Type 2 diabetes, obesity, and related metabolic diseases all involve abnormalities in glucose homeostasis, pancreatic endocrine function, food intake, and energy balance. GLP-1R can simultaneously regulate insulin secretion, glucagon, gastric emptying, and feeding behavior and has therefore become an important metabolic regulatory target. Semaglutide, Liraglutide, Exenatide, Lixisenatide, Dulaglutide, and nonpeptide small molecules can all activate GLP-1R, but differences in molecular structure, half-life extension strategies, and in vivo exposure patterns alter the duration of receptor stimulation and the relative contribution of different physiological effects, ultimately producing distinct pharmacological profiles.

Keywords: GLP-1 receptor agonists; GLP-1R; Semaglutide; Liraglutide; Exenatide; Dulaglutide; cAMP

1 Metabolic Regulatory Basis of GLP-1R

1.1 Production and Inactivation of Endogenous GLP-1

GLP-1 is generated from proglucagon processing, and intestinal L cells are an important source of postprandial GLP-1. Glucose, fat, and some amino acids entering the intestine can promote GLP-1 release, among which GLP-1(7-36)amide is one of the major active forms. After entering the circulation, active GLP-1 can be rapidly cleaved by DPP-4. After alteration of the N-terminal structure, GLP-1R agonist activity decreases markedly. Therefore, native GLP-1 is suitable for short-term postprandial regulation but is difficult to maintain as a sustained systemic GLP-1R stimulus.

1.2 How GLP-1R Recognizes Different Ligands

(1) Recognition of Peptide Ligands

GLP-1R belongs to the Class B1 G protein-coupled receptor family and consists of an extracellular domain and a seven-transmembrane domain. GLP-1 and most peptide agonists interact with the receptor through multiple regions, allowing ligand positioning and promoting the transmembrane region into an activated conformation. The peptide N-terminus is closely related to receptor activation, whereas the middle and C-terminal regions participate in binding stabilization and maintenance of the overall conformation. Therefore, amino acid substitutions or chemical modifications at different sites can separately alter receptor activity, enzymatic stability, and pharmacokinetics.

(2) Recognition of Nonpeptide Small Molecules

Nonpeptide GLP-1R agonists do not need to completely mimic the GLP-1 peptide chain and can stabilize the activated receptor conformation through specific interactions within the transmembrane region. This binding mode expands the range of chemical structures that GLP-1R can recognize and also allows some small molecules to show features different from peptide agonists in receptor conformation, signal output, and species selectivity.

1.3 Core Signaling Pathways of GLP-1R

(1) Gs-AC-cAMP Signaling

After GLP-1R activation, coupling to Gs proteins is mainly promoted. Gsα further activates Adenylate Cyclase, increasing intracellular cAMP levels. cAMP is the most typical proximal second messenger of GLP-1R and is also an important signaling node connecting receptor activation with subsequent cellular functions.

(2) PKA, EPAC, and Ca²⁺

Elevated cAMP can activate PKA and EPAC. PKA regulates ion channels and secretion-related proteins through protein phosphorylation, while EPAC participates in regulation of small GTPases and the secretory machinery. After glucose metabolism increases the ATP/ADP ratio in β cells and promotes membrane depolarization and Ca²⁺ entry, PKA and EPAC further enhance Ca²⁺-dependent recruitment and exocytosis of insulin granules. Thus, GLP-1R mainly acts by amplifying glucose-stimulated insulin secretion.

(3) β-Arrestin and Receptor Trafficking

Sustained GLP-1R activation can also promote receptor phosphorylation, β-arrestin recruitment, and receptor internalization. Internalized receptors can maintain a certain degree of intracellular signaling and are subsequently recycled or enter degradation pathways. Different agonists are not completely identical in G-protein activation, β-arrestin recruitment, internalization, and recycling, giving GLP-1R pharmacology not only differences in activation intensity but also clear temporal and spatial characteristics.

1.4 Major Pharmacological Effects After GLP-1R Activation

(1) Promotion of Glucose-Dependent Insulin Secretion

GLP-1R activation enhances the response of β cells to glucose stimulation and improves the efficiency of insulin granule exocytosis. As glucose levels decrease, the insulinotropic effect is correspondingly reduced. This glucose dependence is an important basis of the glucose-lowering pharmacology of GLP-1R.

(2) Regulation of Glucagon Secretion

GLP-1R activation can reduce inappropriate glucagon release under hyperglycemic conditions, thereby decreasing excessive hepatic glucose output. This process is also influenced by Insulin, Somatostatin, and other pancreatic islet paracrine signals and cannot be simply attributed to direct GLP-1R activation in α cells.

(3) Delayed Gastric Emptying

GLP-1R activation can reduce the rate at which gastric contents empty into the small intestine, thereby slowing the entry of nutrients and glucose into the circulation. This effect mainly influences early postprandial glucose changes and is more prominent with some short-acting GLP-1R agonists.

(4) Suppression of Food Intake

GLP-1R participates in central and peripheral satiety regulation, and sustained activation can reduce food and total energy intake. Long-term reduction in energy intake produces a negative energy balance and is an important basis for reductions in body weight and Fat Mass caused by GLP-1R agonists.

2 From Native GLP-1 to Drug-Like GLP-1R Agonists

2.1 Improving DPP-4 Stability

Native GLP-1 is readily and rapidly inactivated by DPP-4, so improving peptide stability is the first issue that needs to be addressed in the development of GLP-1R agonists. Human GLP-1 analogs can reduce enzymatic cleavage by altering DPP-4-sensitive sites, whereas Exendin-4-derived molecules obtain relatively high DPP-4 stability through sequences that differ from native GLP-1. Both approaches can extend the duration for which active ligands act on GLP-1R.

2.2 Fatty Acid Modification and Albumin Binding

Improving enzymatic stability alone is still insufficient to generate exposure lasting for days or weeks. Semaglutide and Liraglutide further use fatty acid modification to enhance reversible binding to Albumin. Albumin binding reduces the proportion of free drug in the circulation and slows clearance, while differences in fatty acid chain length, linker structure, and modification site further alter the duration of in vivo exposure.

2.3 Exendin Scaffold

Exendin-4 is not a direct analog of human GLP-1. Its own sequence has relatively high DPP-4 stability while retaining effective GLP-1R agonist activity. Exenatide and Lixisenatide are both based on the Exendin scaffold, but their classical formulations remain relatively short-acting molecules and therefore more readily produce phase-specific GLP-1R stimulation.

2.4 Fc Fusion

Dulaglutide increases molecular size by fusing a GLP-1-like functional region with an Fc structure, reducing rapid renal filtration and clearance. Fc fusion can produce systemic exposure lasting several days, but macromolecularization also alters tissue penetration and local drug accessibility, creating a half-life extension pathway distinct from lipidated GLP-1 analogs.

2.5 Nonpeptide Small Molecules

Nonpeptide small-molecule GLP-1R agonists stabilize the activated receptor conformation through binding within the transmembrane region and no longer depend on peptide stabilization, lipidation, or macromolecular fusion. This approach expands the chemical space of GLP-1R ligands while introducing new pharmacological features such as receptor conformation, signaling kinetics, and species selectivity.

3 Lipidated GLP-1 Analogs: Semaglutide and Liraglutide

3.1 Semaglutide

(1) Structure and Half-Life Extension

Semaglutide is based on the human GLP-1 sequence. It improves DPP-4 stability through amino acid substitution at key sites and enhances Albumin binding by attaching a C18 fatty diacid side chain to Lys26. Resistance to enzymatic degradation and Albumin binding together slow drug clearance, resulting in a terminal half-life on the order of approximately 1 week and sustained GLP-1R exposure.

(2) Sustained Receptor Stimulation

Semaglutide still acts mainly through Gs-cAMP-PKA/EPAC and Ca²⁺-related signaling. Its prominent characteristic is the ability to maintain effective GLP-1R stimulation for a prolonged period. Sustained exposure allows pancreatic endocrine and feeding regulation to maintain relatively stable pharmacological effects, while the influence of receptor internalization, intracellular signaling, and recycling becomes more prominent during long-term stimulation.

(3) Pharmacological Characteristics

The long-term pharmacology of Semaglutide is importantly composed of sustained glucose-dependent pancreatic islet regulation and food-intake suppression. Gastric emptying is also affected after administration, but this effect may gradually adapt during sustained GLP-1R stimulation. Therefore, long-term body-weight reduction and metabolic improvement cannot be simply explained by continuously delayed gastric emptying.

3.2 Liraglutide

(1) Structure and Exposure

Liraglutide is a lipidated human GLP-1 analog. The Lys34→Arg substitution prevents acylation at this position, whereas attachment of a C16 fatty acid side chain to Lys26 through a glutamate spacer enhances Albumin binding. Lipidation also gives the molecule a certain degree of self-association, which can simultaneously slow release from the injection site and systemic clearance. Its terminal half-life is approximately 13 hours.

(2) Pharmacological Characteristics

Liraglutide shares core receptor mechanisms such as GLP-1R-cAMP with Semaglutide, but its effective exposure duration is markedly shorter, producing repeated daily periods of prolonged GLP-1R stimulation. Its pancreatic islet, feeding, and gastric-emptying effects all arise from classical GLP-1R pharmacology, while the duration of each functional effect is influenced by the daily exposure cycle.

3.3 Why the Two Lipidation Strategies Show Different Pharmacological Profiles

Semaglutide and Liraglutide both extend their duration of action through fatty acid modification, but differences in fatty acid structure, linkage, and Albumin-binding characteristics allow the former to produce week-scale sustained exposure while the latter retains a more pronounced daily exposure cycle. Their core GLP-1R signaling is not fundamentally different; the primary factor separating their pharmacological profiles is how and for how long effective drug concentrations are maintained in vivo.

4 Exendin-4-Derived Agonists: Exenatide and Lixisenatide

4.1 Characteristics of the Exendin-4 Scaffold

Exendin-4-derived peptides have sequences and C-terminal structures different from human GLP-1 while retaining relatively high DPP-4 stability and GLP-1R agonist activity. Exenatide and Lixisenatide both use this scaffold and therefore do not require the same lipidation strategies used by Semaglutide and Liraglutide, although their specific sequences, drug exposure, and receptor kinetics still differ.

4.2 Exenatide

(1) Short-Acting Exposure

Classical short-acting Exenatide has a terminal half-life of approximately 2.4 hours. It produces a relatively concentrated window of GLP-1R stimulation after administration, followed by a marked decline in drug exposure. Its core receptor signaling still includes Gs-cAMP, PKA, EPAC, and Ca²⁺-related secretory processes.

(2) Pharmacological Characteristics

Phase-specific GLP-1R stimulation gives Exenatide a relatively prominent post-dose inhibition of gastric emptying while also enhancing glucose-dependent insulin secretion and regulating glucagon. Its postprandial metabolic effects are jointly formed by gastrointestinal motility and pancreatic endocrine regulation and cannot be explained solely by changes in β-cell secretion.

4.3 Lixisenatide

(1) Short-Acting Exposure

Lixisenatide is an Exendin-4-derived GLP-1R agonist with a terminal half-life of approximately 3 hours and likewise produces clear phase-specific receptor stimulation. Although it shares the Exendin scaffold with Exenatide, its specific sequence and receptor-binding, internalization, and recycling kinetics are not completely identical.

(2) Pharmacological Characteristics

Lixisenatide has a relatively pronounced acute inhibitory effect on gastric emptying while retaining glucose-dependent pancreatic islet regulation. Lower drug exposure between two administrations allows recovery time for gastrointestinal effects, so repeated administration can still produce relatively pronounced gastric-emptying effects.

4.4 Why Short-Acting Exendin Compounds Show More Prominent Gastric-Emptying Effects

During sustained GLP-1R stimulation, inhibition of gastric emptying can show a certain degree of rapid adaptation. Short-acting Exenatide and Lixisenatide have low-exposure periods between two administrations, allowing this physiological effect to partially recover. Therefore, the difference between short- and long-acting GLP-1R agonists is not simply a difference in half-life, but reflects different degrees of adaptation of individual physiological functions to sustained receptor stimulation.

5 Fc-Fused GLP-1R Agonist: Dulaglutide

5.1 Fc Fusion and Sustained Exposure

Dulaglutide is formed by fusing a GLP-1-like functional region with a modified IgG4 Fc region. The GLP-1 functional region is responsible for receptor recognition and activation, whereas the Fc structure increases molecular size and reduces rapid clearance, resulting in a terminal half-life of approximately 5 days. Its half-life extension does not depend on Albumin binding but instead changes in vivo clearance through macromolecularization.

5.2 Sustained GLP-1R Pharmacology

After activating GLP-1R, Dulaglutide still mainly regulates glucose-dependent insulin secretion, glucagon, food intake, and gastric emptying through classical pathways such as Gs-cAMP-PKA/EPAC. Sustained exposure allows pancreatic islet and feeding effects to be maintained over a longer period, whereas gastric-emptying effects may gradually adapt during sustained GLP-1R stimulation.

5.3 Macromolecular Structure and Tissue Distribution

Fc fusion markedly increases the molecular size of Dulaglutide and, while extending systemic exposure, also alters tissue penetration and barrier-crossing capacity. High systemic exposure does not mean that all tissues have the same local drug accessibility. Therefore, systemic metabolic improvements produced by Dulaglutide need to be distinguished from direct GLP-1R effects in specific tissues.

6 Nonpeptide Small-Molecule GLP-1R Agonists

6.1 How Small Molecules Activate GLP-1R

Nonpeptide small molecules such as PF-06882961 do not rely on an intact GLP-1 peptide chain but instead stabilize the activated receptor conformation through specific interactions within the transmembrane region of GLP-1R. GLP-1R has therefore expanded from a traditional peptide-ligand target to small-molecule pharmacology, providing a receptor-regulation strategy distinct from peptide modification and macromolecular fusion.

6.2 Signal Output and Species Selectivity

(1) Differences in Signal Output

Nonpeptide small molecules can also produce Gs-cAMP signaling, but cAMP cannot fully describe their GLP-1R pharmacology. Different ligands may differ in β-arrestin recruitment, receptor internalization, and sustained signaling. Therefore, similar cAMP potency does not mean that they produce completely identical receptor conformations and cellular responses.

(2) Species Dependence

Some nonpeptide GLP-1R agonists show clear species selectivity, and effective activation of human or primate GLP-1R cannot be directly extrapolated to conventional rodent receptors. For these molecules, receptor species background itself is an important factor determining whether the drug can produce the expected GLP-1R pharmacology.

6.3 Pharmacological Characteristics of Nonpeptide Small Molecules

When nonpeptide small molecules can effectively activate the target GLP-1R and achieve sufficient in vivo exposure, they can also produce classical GLP-1R effects such as glucose-dependent pancreatic islet regulation and food-intake suppression. Compared with peptide agonists, their more distinctive pharmacological features are concentrated in transmembrane binding modes, receptor conformation, signaling bias, and species selectivity.

7 Why the Same GLP-1R Produces Different Pharmacological Profiles

7.1 Exposure Patterns Alter the Mode of Receptor Stimulation

Exenatide and Lixisenatide produce pronounced peak-and-trough exposure, Liraglutide produces repeated daily stimulation, whereas Semaglutide and Dulaglutide can maintain effective GLP-1R stimulation for several days or even on a week-scale. Whether the receptor experiences phase-specific or sustained activation alters the relative contributions of gastric emptying, pancreatic endocrine regulation, and feeding effects to overall pharmacology. Therefore, half-life is not only a pharmacokinetic parameter but also determines the temporal pattern of GLP-1R stimulation.

7.2 Different Physiological Functions Adapt Differently to Sustained Stimulation

Gastric emptying is relatively prone to adaptation during sustained GLP-1R stimulation, whereas pancreatic endocrine and feeding regulation can be maintained over a longer period. Because short-acting agonists have recovery periods between two stimuli, their post-dose gastric-emptying effects are usually more pronounced. Long-acting agonists instead show more sustained pancreatic and energy-intake regulation. Different GLP-1R agonists do not simply prolong or shorten the same set of pharmacological effects but alter the relative weight of different effects within overall pharmacology.

7.3 Receptor Kinetics and Tissue Accessibility Further Expand the Differences

GLP-1R activation involves not only Gs-cAMP but also β-arrestin recruitment, receptor internalization, intracellular signaling, and recycling, and different ligands can produce distinct sustained-signaling characteristics. At the same time, lipidated peptides, Exendin-class peptides, Fc-fusion proteins, and nonpeptide small molecules differ in protein binding, molecular size, and tissue distribution. The final in vivo pharmacological profile is jointly determined by effective exposure duration, receptor-stimulation kinetics, and tissue accessibility rather than by a single in vitro EC₅₀.

8 Key Decision Points in GLP-1R Agonist Research

8.1 Increased cAMP Does Not Equal GLP-1R Specificity

cAMP is a commonly used proximal signal readout for GLP-1R, but other Gs-coupled receptors can also promote cAMP production. In addition to observing concentration dependence, GLP-1R mechanism studies need to combine receptor-blocking tools such as Exendin(9-39) or other evidence of receptor dependence to avoid assigning mechanism solely on the basis of second-messenger changes.

8.2 Postprandial Glucose Lowering Requires Distinguishing Pancreatic Effects From Gastric Emptying

GLP-1R agonists can enhance glucose-dependent insulin secretion, but blood-glucose changes after oral glucose are also strongly affected by gastric emptying. For short-acting molecules such as Exenatide and Lixisenatide, marked postprandial glucose reduction cannot be directly equated with enhanced β-cell function; the rate at which gastric contents enter the small intestine also needs to be considered.

8.3 Tissue Improvement Does Not Equal Direct Tissue Action

After GLP-1R agonists reduce food intake, secondary changes can include body-weight loss, reduced hepatic lipids, improved insulin sensitivity, and decreased inflammation. Determining whether direct GLP-1R action occurs in the liver, adipose tissue, or other tissues requires simultaneous consideration of receptor expression, drug accessibility, proximal receptor signaling, and the effects of changes in food intake and body weight.

8.4 Comparisons Among Different Drugs Need to Consider Exposure and Species

Different GLP-1R agonists differ markedly in half-life, protein binding, molecular weight, and tissue distribution. The same mg/kg dose therefore does not represent the same level of receptor stimulation. Short-acting drugs are also highly dependent on the interval between administration and measurement, while some nonpeptide small molecules show clear species selectivity. Both exposure conditions and receptor species background can affect cross-drug comparisons.

9 GLP-1R Agonists and Related Research Products

9.1 GLP-1R Agonists and Semaglutide-Related Products

9.2 GLP-1R Blockade and GLP-1/DPP-4 Mechanism Research Tools

9.3 GLP-1R Signaling and Metabolic Function Detection

GLP-1R agonists enhance glucose-dependent insulin secretion through Gs-cAMP-PKA/EPAC and Ca²⁺-related signaling and jointly affect blood glucose and body weight through glucagon regulation, gastric emptying, and control of food intake. Semaglutide and Liraglutide extend exposure through lipidation and Albumin binding, Exenatide and Lixisenatide retain the short-acting characteristics of Exendin-class molecules with more pronounced effects on gastric emptying, Dulaglutide achieves sustained action through Fc fusion, and nonpeptide small molecules produce distinct receptor-binding and species-dependent pharmacology.

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