All three descend from the incretin family, the gut derived signaling peptides studied for their effect on insulin release in animal and cell models. Semaglutide, CAS 910463-68-2, is built on the glucagon like peptide 1 backbone with substitutions that blunt enzymatic clipping at the dipeptidyl peptidase 4 site. Tirzepatide, CAS 2023788-19-2, is built instead on the glucose dependent insulinotropic polypeptide backbone and is characterised in the literature as engaging both the GIP and the GLP-1 receptor. Retatrutide, CAS 2381089-83-2 and also catalogued as LY3437943, extends that logic to a third target, the glucagon receptor, and is described as a GGG tri-agonist.
Recorded formulas separate them cleanly. Semaglutide is C187H291N45O59 at an average mass near 4113.58, tirzepatide is C225H348N48O68 near 4813.45, and retatrutide is C256H392N64O79 near 4731.2. Retatrutide carries the largest carbon and nitrogen counts of the three while sitting slightly below tirzepatide in total mass, so mass alone is a poor proxy for structural complexity. Full residue sequences are not held in our molecular dataset for any of the three, so each is described here by backbone class and modification rather than position by position.
Receptor coverage is the real dividing line, and it changes how many readouts an experiment needs. A single agonist can be profiled against one cyclic AMP accumulation curve. A dual agonist requires paired curves in cells expressing each receptor separately, because potency is rarely balanced across the two. A tri-agonist requires three, plus an argument about what the glucagon receptor arm contributes in the particular model being used. Published preclinical work on these compounds reports differences in potency ratio between receptors rather than a simple additive relationship.
Handling is similar across the three because the chemistry is similar. Each carries a fatty diacid chain attached through a linker, a modification described in the literature as promoting albumin association and slowing clearance in animals. That lipid tail also promotes self association, so reconstituted material is swirled gently rather than shaken or vortexed, since foaming drives surface denaturation. Lyophilized powder is held at minus 20 degrees Celsius protected from light and moisture. Once in solution, material is refrigerated at 2 to 8 degrees Celsius, aliquoted to avoid repeated freeze thaw, and inspected for haze before it goes into an assay.
Selection between them in a laboratory setting usually follows the question rather than the molecule. Work that isolates a single incretin pathway uses the single agonist so that any signal has one explanation. Work that asks whether two incretin receptors interact uses the dual agonist and compares it against the single agonist run in parallel. Work on the glucagon arm needs the tri-agonist, and needs the other two on the bench as controls, because a three receptor result cannot otherwise be attributed to any one receptor.
What remains unsettled is substantial. Potency ratios reported between receptors vary with the assay system and the receptor species used, so cross study comparison is difficult. The contribution of the glucagon receptor arm in rodent models is still debated in the published literature, and reported receptor desensitisation profiles differ between groups. None of that is resolved by structure alone, which is why all three are supplied for controlled laboratory comparison rather than for assumption.