Retatrutide

Retatrutide (LY-3437943) is a first-in-class triple agonist peptide that simultaneously activates the glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1), and glucagon receptors. Developed by Eli Lilly, it represents an evolution beyond dual GIP/GLP-1 agonists like tirzepatide by incorporating glucagon receptor activation, which adds thermogenic energy expenditure and hepatic lipid metabolism to the weight loss and glycemic control mechanisms of incretin agonism.

The peptide is a modified 39-amino acid chain with a C20 fatty diacid moiety that enables albumin binding and once-weekly dosing. As of 2024, it is in Phase 3 clinical trials for obesity and type 2 diabetes, with prior Phase 2 results generating substantial attention for producing the largest weight reductions seen in clinical trials at the time of publication.

Retatrutide activates three G protein-coupled receptors with distinct downstream effects. GLP-1 receptor agonism reduces appetite via hypothalamic satiety centers, slows gastric emptying, and enhances glucose-dependent insulin secretion from pancreatic beta cells. GIP receptor activation complements these effects by potentiating insulin secretion and appears to enhance fat tissue responsiveness to weight-loss signals, though the precise mechanism of GIP's contribution to weight loss remains under investigation.

The distinguishing feature of retatrutide is its glucagon receptor agonism. Glucagon increases hepatic glucose output acutely, but chronically it stimulates energy expenditure through thermogenesis, promotes lipolysis, reduces hepatic lipid accumulation, and increases amino acid catabolism. The GLP-1 component counterbalances glucagon's hyperglycemic effect, allowing the metabolic benefits of glucagon signaling without compromising glycemic control. This three-receptor synergy produces greater weight loss and metabolic improvement than either single or dual agonist approaches in preclinical models.