Retatrutide: The Triple-Receptor Agonist Behind 24% Weight Loss in Phase 2

Retatrutide (research code LY3437943) is an investigational peptide developed by Eli Lilly that activates three receptors simultaneously: glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), and the glucagon receptor. It belongs to the same broad family as semaglutide (GLP-1 only) and tirzepatide (GLP-1 + GIP), but the addition of glucagon-receptor agonism is the defining innovation. The molecule is engineered with non-coded amino acid residues and a C20 fatty diacid that binds albumin, extending its half-life enough to support weekly subcutaneous administration in clinical trials.

Glucagon's reputation in metabolic medicine is complicated. As a counter-regulatory hormone, it raises blood glucose — historically a reason to avoid activating its receptor. But glucagon also promotes hepatic fat oxidation and increases resting energy expenditure. The hypothesis behind triple agonism is that pairing glucagon-receptor activity with the incretin axis (GLP-1 + GIP) lets researchers harness the energy-expenditure benefits of glucagon while the GLP-1 and GIP signaling components offset any glucose-raising effect. The Phase 2 data so far appears consistent with this model.

The pivotal Phase 2 obesity trial was published in the New England Journal of Medicine in 2023 by Jastreboff and colleagues. Participants without diabetes were randomized to one of several retatrutide doses or placebo for 48 weeks. Mean weight reductions at the highest dose (12 mg weekly) reached approximately 24% of baseline body weight, a magnitude not previously observed for a peptide therapeutic in this duration of an obesity trial. Reductions at lower doses were also large and dose-dependent.

A parallel Phase 2 trial in patients with type 2 diabetes, published in The Lancet later that year by Rosenstock et al., demonstrated dose-dependent improvements in HbA1c and weight, with comparable safety signaling to existing incretin therapies. Across both studies, the most common treatment-related adverse events were gastrointestinal — nausea, diarrhea, vomiting — mirroring the GLP-1 class.

Phase 3 trials are currently underway. Until those read out, retatrutide remains investigational and not approved by any regulatory authority for any human indication.

The C20 fatty diacid moiety allows retatrutide to bind reversibly to serum albumin, slowing renal clearance and protease degradation. The published half-life supports once-weekly dosing in the clinical trial design. The non-coded residues confer additional protection against DPP-4 cleavage at the N-terminus, which would otherwise rapidly inactivate native incretin sequences. These engineering choices follow the same blueprint that made semaglutide and tirzepatide commercially viable.

Retatrutide is supplied lyophilized and is typically stored at -20°C, dry and dark, with minimal freeze-thaw cycling. Researchers should consult published protocols specific to their model system.

This product is supplied strictly for in vitro and preclinical research use. It is not approved by the FDA for human therapeutic use. Phase 3 results, long-term safety, and durability of effect have not yet been reported in the peer-reviewed literature. Any interpretation of the Phase 2 data should account for the relatively short study duration and the selected trial population.