Rezdiffra (Resmetirom): History, Development, and How It Works

The Decades-Long Search for a MASH Drug

For more than 20 years, MASH (formerly called NASH, or non-alcoholic steatohepatitis) stood as one of the largest unmet needs in hepatology. No drug carried an FDA indication for the disease. Patients progressed from fat accumulation and liver inflammation to fibrosis and, in some cases, cirrhosis or hepatocellular carcinoma with no approved pharmacotherapy to alter that trajectory.

By 2020, an estimated 16 million adults in the United States had MASH, and the global prevalence of its precursor, metabolic dysfunction-associated steatotic liver disease (MASLD), exceeded 30% of the adult population. Multiple drug candidates failed in phase 3 trials during the 2010s. Obeticholic acid (Intercept Pharmaceuticals) received a Complete Response Letter from the FDA in June 2020 after the REGENERATE trial raised benefit-risk concerns, including pruritus in nearly 50% of patients and unfavorable LDL cholesterol changes [2]. Elafibranor (Genfit) missed its primary endpoint in the RESOLVE-IT trial in May 2020 [3]. Selonsertib (Gilead) failed two phase 3 studies in 2019. Each setback reinforced the difficulty of the target.

That backdrop explains why Madrigal's resmetirom program attracted intense attention when it advanced past phase 2. The drug took a different pharmacological approach entirely.

Origins of Thyroid Hormone Receptor Research

The scientific rationale for resmetirom stretches back to observations in the 1990s about thyroid hormone and hepatic lipid metabolism. Thyroid hormones accelerate fatty acid oxidation in the liver, reduce hepatic triglyceride content, and lower LDL cholesterol through upregulation of LDL receptor expression. Hypothyroid patients develop hepatic steatosis at high rates. The problem was selectivity. Systemic thyroid hormone excess causes tachycardia, bone loss, and muscle wasting through activation of thyroid hormone receptor alpha (THR-α) in cardiac and skeletal tissues.

Research teams at institutions including the University of California, San Francisco and later at companies such as KaroBio AB explored whether a molecule could be designed to preferentially activate THR-β (concentrated in the liver) while sparing THR-α (dominant in the heart and bone). Early work published in the Proceedings of the National Academy of Sciences confirmed that THR-β selectivity was structurally achievable, and several first-generation compounds entered preclinical development.

VIA Pharmaceuticals (later acquired by Madrigal Pharmaceuticals) synthesized MGL-3196, the compound now known as resmetirom, in the early 2010s. The molecule demonstrated approximately 28-fold selectivity for THR-β over THR-α in binding assays.

Madrigal Pharmaceuticals and the Resmetirom Program

Madrigal Pharmaceuticals, a small clinical-stage biopharmaceutical company based in West Conshohocken, Pennsylvania, acquired VIA Pharmaceuticals in 2016 and inherited the MGL-3196 asset. At the time, Madrigal's market capitalization was modest, and the MASH field was littered with clinical failures.

CEO Paul Friedman, M.D., had prior experience with thyroid hormone biology from his research career. Under his leadership, Madrigal redesigned the clinical development strategy around liver histology endpoints rather than relying solely on imaging or serum biomarkers. That decision proved important. The FDA had signaled through its 2018 draft guidance on NASH drug development that it would consider accelerated approval based on histological improvement (NASH resolution without worsening fibrosis, or fibrosis improvement without worsening NASH) at an interim biopsy, with a requirement for confirmatory outcomes data [4].

Resmetirom entered its first human study in 2013. A phase 1 trial established safety, tolerability, and pharmacokinetics in healthy volunteers. No clinically significant cardiac effects were observed at therapeutic doses, supporting the THR-β selectivity hypothesis.

Phase 2: Proof of Concept

The phase 2 trial (NCT02912260) randomized 125 patients with biopsy-confirmed NASH to resmetirom 80 mg daily or placebo for 36 weeks. Results published in The Lancet Gastroenterology & Hepatology in 2019 showed a statistically significant reduction in hepatic fat fraction measured by MRI-proton density fat fraction (MRI-PDFF). The resmetirom group achieved a relative reduction of approximately 36% in hepatic fat at week 12 compared to 9% with placebo (P<0.001).

Secondary endpoints were encouraging. LDL cholesterol fell by roughly 12% in the resmetirom arm. Liver enzyme levels (ALT and AST) improved. Adverse events were mild and predominantly gastrointestinal: diarrhea and nausea occurred more frequently with resmetirom than placebo but rarely led to discontinuation.

The trial was not powered for histological endpoints, but an optional week-36 biopsy substudy showed numerical improvements in steatohepatitis activity scores. These signals, combined with the biomarker data, justified advancement to phase 3.

MAESTRO-NASH: The Key Phase 3 Trial

MAESTRO-NASH (NCT03900429) was a randomized, double-blind, placebo-controlled trial enrolling 966 adults with biopsy-confirmed NASH and liver fibrosis stage F1B, F2, or F3. Patients were randomized 1:1:1 to resmetirom 80 mg, resmetirom 100 mg, or placebo, taken once daily. The trial was conducted across 200 sites in 14 countries [1].

The dual primary endpoints at the 52-week interim biopsy were:

1. NASH resolution with no worsening of fibrosis (defined as a NAFLD Activity Score of 0 or 1 for both lobular inflammation and hepatocellular ballooning, with at least a 2-point reduction in NAS from baseline).
2. At least one stage of fibrosis improvement with no worsening of NASH (defined as a reduction in NAS of at least 1 point for either lobular inflammation or ballooning).

Results published in the New England Journal of Medicine in February 2024 demonstrated that both primary endpoints were met. For NASH resolution: 25.9% in the 80 mg group and 29.9% in the 100 mg group achieved this endpoint, compared with 9.7% in the placebo group (P<0.001 for both comparisons). For fibrosis improvement: 24.2% (80 mg) and 25.9% (100 mg) met the endpoint, versus 14.2% with placebo (P<0.002 for both) [1].

The response rates may appear modest in absolute terms, but they are clinically meaningful in context. Prior to MAESTRO-NASH, no randomized trial had demonstrated this degree of histological improvement with an acceptable safety profile. The placebo response rate for NASH resolution (9.7%) reflects the natural variability in serial liver biopsies and regression to the mean.

Safety Profile in MAESTRO-NASH

The adverse event profile was generally consistent with the phase 2 findings. Diarrhea occurred in approximately 27% of patients on resmetirom 100 mg versus 16% on placebo, but most episodes were mild and transient. Nausea affected about 19% on the 100 mg dose versus 12% on placebo.

A signal of interest was the occurrence of drug-induced liver injury. One case of serious DILI was reported in the resmetirom 100 mg group. The FDA label includes a warning about hepatotoxicity, and the REMS program requires liver function monitoring. Thyroid function tests remained stable, and there were no excess cardiovascular adverse events, bone fractures, or muscle-related events compared to placebo [1].

The MAESTRO Clinical Program: Beyond MAESTRO-NASH

Madrigal ran three phase 3 studies under the MAESTRO umbrella:

MAESTRO-NASH (described above) provided the histological efficacy data for accelerated approval.

MAESTRO-NAFLD-1 (NCT04197479) was a 52-week safety study enrolling approximately 700 patients with NAFLD (not all biopsy-confirmed NASH). It was designed to expand the safety database, as FDA guidance recommends exposure data in at least 1,500 patients for chronic-use drugs. Results confirmed the safety profile seen in MAESTRO-NASH, with no new safety signals.

MAESTRO-NAFLD-Open-Label-Extension allowed patients from MAESTRO-NASH to continue on resmetirom for an extended period. The 54-month biopsy data from this extension will serve as the confirmatory evidence required to convert the accelerated approval to full (traditional) approval. The FDA set an expected completion date of 2028 for this confirmatory endpoint.

FDA Accelerated Approval: March 14, 2024

On March 14, 2024, the FDA approved Rezdiffra (resmetirom) for the treatment of adults with noncirrhotic NASH with moderate to advanced liver fibrosis (consistent with stages F2 to F3), in conjunction with diet and exercise. This represented a watershed moment in hepatology. No prior drug had received an FDA indication for MASH or its predecessor diagnosis, NASH.

The FDA approval letter and prescribing information specified two dosage strengths: 80 mg and 100 mg once daily, with dosing based on body weight. Patients weighing less than 100 kg receive 80 mg; those at or above 100 kg receive 100 mg. Both are taken in the morning, at least 30 minutes before the first meal.

The approval came through the accelerated pathway, meaning it was based on the surrogate histological endpoints rather than long-term clinical outcomes such as progression to cirrhosis, liver transplantation, or death. Madrigal is required to complete the confirmatory trial demonstrating clinical benefit; failure to do so could result in withdrawal of the approval.

The FDA Advisory Committee had voted 11-1 in favor of approval at its November 2023 meeting, reflecting strong consensus that the histological improvements were reasonably likely to predict clinical benefit.

How Resmetirom Works: Mechanism of Action

Resmetirom is a liver-directed, orally active, selective agonist of thyroid hormone receptor beta (THR-β). Understanding the mechanism requires context on thyroid hormone receptor biology.

Two main thyroid hormone receptor isoforms exist: THR-α and THR-β. THR-α is expressed predominantly in the heart, bone, and central nervous system. THR-β is expressed predominantly in the liver, and to a lesser extent in the pituitary and kidney. Natural thyroid hormones (T3 and T4) activate both receptor types. The clinical effects of systemic thyroid hormone excess (hyperthyroidism) including tachycardia, atrial fibrillation, osteoporosis, and anxiety are mediated largely through THR-α [5].

Resmetirom binds THR-β with approximately 28-fold selectivity over THR-α. After oral absorption, it concentrates in hepatocytes through first-pass hepatic uptake, further enhancing liver selectivity. Inside the hepatocyte nucleus, resmetirom-bound THR-β activates transcription of genes involved in:

• Mitochondrial fatty acid β-oxidation: increasing the rate at which hepatocytes burn stored triglycerides for energy, directly reducing steatosis.
• Lipid export pathways: activating genes such as CPT1A (carnitine palmitoyltransferase 1A) that shuttle fatty acids into mitochondria.
• LDL receptor upregulation: increasing clearance of LDL cholesterol from the bloodstream, producing a secondary cardiovascular benefit.
• Reduction of lipotoxic intermediates: by clearing excess hepatic fat, resmetirom reduces the production of toxic lipid metabolites (diacylglycerols, ceramides) that drive hepatocyte injury, inflammation, and fibrogenesis.

The net pharmacological effect is a reduction in the substrate that drives MASH pathology. Less hepatic fat means less lipotoxicity, less inflammation, less hepatocyte ballooning, and over time, less stimulus for stellate cell activation and fibrosis progression.

"Resmetirom addresses the upstream metabolic driver of MASH rather than targeting downstream inflammatory or fibrotic pathways," stated Dr. Stephen Harrison, gastroenterologist and principal investigator at Pinnacle Clinical Research, in commentary accompanying the MAESTRO-NASH publication [1].

What Makes Resmetirom Different from Failed MASH Drugs

Several features distinguish resmetirom from the MASH drug candidates that did not succeed.

The mechanism is metabolic, not anti-inflammatory or anti-fibrotic. Obeticholic acid (FXR agonist), cenicriviroc (CCR2/5 antagonist), and selonsertib (ASK1 inhibitor) all targeted downstream pathways in the inflammatory-fibrotic cascade. Resmetirom addresses the root metabolic derangement: excess hepatic triglyceride accumulation and impaired mitochondrial fat oxidation [6].

The safety profile favors chronic use. Obeticholic acid caused pruritus in 51% of patients at 25 mg and raised LDL cholesterol, a problematic effect in a patient population already at high cardiovascular risk. Resmetirom lowers LDL cholesterol by approximately 14% to 22% and does not cause pruritus [1].

The oral, once-daily dosing aligns with real-world adherence patterns for a chronic metabolic disease. Injectable therapies or complex dosing regimens would face adoption barriers in a population that already manages multiple cardiometabolic conditions.

The Road Ahead: Confirmatory Data and Expanded Indications

Madrigal Pharmaceuticals is conducting the ongoing MAESTRO-NASH 54-month biopsy phase to generate the clinical outcomes data required for full FDA approval. The company expects top-line results from this confirmatory analysis by 2028.

Several open clinical questions remain. Whether resmetirom prevents progression to cirrhosis, reduces hepatocellular carcinoma incidence, or decreases liver-related mortality has not yet been proven in a randomized trial. The accelerated approval was granted on the basis of histological surrogate endpoints, and the medical community awaits the longer-term data.

Combination therapy is an active area of investigation. The GLP-1 receptor agonist semaglutide showed NASH resolution rates of 59% at 72 weeks in the phase 2 trial by Newsome et al. (N=320), though it did not significantly improve fibrosis [7]. Researchers are examining whether combining a THR-β agonist (addressing hepatic fat metabolism) with a GLP-1 agonist (addressing systemic metabolic dysfunction and body weight) could produce additive or complementary histological benefits. No phase 3 combination data exist yet.

"The approval of resmetirom opens the door to rational combination regimens for MASH, much as combination therapy transformed HIV and hepatitis C treatment," noted Dr. Rohit Loomba, Director of the MASLD Research Center at the University of California San Diego, in a 2024 editorial in Hepatology.

The commercial launch of Rezdiffra began in April 2024. Madrigal reported a wholesale acquisition cost of approximately $47,400 per year. Insurance coverage and prior authorization requirements vary, and many commercial payers initially required documentation of biopsy-confirmed MASH with F2-F3 fibrosis, consistent with the FDA label.

Resmetirom's 2024 approval ended a decades-long drought in MASH drug development. Whether it becomes the standard of care or the foundation of future combination regimens depends on the confirmatory trial data expected within the next two years.