Tirzepatide mechanism of action: dual GIP and GLP-1 receptor agonism

The short version

Tirzepatide is a single 39-residue peptide that binds and activates two distinct G-protein coupled receptors at the same time: the GIP receptor (GIPR) and the GLP-1 receptor (GLP-1R). It is a full agonist at the GIP receptor and an imbalanced (biased) agonist at the GLP-1 receptor, where it preferentially drives cAMP signaling while attenuating β-arrestin recruitment relative to native GLP-1.^[2]

The incretin axis in one paragraph

GIP and GLP-1 are gut-derived peptide hormones released after a meal. They are known collectively as incretin hormones, and they are responsible for the incretin effect: the observation that an oral glucose load produces a much larger insulin response than an equivalent intravenous glucose load. Both peptides are rapidly degraded in circulation by the protease DPP-4, which is why native incretins are unsuitable as drugs without modification.

Why dual agonism?

GLP-1 receptor agonists are well-established for type 2 diabetes and obesity. The rationale for adding GIP receptor agonism was, in part, contrarian: for many years GIP receptor signaling was considered to promote weight gain. Later work demonstrated that chronic, supraphysiological GIP receptor agonism can produce additive, and in some measures synergistic, glucose-lowering and weight-reducing effects when combined with GLP-1 receptor agonism.^[1]

• GLP-1R activation: glucose-dependent insulin secretion, suppression of inappropriate glucagon, slowed gastric emptying, and central appetite suppression via hindbrain and hypothalamic circuits.
• GIPR activation: potentiates β-cell insulin secretion, influences adipocyte lipid handling and energy expenditure, and contributes to additional appetite modulation through hypothalamic GIP receptor expression.

Biased agonism at the GLP-1 receptor

G-protein coupled receptors couple to multiple downstream effector pathways. Native GLP-1 activates both the cAMP / protein kinase A axis (driving insulin secretion) and recruits β-arrestins (which trigger receptor internalization and signal attenuation). Tirzepatide produces a different downstream signature at the GLP-1 receptor: it stimulates cAMP comparably to native GLP-1 but recruits β-arrestin far less efficiently. The net effect is sustained surface availability of the receptor and prolonged signaling per molecule of agonist bound.^[2]

At the GIP receptor, by contrast, tirzepatide behaves more like a full, unbiased agonist with affinity comparable to native GIP. The molecule is therefore described as imbalanced: full agonism at GIPR, biased agonism at GLP-1R.

From receptor to clinical effect

The combination of effects observed clinically (substantial glucose lowering, large weight reductions, and improvements in lipid parameters) follows from the integrated downstream signaling at both receptors. Insulin sensitivity also improves, although the extent attributable to receptor-level effects versus weight loss is debated.

Open mechanistic questions

• How much of the weight effect is incremental over a high-dose GLP-1 receptor agonist at matched exposure? Direct comparisons across the SURMOUNT and STEP programs are confounded by differences in dose, trial design, and baseline characteristics.
• What is the long-term effect of chronic GIP receptor agonism on adipose tissue biology, and does it differ across patient subgroups?
• Does biased agonism at GLP-1R translate to a tolerability advantage, or does the extended signaling re-introduce GI side effects through different kinetics?