Resmetirom and Accelerated Approval for MASH

In March 2024, the US Food and Drug Administration approved resmetirom, sold as Rezdiffra, alongside diet and exercise for adults with noncirrhotic steatohepatitis and moderate to advanced fibrosis consistent with stages F2 to F3. The condition is now commonly called metabolic dysfunction-associated steatohepatitis, or MASH. The US label uses the older term NASH.

This was the first FDA approval for the disease. It was an accelerated approval based on improvements in liver-biopsy endpoints judged reasonably likely to predict benefit, and it did not yet establish that resmetirom prevents cirrhosis, liver decompensation, transplantation, or death. Those patient outcomes are the confirmatory question.

Why the approval pathway matters#

FDA's accelerated-approval pathway is intended for serious conditions with unmet need; it permits approval using a surrogate or intermediate endpoint that is reasonably likely to predict how a person feels, functions, or survives. The evidentiary bargain is explicit: earlier availability in exchange for required confirmation.

An accelerated approval is a real approval, not an experimental-use designation. It is also not the same as confirmation of long-term clinical benefit. FDA can revise or withdraw an indication if required studies do not verify benefit, cannot be completed with due diligence, or change the benefit-risk assessment.

Resmetirom remains listed by FDA as an ongoing accelerated approval. FDA's current list identifies a 54-month randomized, double-blind, placebo-controlled requirement within study MGL-3196-11 to test a composite of clinically consequential liver outcomes. The trial record is NCT03900429.

What resmetirom is designed to do#

Resmetirom is an oral agonist that preferentially activates thyroid hormone receptor beta in the liver, a receptor that participates in lipid metabolism, and the drug lowers liver fat and atherogenic lipids without being ordinary thyroid-hormone replacement.

Mechanism supports plausibility but does not prove patient benefit. MASH arises within a broader metabolic and inflammatory process, and fibrosis is the feature most closely tied to liver-related risk: a useful therapy therefore needs evidence in people with meaningful fibrosis, not only changes in liver enzymes or imaging.

The approved population does not include cirrhosis. The label says to avoid use in decompensated cirrhosis, and it also does not present resmetirom as a substitute for nutrition, physical activity, weight management, diabetes care, lipid management, or treatment of other cardiovascular risks.

What MAESTRO-NASH tested#

MAESTRO-NASH is an ongoing phase 3 trial. Its month-12 biopsy analysis enrolled adults with biopsy-confirmed steatohepatitis and fibrosis. The FDA efficacy population included 888 participants with F2 or F3 fibrosis and a qualifying disease-activity score. Participants were assigned to resmetirom 80 mg, resmetirom 100 mg, or placebo once daily.

The trial had two primary histologic endpoints:

1. resolution of steatohepatitis without worsening of fibrosis;
2. improvement in fibrosis by at least one stage without worsening of steatohepatitis.

For MASH resolution without fibrosis worsening, the reported proportions were 25.9% with 80 mg, 29.9% with 100 mg, and 9.7% with placebo. For fibrosis improvement without MASH worsening, they were 24.2%, 25.9%, and 14.2%, respectively. Both doses outperformed placebo on both endpoints.

Those percentages need their denominators and their counterfactual before you can use them. They show an increased chance of histologic response, not that every treated liver improved. Roughly seven in ten participants did not meet the resolution endpoint at one year, even at 100 mg. A placebo response also occurred, which can reflect biological variability, background care, sampling, and interpretation of biopsy tissue.

Why biopsy is useful and limited#

Fibrosis regression and resolution of steatohepatitis are biologically relevant. Waiting years for decompensation or death before allowing any treatment would delay access in a disease with few prior options. Histology also provides direct tissue information that a liver-enzyme change cannot.

Yet a biopsy samples a tiny portion of a large, heterogeneous organ. Fibrosis and inflammation can vary within the liver. Tissue adequacy and reader interpretation affect classification. The coprimary definitions also combine several scores, so a person just above a cutoff may be classified differently after a small change.

Most importantly, histologic improvement is not itself the clinical outcome patients are trying to avoid. The confirmatory phase must determine whether the observed biopsy changes translate into fewer events such as progression to cirrhosis, hepatic decompensation, transplantation, or death.

Read the one-year findings in context#

The randomized, placebo-controlled design supports a causal conclusion about the measured endpoints over the analyzed period. Blinded central pathology review reduces, but does not eliminate, subjectivity. The two doses and two prespecified primary endpoints clarify dose response and protect the interpretation from relying on one selected result.

Several limits remain. The month-12 analysis is short relative to the natural history of fibrosis. Trial participants met specific criteria and received structured follow-up. Results may differ in people with other causes of liver disease, cirrhosis, substantial comorbidity, or medication patterns not represented in the trial.

Clinical outcomes and longer-term adverse events require more time and more events. The confirmatory analysis is therefore not paperwork after the “real” result. It answers a different and more consequential question.

Safety and interaction evidence belongs in the reading#

The current prescribing information warns about hepatotoxicity and gallbladder-related adverse reactions. Symptoms or laboratory findings suggesting liver injury require evaluation under the label. Cholelithiasis and cholecystitis were observed more often with treatment, and suspected acute gallbladder events warrant assessment.

Resmetirom has clinically relevant interactions. Strong CYP2C8 inhibitors such as gemfibrozil are not recommended; a dose adjustment is described with a moderate inhibitor such as clopidogrel. The drug can increase concentrations of some statins, so the label specifies dose limits and monitoring considerations for statin-related adverse effects.

These facts do not mean the medicine is broadly unsafe. They show why a benefit-risk decision is tied to the approved population, concurrent medicines, liver status, and follow-up. A trial percentage cannot make that decision for you.

“First” no longer means “only”#

Resmetirom was the first FDA-approved treatment for this disease in March 2024. The field has continued to change. FDA's accelerated-approval list now also includes a 2025 MASH indication for semaglutide. Historical precedence and current treatment choice are separate facts.

Comparisons across different trials can mislead you, because populations, endpoints, estimands, follow-up, background care, and missing-data rules all differ. A larger percentage in one report does not establish superiority over another medicine without a suitable direct comparison.

A practical evidence checklist#

Start by confirming that the exact labeled population is the one you are asking about, and that the terminology matches. Separate biopsy response from clinical benefit. Read both primary endpoints, their placebo rates, confidence intervals, and missing-biopsy handling. Review safety and interaction sections rather than only the efficacy table. Then check the status and design of the confirmatory requirement.

The durable question is not whether a surrogate moved. It is whether treatment helps the intended population avoid meaningful liver outcomes with acceptable harm over time.

References#

1. FDA approval announcement for resmetirom
2. FDA Drug Trials Snapshot for Rezdiffra
3. Current FDA prescribing information for Rezdiffra
4. MAESTRO-NASH randomized phase 3 report
5. FDA list of ongoing non-oncology accelerated approvals
6. MAESTRO-NASH trial record, NCT03900429