MAESTRO-NASH Trial: A Plain-English Overview of What It Established

At a glance

| Detail | Value | |---|---| | Trial name | MAESTRO-NASH (NCT03900429) | | N randomized | 966 | | Intervention | Resmetirom 80 mg or 100 mg once daily | | Comparator | Placebo | | Duration | 52 weeks (primary analysis); 54 months planned | | Primary endpoints | (1) NASH resolution with no worsening of fibrosis; (2) Improvement in fibrosis by ≥1 stage with no worsening of NAS | | Key result | Both co-primary endpoints met at both doses (p <0.001 for each comparison) | | Publication | Harrison SA et al., N Engl J Med 2024; 390:497-509 |

Why This Trial Exists

For decades, clinicians managing what was formerly called nonalcoholic steatohepatitis (NASH, now MASH) had no approved pharmacotherapy. Patients were told to lose weight and exercise. Some received off-label vitamin E or pioglitazone. Neither carried an FDA indication for the disease. The unmet need was enormous: an estimated 6 to 8 million Americans have MASH with significant fibrosis, and progression to cirrhosis, liver failure, or hepatocellular carcinoma is well-documented.

Resmetirom targets thyroid hormone receptor beta (THR-β), which is selectively expressed in the liver. Activating THR-β increases hepatic fat metabolism, reduces lipotoxic lipid species, and lowers circulating lipoproteins. Earlier phase 2 data (Harrison et al., Lancet 2019) showed significant reductions in hepatic fat fraction by MRI-PDFF, giving the rationale for a large phase 3 trial powered on histological endpoints.

Who Was Enrolled (and Who Was Not)

MAESTRO-NASH enrolled adults aged 18 and older with biopsy-confirmed MASH and a NAFLD Activity Score (NAS) of 4 or higher, with at least 1 point in each component (steatosis, lobular inflammation, hepatocyte ballooning). Fibrosis stage had to be F1B, F2, or F3 on the NASH CRN scoring system.

Key exclusions:

• Compensated or decompensated cirrhosis (F4)
• Other chronic liver diseases (viral hepatitis, autoimmune hepatitis, Wilson disease)
• Uncontrolled thyroid disease or TSH outside the normal range
• Bariatric surgery within 5 years
• Use of GLP-1 receptor agonists, pioglitazone, or vitamin E >400 IU/day unless stable for ≥6 months

This last point matters. The trial population was relatively "clean" compared to real-world MASH patients, many of whom take GLP-1 agonists or have diabetes managed with multiple agents. Baseline characteristics showed a mean BMI around 36 kg/m², roughly 65% of participants had type 2 diabetes, and mean NAS was approximately 5.

What Participants Received

Randomization was 1:1:1 to resmetirom 80 mg daily, resmetirom 100 mg daily, or matching placebo. All tablets were taken once daily with food. The trial protocol did not mandate specific dietary or exercise counseling beyond standard of care, which makes the drug effect interpretable against a background of routine clinical management rather than an intensive lifestyle intervention.

Paired liver biopsies were performed at baseline and week 52. A single, centralized pathology reading service scored biopsies to reduce inter-reader variability, a methodological strength not present in all NASH/MASH trials.

The Two Co-Primary Endpoints, Explained

The FDA required two histological co-primary endpoints, each assessed at 52 weeks:

1. NASH resolution: defined as a ballooning score of 0, a lobular inflammation score of 0 or 1, and no worsening of fibrosis stage from baseline.
2. Fibrosis improvement: defined as at least a 1-stage reduction in fibrosis score with no worsening of NAFLD Activity Score.

Both had to be statistically significant for the trial to succeed. This dual-gating is intentional. Reducing inflammation alone does not help if fibrosis keeps advancing. Improving fibrosis alone does not help if the underlying steatohepatitis remains active. The FDA wanted evidence of benefit on both axes.

Interpreting Histological Endpoints: A Practical Framework

Histological endpoints in MASH trials are sometimes misunderstood. "NASH resolution" does not mean the liver is normal. It means the inflammatory pattern that defines steatohepatitis (ballooning plus lobular inflammation) is no longer present on biopsy. Steatosis (fat) can persist. Similarly, a 1-stage fibrosis improvement (say, F3 to F2) is clinically meaningful because fibrosis stage is the strongest histological predictor of liver-related mortality (Taylor et al., Gastroenterology 2020). Each stage reduction is associated with a measurable decrease in long-term risk of decompensation and death.

Still, these are surrogate endpoints. The trial was not powered to show reductions in cirrhosis, liver transplant, or death. A 54-month extension is intended to collect those harder outcomes.

Results: What the Numbers Actually Show

NASH Resolution (Co-Primary Endpoint 1)

| Group | Responders | Rate | Difference vs Placebo | p-value | |---|---|---|---|---| | Resmetirom 80 mg | 78/322 | 25.9% | +13.6 pp | <0.001 | | Resmetirom 100 mg | 81/321 | 29.9% | +17.6 pp | <0.001 | | Placebo | 36/321 | 9.7% |, |, |

Fibrosis Improvement (Co-Primary Endpoint 2)

| Group | Responders | Rate | Difference vs Placebo | p-value | |---|---|---|---|---| | Resmetirom 80 mg | 75/322 | 24.2% | +8.1 pp | <0.001 | | Resmetirom 100 mg | 77/321 | 25.9% | +9.8 pp | <0.001 | | Placebo | 49/321 | 14.2% |, |, |

The placebo response rates deserve attention. A 9.7% NASH resolution rate and 14.2% fibrosis improvement rate in the placebo arm are consistent with prior MASH trials and partly reflect sampling variability in paired biopsies, regression to the mean, and possibly modest lifestyle changes triggered by trial participation. The drug effect is measured on top of this background.

Secondary Endpoints

Resmetirom produced large reductions in LDL cholesterol (approximately 14-16% reduction) and hepatic fat fraction by MRI-PDFF. Liver enzyme levels (ALT, AST) decreased. These biomarker changes were apparent by week 12 and preceded the histological improvements assessed at week 52, consistent with a model where reduced lipotoxicity gradually resolves inflammation and, over time, fibrosis.

Safety and Tolerability

The overall safety profile was favorable enough for FDA accelerated approval in March 2024. The most common adverse events were gastrointestinal: diarrhea (27% with 100 mg vs 16% placebo) and nausea (19% vs 10%). Most GI events were mild to moderate and concentrated in the first 4 to 8 weeks.

Serious adverse events occurred at similar rates across groups (roughly 11-12%). There were no signals of clinically significant thyrotoxicosis. TSH levels remained within normal limits in the vast majority of participants, which is expected given resmetirom's selectivity for THR-β over THR-α (the receptor subtype responsible for cardiac and bone effects of thyroid hormone).

Drug-induced liver injury (DILI) is always a concern with any hepatic-targeted agent. The trial reported rare ALT elevations >5x upper limit of normal, but rates were not statistically different between groups. Post-marketing surveillance continues.

Limitations the Investigators Acknowledged

The original publication and subsequent editorials flagged several limitations:

• Surrogate endpoints only. No data yet on hard outcomes (cirrhosis, decompensation, transplant, death). The accelerated approval pathway explicitly requires confirmatory outcome data.
• 52-week snapshot. Fibrosis regression is slow. Some patients may need longer treatment to show a histological response.
• Limited diversity. Approximately 4% of participants were Black, despite MASH affecting Black populations. Hispanic representation was also below proportional burden.
• Concomitant medication restrictions. Patients on GLP-1 agonists or high-dose vitamin E were underrepresented, so the trial does not directly inform combination strategies that are now common in practice.
• Biopsy sampling error. Liver biopsies sample roughly 1/50,000th of the organ. Paired-biopsy designs mitigate but do not eliminate this noise. A patient scored as "improved" on one biopsy might not be improved in another region of the liver.

What This Means for Clinical Practice

Resmetirom (brand name Rezdiffra) received FDA accelerated approval in March 2024 for adults with non-cirrhotic MASH and moderate to advanced fibrosis (F2-F3). It is the first drug approved specifically for MASH. The approval carries a requirement for a confirmatory trial demonstrating benefit on clinical outcomes.

Practically, this changes the conversation between hepatologist and patient. Before MAESTRO-NASH, the only evidence-based recommendation was weight loss of 7-10% of body weight, which fewer than 10% of patients sustain. Now there is a prescription option. Current AASLD guidance acknowledges resmetirom as a treatment option for the approved population while emphasizing that lifestyle modification remains foundational.

Several open questions remain. Can resmetirom be combined with GLP-1 agonists for additive benefit on both liver histology and weight? Will the fibrosis improvements observed at 52 weeks translate into reduced rates of cirrhosis over 5 to 10 years? How should clinicians monitor response without serial biopsies, perhaps using noninvasive biomarkers like FibroScan or the Enhanced Liver Fibrosis (ELF) test? These are active areas of investigation.

The cost of Rezdiffra (approximately $47,000 per year at launch) also introduces access questions. Insurance coverage varies, and prior authorization requirements are common. For the subset of patients with confirmed F2-F3 fibrosis who meet the label criteria, the drug represents a meaningful therapeutic advance. For the much larger population with earlier-stage disease or isolated steatosis, it is not indicated.

Frequently asked questions

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References

1. Harrison SA, Bedossa P, Guy CD, et al. A phase 3, randomized, controlled trial of resmetirom in NASH with liver fibrosis. N Engl J Med. 2024;390(6):497-509. PubMed
2. Harrison SA, Bashir MR, Guy CD, et al. Resmetirom (MGL-3196) for the treatment of non-alcoholic steatohepatitis: a multicentre, randomised, double-blind, placebo-controlled, phase 2 trial. Lancet. 2019;394(10213):2012-2024. PubMed
3. Taylor RS, Taylor RJ, Bayliss S, et al. Association between fibrosis stage and outcomes of patients with nonalcoholic fatty liver disease: a systematic review and meta-analysis. Gastroenterology. 2020;158(6):1611-1625.e12. PubMed
4. Rezdiffra (resmetirom) prescribing information. U.S. Food and Drug Administration. 2024. FDA Label
5. Rinella ME, Lazarus JV, Ratziu V, et al. A multisociety Delphi consensus statement on new fatty liver disease nomenclature. Hepatology. 2023;78(6):1966-1986. PubMed