Tirzepatide (LY3298176) is a 39–amino-acid synthetic peptide based primarily on the native GIP sequence, engineered as a fatty-acid–modified dual incretin receptor agonist for once-weekly subcutaneous administration. Following injection, the molecule circulates bound to albumin via a C20 fatty di-acid moiety enabling prolonged systemic exposure, distributing to tissues that express the glucose-dependent insulinotropic polypeptide receptor (GIPR) and the glucagon-like peptide-1 receptor (GLP-1R) — principally the pancreas, adipose tissue, and discrete brain regions. The peptide's in vivo stability is further shaped by two non-coded amino acid residues at positions 2 and 13 (Aib, α-aminoisobutyric acid) and by acylation at Lys20 with a γGlu-2×OEG linker and fatty diacid moiety, features that collectively determine the mode of action of this multireceptor agonist.

Imbalanced dual receptor engagement. High-resolution cryo-EM complexes show that tirzepatide adopts an α-helical conformation with its N terminus penetrating deep within the transmembrane core of both GIPR and GLP-1R. At GIPR the interaction is native-like: Tyr1 is buried in the TM core of the GIPR, stabilised by hydrogen bonding and aromatic contacts with conserved residues. At GLP-1R, weaker complementarity arises because Arg299 ECL2, which mediates polar interactions with various GLP-1R ligands, does not form such contacts with tirzepatide. This structural asymmetry translates directly into pharmacology: tirzepatide binds GIPR with affinity equal to native GIP but engages GLP-1R with approximately 5-fold weaker affinity than native GLP-1, producing an imbalanced occupancy profile that skews functional engagement toward GIPR.

Biased GLP-1R signalling. Beyond affinity, tirzepatide differentiates itself through receptor-regulatory behaviour. It is less effective at internalising GLP-1R relative to GLP-1 (<40% Emax), indicating reduced trafficking into desensitising endocytic pathways. Consistent with this, tirzepatide shows a limited ability to cause GLP-1R desensitisation through GRK2/β-arrestin recruitment. The net result is a bias toward sustained cAMP signalling at GLP-1R with attenuated β-arrestin–dependent receptor downregulation — a configuration that can preserve insulinotropic output despite weaker receptor affinity.

β-cell insulinotropic and secretory-processing effects. In pancreatic islets both receptors contribute functionally: antagonising GIPR activity consistently decreases the insulin response to tirzepatide in human islets, confirming that GIPR engagement is not redundant. Downstream, treatment increases HOMA2-B indices and reduces intact proinsulin, indicating improved secretory capacity and proinsulin processing. In vivo physiology confirms enhanced insulin secretion relative to sensitivity, reflected in an increase in the clamp disposition index.

Adipose-tissue GIPR-forward remodelling. Adipose expresses GIPR but not GLP-1R, making it a tissue where GIPR agonism drives metabolic substrate partitioning selectively. In differentiated human adipocytes, tirzepatide suppresses insulin-stimulated de novo lipogenesis while stimulating glycerol release, counter-regulating insulin-driven lipid storage. It concurrently drives lipid clearance by increasing lipoprotein lipase expression, secretion, activity, and fatty-acid uptake, with additive effects when combined with insulin. In GLP-1R–deficient mice, tirzepatide selectively enhances insulin-stimulated glucose uptake in white adipose tissue but not in skeletal muscle. Brown adipose tissue undergoes transcriptional remodelling toward catabolic programmes for glucose, lipids, and branched-chain amino acids (BCAAs), aligned with reductions in circulating BCAA levels (leucine, isoleucine, and valine). In parallel, tirzepatide mitigates infiltration of pro-inflammatory M1 adipose tissue macrophages, linking immunometabolic tone to improved insulin sensitivity.

Central appetite and energy-balance circuitry. Chronic administration decreases body weight and food intake more than a selective GLP-1 receptor agonist in preclinical models. Systemically administered fluorescent-labelled tirzepatide reaches the median eminence and area postrema — circumventricular brain regions accessible to circulating peptides — providing a plausible anatomical route for central satiety signalling. The additive appetite reduction requires GABAergic, GIPR-expressing neurons in the CNS, indicating a circuit-level integration mechanism beyond peripheral incretin action.

Clinical translation. These converging molecular events produce concurrent improvements in β-cell function, insulin sensitivity, and glucagon secretion that together explain tirzepatide's glycaemic efficacy. Insulin sensitisation has both weight-dependent and weight-independent components: weight loss explained only 13–21% of the variation in HOMA2-IR improvement at higher doses, implicating direct adipose-tissue remodelling. Suppression of islet hormone imbalance is reflected in glucose-adjusted glucagon decreases of 28–36% across therapeutic doses, reducing hepatic glucose output. Taken together, tirzepatide's structural engineering — imbalanced receptor affinity, biased GLP-1R cAMP signalling, GIPR-forward adipose programming, and CNS GABAergic engagement — converges on a multi-tissue mechanism that produces greater glycaemic and weight-loss efficacy than can be attributed to either incretin pathway alone.