Retatrutide

Retatrutide engages three distinct G protein-coupled receptors simultaneously: the GLP-1 receptor (GLP-1R), the GIP receptor (GIPR), and the glucagon receptor (GCGR). All three are class B1 GPCRs that signal primarily through Gs-coupled adenylyl cyclase activation, generating intracellular cAMP and activating downstream protein kinase A (PKA) cascades. The concurrent engagement of all three receptors is the defining pharmacological characteristic under study.

The GLP-1R component contributes insulinotropic signalling in pancreatic beta-cells (glucose-dependent), suppression of glucagon from alpha-cells, and central appetite modulation through hypothalamic and brainstem GLP-1R-expressing circuits. The GIPR component provides additional pancreatic insulinotropic input and adipocyte signalling through GIPR expressed on fat cells, studied for effects on lipid metabolism and adipose tissue remodelling. These two components are shared with dual GLP-1/GIP agonists.

The distinguishing feature of retatrutide is the addition of glucagon receptor (GCGR) agonism. GCGR is expressed predominantly in hepatocytes, where glucagon signalling drives glycogenolysis and gluconeogenesis — normally an opposing action to insulin. In the context of multi-receptor agonism, hepatic GCGR activation is studied specifically for its effect on fatty acid oxidation, thermogenesis, and energy expenditure, which are distinct from the hepatic glucose output typically associated with isolated GCGR activation. The combination of all three receptor pathways is studied for whether it produces metabolic modulation beyond what is achievable through incretin axes alone.

Hypothalamic co-expression of GLP-1R, GIPR, and GCGR has been documented neuroanatomically, providing a mechanistic basis for studying central appetite and energy balance effects through all three receptors in the same neural circuits.