Tirzepatide mechanism of action.

Tirzepatide (development code LY3298176) is a 39-residue synthetic peptide engineered to activate two class-B G-protein-coupled receptors simultaneously: the glucose-dependent insulinotropic polypeptide receptor (GIPR) and the glucagon-like peptide-1 receptor (GLP-1R). Rather than modify a single endogenous hormone, the backbone was designed from a GIP-like starting point and tuned to add balanced GLP-1R activity (Coskun et al., 2018).

Both target receptors are members of the secretin family of GPCRs. They share a large N-terminal extracellular domain that binds the C-terminal helix of the peptide agonist and a seven-transmembrane bundle that engages the N-terminal residues to initiate signalling. In each case the dominant downstream coupling is via the stimulatory Gαs protein, activation of adenylyl cyclase and elevation of intracellular cyclic AMP — leading to PKA and EPAC pathway activation (Drucker, 2018; Campbell & Drucker, 2013).

A C20 fatty-diacid chain is conjugated to the peptide backbone. This does not participate in receptor engagement itself; it promotes reversible albumin binding to extend circulating half-life and support the once-weekly regimens reported in the clinical literature (Frías et al., 2018).

The GLP-1 receptor is expressed on pancreatic β-cells, on neurons in the hypothalamus and brainstem, and in gastric, cardiovascular and immune tissues. Activation raises β-cell cAMP, potentiating glucose-dependent insulin secretion, and engages central satiety circuitry that reduces food intake and delays gastric emptying (Müller et al., 2019).

Willard et al. (2020) reported that tirzepatide is a biased and imbalanced agonist at GLP-1R — favouring cAMP accumulation over β-arrestin recruitment relative to native GLP-1(7–37). In functional cAMP assays its potency at GLP-1R is lower than at GIPR, which is the source of the "imbalanced" descriptor in the literature. This bias is a distinguishing mechanistic feature of tirzepatide relative to single GLP-1 agonists such as semaglutide.

GIP is the second major incretin hormone. Its receptor is expressed on pancreatic β-cells, adipocytes, bone and specific CNS regions. β-cell GIPR activation potentiates glucose-dependent insulin release in a manner complementary to GLP-1R engagement, while adipose GIPR activity is implicated in lipid handling and central GIPR signalling in appetite regulation (Campbell & Drucker, 2013).

Tirzepatide behaves as a "GIP-like" agonist at GIPR, with a potency profile close to native GIP(1–42) (Willard et al., 2020). This is the arm that separates tirzepatide most cleanly from single GLP-1 agonists and is the defining pharmacological addition of dual agonism.

Simultaneous engagement of GIPR and GLP-1R on the same cell — for example a pancreatic β-cell — produces additive Gαs coupling that raises intracellular cAMP more effectively than either receptor engaged in isolation at matched exposure. Downstream, this translates to PKA-driven potentiation of glucose-dependent insulin secretion and EPAC-mediated effects on exocytosis (Coskun et al., 2018).

Because β-arrestin recruitment at GLP-1R is reduced relative to native GLP-1 (Willard et al., 2020), tirzepatide is associated with reduced receptor internalisation and desensitisation at that arm — a proposed mechanistic contributor to sustained signalling over the dosing interval.

Single GLP-1 agonists access one incretin axis. Adding balanced GIPR activity engages the second incretin receptor at the same cell and expands the pharmacology beyond what GLP-1R alone can reach. The mechanistic result is a receptor signature that cannot be reproduced with GLP-1R-selective peptides, and that requires two-receptor assay coverage to characterise correctly.

For laboratory research, the practical implication is that assay design must span both incretin receptors: cAMP accumulation and β-arrestin recruitment in GIPR- and GLP-1R-expressing lines, ideally as a matched panel. Restricting characterisation to a single receptor systematically under-reports the peptide's pharmacology.