Retatrutide

Retatrutide (LY3437943) is a first-in-class triple-agonist peptide that simultaneously activates the glucagon-like peptide-1 receptor (GLP-1R), glucose-dependent insulinotropic polypeptide receptor (GIPR), and glucagon receptor (GCGR). It is a 39-amino-acid synthetic peptide constructed on a GIP backbone with engineered GLP-1R and GCGR activity. A C20 fatty diacid is attached to enable albumin binding and weekly dosing. The compound was developed by Eli Lilly. CAS number: 2381089-83-2. Retatrutide has balanced agonism at all three receptors, distinguishing it from tirzepatide (dual GIP/GLP-1) and semaglutide (selective GLP-1).

The triple-agonist mechanism of retatrutide engages three distinct metabolic pathways. GLP-1R activation in the hypothalamus and brainstem suppresses appetite via POMC neuron stimulation and NPY/AgRP inhibition, slows gastric emptying, and potentiates glucose-dependent insulin secretion. GIPR activation enhances adipose tissue lipid buffering, improves beta-cell function, and may amplify central anorectic signaling. The third component -- glucagon receptor activation -- is what fundamentally differentiates retatrutide. GCGR signaling in hepatocytes activates adenylyl cyclase, increasing cAMP and driving glycogenolysis, gluconeogenesis, and critically, hepatic fatty acid oxidation via CPT1a upregulation. In brown and beige adipose tissue, glucagon signaling increases uncoupling protein 1 (UCP1) expression and thermogenesis, directly raising resting energy expenditure. This means retatrutide attacks obesity from both sides: reducing caloric intake (GLP-1R/GIPR) while simultaneously increasing caloric expenditure (GCGR).

The Phase 2 dose-finding trial (N=338), published in the New England Journal of Medicine in 2023 (DOI: 10.1056/NEJMoa2301972), randomized participants with obesity (BMI 30-50) to retatrutide at doses ranging from 0.5mg to 12mg weekly or placebo over 48 weeks. At the 12mg dose, mean body weight reduction was 24.2%, the highest ever recorded for any anti-obesity pharmacotherapy. 100% of participants at the highest dose lost at least 5% body weight. 83% lost at least 15%. 63% lost at least 20%. Mean liver fat reduction was 82.4% by MRI-PDFF, with 86% of participants who had MASLD (metabolic-associated steatotic liver disease) at baseline achieving complete resolution (liver fat below 5%). This liver fat data has generated significant interest for NASH/MASH research.

Retatrutide also demonstrated strong metabolic improvements beyond weight. A1C decreased by 1.3% in a parallel Phase 2 diabetes study. Fasting triglycerides decreased by up to 35%. Systolic blood pressure decreased by 7-10 mmHg. These metabolic benefits were dose-dependent and correlated with weight loss magnitude. Phase 3 trials (TRIUMPH program) are now enrolling to confirm these results in larger populations and evaluate cardiovascular outcomes.

Retatrutide has an estimated plasma half-life of approximately 6 days, supporting once-weekly subcutaneous dosing. The C20 fatty diacid modification drives albumin binding (greater than 99% bound), protecting the peptide from DPP-4 degradation and renal clearance. Peak plasma concentrations are reached approximately 24-72 hours post-injection. Steady state is achieved by weeks 4-6 with weekly dosing. Metabolism occurs via proteolytic degradation and fatty acid beta-oxidation, similar to other acylated peptides in this class.

Lyophilized retatrutide should be stored at -20C prior to reconstitution. Reconstitute with bacteriostatic water by directing the stream along the vial wall and gently swirling until dissolved. Do not vortex or shake. Reconstituted solution should be stored at 2-8C and used within 28 days. Protect from light and avoid freeze-thaw cycles. The peptide is synthesized using solid-phase peptide synthesis (SPPS) with Fmoc chemistry, and each batch is verified by LC-MS for sequence accuracy and reverse-phase HPLC for purity confirmation.

Retatrutide is studied in research on triple incretin pharmacology, hepatic fat metabolism, NASH/MASH resolution, energy expenditure physiology, brown adipose tissue activation, and multi-receptor agonist design principles. It is particularly valuable for researchers studying the interplay between glucagon signaling, hepatic lipid oxidation, and thermogenesis -- areas where single- and dual-agonists provide limited data.

In the Phase 2 trial, the most common adverse events were gastrointestinal: nausea (up to 25.6% at higher doses), diarrhea (up to 22.0%), vomiting (up to 12.2%), and constipation (up to 11.0%). These were predominantly mild to moderate and concentrated during dose escalation. Discontinuation due to adverse events was 6.1-9.8% across dose groups versus 0% placebo. Heart rate increased by a mean of 3-4 bpm, consistent with the GLP-1 class effect. No pancreatitis, medullary thyroid carcinoma, or other serious class-specific events were reported.