Tirzepatide

Tirzepatide is a first-in-class dual glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptor agonist developed by Eli Lilly. It is a 39-amino acid synthetic peptide based on the native GIP sequence with modifications that enable dual receptor agonism and a C20 fatty diacid side chain for albumin binding and once-weekly dosing. Tirzepatide is approved for type 2 diabetes (Mounjaro) and chronic weight management (Zepbound).

The peptide has produced the largest weight reductions seen in pharmaceutical obesity trials. In the SURMOUNT-1 trial, the highest dose group achieved mean weight loss exceeding 20% — approaching the efficacy of bariatric surgery for the first time with a pharmacological agent. Its dual agonist mechanism represents a conceptual advance over single-target GLP-1 agonists, though the precise contribution of GIP receptor activation to its superior efficacy remains an active area of investigation.

Tirzepatide simultaneously activates two incretin receptors with an imbalanced agonism profile — it is a full GIP receptor agonist but a biased, partial GLP-1 receptor agonist. GLP-1R activation provides the established benefits of appetite suppression, glucose-dependent insulin secretion, glucagon suppression, and delayed gastric emptying. The addition of GIP receptor agonism produces several complementary effects that are not fully understood but appear critical to tirzepatide's superior efficacy.

GIP receptor activation in adipose tissue appears to improve fat tissue metabolism and insulin sensitivity. In the central nervous system, GIPR signaling in hypothalamic neurons may enhance the anorexigenic effects of GLP-1R activation through converging intracellular pathways. The dual mechanism also produces greater improvements in insulin sensitivity than GLP-1 agonism alone, potentially through direct effects on adipocyte lipid handling and hepatic glucose production. The apparent paradox — that GIP is hyperactive in obese states yet therapeutic when given exogenously alongside GLP-1 agonism — suggests that the two incretin systems interact synergistically in ways that monotherapy with either cannot replicate.