Rezdiffra (Resmetirom) Regulatory Status: US, EU, Canada, and UK

United States: First-in-Class Accelerated Approval

The FDA granted Rezdiffra accelerated approval on March 14, 2024, making resmetirom the first therapy ever approved specifically for MASH [1]. The approval covers adults with non-cirrhotic MASH and moderate-to-advanced liver fibrosis (stages F2 and F3), based on a surrogate endpoint of histological improvement rather than long-term clinical outcomes like cirrhosis prevention.

The MAESTRO-NASH Trial That Drove Approval

The accelerated approval rested primarily on the MAESTRO-NASH phase 3 trial, a randomized, double-blind, placebo-controlled study enrolling 966 patients with biopsy-confirmed MASH and fibrosis stages F1B through F3 [1]. At 52 weeks, 25.9% of patients receiving resmetirom 80 mg and 29.9% receiving 100 mg achieved MASH resolution with no worsening of fibrosis, compared with 9.7% on placebo [1]. Fibrosis improvement by at least one stage (with no worsening of NAFLD activity score) occurred in 24.2% on 80 mg and 25.9% on 100 mg, versus 14.2% on placebo [1].

Surrogate Endpoints and What Full Approval Requires

The FDA used its accelerated pathway because MASH resolution and fibrosis improvement on biopsy are reasonably likely to predict clinical benefit but have not yet been formally validated as surrogate endpoints for outcomes like transplant-free survival. Madrigal must submit 54-month clinical outcomes data from the ongoing MAESTRO-NASH extension to convert the accelerated approval to traditional (full) approval. If those data do not confirm benefit, the FDA can initiate proceedings to withdraw the drug from the market under 21 CFR 314.530 [2].

Prescribing and Access in the US

Rezdiffra launched commercially in the US in April 2024. The wholesale acquisition cost was set at approximately $47,400 per year. Weight-based dosing applies: patients weighing less than 100 kg receive 80 mg once daily, while those at or above 100 kg receive 100 mg once daily. The FDA label carries warnings for drug-induced liver injury, gallbladder-related adverse events, and drug interactions with strong CYP2C8 inhibitors [2].

European Union: EMA Review in Progress

The European Medicines Agency accepted Madrigal's marketing authorization application (MAA) for resmetirom, with the Committee for Medicinal Products for Human Use (CHMP) conducting its assessment. No conditional or full marketing authorization has been granted in the EU as of May 2026.

Key Differences in the European Regulatory Pathway

The EMA does not have a direct equivalent of the FDA's accelerated approval. Two pathways could apply: conditional marketing authorization (CMA), which permits approval based on less-than-complete data with an obligation to submit confirmatory evidence, and standard authorization. A CMA would mirror the FDA's approach by granting market access while awaiting long-term outcomes data. The CHMP's assessment will weigh the same MAESTRO-NASH biopsy endpoints the FDA reviewed, but European regulators have historically applied stricter evidentiary thresholds for liver disease endpoints.

Timeline Considerations for EU Access

European marketing authorization reviews typically take 12 to 18 months from validation of the application. Pricing and reimbursement negotiations occur at the national level after EMA approval, adding 6 to 24 additional months before patients in individual EU member states gain practical access. Germany's AMNOG process and France's HAS transparency commission evaluations will each independently assess added therapeutic benefit.

Canada: Health Canada Submission Under Review

Madrigal Pharmaceuticals filed a New Drug Submission (NDS) with Health Canada for resmetirom. The review is ongoing, with no Notice of Compliance (NOC) or NOC with conditions (NOC/c) issued as of mid-2026.

How the Canadian Review Differs

Health Canada's NOC/c pathway functions similarly to the FDA's accelerated approval, permitting market entry based on promising evidence with mandatory post-market confirmatory studies [3]. If resmetirom receives an NOC/c, Madrigal would need to fulfill conditions including submission of the MAESTRO-NASH long-term outcomes data. The standard Health Canada review target is 300 days for priority submissions, though actual timelines vary.

CADTH and Provincial Formulary Implications

Even after Health Canada approval, Canadian patients face a second gate: the Canadian Agency for Drugs and Technologies in Health (CADTH) must issue a positive reimbursement recommendation before provincial formularies will cover the drug. Given resmetirom's anticipated annual cost and the histology-based efficacy evidence, CADTH's Canadian Drug Expert Committee (CDEC) will likely scrutinize the cost-effectiveness ratio relative to the surrogate endpoint uncertainty. Patients with private insurance may gain access sooner than those on public drug plans.

United Kingdom: MHRA and NICE Pathway

The UK Medicines and Healthcare products Regulatory Agency (MHRA) has not granted marketing authorization for resmetirom as of May 2026. Post-Brexit, the MHRA operates independently from the EMA, though it can use international recognition pathways to expedite reviews of drugs already approved by trusted regulators.

The ILAP Pathway

The MHRA's Innovative Licensing and Access Pathway (ILAP) allows manufacturers to request an Innovation Passport for drugs addressing unmet medical needs. Whether Madrigal has pursued an ILAP designation for resmetirom has not been publicly confirmed. An Innovation Passport would grant access to a target development profile and rolling regulatory engagement, potentially shortening the time to marketing authorization.

NICE Appraisal and NHS Access

Approval by the MHRA alone does not guarantee NHS coverage. The National Institute for Health and Care Excellence (NICE) must conduct a technology appraisal to determine cost-effectiveness. NICE typically applies a willingness-to-pay threshold of £20,000 to £30,000 per quality-adjusted life year (QALY). Because MASH progression to cirrhosis occurs over decades, modeling the QALY gain from a histological surrogate endpoint introduces substantial uncertainty. NICE may require a managed access agreement or patient access scheme to offset this uncertainty.

How Resmetirom Works: Mechanism of Action

Resmetirom is a selective agonist of thyroid hormone receptor beta (THR-beta), the predominant thyroid hormone receptor isoform in hepatocytes [4]. THR-beta activation in the liver promotes mitochondrial fatty acid beta-oxidation and reduces hepatic de novo lipogenesis, directly addressing the lipotoxic environment that drives MASH progression.

Why Selectivity for THR-Beta Matters

Non-selective thyroid hormone receptor activation (hitting both THR-alpha and THR-beta) would cause cardiac effects including tachycardia and bone loss, effects mediated primarily by THR-alpha in the heart and skeleton. Resmetirom's approximately 28-fold selectivity for THR-beta over THR-alpha allows hepatic metabolic correction without clinically meaningful cardiac or skeletal toxicity at therapeutic doses [4]. In the MAESTRO-NASH trial, heart rate increases were minimal and bone mineral density remained stable over 52 weeks [1].

Effects Beyond the Liver

THR-beta agonism with resmetirom also reduced LDL cholesterol by approximately 14% to 16%, apolipoprotein B, and triglycerides in the MAESTRO-NASH population [1]. These lipid effects are consistent with THR-beta's known role in hepatic LDL receptor upregulation and VLDL clearance. Whether these lipid changes translate into cardiovascular risk reduction in the MASH population remains an open question that the ongoing extension study may help answer.

Safety Profile Across Regulatory Submissions

The safety database regulators are reviewing draws primarily from MAESTRO-NASH and earlier phase 2 studies. In MAESTRO-NASH, the most common adverse events with resmetirom included diarrhea (27% with 100 mg vs. 16% placebo) and nausea (22% vs. 13%) [1]. Most gastrointestinal events were mild to moderate and occurred early in treatment.

Liver Safety Monitoring

The FDA label includes a warning about drug-induced liver injury (DILI). In clinical trials, ALT elevations exceeding 5 times the upper limit of normal occurred in a small number of resmetirom-treated patients [2]. The FDA recommends hepatic function testing before initiation, during dose titration, and periodically thereafter. European and Canadian regulators are likely to impose similar or stricter monitoring requirements given the hepatotoxic signal in a drug targeting liver disease.

Drug Interaction Profile

Resmetirom is a substrate of CYP2C8 and CYP3A4. Co-administration with gemfibrozil (a strong CYP2C8 inhibitor) is contraindicated because it roughly doubles resmetirom exposure [2]. Statins, which many MASH patients take, are permitted but warrant monitoring given the additive LDL-lowering effect. Regulators in each jurisdiction will evaluate whether additional interaction contraindications are necessary based on local prescribing patterns.

The Unmet Need Driving Global Review

MASH affects an estimated 3% to 5% of the global adult population, with prevalence increasing in parallel with obesity and type 2 diabetes [5]. Before resmetirom's FDA approval, no pharmacotherapy carried a MASH-specific indication. Patients received off-label pioglitazone or vitamin E with limited and inconsistent histological benefit.

Fibrosis Stage Restriction and Future Expansion

All current regulatory filings restrict resmetirom to non-cirrhotic MASH with F2-F3 fibrosis. Patients with F1 fibrosis (the largest MASH population by number) and those with compensated cirrhosis (F4) fall outside the current indication. Madrigal has indicated interest in label expansion, but additional trial data in these populations would be required for any regulator to broaden the indication.

Competitive Field

Several other MASH drugs are in late-stage development, including semaglutide (Novo Nordisk's ESSENCE trial showed MASH resolution in 62.9% of patients at 72 weeks vs. 34.7% on placebo) [6], survodutide, and combinations targeting FXR and other pathways. Regulatory agencies in Europe, Canada, and the UK may weigh resmetirom's benefit-risk profile against these emerging competitors when setting reimbursement terms, even if approval decisions themselves are drug-specific.

Tracking Regulatory Milestones

For physicians and patients monitoring resmetirom's global rollout, the key milestones to watch include: the CHMP's opinion on the EU marketing authorization application (expected within the standard review timeline), Health Canada's NOC or NOC/c decision, any MHRA Innovation Passport or marketing authorization filing, and Madrigal's submission of the MAESTRO-NASH 54-month confirmatory data to the FDA. Each of these events will shift the access picture for the estimated 115 million adults living with MASH worldwide [5].

The FDA's post-marketing requirement specifies that Madrigal must submit confirmatory clinical outcomes data. If the 54-month results demonstrate that histological improvements on resmetirom translate into reduced progression to cirrhosis, liver transplant, or liver-related mortality, full approval in the US and positive reimbursement decisions in other jurisdictions become substantially more likely. If outcomes data disappoint, regulatory agencies may narrow, condition, or revoke existing authorizations.