Retatrutide Phase 1: First-in-Human Safety Study

Retatrutide, originally identified by the development code LY3437943, entered human clinical research through an early Phase 1 study designed primarily to assess safety, tolerability and pharmacokinetics. This first-in-human research represented an important starting point in the compound’s wider investigational development programme.

The study, registered as NCT03841630, was sponsored by Eli Lilly and Company. It began in March 2019 and was completed in July 2019, enrolling 45 healthy participants. Researchers used a single-ascending-dose design, meaning different groups received increasing doses of LY3437943 or a matching placebo.

What Did the Phase 1 Study Investigate?

The main objective was to determine how participants tolerated a single subcutaneous injection and to identify any treatment-emergent or serious adverse events. Blood samples were also collected to investigate pharmacokinetic characteristics, including how quickly LY3437943 entered the bloodstream, its maximum concentration and how long it remained in the body.

This type of early research is essential before a new investigational compound progresses into larger studies. Readers interested in the wider development programme can explore our timeline of retatrutide’s investigational development.

Early Safety and Pharmacokinetic Findings

Development documents describing the study reported that gastrointestinal events, including nausea, abdominal distension and vomiting at higher doses, were among the more commonly observed treatment-emergent events. These effects were generally dose dependent and mostly mild. Temporary dose-related changes in heart rate and systolic blood pressure were also observed.

Pharmacokinetic observations indicated that maximum blood concentrations generally occurred around one to three days after administration. The reported terminal half-life was approximately five to seven days, providing early evidence supporting investigation of once-weekly dosing in subsequent studies.

Why Was the First-in-Human Study Important?

The Phase 1 programme established the early safety and pharmacokinetic foundation required for further investigation of retatrutide. Later trials moved beyond healthy volunteers to participants with metabolic conditions and evaluated repeated dosing, efficacy and longer-term safety.

Retatrutide is designed to activate GIP, GLP-1 and glucagon receptors. For more background, see our guide to the triple-agonist hypothesis and our overview of how incretin receptors are studied in vitro.

Subsequent research progressed into dose-ranging and large-scale trials. You can continue exploring the programme through our retatrutide clinical studies section or visit the Knowledge Centre for further research-focused articles.

Research note: Retatrutide remains an investigational compound. Clinical trial information should not be interpreted as medical advice or as evidence of regulatory approval for personal use.