Testing the triple hormone drug retatrutide for fatty liver disease linked to metabolism problems: a phase 2a trial

Nonalcoholic fatty liver disease, now termed metabolic dysfunction-associated steatotic liver disease (MASLD), is one of the most common chronic liver diseases in the world^1,2. The global prevalence of MASLD has increased dramatically, from 25% in 1990–2006 to 38% in 2016–2019. Meta-regression analyses have demonstrated that the increased prevalence of obesity is a major contributor to the growing burden of MASLD². At least half of patients with MASLD are estimated to have obesity³.

Insulin resistance is an important pathophysiologic driver for MASLD. Insulin resistance in adipocytes contributes to dysregulated lipolysis, resulting in excessive delivery of fatty acids to the liver. Substrate overload can drive hepatic de novo lipogenesis. Hepatic steatosis can trigger inflammation causing hepatocyte injury, apoptosis and necrosis, eventually leading to liver fibrosis, although numerous factors contribute to the heterogeneity of disease progression⁴. These features characterize the more progressive form of MASLD, which is called nonalcoholic steatohepatitis or metabolic dysfunction-associated steatohepatitis (MASH)¹. Over the course of several years, worsening fibrosis may progress to cirrhosis, the development of portal hypertension and ultimately hepatic decompensation. By 2019, MASH became the second most common indication for liver transplant in the United States and was the fastest increasing indication⁵.

Currently, no treatments for MASH are approved in the United States or Europe. Many potential pharmacological therapies are in clinical development, harnessing several different mechanisms of action. Among these are the incretin-based therapies that target glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) receptors and induce weight loss. Semaglutide, a GLP-1 receptor mono-agonist, is in phase 3 development for noncirrhotic MASH, having demonstrated histological improvements in a phase 2 MASH trial⁶. The GIP/GLP-1 dual agonist, tirzepatide, reduced liver fat and improved biomarkers of MASH and fibrosis in patients with type 2 diabetes (T2D); a phase 2 trial in MASH is ongoing^7,8. The addition of glucagon (GCG) agonist activity to GLP-1 agonism has shown promise for providing greater reduction of hepatic fat, an early marker of improvement in MASH. Efocipegtrutide (HM15211), a triple GLP-1/GIP/GCG receptor agonist, has demonstrated significant liver fat reduction after 12 weeks in participants with MASLD and is now in phase 2 development^9,10.

Retatrutide (RETA; LY3437943) is a single protein conjugated to a fatty diacid moiety that activates human GIP, GLP-1 and GCG receptors. On the basis of cell culture studies, retatrutide is less potent than endogenous ligands of the human GCG and GLP-1 receptors (0.3 and 0.4 times as active, respectively) and is more potent at the human GIP receptor (by a factor of 8.9)¹¹. The pharmacokinetics of retatrutide are dose proportional; it has a half-life of approximately 6 days, enabling weekly subcutaneous administration¹². In a phase 2 study of retatrutide in people with obesity who did not have T2D, a weight reduction up to 24.2% was observed after 48 weeks. Treatment was also associated with improvements in blood pressure (BP), lipids and glycemia¹³. In this Article, we report the results of a substudy of that trial that evaluated changes in liver fat and biomarkers of MASH and fibrosis in people with MASLD.

In this substudy of participants with MASLD who were included in a phase 2 study of people with obesity or overweight with weight-related complications, treatment for 24 weeks with the GIP/GLP-1/GCG triple receptor agonist, retatrutide, was associated with significant reductions in liver fat content at all doses. At the two highest doses, 80% or more of participants achieved ≥70% relative reduction in liver fat and more than 85% achieved resolution of steatosis, defined as <5% total liver fat content. Near-maximal liver fat reduction was achieved at an approximately 20% reduction in body weight. Reductions in liver fat were strongly associated with reductions in body weight, ASAT and VAT, and with improvements in markers of insulin sensitivity and lipid metabolism.

The 86% relative liver fat reduction observed with retatrutide 12 mg at 48 weeks is among the largest treatment effects reported so far, although differences in populations and study design across trials limit direct comparisons. In phase 1 and 2 studies in patients with MASLD or MASH, liver fat reductions have been greatest with GLP-1/GIP/GCG triple agonists, GLP-1/GCG dual agonists and FGF21 analogs. The GLP-1/GIP/GCG triple agonist, efocipegtrutide, demonstrated up to 81% liver fat reduction after 12 weeks in participants with MASLD⁹. After 24 weeks, the dual GLP-1/GCG agonists, efinopegdutide and pemvidutide, reduced liver fat by up to 73% and 76%, respectively, in participants with MASLD^14,15. In patients with biopsy-proven MASH, efruxifermin reduced liver fat by up to 72% after 12 weeks and pegozafermin reduced liver fat by up to 48% after 24 weeks (both FGF21 analogs)^16,17. Reported liver fat reductions with GLP-1 mono-agonists and the GIP/GLP-1 dual agonist, tirzepatide, were lower than those observed with retatrutide, ranging from 32% after 24 weeks with dulaglutide and 47% after 52 weeks with tirzepatide, to approximately 50% after 72 weeks treatment with semaglutide^8,18,19. Rapid reductions of liver fat were reported in a study of 50 patients undergoing a very low-calorie diet for 2–3 weeks followed by bariatric surgery; mean liver fat content was reduced from a baseline of 18.1 to 14.9% after very low-calorie diet and then to 9.3%, 6.5% and 4.9% at 1, 3 and 6–10 months of postsurgical follow-up. Overall, liver fat was normalized (to <5% liver fat) in 32 (64%) of 50 participants after a mean of 22.5 weeks²⁰. In the current study, more than 85% of participants treated with retatrutide 12 mg achieved normal levels of liver fat after 24 and 48 weeks. Of note, in clinical studies, a relative liver fat reduction of ≥30% has been associated with histological improvement in patients with MASH²¹. Greater reductions in liver fat may result in higher odds of histological improvement, particularly if there is complete resolution of steatosis.

The additional liver fat lowering observed with retatrutide compared with GLP-1 mono-agonists and tirzepatide may be related to the greater weight reduction achieved with retatrutide, direct hepatic effects of glucagon receptor agonism or both^13,22. In preclinical models, retatrutide promoted weight loss by reducing food intake and increasing energy expenditure compared with calorie intake-matched animals. In addition, its glucagon activity may reduce liver fat by stimulating hepatic fatty acid oxidation and reducing hepatic lipogenesis^11,22. In the present study, levels of β-hydroxybutyrate, a biomarker of fatty acid oxidation, increased two to threefold in a dose-related pattern with retatrutide doses 4 mg and higher. The largest increases in β-hydroxybutyrate were apparent by week 24 when most of the reduction in liver fat had occurred and percent changes in β-hydroxybutyrate and liver fat were significantly correlated. In a recent 24-week study, the dual GLP-1/GCG agonist, efinopegdutide, demonstrated greater liver fat lowering efficacy than semaglutide for the same degree of weight loss. For weight loss categories of ≤5%, >5% to ≤10%, and >10%, liver fat reduction with efinopegdutide (52.4%, 76.6% and 86.2%, respectively) was greater than with semaglutide (13.4%, 39.6% and 64.2%, respectively)¹⁴. These data suggest that glucagon agonism provides additional liver fat reducing efficacy beyond what is expected from weight loss alone. In this regard, it is noteworthy that a selective glucagon receptor antagonist increased hepatic fat in patients with T2D²³. FGF21, a liver-secreted hormone, has been proposed as a potential mediator of GCG actions to regulate hepatic metabolism²⁴. Previous studies have shown that chronic GCG receptor agonism increased circulating FGF21 in mice and acute glucagon administration transiently increased FGF21 levels in humans²⁵. Notably, FGF21 analogs have reduced insulin resistance or decreased liver fat in clinical trials^16,26; however, administration of retatrutide at higher doses significantly reduced circulating FGF21 levels, suggesting that retatrutide efficacy in MASLD is unlikely to be attributable to increased FGF21 levels.

Reductions in liver fat with retatrutide treatment were significantly related to changes in body weight, ASAT and VAT. A near-maximal liver fat reduction of approximately 75% was achieved coincident with an approximately 20% reduction in body weight. Thus, almost all the reduction in liver fat occurred within the first 24 weeks. In contrast, significant further reductions in ASAT and VAT continued beyond 24 weeks in parallel with continued weight loss. This finding is consistent with a ‘floor effect’ for hepatic fat, which is unlikely to be reduced below the median of 2.1% observed in the large UK Biobank study²⁷. Therefore, with weight loss greater than 20%, the relative reduction of liver fat approaches an asymptote at the maximal achievable liver fat loss. These data also suggest that under conditions favoring fat mobilization and oxidation, hepatic fat is preferentially mobilized over fat from adipose tissue depots.

In addition to reductions in body weight, abdominal fat and liver fat, retatrutide treatment was associated with improvements in insulin sensitivity, lipid metabolism and adipocyte hormones. Higher doses of retatrutide reduced biomarkers of insulin resistance, including fasting insulin, fasting C-peptide and HOMA2-IR, by up to 50% or more from baseline. Time- and dose-related improvements in insulin sensitivity were similar to the temporal patterns of body weight loss with retatrutide in both the MASLD cohort reported here and in the full cohort of people with obesity¹³. Retatrutide-mediated reductions in markers of insulin resistance were closely associated with decreased liver fat. Retatrutide improved fasting lipid profiles including triglycerides, very low-density lipoprotein cholesterol and non-high-density lipoprotein cholesterol in clinical trials in people with T2D or obesity, while low-density lipoprotein cholesterol was reduced in people with obesity^12,13,28. In the current analysis in people with MASLD, fasting triglycerides were reduced by greater than 40% by retatrutide 8 mg and 12 mg doses after 48 weeks, and these reductions were significantly associated with reduced liver fat. Activation of both GIP and GLP-1 receptors by retatrutide probably contributed to the observed changes in circulating triglycerides, as treatment with the dual GIP/GLP-1 receptor agonist tirzepatide significantly reduced triglyceride levels by up to 29% at 52 weeks in a clinical trial in people with T2D and MASLD⁸. Retatrutide also reduced liver triglyceride levels in preclinical studies of obese mice¹¹. The observed reductions in leptin levels with higher doses of retatrutide may reflect a function of this hormone as a circulating indicator of fat stores²⁹. Adiponectin levels increased in a time-related manner with higher doses of retatrutide, consistent with progressive adipose tissue reductions and improvements in insulin sensitivity³⁰.

Several MASH-related biomarkers were evaluated in this study. Significant reductions were observed for K-18 with the 8 mg and 12 mg doses of retatrutide (up to 49.6%) and for pro-C3 with all doses of retatrutide (up to 26.4%). K-18 is released during hepatocyte apoptosis and has been proposed as a marker of cell death in the liver³¹. A recent meta-analysis proposed K-18 values between 127–191 U l⁻¹ for ruling-out MASH at fixed sensitivity levels and 304–399 U l⁻¹ to rule-in MASH at fixed specificity levels³². In this study, mean baseline K-18 levels ranged from 263 to 311 U l⁻¹. The magnitude of decrease in K-18 observed with retatrutide 8 mg and 12 mg has been associated with greater odds of histological improvement in previous studies of patients with MASH³³. Pro-C3 is an epitope that is generated during procollagen type III cleavage and reflects fibrogenic drive³⁴. Plasma pro-C3 levels have been shown to correlate with severity of steatohepatitis and fibrosis stage in patients with MASLD³⁵. In a longitudinal study of such patients, mean pro-C3 increased with worsening of fibrosis and decreased with fibrosis improvement³⁶. Changes in pro-C3 have also been shown to correlate with changes in fibrosis stage in patients with T2D and MASLD after 18 months of treatment with pioglitazone, vitamin E or placebo³⁷. FIB-4 and ELF score are two well-validated biomarkers that assess the risk for liver fibrosis in patients with MASLD. A FIB-4 index of <1.3 and an ELF score of <9.8 have both been used to rule-out advanced fibrosis^38,39. In this study, mean baseline FIB-4 index ranged from 0.7 to 0.9 and mean baseline ELF score ranged from 7.8 to 8.3. Only seven participants (7.1%) had either a FIB-4 index higher than 1.3 or an ELF score higher than 9.8. Thus, it is likely that most participants in this study had simple steatosis or MASH with mild fibrosis. Therefore, significant improvements in FIB-4 and ELF would not be expected in this population. Mean ALT and AST levels were also normal at baseline; thus, the absence of consistent change in these enzymes after retatrutide treatment is neither surprising nor determinant. The reasons for the discrepancy between pro-C3 and ELF results are not entirely clear but may reflect the fact that the ELF test is a proprietary algorithm based on three fibrosis biomarkers (hyaluronic acid, procollagen III amino acid terminal peptide and tissue inhibitor of metalloproteinase 1), whereas pro-C3 is a single biomarker targeting the N-terminal pro-peptide of type III collagen by a different epitope than the procollagen III amino acid terminal peptide assay included in the ELF test. A potential explanation of the difference in response between the two tests is that pro-C3 may be more indicative of active fibrogenesis, whereas ELF may correlate more with the severity of histological liver fibrosis^36,40.

The safety profile of retatrutide in people with MASLD was similar to that observed in the broader trial population of people with obesity^13,28. Transient, mostly mild-to-moderate gastrointestinal events were the most frequently reported adverse events, occurring primarily during dose escalation. The frequency of these adverse events was higher in the 8 mg and 12 mg dose groups than in the other dose groups. There were no hepatotoxicity signals in the overall obesity trial population or in the subset of participants with MASLD through 48 weeks. These safety findings are similar to those reported for therapies based on GLP-1 or GIP/GLP-1 agonism for the treatment of type 2 diabetes or obesity^41–45. This study did not include patients with advanced fibrosis or cirrhosis; thus, potential safety concerns with use of retatrutide in such patients cannot be assessed from these data.

The strengths of this trial include its prospective design with a priori defined endpoints, the inclusion of approximately equal percentages of men and women and 41.8% of participants identifying as Hispanic or Latino, the evaluation of liver fat and body composition by MRI and the measurement of several biomarkers to indirectly assess progressive MASLD and suggest potential mechanistic insights. The limitations include the relatively small sample size of the MASLD substudy, the geographic and racial homogeneity of the sample (United States only and majority white), the exclusion of patients with T2D, the absence of liver histology, the lack of enrichment for MASH or significant fibrosis, the lack of multiplicity control given the large number of statistical assessments and the absence of 48-week MRI data for 56.1% of participants, which limits interpretation of the dose-response relationship for liver fat reduction at 48 weeks. However, despite this latter limitation, the 48-week data demonstrate that the defatting of the liver that occurred by 24 weeks was maintained at 48 weeks. The results of this phase 2 substudy should be considered as hypothesis-generating and not definitive.

In this phase 2 trial in participants with obesity and MASLD, once-weekly treatment with the GIP/GLP-1/GCG triple agonist retatrutide resulted in substantial reductions in liver fat, body weight, ASAT and VAT, which were associated with improvements in insulin sensitivity, serum lipids, K-18 and pro-C3. Hepatic steatosis resolved in more than 85% of participants in the two highest dose groups. These results suggest that retatrutide may be an effective therapeutic agent for treatment of MASLD. Further studies are warranted to determine whether retatrutide treatment can reduce the severity of fibrotic MASH and reduce the risk of major adverse liver outcomes in people with MASLD.

Any methods, additional references, Nature Portfolio reporting summaries, source data, extended data, supplementary information, acknowledgements, peer review information; details of author contributions and competing interests; and statements of data and code availability are available at 10.1038/s41591-024-03018-2.