distension [3].
Tirzepatide is now an approved medicine, marketed as Mounjaro and Zepbound. The TZ-2 material in the Northbridge Research Labs catalog is a research compound referenced to the tirzepatide literature. It is not either of those products and is not made or tested to pharmaceutical standards for human use.
At first glance, glucagon seems like an odd partner for an anti-obesity or glucose-lowering peptide, since glucagon raises hepatic glucose output. The rationale is that glucagon receptor signaling also increases energy expenditure, and that the insulin-promoting effects of GLP-1 and GIP agonism can offset its effect on glucose.
LY3437943, the compound named retatrutide, was designed around that idea. In vitro it showed balanced glucagon and GLP-1 receptor activity with greater GIP receptor activity. In obese mice it lowered body weight and improved glycemic control, and the authors attributed the extra weight loss to glucagon receptor-driven increases in energy expenditure added on top of the reduced food intake produced by GIP and GLP-1 receptor activity. In a phase 1 single-ascending study, a reduction in body weight persisted up to day 43 after a single administration [4].
A phase 2, double-blind, placebo-controlled trial then randomized 338 adults with obesity, or with overweight plus a weight-related condition, to several retatrutide arms or placebo for 48 weeks. At 48 weeks the least-squares mean change in body weight ranged from -8.7% in the lowest arm to -24.2% in the highest arm, compared with -2.1% on placebo. Gastrointestinal events were the most common adverse events and were mostly mild to moderate, and increases in heart rate peaked at 24 weeks and declined afterward [5]. These are the findings of one trial in one population, funded by the developer, and they describe that study rather than any expected outcome. In our catalog, the corresponding research material is listed as RT-3.
Amylin is a 37-amino-acid peptide co-secreted with insulin by pancreatic beta cells. Its receptors are unusual: each is formed by the calcitonin receptor combined with one of the receptor activity-modifying proteins (RAMPs), giving several receptor subtypes with different pharmacology [6]. A review of rodent and human work describes amylin as a glucose-regulating hormone that acts principally in the circumventricular organs of the brain and interacts with other metabolic signals, including cholecystokinin, leptin, and estradiol [6].
Human amylin readily aggregates into amyloid fibrils, which makes the native peptide difficult to work with. Pramlintide, an amylin analog carrying proline substitutions that reduce this tendency, has been approved for use alongside insulin in type 1 and type 2 diabetes, and clinical studies in obesity suggest amylin agonists may be most useful in combination with other agents [6]. That combination logic is why amylin receptor agonists are now studied alongside incretin-based peptides.
AOD-9604 is a synthetic peptide based on the C-terminal region of human growth hormone. In a 2001 study, obese mice given AOD-9604 or growth hormone for 14 days lost body weight and body fat, and expression of beta-3 adrenergic receptor RNA in fat tissue rose toward the level seen in lean mice. In beta-3 adrenergic receptor knockout mice, however, long-term administration did not produce the same weight change, while an acute experiment still showed increased energy expenditure and fat oxidation. The authors concluded that the lipolytic effect is not mediated directly through that receptor [7]. The evidence summarized here is from mice.
MOTS-c is a 16-amino-acid peptide encoded by a short open reading frame within the mitochondrial 12S rRNA gene. Its discoverers reported that its primary target appears to be skeletal muscle, where it inhibits folate metabolism and the de novo purine synthesis tied to it, leading to activation of AMPK, the cell's energy sensor. In mice, MOTS-c administration prevented both age-dependent and high-fat-diet-induced insulin resistance and diet-induced obesity [8]. These findings come from cell and rodent work.
Note: All compounds discussed here, including RT-3 and TZ-2, are sold by Northbridge Research Labs strictly for laboratory research use. They are not drugs, are not approved for human or veterinary use, and are not the approved medicines named in this article. Clinical results are reported as the published findings of those trials.
How a single trans-3-hexenoyl group turns native GHRH into a longer-lived analogue, what the phase 3 and liver-fat trials actually found, what preclinical work showed, and how to handle tesamorelin in the lab.
What sermorelin acetate is, where the 29-residue fragment came from, how it acts at the GHRH receptor, how it compares with native GHRH, tesamorelin, CJC-1295 and ipamorelin, and how to handle it in the lab.
How retatrutide-type triple agonism at the GLP-1, GIP and glucagon receptors was designed, what the published rodent and phase 2 literature reports, and which questions remain open for laboratory study.
Research Use Only: The information in this article is for educational and research purposes only. All products mentioned are intended for laboratory research use only and are not approved for human or veterinary use.