Retatrutide, Semaglutide and Tirzepatide: A Receptor Mechanism Comparison

Comparing by mechanism, not outcome

Three peptides are often mentioned together in discussions of metabolic research: semaglutide, tirzepatide and retatrutide. The most useful and accurate way to compare them is by their pharmacology, specifically how many and which receptors each one activates. That is what this article does.

A few points of context first. Of the three, only retatrutide is in the Axiom catalogue, and it is supplied strictly as a reagent for in vitro laboratory research. Semaglutide and tirzepatide are referenced here only to explain where retatrutide sits scientifically. This article does not compare them as treatments, does not describe human use, and does not suggest outcomes anyone should pursue. It compares mechanisms.

The incretin system in brief

To understand all three, it helps to know two hormones. According to PubMed, the incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) both play a role in the physiology of glucose handling and body-weight regulation, and the medications in this area were developed to act on the receptors for these hormones DOI.

The three peptides below differ in how many of these receptor systems, plus the glucagon receptor, they engage.

Semaglutide: a single-receptor agonist

Semaglutide acts on one receptor: the GLP-1 receptor. According to PubMed, it is described in the literature as a selective GLP-1 receptor agonist, and serves as the single-receptor reference point against which the multi-receptor compounds are compared DOI.

Tirzepatide: a dual-receptor co-agonist

Tirzepatide adds a second receptor. According to PubMed, it is a dual GIP and GLP-1 receptor co-agonist, engineered to activate both incretin receptors, and in clinical trials it was reported to reduce HbA1c and body weight more than the selective GLP-1 receptor agonist semaglutide. The same review notes that important mechanistic questions about how GIP receptor agonism contributes remain a matter of active research DOI.

Retatrutide: a triple-receptor agonist

Retatrutide adds a third. According to PubMed, it is an agonist of the GIP, GLP-1 and glucagon receptors, the only one of the three to engage the glucagon receptor as well as the two incretin receptors DOI.

Crucially, and unlike semaglutide and tirzepatide, retatrutide is investigational. According to PubMed, a 2024 review described it explicitly as an investigational drug with Phase 3 trials begun in 2023 and evidence at that point limited to Phase 2 studies DOI.

What the progression means

Read across the three, there is a clear pharmacological progression: one receptor, then two, then three. Semaglutide acts on GLP-1; tirzepatide on GLP-1 and GIP; retatrutide on GLP-1, GIP and glucagon.

This progression is the scientific reason retatrutide is of such interest as a research compound: it represents the next step in a sequence of increasingly multi-targeted molecules, and the biology of engaging three receptors at once is an active research question rather than a settled one.

An important difference in status

There is one difference that matters more than the receptor count. Semaglutide and tirzepatide are approved medicines. Retatrutide is not. It has not been approved by the MHRA, the FDA or any other regulator, and remains in clinical development.

That is why, of the three, retatrutide is handled as a research compound. Axiom supplies it for laboratory research use only. It is not a medicine, not a licensed treatment, and not a supplement, and the comparison above is offered as scientific context, not as guidance of any other kind.

Summary

The clearest way to distinguish these three peptides is by receptor mechanism: semaglutide is a single GLP-1 receptor agonist, tirzepatide a dual GIP and GLP-1 co-agonist, and retatrutide a triple GIP, GLP-1 and glucagon receptor agonist. The progression from one receptor to three is what places retatrutide at the frontier of this research area, while its investigational status is what keeps it firmly in the research-use-only category. For more on retatrutide specifically, see our full research overview.