GLP-1 vs GLP-2 vs GLP-3: Semaglutide, Tirzepatide & Retatrutide Explained

The GLP-1/GLP-2/GLP-3 naming convention used in research shorthand refers to the generational progression of incretin receptor agonists: from the original GLP-1 monoagonist class (Semaglutide) through dual agonism (Tirzepatide/GIP+GLP-1) to triple receptor engagement (Retatrutide/GIP+GLP-1+GcgR). Understanding the distinctions between these compounds is essential for anyone working in metabolic research.

The Incretin Receptor Landscape

Three receptors form the core of the incretin and related metabolic signaling systems relevant to this compound class:

• GLP-1R (Glucagon-Like Peptide-1 Receptor): Expressed in pancreas, brain, gut, heart, and kidney. GLP-1R agonism enhances glucose-dependent insulin secretion, suppresses glucagon, slows gastric emptying, and produces CNS-mediated satiety. The foundational target of all three compound classes.
• GIPR (Glucose-Dependent Insulinotropic Polypeptide Receptor): Expressed in pancreas, adipose tissue, bone, and brain. GIP enhances insulin secretion in a glucose-dependent manner and may improve GLP-1R agonist tolerability by counteracting GLP-1R-driven nausea through separate pathways.
• GcgR (Glucagon Receptor): Expressed in liver, kidney, heart, and adipose tissue. Glucagon receptor agonism increases hepatic glucose output (seemingly counterproductive) but critically also increases energy expenditure and promotes hepatic lipid clearance — adding a metabolic rate dimension absent from the dual agonist class.

Semaglutide (GLP-1): The Reference Standard

Semaglutide is a GLP-1 monoagonist that established the modern era of peptide-based metabolic research. Modified with C18 fatty acid albumin binding for a 7-day half-life, it became the first GLP-1 agent to demonstrate clinically significant weight reduction in large-scale trials.

Key Trial Data

• STEP 1 (Wilding et al., NEJM 2021): 2.4mg weekly semaglutide → mean 14.9% weight reduction at 68 weeks in non-diabetic adults with obesity. 86% of subjects achieved ≥5% weight loss. The benchmark for GLP-1 monoagonism.
• SUSTAIN-6: 26% reduction in MACE (major adverse cardiovascular events) vs placebo — establishing the GLP-1 cardiovascular benefit that applies across the class.
• NASH/NAFLD research: Semaglutide reduces hepatic steatosis and liver inflammation markers in non-alcoholic fatty liver disease models — likely through direct hepatic GLP-1R effects beyond weight loss-mediated improvement.

Tirzepatide (GLP-2 in research shorthand): The Dual Agonist

Tirzepatide targets GLP-1R and GIPR simultaneously. The GIP coagonism was initially controversial — GIP was considered a "failed" target because GIPR knockout mice were lean, suggesting antagonism might be beneficial. Tirzepatide resolved this paradox by showing that GIPR agonism, not antagonism, produces favorable metabolic effects when combined with GLP-1R agonism, possibly because GIP receptor dynamics in adipose tissue differ from those in the pancreas under chronic agonism.

Key Trial Data

• SURPASS-2 (Frías et al., NEJM 2021): Tirzepatide 15mg → HbA1c reduction of 2.46% vs semaglutide 1mg (1.86%) in T2D. Superior to GLP-1 monoagonism on the primary glycemic endpoint.
• SURMOUNT-1 (Jastreboff et al., NEJM 2022): 22.5% mean weight reduction at 72 weeks with tirzepatide 15mg in non-diabetic obesity — surpassing all prior incretin trial results at time of publication. 87% achieved ≥5%, 57% achieved ≥20% weight loss.
• GI tolerability: Despite greater efficacy, tirzepatide's GI adverse event rates were comparable or lower than expected for equivalent GLP-1 monoagonist doses — supporting the hypothesis that GIPR coagonism attenuates GLP-1R-driven nausea.

Retatrutide (GLP-3 in research shorthand): The Triple Agonist

Retatrutide adds glucagon receptor (GcgR) agonism to the GIP/GLP-1 dual mechanism, creating a triple receptor profile. The GcgR component is the key pharmacological innovation — glucagon receptor agonism increases hepatic energy expenditure, promotes hepatic lipid clearance, and raises basal metabolic rate, effects that operate through mechanisms distinct from the satiety-focused GLP-1 axis.

Key Trial Data

• Jastreboff et al. (NEJM, 2023): Phase 2 retatrutide in obesity. At 12mg dose, 48-week mean weight reduction of ~24.2% — the largest weight reduction in any obesity pharmacotherapy Phase 2 trial at time of publication. Over 90% of participants achieved ≥5% weight loss.
• Glycemic data (T2D): Phase 2 in type 2 diabetes: ~2.0% HbA1c reduction at higher doses, with fasting glucose improvements comparable to tirzepatide.
• Heart rate: Slight elevation (4–7+ bpm) noted, somewhat higher than tirzepatide — attributed to the glucagon receptor component's effect on cardiac chronotropy. An active area of monitoring in Phase 3 programs.

Head-to-Head Comparison

Research Applications

Each compound offers distinct utility for metabolic research:

• Semaglutide is the established reference compound for GLP-1R pharmacology — any new compound in this class is benchmarked against it.
• Tirzepatide enables investigation of GIP/GLP-1 receptor synergy and the mechanistic role of GIPR coagonism in metabolic outcomes.
• Retatrutide allows investigation of triple receptor engagement and the specific contribution of GcgR activity to energy expenditure, hepatic lipid metabolism, and weight outcomes.