Tirzepatide (15mg)

Tirzepatide exerts effects through multiple mechanisms: (1) Imbalanced Dual GIP/GLP-1 Receptor Agonism - binds GIPR with affinity comparable to native GIP but engages GLP-1R with ~5-fold weaker affinity than native GLP-1 (Willard 2020); greater engagement of GIPR than GLP-1R at clinical doses; GIPR activation enhances glucose-dependent insulin secretion and modulates adipocyte function; GLP-1R activation promotes glucose-dependent insulin release, glucagon suppression, delayed gastric emptying, and central appetite regulation; dual mechanism produces synergistic effects on insulin and glucagonostatic responses exceeding either incretin receptor agonist alone; (2) Biased GLP-1R Signaling - favors Gs-coupled cAMP generation over beta-arrestin recruitment; reduced GLP-1R internalization compared to native GLP-1 and other GLP-1 agonists; cryo-EM structure (Sun 2022, 2.9Å GLP-1R/tirzepatide) revealed Tyr1 (vs. His1 in GLP-1) and C20 lipid moiety destabilize interactions between transmembrane domain 5 (TM5) and extracellular loop 3 (ECL3), weakening receptor-effector coupling for beta-arrestin pathways; beta-arrestin-1 normally limits insulin response to GLP-1; by avoiding beta-arrestin recruitment, tirzepatide achieves enhanced insulin secretion; reduced receptor internalization sustains receptor availability at cell surface, contributing to prolonged efficacy; (3) GIPR-Mediated Weight-Independent Insulin Sensitization - improves insulin sensitivity through both weight-dependent and weight-independent mechanisms (Samms 2021); using GLP-1R-knockout mice, GIPR agonism independently enhances insulin sensitivity by promoting glucose disposal in white adipose tissue (WAT); in absence of GLP-1R-induced weight loss, GIPR agonism alone improved insulin sensitivity in obese mice; enhanced glucose, lipid, and branched-chain amino acid (BCAA) catabolism in brown adipose tissue; circulating BCAA and ketoacid levels reduced (markers of improved metabolic health); weight-independent benefits suggest greater treatment durability vs. agents relying solely on weight loss; (4) Central Appetite Regulation - both GIPR and GLP-1R expressed in brain regions regulating appetite (hypothalamus, brainstem); selectively reduces preference for high-fat foods in rodent models (dual receptor engagement in reward and satiety circuits); combination of peripheral metabolic signaling and central appetite suppression contributes to substantial body weight reductions; (5) Engineered Stability - Aib residues at positions 2 and 13 eliminate DPP-4 cleavage (native incretins have half-lives 2-7 minutes); C20 fatty diacid enables >99% reversible albumin binding, half-life ~5 days (116-120 hours) enabling once-weekly dosing; differential conformational behavior: compact conformation (radius of gyration ~6.8Å) at GIPR, extended conformation (~7.6Å) at GLP-1R (molecular dynamics simulations, Sun 2022).