SYNERGY-OUTCOMES: Retatrutide and Tirzepatide in MASLD/MASH

Study Overview

SYNERGY-OUTCOMES is a Phase 3, randomized, master protocol clinical trial evaluating both retatrutide and tirzepatide versus placebo for the prevention of Major Adverse Liver Outcomes (MALO) in adults with high-risk metabolic dysfunction-associated steatotic liver disease (MASLD), including those with metabolic dysfunction-associated steatohepatitis (MASH). Registered as NCT07165028, the trial plans to enroll approximately 4,500 participants and run for approximately 224 weeks (4.3 years).

This is one of the most ambitious liver disease trials ever designed — the first event-driven outcomes trial in MASLD/MASH using incretin-based therapies. Its results could fundamentally reshape the treatment landscape for a disease that affects an estimated 30% of the global adult population and currently has extremely limited pharmacotherapy options.

Why This Trial Matters

The First Event-Driven MASLD Outcomes Trial

Previous MASLD/MASH drug trials have relied on histological endpoints — liver biopsy changes in steatosis, inflammation, and fibrosis scores. While these endpoints are clinically meaningful, they are surrogate markers. No drug trial has yet demonstrated that treating MASLD prevents the clinical events that matter most to patients: liver failure, liver transplantation, hepatocellular carcinoma, and death from liver disease.

SYNERGY-OUTCOMES changes this paradigm. By using Major Adverse Liver Outcomes (MALO) as the primary endpoint, the trial directly measures whether treatment prevents the devastating consequences of progressive liver disease. This event-driven design is analogous to the cardiovascular outcomes trials (CVOTs) that transformed diabetes drug development — and it brings the same level of evidence rigor to the liver disease field.

The MALO Composite Endpoint

The Major Adverse Liver Outcomes (MALO) endpoint is expected to include:

• Progression to cirrhosis (histological or clinical)
• Hepatic decompensation events (variceal bleeding, ascites, hepatic encephalopathy)
• Liver transplantation
• Hepatocellular carcinoma
• Death from liver-related causes

This composite captures the full spectrum of clinically relevant liver disease progression and sets a high bar for demonstrating therapeutic benefit.

Phase 2a Foundation: 82% Liver Fat Reduction

The rationale for SYNERGY-OUTCOMES is built on extraordinary Phase 2a data. In the retatrutide Phase 2 liver substudy (Sanyal et al., Nature Medicine 2024), retatrutide at the 12 mg dose reduced liver fat content by approximately 82% as measured by MRI-PDFF (magnetic resonance imaging proton density fat fraction).

Key Phase 2a liver findings:

• 82% mean reduction in liver fat at 12 mg (48 weeks)
• 86% of participants achieved normalization of liver fat (under 5% liver fat content)
• Meaningful reductions in ALT and AST (liver enzymes indicating hepatocellular injury)
• Improvements in non-invasive fibrosis markers

These results represent the most dramatic liver fat reduction ever reported for any pharmacological agent in a clinical trial. For context, resmetirom (the first FDA-approved MASH drug, marketed as Rezdiffra) reduced liver fat by approximately 40-50% in its Phase 3 program.

Master Protocol Design

SYNERGY-OUTCOMES employs a master protocol design, evaluating both retatrutide and tirzepatide within the same trial framework. This offers several advantages:

• Shared placebo arm: Reduces the total number of participants needed and provides a common comparator
• Consistent endpoints and methodology: Enables valid cross-drug comparisons within the same trial infrastructure
• Operational efficiency: Shared study sites, laboratory standards, and adjudication processes
• Regulatory efficiency: A single trial can support regulatory submissions for multiple compounds

The inclusion of tirzepatide (already approved for obesity and T2D) alongside retatrutide provides a comparison between dual (GIP/GLP-1) and triple (GIP/GLP-1/glucagon) agonism in liver disease. Whether the glucagon receptor component of retatrutide provides additional hepatic benefit is a key scientific question this trial can address.

Trial Design

Population

The trial targets a high-risk MASLD/MASH population — patients with significant hepatic fibrosis who are at elevated risk for progression to cirrhosis and clinical liver events. This population is selected because:

• Event rates in low-risk MASLD are too low for a feasible outcomes trial
• High-risk patients (typically F2-F3 fibrosis) have the most to gain from effective therapy
• Regulatory agencies have emphasized the need for outcomes data in this population

Non-Invasive Biomarker Endpoints

While the primary endpoint is event-driven, SYNERGY-OUTCOMES is also expected to incorporate non-invasive biomarker endpoints as key secondary measures:

• MRI-PDFF for liver fat quantification
• FibroScan (VCTE) for liver stiffness as a surrogate for fibrosis
• Enhanced Liver Fibrosis (ELF) score
• FIB-4 index
• NAFLD Activity Score components (where biopsy data are available)

These non-invasive endpoints reduce the need for serial liver biopsies and align with the field’s shift toward imaging and serum-based assessment of liver disease.

The Glucagon Receptor Advantage in Liver Disease

Retatrutide’s glucagon receptor agonism may be particularly relevant in MASLD/MASH:

• Glucagon promotes hepatic lipid oxidation, directly reducing liver fat content
• Glucagon increases hepatic autophagy, which clears damaged organelles and lipid droplets
• The 82% liver fat reduction seen in Phase 2a substantially exceeded what GLP-1-only agonists (semaglutide: ~40-50% reduction) have achieved, suggesting the glucagon component contributes meaningfully to hepatic effects
• Glucagon modulates hepatic glucose output, which may improve the metabolic dysregulation underlying MASLD

Whether these mechanistic advantages translate to superior prevention of clinical liver events compared to tirzepatide (which lacks glucagon receptor activity) is precisely the question SYNERGY-OUTCOMES is designed to answer.

Challenges and Considerations

Trial Duration and Attrition

A 4.3-year trial carries substantial operational challenges:

• Participant retention over more than 4 years is difficult, particularly for a disease that is often asymptomatic until advanced stages
• Event rates must be sufficient to achieve statistical power — if the enrolled population is lower risk than anticipated, the trial may need to extend or expand
• Competing treatments may become available during the trial period, complicating interpretation

Regulatory Implications

A positive SYNERGY-OUTCOMES result would be transformative:

• It would be the first demonstration that any drug prevents clinical liver events in MASLD/MASH
• It could support a MASLD/MASH indication for retatrutide (and potentially tirzepatide) beyond the obesity and diabetes indications
• It would establish the MALO endpoint as a viable primary endpoint for future MASLD trials

Comparison with Resmetirom

Resmetirom (Rezdiffra), a thyroid hormone receptor beta agonist, was approved by the FDA in March 2024 for MASH with moderate to advanced fibrosis based on histological improvement at 52 weeks. It was not approved based on clinical outcomes data. SYNERGY-OUTCOMES, by targeting MALO prevention, sets a higher evidentiary bar and could position retatrutide as a treatment with demonstrated outcomes benefit — a distinction that would carry significant weight with guideline committees and payers.

Timeline

Given the approximately 224-week duration and recruitment starting in 2025-2026, primary results from SYNERGY-OUTCOMES are not expected until 2029-2030 at the earliest. Interim analyses may provide earlier signals, but the event-driven design requires sufficient accumulated events to reach statistical significance.

Summary

SYNERGY-OUTCOMES is a landmark Phase 3, event-driven outcomes trial evaluating retatrutide and tirzepatide for prevention of Major Adverse Liver Outcomes in high-risk MASLD/MASH. With 4,500 participants, a 4.3-year duration, and a master protocol design, it represents the most ambitious liver disease trial ever conducted with incretin-based therapies. The trial builds on Phase 2a data showing 82% liver fat reduction with retatrutide at 12 mg and aims to demonstrate what no drug has yet shown: that pharmacotherapy can prevent cirrhosis, liver failure, transplantation, and death in MASLD/MASH. Results are expected in 2029-2030.