Retatrutide 2026: Phase 3 Trials and What Researchers Should Know

The Phase 2 trial of Retatrutide published in The New England Journal of Medicine (Jastreboff et al., 2023) was a 48-week, randomised, double-blind, placebo-controlled dose-ranging study in adults with obesity (BMI 27–50) without type 2 diabetes. Five weekly dose cohorts were evaluated against placebo.

The trial established a clear dose-response gradient. At the 12mg/week dose, mean body weight reduction was approximately 24.2% at 48 weeks — placing Retatrutide above semaglutide (~14.9% at 68 weeks, STEP 1) and broadly comparable to or above tirzepatide (~20.9% at 72 weeks, SURMOUNT-1) in historical comparison. No direct head-to-head trials have been conducted and protocol differences limit direct comparison.

Secondary endpoints at 12mg: significant reductions in waist circumference (~22 cm), fasting glucose, triglycerides (~41%), blood pressure, and liver fat fraction measured by MRI in a dedicated sub-study. The hepatic fat findings were notable — glucagon receptor activation in the liver produces fat mobilisation not produced by GLP-1/GIP dual agonism alone. Adverse events were predominantly gastrointestinal (nausea, vomiting, diarrhoea, constipation) — dose-dependent and mostly mild-to-moderate, most frequent during escalation phases. Profile consistent with the GLP-1 class.

The central mechanistic question in Retatrutide research is the incremental contribution of glucagon receptor (GCGR) co-agonism beyond GLP-1R/GIPR dual agonism (tirzepatide). This distinction is essential for interpreting Phase 3 data and for designing mechanistic protocols that isolate the glucagon receptor's metabolic contribution.

GCGR activation drives effects not produced by GLP-1R/GIPR agonism alone:

• Hepatic fat mobilisation — GCGR activation in the liver promotes fatty acid oxidation and ketogenesis. Combined with concurrent GLP-1R/GIPR-mediated caloric restriction, this produces net hepatic fat reduction beyond what GLP-1/GIP achieves. The Phase 2 liver fat sub-study results are the primary evidence, and form the basis for Retatrutide's research interest in NASH/MASH models.
• Thermogenesis — glucagon receptor stimulation increases resting energy expenditure in preclinical and clinical models. This thermogenic component is thought to contribute to Retatrutide's weight outcomes and may explain the incremental signal over tirzepatide observed in Phase 2 historical comparison.
• Counter-regulatory buffer — glucagon's glycogenolytic activity may provide an inherent buffer against hypoglycaemia in the context of GLP-1-mediated insulin secretion enhancement, particularly relevant for the T2D cohort in TRIUMPH-2.

Retatrutide's potency profile across the three receptors is: GLP-1R (high) ≥ GIPR (moderate) > GCGR (moderate). The glucagon component is co-agonist rather than dominant — contributing to but not overwhelming the metabolic profile.

Eli Lilly is conducting Retatrutide's Phase 3 evaluation under the TRIUMPH programme (TRIple-hormone receptor AgonIst for Metabolic diseases), evaluating multiple patient populations:

• TRIUMPH-1 — adults with obesity (BMI ≥30, or ≥27 with at least one weight-related comorbidity) without type 2 diabetes. 2,339 participants were randomised to retatrutide 4mg, 9mg or 12mg weekly (titrated from 2mg in four-weekly steps) or placebo. Reported 21 May 2026: mean weight loss at the 80-week primary endpoint was 19.0% (4mg), 25.9% (9mg) and 28.3% (12mg) versus 2.2% for placebo; a higher-BMI subgroup escalated to the maximum tolerated dose reached up to ~30% by week 104. See the full TRIUMPH-1 results table.
• TRIUMPH-2 — adults with obesity and type 2 diabetes. Co-primary endpoints: body weight change and HbA1c change from baseline. Evaluates the metabolic and glycaemic dimensions of triple agonism — of particular interest given glucagon receptor activation's interaction with hepatic glucose output in T2D.
• TRIUMPH-3 (cardiovascular outcomes) — long-term safety in patients with established cardiovascular disease or high cardiovascular risk. Primary endpoint: MACE (major adverse cardiovascular events). Modelled on SELECT (semaglutide) and SURMOUNT-CVOT designs. Last trial in the programme to complete.

TRIUMPH-1 reported its primary 80-week results on 21 May 2026 (above). Remaining sub-trials are progressing with primary completion estimated 2026–2027; regulatory submissions are expected to follow pending the full programme readout and review timelines.

TRIUMPH's design closely mirrors tirzepatide's SURMOUNT programme, enabling historical comparison as data emerges:

The protocol alignment is deliberate — matching duration and population enables comparison with published SURMOUNT data. A direct head-to-head Retatrutide vs tirzepatide trial has not been announced; Phase 3 comparison will be observational across separate studies.

Phase 3 data will address several questions Phase 2 could not fully answer. These are the mechanistic questions most relevant to preclinical and translational researchers following the programme:

Weight trajectory beyond 48 weeks — now partly answered
Phase 2 was capped at 48 weeks — insufficient to observe the full weight nadir and plateau. TRIUMPH-1 (reported 21 May 2026) extended the observation window to 80 weeks and showed weight reduction continuing well past the Phase 2 endpoint, reaching 28.3% at 12mg, with a higher-BMI subgroup reaching up to ~30% by week 104. The precise shape of the plateau and how it compares dose-for-dose with tirzepatide's trajectory will be clearer once the full peer-reviewed dataset and remaining TRIUMPH readouts are published.

NASH/MASH hepatic outcomes
The Phase 2 liver fat MRI sub-study showed significant hepatic fat reduction at 8mg and 12mg doses. Whether this translates to histological NASH resolution (fibrosis regression, lobular inflammation reduction) will be evaluated in dedicated sub-studies. Glucagon receptor-mediated hepatic fat mobilisation is the mechanistic differentiator from GLP-1/GIP compounds in this indication.

Weight regain on discontinuation
SURMOUNT-4 demonstrated approximately 14 percentage points of lost weight regained at 52 weeks post-tirzepatide discontinuation. Whether Retatrutide's additional glucagon receptor-mediated metabolic adaptations alter the discontinuation profile is an open question likely addressed in extension arms.

Cardiovascular outcomes with triple agonism
SELECT established semaglutide reduces MACE by approximately 20% in non-diabetic high-risk subjects. Glucagon receptor activation has complex cardiovascular implications — increasing heart rate and having context-dependent cardiac effects. TRIUMPH-3 will characterise whether triple agonism's net cardiovascular profile is equivalent to, superior to, or different in character from GLP-1 monotherapy.

For preclinical and mechanistic researchers using research-grade Retatrutide, the Phase 3 programme has practical implications for protocol design:

Dose reference: The 12mg/week Phase 3 dose is the current reference dose. Published Phase 2 data provides dose-response characterisation across four active dose levels — enabling researchers to select doses appropriate to specific mechanistic questions, from GLP-1/GIP-dominated effects at lower doses to full triple-agonist phenotype at 12mg equivalents.

Glucagon receptor isolation methodology: Retatrutide paired with a tirzepatide control (GLP-1/GIP, no glucagon agonism) allows quantitative attribution of observed metabolic effects to glucagon receptor co-agonism — a methodology increasingly used in academic metabolic research labs studying the individual receptor contributions of incretin-based compounds.

RetaLABS stocks research-grade Retatrutide in 20mg and 30mg vials. All products are for laboratory research use only.

Continue across the Retatrutide research cluster: Retatrutide Research Guide · Retatrutide Statistics 2026 · buy Retatrutide in Australia.