Semaglutide, Tirzepatide, Retatrutide — the GLP-1, dual, and triple receptor agonist trio

Why a structural comparison and not an outcome comparison

The three peptides — Semaglutide (CP-001), Tirzepatide (CP-002), and Retatrutide (CP-003) — share commercial shorthand and appear together in nearly every procurement conversation about the GLP-1 family. Their structures sit on a clean progression: mono-receptor agonist, dual-receptor agonist, triple-receptor agonist. The catalog framing — comparison by sequence length, residue substitutions, fatty-acid acylation chemistry, molecular weight, and HPLC-MS identity markers — is the right one for in-vitro reference standard work. Outcome framing belongs in clinical journals, not in supplier documentation.

This article walks the three peptides side-by-side at the chemistry-and-receptor level so a procurement lead, postdoc, or analytical chemist evaluating which lot to characterise next can read the structural differences without wading through outcome-led marketing literature.

The receptor lineup that defines the trio

Three peptide-hormone receptors in the incretin family anchor the comparison:

• GLP-1 receptor (GLP-1R) — glucagon-like peptide-1 receptor; the canonical incretin target
• GIP receptor (GIPR) — glucose-dependent insulinotropic polypeptide receptor; the second incretin target
• Glucagon receptor (GCGR) — the energy-mobilisation counterweight to GLP-1; mechanistically distinct but pharmacologically engaged by the same backbone with the right substitutions

Semaglutide (CP-001) — the mono-agonist baseline

Semaglutide is a synthetic 31-amino-acid analogue of GLP-1(7-37). The sequence carries two engineered modifications: an α-aminoisobutyric acid (Aib) at position 8 (replacing the canonical alanine, conferring protease resistance) and a lysine-26 acylation with a C18 fatty-diacid spacer through a γ-Glu linker.

• Receptor target: GLP-1R only (a mono-agonist)
• Molecular weight: 4113.6 Da
• Backbone: GLP-1(7-37)
• Key modifications: Aib-8 + C18 fatty-diacid on Lys-26
• HPLC-MS identity: confirmed within ±0.5 Da of 4113.6 Da

Tirzepatide (CP-002) — the dual agonist

Tirzepatide is a synthetic 39-amino-acid peptide built on the GIP backbone with engineered cross-reactivity at the GLP-1 receptor. The longer chain (39 vs 31 residues) reflects the GIP origin.

• Receptor targets: GIPR + GLP-1R (a dual agonist)
• Molecular weight: 4813.5 Da
• Backbone: GIP
• Key modifications: Aib-2 + C20 fatty-diacid on Lys-20 through a γ-Glu spacer
• HPLC-MS identity: confirmed within ±0.5 Da of 4813.5 Da

Retatrutide (CP-003) — the triagonist (informally "GLP-3")

Retatrutide is a synthetic 39-amino-acid peptide that extends the dual-agonist design framework to include the glucagon receptor. Some literature and informal discussion refer to the triple-receptor framework as the "GLP-3" class — though the formal nomenclature is "triagonist" or "triple-receptor agonist", not "GLP-3" as a distinct hormone.

• Receptor targets: GLP-1R + GIPR + GCGR (a triagonist)
• Molecular weight: 4731.4 Da
• Backbone: GIP-like with engineered glucagon-receptor cross-reactivity
• Key modifications: Aib substitutions at the N-terminal positions; C20 fatty-diacid through a γ-Glu spacer
• HPLC-MS identity: confirmed within ±0.5 Da of 4731.4 Da

Why "GLP-3" is informal shorthand

In casual procurement-lab conversation and some online sources, Retatrutide is referred to as a "GLP-3 peptide" or "GLP-3 reference standard". The terminology is shorthand for "the third generation in the GLP-1 lineage" — not a reference to a distinct hormone receptor named GLP-3 (no such receptor exists in the standard incretin nomenclature). The formal description is "triagonist" or "triple-receptor agonist (GLP-1R / GIPR / GCGR)". Both terms appear in supplier literature; the formal term is preferred for analytical and regulatory documentation, while the informal "GLP-3" term is widely used in colloquial procurement search queries.

For Canada Peptides catalog purposes: Retatrutide (CP-003) is described as a triagonist; the informal "GLP-3" terminology is acknowledged here for searchability but is not used in the COA, the PDP body, or the structural identity line.

Analytical implications for COA review

The three peptides resolve cleanly on reversed-phase HPLC at 220 nm under matched gradient conditions, but their retention behaviour differs. Semaglutide elutes earlier on a typical C18 gradient than the other two, reflecting its shorter backbone and slightly smaller fatty-diacid modification. Tirzepatide and Retatrutide elute closer together because their length and fatty-acid chain length are matched; the difference in residue identity at specific positions creates a small but measurable retention-time gap.

All three show clean main peaks on a well-controlled synthesis, with impurity profiles dominated by deletion sequences (one residue short — mass shift of ~110-130 Da depending on the missing residue), methionine oxidation (+16 Da, when methionine is present), and fatty-acid acylation incompleteness (mass shift of the entire C18 or C20 chain — major peak shift, easy to spot).

The article on reading an HPLC chromatogram covers the impurity-profile interpretation framework. The article on counter-ions on a peptide COA covers the salt-form arithmetic that affects net mass per vial — relevant for all three because they are commonly supplied as the TFA salt with multi-counter-ion stoichiometry.

Summary

• The trio progresses cleanly: Semaglutide (mono-agonist, 31 residues, ~4114 Da, GLP-1R only) → Tirzepatide (dual agonist, 39 residues, ~4814 Da, GIPR + GLP-1R) → Retatrutide (triagonist, 39 residues, ~4731 Da, GLP-1R + GIPR + GCGR)
• The structural fingerprint that distinguishes them on a COA is: residue count + molecular weight + Aib position + fatty-acid chain length (C18 vs C20) + receptor-engagement profile
• Informal terminology — "GLP-3 peptide" for Retatrutide — appears in colloquial search queries; the formal term is "triagonist"
• For chromatographic comparator runs, all three resolve cleanly on a standard C18 gradient at 220 nm; the impurity-profile signature is what to monitor across release lots

Frequently asked questions