The incretin system — the network of gut-derived hormones that modulate the body’s response to nutrient ingestion — has been one of the most intensively studied areas of metabolic biology over the past four decades. The identification of glucagon-like peptide-1 (GLP-1) as a potent insulinotropic agent in the 1980s, followed by the characterisation of glucose-dependent insulinotropic polypeptide (GIP) and glucagon receptor signalling, laid the groundwork for what has become a rapidly evolving field of receptor pharmacology.

This article provides an academic overview of the three principal receptor systems now at the centre of metabolic research: the GLP-1 receptor, the GIP receptor, and the glucagon receptor. It traces the mechanistic rationale behind single, dual, and triple receptor agonism as research models, and summarises the key findings from the clinical literature to date.

1. The GLP-1 Receptor

Glucagon-like peptide-1 is an incretin hormone secreted by L-cells of the small intestine in response to nutrient ingestion. It was first isolated and characterised by Mojsov, Heinrich, and Habener in 1987, with subsequent studies establishing its role as a potent stimulator of glucose-dependent insulin secretion. The GLP-1 receptor (GLP-1R), a class B G protein-coupled receptor, is expressed not only in pancreatic beta cells but also in the central nervous system, gastrointestinal tract, heart, and kidneys — a distribution that accounts for the breadth of physiological effects observed in GLP-1R agonism research.

The primary actions documented in the literature include: augmentation of glucose-stimulated insulin secretion, suppression of glucagon release from pancreatic alpha cells, deceleration of gastric emptying (which attenuates postprandial glucose excursions), and appetite suppression mediated through hypothalamic and brainstem GLP-1 receptor signalling. The appetite-suppressive effect in particular has been the subject of extensive preclinical and clinical investigation, with the STEP trial programme representing the most comprehensive clinical dataset produced for a GLP-1 receptor agonist compound to date.

Key Research Findings — GLP-1 Monoagonism

The STEP clinical programme (2019–2021) investigated a GLP-1 receptor monoagonist at escalating doses over 68-week periods. STEP 1 reported a mean body weight reduction of 14.9% versus 2.4% with placebo in adults with obesity, representing the most significant weight reduction outcome recorded for a pharmacological agent at that point. Subsequent STEP trials confirmed the effect across diverse populations and established a safety profile that informed later regulatory assessments.

2. The GIP Receptor

Glucose-dependent insulinotropic polypeptide (GIP) is the other principal incretin hormone, secreted by K-cells in the duodenum and proximal jejunum. First characterised by Brown and colleagues in the 1970s, GIP was initially understood primarily as an insulinotropic signal. Subsequent research revealed a more complex receptor biology: GIP receptors are expressed in adipose tissue, bone, and the central nervous system, and GIP signalling appears to modulate adipogenesis, bone turnover, and neurobiological functions in ways that are still being characterised.

The rationale for combining GLP-1R and GIPR agonism in a single research molecule emerged from observations that the two pathways, while overlapping in their insulinotropic effects, engage distinct downstream mechanisms. GIP receptor agonism in preclinical models attenuated the gastrointestinal side effects commonly associated with isolated GLP-1R agonism — an observation that gained clinical significance in the SURMOUNT trial programme, where the dual agonist molecule demonstrated superior efficacy to GLP-1 monoagonism with a tolerability profile that was non-inferior or superior across most measures.

3. The Glucagon Receptor

Glucagon, the pancreatic alpha-cell hormone responsible for hepatic glucose production during fasting states, has historically been regarded as a target for suppression rather than activation in metabolic research contexts. The reconceptualisation of glucagon receptor agonism as a potentially beneficial component of a multi-receptor approach emerged from observations of its distinct role in energy expenditure and direct lipolytic activity.

Glucagon receptor activation increases hepatic glucose output and, importantly for energy metabolism research, stimulates thermogenesis through brown adipose tissue activation and directly mobilises stored lipid through glucagon-mediated lipolytic signalling in adipocytes. The theoretical concern — that glucagon receptor agonism would increase plasma glucose to a clinically problematic degree — was addressed in preclinical research demonstrating that co-administration of GLP-1 receptor agonism is sufficient to counteract the hyperglycaemic effect of glucagon signalling, leaving the thermogenic and lipolytic actions intact.

Triple Receptor Agonism — Early Phase Research

Phase 2 clinical research into a triple GLP-1/GIP/glucagon receptor agonist published in the New England Journal of Medicine in 2023 reported mean body weight reductions of up to 24.2% at 48 weeks — the highest figure recorded in any randomised controlled trial of a pharmacological compound in the obesity research literature. The addition of glucagon receptor agonism to the dual incretin receptor background appeared to produce additive effects on energy expenditure and fat mobilisation beyond what dual agonism alone achieved.

It is important to note that long-term safety data for triple receptor agonism remains limited relative to the monoagonist and dual agonist literature, and ongoing Phase 3 programmes are expected to provide the extended datasets necessary for a comprehensive safety characterisation.

4. Comparative Mechanistic Summary

5. Research Implications and Future Directions

The trajectory from single to dual to triple receptor agonism in incretin-based research represents one of the clearest examples in modern pharmacology of mechanistic rationale driving iterative compound design. Each step has been informed by both preclinical receptor biology and accumulating clinical datasets, with each generation demonstrating improvements in the primary research endpoints while maintaining acceptable tolerability profiles.

Current areas of active research interest include the tissue-specific distribution of incretin receptors beyond metabolic organs, potential cardiovascular effects observed in secondary endpoints of major trials, and the neurobiological mechanisms underlying appetite suppression — particularly the role of hypothalamic and brainstem GLP-1 receptor populations in reward-based eating behaviour.

The field continues to evolve rapidly. The publication of Phase 3 data for triple agonist molecules, alongside emerging research into receptor-selective agonism and tissue-targeted delivery modalities, suggests that the incretin receptor biology space will remain one of the most productive areas of metabolic research through the remainder of the decade.