Tirzepatide (LY3298176): verified identity and the evidence ledger

Verified against PubChem PUG-REST (CID 166567236, formula and synonym list); DailyMed MOUNJARO prescribing information; PubMed. Fields marked “not verified” are ones we could not confirm against a primary chemical database — we leave them blank rather than guess.

Verified identity and structural features

Tirzepatide, development code LY3298176, is a synthetic peptide of 39 amino acids built on a GIP-based sequence. FDA labeling for the marketed article states the empirical formula C225H348N48O68 and a molecular weight of 4,813.53 Da, and describes the modifications precisely: aminoisobutyric acid (Aib) at positions 2 and 13, a C-terminal amide, and a lysine at position 20 attached to 1,20-eicosanedioic acid through a linker. The two Aib substitutions are non-coded residues; the eicosanedioic acid is the C20 fatty diacid.

PubChem corroborates the formula independently. A name query returns several depositor records rather than one, which is worth noting for anyone matching identifiers: CID 166567236 is the free-base entry, carrying CAS 2023788-19-2 alongside both marketed trade names and the JAN/USAN designation, while other CIDs under the same CAS are acetate-form records. A CAS number alone therefore does not distinguish salt form for this compound.

The full 39-residue sequence is not printed in the labeling sections or database records consulted for this page, so no sequence is published here. The structural features listed above are what the primary documents state, and nothing has been reconstructed from secondary sources to fill the gap.

Two receptors, and what was measured at each

Coskun and colleagues characterised the molecule in cell lines expressing recombinant or endogenous incretin receptors and reported activation of both GIP and GLP-1 receptor signalling, followed in mice by glucose-dependent insulin secretion and improved glucose tolerance acting through both receptors. Chronic administration in mice decreased body weight and food intake, and the report described that decrease as significantly greater than the effect of a GLP-1 receptor agonist comparator (Mol Metab, 2018).

Willard and colleagues then reported an asymmetry between the two receptors. Calculating occupancy at clinically studied exposures, they described greater engagement of the GIP receptor than the GLP-1 receptor and characterised the mechanism as imbalanced. At the GIP receptor the compound mimicked native GIP. At the GLP-1 receptor it showed bias, favouring cAMP generation over beta-arrestin recruitment, with weaker GLP-1 receptor internalisation than GLP-1 itself. In primary islets, beta-arrestin1 limited the insulin response to GLP-1 but not to GIP or to tirzepatide (JCI Insight, 2020).

An independent group working in HEK293 cells expressing human GIP or GLP-1 receptors reported a different bias axis: relative to the endogenous ligands, LY3298176 was biased toward ERK1/2 phosphorylation over cAMP accumulation at both receptors (Biochem Pharmacol, 2020). These are in-vitro and rodent findings. They describe receptor behaviour in engineered and primary cell systems, and are logged in the ledger at that grade rather than as human evidence.

The human trial record

The first-in-human programme was reported alongside the preclinical work. A phase 1 study in three parts - single ascending dose and four-week multiple ascending dose in healthy subjects, then a four-week phase 1b in participants with type 2 diabetes - enrolled 142 subjects across tirzepatide, dulaglutide and placebo. In the diabetes portion, the 10 mg and 15 mg groups showed reductions in fasting serum glucose against placebo of minus 49.12 mg/dL and minus 43.15 mg/dL respectively (Mol Metab, 2018).

SURPASS-2 was an open-label 40-week phase 3 trial in which 1,879 participants with type 2 diabetes were randomised across three tirzepatide doses and semaglutide 1 mg. Estimated mean HbA1c change from a baseline of 8.28 percent was minus 2.01, minus 2.24 and minus 2.30 percentage points across the tirzepatide groups against minus 1.86 for semaglutide, with body weight differences of minus 1.9, minus 3.6 and minus 5.5 kg (N Engl J Med, 2021).

SURMOUNT-1 randomised 2,539 adults with obesity, or with a body-mass index of 27 or more plus a weight-related complication and without diabetes, to three tirzepatide doses or placebo for 72 weeks. Mean weight change at week 72 was minus 15.0, minus 19.5 and minus 20.9 percent against minus 3.1 percent on placebo (N Engl J Med, 2022). Gastrointestinal adverse events were the most common in both trials, concentrated during dose escalation.

Pharmacokinetics and the role of the C20 diacid

The fatty diacid is often described in secondary sources as improving stability. The labeling describes a different and more specific role. Tirzepatide is stated to be 99 percent bound to plasma albumin, with a mean absolute bioavailability after subcutaneous administration of 80 percent and an elimination half-life of approximately five days, which the document ties directly to once-weekly dosing. Steady-state plasma concentrations were reached after four weeks of once-weekly administration.

Clearance runs through proteolytic cleavage of the peptide backbone, beta-oxidation of the C20 fatty diacid, and amide hydrolysis. Metabolites are excreted in urine and faeces, and the labeling states that intact tirzepatide is not observed in either. The diacid is therefore itself a metabolic substrate, not an inert protective cap.

The documented function of the modification is albumin binding and the resulting extension of circulating half-life, which is a pharmacokinetic property measured in people. That is a different claim from chemical stability in a vial, which is a formulation property, and no published measurement of the latter was located for this molecule. Secondary sources frequently conflate the two.

The approved article is a solution, not a lyophilisate

Every FDA-approved presentation of tirzepatide is a ready-to-use sterile solution, described in the labeling as clear and colorless to slightly yellow, at pH 6.5 to 7.5. Single-dose pens and vials list sodium chloride, sodium phosphate dibasic heptahydrate and water for injection as the inactive ingredients. Multi-dose vials and pens add benzyl alcohol, glycerin and phenol as preservatives. None of these presentations is reconstituted before use.

Storage is specified at 2 to 8 degrees C, with room-temperature excursions not exceeding 30 degrees C permitted for up to 21 days for single-dose presentations and 30 days for multi-dose presentations. The labeling states that the product must not be frozen and must not be used if it has been frozen.

This matters for reading any handling guidance about a lyophilised powder reconstituted with bacteriostatic water. No approved lyophilised tirzepatide product exists, and a PubMed search for stability or beyond-use data on reconstituted tirzepatide returns zero records. The multi-dose approved formulation happens to use benzyl alcohol and phenol as its preservative system, which is chemically adjacent to bacteriostatic water, but that is an observation about the approved solution rather than evidence for any window applied to a reconstituted powder. No published figure supports such a window at any temperature.

What is not known

The receptor pharmacology that explains tirzepatide's profile rests almost entirely on in-vitro and rodent work, and the two published bias analyses do not fully agree with one another: one reports bias at the GLP-1 receptor toward cAMP over beta-arrestin, the other reports bias toward ERK1/2 over cAMP at both receptors relative to endogenous ligands. Which signalling axis accounts for the clinical difference against selective GLP-1 receptor agonists has not been established in people. The phase 3 trials were 40 to 72 weeks; durability beyond that, and what happens after discontinuation, is not covered by the trials cited here. SURMOUNT-1 excluded participants with diabetes and SURPASS-2 was open-label, which limits what each can say about the other's population. Cardiovascular and renal outcome data, long-term safety beyond the trial windows, pregnancy and lactation data, and paediatric data are outside the evidence logged on this page. On the chemistry side, nothing is published on the physical stability of tirzepatide outside the approved solution formulation: no forced-degradation study, no lyophilisate characterisation, and no reconstituted-solution stability data at any temperature. The full 39-residue sequence was not verified against a primary source for this page and is therefore not printed.

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