Rezdiffra (Resmetirom) vs Actos (Pioglitazone): Head-to-Head Efficacy for MASH

Why These Two Drugs Get Compared

Clinicians treating metabolic dysfunction-associated steatohepatitis (MASH, formerly NASH) have operated for over a decade with no approved pharmacotherapy. Pioglitazone filled part of that gap as the most studied off-label option, backed by the 2010 PIVENS trial [1]. Resmetirom changed the field in March 2024 when the FDA granted it accelerated approval as the first drug specifically indicated for noncirrhotic MASH with moderate to advanced fibrosis (stages F2-F3) [2].

No randomized trial has directly compared these two agents. Any cross-trial comparison requires caution: patient populations, endpoints, treatment durations, and histological scoring methods differ between MAESTRO-NASH and PIVENS. The comparison below synthesizes the best available data from each program so clinicians and patients can weigh the trade-offs. The American Association for the Study of Liver Diseases (AASLD) 2023 practice guidance lists both agents among pharmacotherapies with evidence supporting use in biopsy-confirmed MASH, though only resmetirom carries an FDA-approved MASH indication [3].

Mechanism of Action: Two Distinct Pathways

Resmetirom activates the thyroid hormone receptor beta (THR-beta), which is predominantly expressed in hepatocytes. THR-beta activation increases hepatic fatty acid oxidation, reduces de novo lipogenesis, and lowers circulating levels of atherogenic lipoproteins [2]. The drug was designed for liver selectivity, sparing the THR-alpha receptor responsible for cardiac and bone effects.

Pioglitazone works through a different axis entirely. As a PPAR-gamma agonist, it improves peripheral insulin sensitivity in adipose tissue and skeletal muscle, which indirectly reduces hepatic fat delivery and inflammation [4]. Its metabolic benefits extend to glucose control, which is why it remains a mainstay for type 2 diabetes. The downstream hepatic effect is real but secondary to the systemic insulin-sensitizing action.

This mechanistic divergence matters clinically. Resmetirom targets the liver directly. Pioglitazone reshapes the metabolic environment that drives liver injury. Both reduce steatohepatitis, but through fundamentally different biology, raising the question of whether combination therapy could one day prove additive.

MAESTRO-NASH: The Resmetirom Key Trial

The MAESTRO-NASH trial (N=966) randomized adults with biopsy-confirmed MASH and fibrosis stages F1B through F3 to resmetirom 80 mg, resmetirom 100 mg, or placebo for 52 weeks [2]. Two co-primary endpoints defined success: MASH resolution with no worsening of fibrosis, and fibrosis improvement by at least one stage with no worsening of the NAFLD Activity Score (NAS).

Results at 52 weeks were statistically significant for both doses on both endpoints. For MASH resolution, 25.9% of the 80 mg group and 29.9% of the 100 mg group met the endpoint, compared with 9.7% on placebo (P<0.001 for both comparisons) [2]. For fibrosis improvement, rates were 24.2% (80 mg) and 25.9% (100 mg) versus 14.2% for placebo (P<0.01 and P<0.001, respectively).

Beyond histology, resmetirom produced meaningful reductions in LDL cholesterol (approximately 14-16%), triglycerides, and liver enzymes (ALT). Patients on resmetirom also experienced modest weight reduction rather than gain, a finding that differentiates it sharply from pioglitazone. The most common adverse events were diarrhea and nausea, mostly mild and concentrated in the first weeks of dosing.

PIVENS: The Pioglitazone Landmark Trial

The PIVENS trial (N=247) tested pioglitazone 30 mg, vitamin E 800 IU, or placebo over 96 weeks in adults with biopsy-confirmed NASH who did not have diabetes [1]. The primary endpoint was a composite improvement in the NAS by at least 2 points, with improvement in at least two components and no worsening of fibrosis.

Pioglitazone did not meet the primary composite endpoint (34% vs 19% placebo, P=0.04 against a prespecified significance threshold of P<0.025 due to two active comparisons) [1]. It did, however, meet the prespecified secondary endpoint of NASH resolution: 47% of pioglitazone-treated patients achieved resolution versus 22% on placebo (P<0.001). Vitamin E also showed benefit on the primary endpoint (43% vs 19%, P=0.001).

Fibrosis improvement was not significant with pioglitazone in PIVENS. Weight gain averaged 4.7 kg over 96 weeks, and this remains the single largest deterrent for many patients and prescribers. Dr. Arun Sanyal, who led the PIVENS trial, noted that "pioglitazone's insulin-sensitizing effects address a root driver of NASH, but the weight gain and fluid retention limit its use in the exact population that needs it most" [1].

Cross-Trial Efficacy: Interpreting Different Numbers

Comparing 29.9% MASH resolution (resmetirom 100 mg, 52 weeks) to 47% NASH resolution (pioglitazone, 96 weeks) requires context. These numbers cannot be directly compared for several reasons.

First, treatment duration differed. PIVENS ran 96 weeks; MAESTRO-NASH reported the 52-week primary analysis. Longer treatment duration with resmetirom may yield higher response rates, and the ongoing MAESTRO-NASH extension will clarify this. Second, the PIVENS population excluded patients with diabetes, while MAESTRO-NASH included them (approximately 60% had type 2 diabetes). Patients with diabetes tend to have more treatment-resistant disease, which could lower observed response rates. Third, histological scoring criteria and central pathology reading protocols evolved between 2010 and 2024.

The AASLD practice guidance acknowledges these cross-trial limitations and does not rank one agent above the other based on resolution rates alone [3]. What can be stated is that both drugs produce histologically meaningful improvements over placebo, and resmetirom is the only one that has demonstrated significant fibrosis improvement in a phase III trial.

Fibrosis: The Endpoint That Matters Most

Liver fibrosis stage is the strongest predictor of liver-related mortality, transplantation, and decompensation events in MASH. A meta-analysis of paired-biopsy studies (N=4,428) published in Hepatology found that each one-stage increase in fibrosis was associated with a 58% increase in all-cause mortality [5].

Resmetirom's fibrosis improvement data (25.9% at 100 mg vs 14.2% placebo) represent a roughly 12-percentage-point absolute benefit, which is clinically meaningful given the natural history of MASH fibrosis progression [2]. Pioglitazone has not demonstrated statistically significant fibrosis improvement in any large randomized trial, including PIVENS [1]. Smaller studies and post-hoc analyses have suggested possible antifibrotic effects, but the evidence remains inconsistent.

This distinction carries weight. For patients with F2-F3 fibrosis who are at genuine risk of progressing to cirrhosis, resmetirom offers evidence of fibrosis regression that pioglitazone does not. For patients with earlier-stage disease or predominant steatohepatitis without advanced fibrosis, pioglitazone's resolution data and low cost may still hold value, particularly in the setting of coexisting type 2 diabetes where pioglitazone provides dual benefit.

Safety and Tolerability

Resmetirom's safety profile in MAESTRO-NASH was generally favorable. Diarrhea occurred in 27% of the 100 mg group versus 16% on placebo, and nausea in 22% versus 13% [2]. These gastrointestinal effects were predominantly grade 1, transient, and rarely led to discontinuation. Gallbladder-related events occurred at slightly higher rates. The FDA label includes monitoring recommendations for thyroid function and hepatic safety markers. No signals of cardiac arrhythmia, bone loss, or muscle toxicity emerged.

Pioglitazone's side-effect profile is well-characterized over two decades of post-market experience. Weight gain (mean 4.7 kg in PIVENS), peripheral edema, and increased fracture risk (particularly in postmenopausal women) are established class effects [1]. The FDA added a black-box warning for congestive heart failure exacerbation. An association with bladder cancer prompted regulatory review, though the 2016 FDA safety communication concluded that the evidence was inconclusive [6]. Pioglitazone's cardiovascular profile is otherwise favorable; the PROactive trial (N=5,238) showed a 16% relative reduction in the composite of all-cause mortality, MI, and stroke [4].

Dr. Zobair Younossi, chair of the Global NASH Council, has stated: "The tolerability advantage of resmetirom is notable because MASH patients often have multiple comorbidities, and adding a drug that causes weight gain and fluid retention to an already metabolically burdened patient is a difficult clinical proposition" [7].

Cost and Access

The price gap between these two drugs is enormous. Rezdiffra carries a wholesale acquisition cost exceeding $47,000 per year [8]. Generic pioglitazone is available for under $30 per month at most pharmacies, making it one of the least expensive options in hepatology.

Insurance coverage for Rezdiffra requires prior authorization, typically including documentation of liver biopsy showing MASH with F2-F3 fibrosis. Many payers have imposed step-therapy requirements. Pioglitazone, by contrast, is covered on virtually all formularies for its approved diabetes indication, and off-label use for MASH is generally not subject to the same access barriers, though formal coverage for the MASH indication is not standard.

For uninsured or underinsured patients, this cost differential often makes the decision. The clinical question then becomes whether the fibrosis benefit of resmetirom justifies the price premium over a drug that achieves comparable steatohepatitis resolution at a fraction of the cost.

Which Patients Benefit From Which Drug

Selecting between these agents depends on fibrosis stage, diabetes status, body composition, cardiovascular risk, and access.

Resmetirom fits best for patients with biopsy-confirmed MASH and F2-F3 fibrosis who need documented fibrosis regression. It is the only agent with an FDA-approved indication for this population and the only one with phase III fibrosis improvement data. Its lipid-lowering and weight-neutral profile adds value in patients with atherogenic dyslipidemia.

Pioglitazone remains a reasonable choice for patients with MASH and coexisting type 2 diabetes, particularly those with earlier fibrosis (F0-F1) where the fibrosis question is less urgent. Its insulin-sensitizing mechanism addresses a root metabolic driver, and its cardiovascular data from PROactive provide reassurance in a high-risk population [4]. Patients who cannot tolerate or access Rezdiffra may also benefit, given pioglitazone's strong NASH resolution data.

Patients who are already overweight with class II or III obesity may tolerate pioglitazone-related weight gain poorly. Those with heart failure (NYHA class III-IV) should not receive pioglitazone. Women with osteoporosis or high fracture risk should also approach pioglitazone with caution.

The Combination Question

No published trial has tested resmetirom and pioglitazone together. The mechanistic rationale for combination therapy is plausible: resmetirom increases hepatic fat oxidation while pioglitazone improves peripheral insulin sensitivity and redirects lipid flux. Whether additive histological benefit would result, and whether it would justify the cost and side-effect burden of dual therapy, remains unknown.

The ongoing MAESTRO-NAFLD and post-marketing studies may provide subgroup data on patients who were concurrently taking pioglitazone, which could offer preliminary signals. Until prospective combination data exist, co-prescribing is a clinician judgment call rather than an evidence-based recommendation.

Monitoring and Follow-Up

Both agents require baseline and periodic liver enzyme monitoring. For resmetirom, the FDA label recommends hepatic function tests before initiation and periodically thereafter [8]. Thyroid function should be assessed at baseline. For pioglitazone, liver enzymes should be checked before starting and periodically during treatment, along with monitoring for signs of heart failure and periodic bone density assessment in at-risk populations [6].

Repeat liver biopsy at 12-18 months is recommended by the AASLD to assess histological response to either therapy, though noninvasive markers such as FibroScan (vibration-controlled transient elastography), FIB-4 index, and Enhanced Liver Fibrosis (ELF) score are increasingly used as interim surrogates [3]. A rising FIB-4 or worsening FibroScan score on therapy should prompt biopsy consideration and potential treatment reassessment.

Patients started on resmetirom 80 mg should be assessed at 12 weeks for tolerability before dose escalation to 100 mg, per the approved dosing schedule [8].