LY3298176, a novel dual GIP and GLP-1 receptor agonist for the treatment of type 2 diabetes mellitus: From discovery to clinical proof of concept
Coskun T, Sloop KW, Loghin C, Alsina-Fernandez J, Urva S, Bokvist KB, Cui X, Briere DA, +8 more
Molecular Metabolism (2018)
The discovery paper for LY3298176 — later named tirzepatide — established the molecule as a single 39-residue, fatty-acid-conjugated peptide engineered to engage both the GIP and GLP-1 receptors with once-weekly pharmacokinetics, framing it explicitly as a unimolecular dual agonist rather than a co-administered combination.
This is the foundational design and proof-of-concept paper for tirzepatide (LY3298176), authored by the Eli Lilly discovery team and published in Molecular Metabolism. Coskun and colleagues describe the molecular engineering rationale, in vitro pharmacology, rodent and primate pharmacodynamics, and Phase 1 human results that justified the molecule's progression into Phase 2 in type 2 diabetes and obesity.
LY3298176 is a 39-amino-acid synthetic peptide derived from the native GIP sequence with a fatty-acid (C20 diacid) modification conjugated through a γ-glutamyl-2× aminoethoxyacetic-acid linker to a lysine residue. The fatty-acid moiety drives albumin binding and is the structural element responsible for the once-weekly half-life in humans. In vitro, the peptide bound and activated the human GIP receptor with potency comparable to native GIP, while binding the human GLP-1 receptor with approximately 5-fold lower affinity than native GLP-1. The pharmacological design intent was unimolecular dual agonism — a single peptide engaging both receptors rather than a combination of separate agonists — to enable a single subcutaneous injection per week and to test the hypothesis that adding GIP-receptor agonism would augment the established metabolic effects of GLP-1-receptor agonism.
In Phase 1, LY3298176 demonstrated dose-dependent reductions in fasting and post-prandial glucose in type 2 diabetes patients, with the higher doses (10–15 mg) producing reductions in body weight that exceeded what selective GLP-1-receptor agonists had achieved at the time. The headline conclusion of the paper — that LY3298176 was suitable for further clinical development for T2D and potentially obesity — has been borne out by the SURPASS and SURMOUNT Phase 3 programs.
This is a discovery-and-Phase-1 paper from the sponsor's discovery team; it should be read as the molecule's design rationale and first human pharmacology, not as a confirmatory efficacy or safety reference. All claims about long-term efficacy, durability, and clinical benefit rest on the subsequent Phase 2 and Phase 3 trials. The framing of LY3298176 as a "balanced" or "equipotent" dual agonist in the early literature has been substantially refined by subsequent receptor-pharmacology work — particularly Willard et al. 2020 in JCI Insight, which characterized the molecule as imbalanced (more GIP-receptor than GLP-1-receptor activity) and biased at the GLP-1 receptor toward cAMP signaling over β-arrestin recruitment.
The "tirzepatide is not just two GLP-1 agonists" framing matters for cross-class comparison. The popular intuition that tirzepatide's weight-loss advantage over semaglutide reflects "more agonism" elides the receptor-pharmacology distinction that the discovery paper and subsequent biased-agonism work make explicit. The corpus addresses this confusion in the same-mechanism myth response.
Industry sponsorship by Eli Lilly is disclosed; the authors are Lilly employees. The paper's preclinical findings have been replicated in academic labs in subsequent receptor-pharmacology work, and the human results have been replicated and substantially extended in the SURPASS and SURMOUNT programs. Reproducibility of the design-stage claims is therefore strong, with the caveat that some quantitative receptor-affinity values were updated by the Willard 2020 analysis.
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