Tirzepatide is an imbalanced and biased dual GIP and GLP-1 receptor agonist
Willard FS, Douros JD, Gabe MBN, Showalter AD, Wainscott DB, Suter TM, Capozzi ME, van der Velden WJC, +10 more
JCI Insight (2020)
Tirzepatide is not a balanced dual GIP / GLP-1 agonist — it engages the GIP receptor with native-ligand potency and the GLP-1 receptor with approximately 5-fold weaker affinity, and at the GLP-1 receptor it is biased toward cAMP signaling over β-arrestin recruitment.
This is the receptor-pharmacology paper that reframed the field's understanding of tirzepatide. Willard and colleagues — a collaboration between Eli Lilly, Duke, the University of Copenhagen, and the Holst laboratory — characterized tirzepatide's signaling profile at both the GIP and GLP-1 receptors using a panel of complementary assays: radioligand binding, cAMP accumulation, β-arrestin recruitment, receptor internalization, and ex vivo pancreatic islet perifusion in genetically modified mice.
At the GIP receptor, tirzepatide demonstrated equipotency compared with native GIP and behaved as a full agonist across signaling outputs. At the GLP-1 receptor, the picture differed in two important respects. First, tirzepatide bound with approximately 5-fold weaker affinity than native GLP-1 and behaved as a partial agonist — not a full agonist matching GLP-1's efficacy. Second, at the GLP-1 receptor, tirzepatide exhibited signaling bias: it favored cAMP generation over β-arrestin recruitment (β-arrestin Emax less than 10% of native GLP-1), and it drove substantially weaker GLP-1 receptor internalization than GLP-1 itself. The authors framed tirzepatide therefore as an "imbalanced and biased" dual agonist — not the "balanced co-agonism" framing that had dominated early commentary.
The mechanistic implication is important. The pharmacology suggests that tirzepatide's clinical profile is driven by stronger GIP-receptor engagement combined with biased, partial GLP-1-receptor agonism that preserves cAMP-mediated metabolic signaling while reducing β-arrestin-mediated receptor desensitization and downregulation. Whether the GLP-1-receptor bias contributes to durability of effect over chronic dosing (less receptor downregulation than full GLP-1 agonists would produce) is an active hypothesis but is not directly demonstrated by this in vitro work. The discovery paper had already noted the differential affinity; Willard et al. characterized the signaling-bias component that the discovery paper did not address in depth.
This is an in vitro and ex vivo receptor-pharmacology paper. The findings characterize tirzepatide's intrinsic molecular properties at recombinant human receptors and in mouse pancreatic islets; they do not directly demonstrate how those properties translate to clinical outcomes. The interpretive bridge between "biased agonism" at the GLP-1 receptor and "tirzepatide produces more weight loss than semaglutide" runs through several layers of inference and additional data, not all of which is settled.
The cell systems used (predominantly HEK293 with defined receptor densities) are standard for pharmacology research but do not recapitulate the native receptor environment in human pancreas, hypothalamus, brainstem, or adipose tissue, where receptor density, accessory proteins, and signaling-pathway compartmentalization differ. Whether tirzepatide's bias at the GLP-1 receptor produces the same pharmacological signature in human hypothalamic neurons (where appetite suppression is mediated) as in HEK293 cells is not tested.
The paper's claim that tirzepatide is "imbalanced" toward GIP-receptor activity has been used in popular commentary to argue that "tirzepatide's benefit is mostly GIP." That inference does not follow cleanly from the pharmacology — the GLP-1 receptor remains potently activated even if partially, and the clinical pharmacology in humans cannot be straightforwardly partitioned between the two receptor contributions from this paper alone. The tirzepatide vs semaglutide mechanism myth response addresses the common misreadings.
Industry sponsorship by Eli Lilly is disclosed; several authors are Lilly employees. The academic collaborators (Holst, Rosenkilde, D'Alessio, Campbell) are independent group leaders with publication records in receptor pharmacology that pre-date and post-date this paper. The findings have been independently replicated in subsequent academic and industry-academic collaborations on tirzepatide and related dual agonists.
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