Rezdiffra (Resmetirom) Hispanic / Latino Documented Efficacy Gaps

Why Hispanic and Latino Patients Face a Different MASH Field

Hispanic and Latino adults in the United States develop metabolic dysfunction-associated steatotic liver disease at rates that exceed every other demographic group. This biological reality makes the efficacy question around resmetirom especially urgent for roughly 19% of the U.S. Population.

The Burden Is Not Hypothetical

Population-based studies using the National Health and Nutrition Examination Survey (NHANES) database show that MASLD prevalence among Hispanic adults reaches 29% to 33%, compared with 23% to 24% in non-Hispanic White adults 1. A 2023 meta-analysis in Hepatology confirmed that Mexican-American ethnicity carries the highest age-adjusted odds ratio for hepatic steatosis among all U.S. Subgroups 2. The overlap between MASLD and type 2 diabetes compounds this disparity. CDC surveillance data show that diagnosed diabetes prevalence among Hispanic adults is 12.5%, compared with 7.5% among non-Hispanic White adults 3. Insulin resistance, a driver of both conditions, tends to manifest earlier and at lower BMI thresholds in Hispanic populations 4.

Genetic Susceptibility Amplifies Risk

The PNPLA3 I148M (rs738409 C>G) variant occurs at an allele frequency of approximately 49% in Hispanic populations, compared with 23% in European-ancestry groups 5. Carriers of the GG genotype have a 3.2-fold increased risk of hepatic steatosis and more rapid fibrosis progression. A second variant, TM6SF2 E167K, also shows elevated frequency in Hispanic cohorts and independently predicts advanced fibrosis 6. These genetic risk factors mean that Hispanic patients who qualify for resmetirom may present with more aggressive underlying disease biology than the average trial participant.

What MAESTRO-NASH Showed (and Did Not Show)

The MAESTRO-NASH trial is the registration study that led to resmetirom's accelerated FDA approval in March 2024. It remains the primary source of efficacy data. The trial demonstrated clear benefit overall but left significant gaps in understanding how different populations respond.

Trial Design and Demographics

MAESTRO-NASH randomized 966 adults with biopsy-confirmed MASH and fibrosis stage F1B through F3 to resmetirom 80 mg, 100 mg, or placebo for 52 weeks 7. The dual primary endpoints were MASH resolution without fibrosis worsening and fibrosis improvement by at least one stage without MASH worsening. At the 100 mg dose, 29.9% of patients achieved MASH resolution (vs. 9.7% placebo), and 25.9% achieved fibrosis improvement (vs. 14.2% placebo) 7.

The Enrollment Gap

Published trial demographics report that the majority of participants were White and non-Hispanic. The FDA's multidisciplinary review for resmetirom noted limited representation of racial and ethnic minorities in the key trials 8. This underrepresentation is not unique to resmetirom. A systematic review in Clinical Gastroenterology and Hepatology found that across 53 NASH clinical trials from 2004 to 2021, Hispanic enrollment averaged just 16%, despite Hispanic adults comprising the highest-risk group 9. The Endocrine Society's 2023 clinical practice guideline on pharmacotherapy for MASLD flagged the need for diversity-enriched trials 10.

What the Subgroup Analyses Do (and Do Not) Tell Us

The MAESTRO-NASH publication includes prespecified subgroup analyses by sex, BMI, diabetes status, and fibrosis stage, but ethnicity-specific forest plots have not been published separately as peer-reviewed data 7. The FDA label does not include ethnicity-specific dose adjustments 8. This absence does not mean "no difference." It means the question has not been answered with adequate statistical power.

Pharmacogenomic Factors That May Modify Resmetirom Response

Resmetirom is a selective thyroid hormone receptor beta (THR-beta) agonist that reduces hepatic fat through activation of lipid metabolism pathways. Its pharmacokinetic profile involves metabolic steps that vary by ancestry.

CYP3A4 Metabolism and Hispanic Variant Frequencies

Resmetirom undergoes oxidative metabolism primarily through CYP3A4, with minor contributions from CYP2C8 8. CYP3A4 activity varies across populations. The CYP3A41B allele, associated with modestly altered enzyme expression, occurs at frequencies of 5% to 9% in Hispanic populations compared with 2% to 4% in European-ancestry groups 11. PharmGKB data catalog additional CYP3A4 and CYP3A5 variants with population-specific frequencies that may influence drug clearance 12. The CYP3A53 loss-of-function allele is common across all groups, but CYP3A5*1 (the active allele) is more frequent in populations with African and Amerindian admixture, including many Hispanic subpopulations 12.

Clinical Implications of Variable Metabolism

Higher CYP3A4/5 activity could theoretically increase resmetirom clearance, leading to lower steady-state plasma concentrations at a fixed dose. The FDA-approved dose range (80 mg for body weight <100 kg, 100 mg for weight ≥100 kg) is based on body weight, not genotype 8. No pharmacogenomic-guided dosing recommendations exist. Clinicians treating Hispanic patients should be aware that weight-based dosing alone may not capture interindividual variability driven by CYP genotype.

The PNPLA3 Question: Does Genotype Predict Drug Response?

Whether PNPLA3 I148M status modifies response to THR-beta agonism is an open question. PNPLA3-driven steatosis operates through a distinct mechanism (impaired lipid droplet remodeling) compared with THR-beta-mediated increases in mitochondrial fatty acid oxidation 13. A post-hoc analysis from a phase 2 resmetirom study showed that LDL-C reduction did not differ significantly by PNPLA3 genotype, but liver fat reduction was not stratified by this variant in published data 14. If PNPLA3 GG carriers (disproportionately Hispanic) derive less hepatic fat reduction from resmetirom, this would represent a clinically meaningful efficacy gap that current trial data cannot rule out.

Insulin Resistance, Diabetes, and the Hispanic Metabolic Phenotype

The interaction between resmetirom efficacy and metabolic comorbidities is relevant because Hispanic patients are more likely to present with concurrent insulin resistance and type 2 diabetes at the time of MASH diagnosis.

Diabetes Subgroup Data from MAESTRO-NASH

In the MAESTRO-NASH trial, patients with type 2 diabetes showed numerically lower rates of MASH resolution compared with those without diabetes, though the interaction was not statistically significant 7. Given that approximately 50% of Hispanic patients with MASH also carry a diabetes diagnosis 15, even a modest attenuation of efficacy in the diabetes subgroup would disproportionately affect Hispanic treatment outcomes.

HbA1c and Hepatic Fat: Linked Outcomes

Resmetirom lowered hepatic fat content by a mean of 7.2 percentage points at week 52 in the 100 mg group, measured by MRI-PDFF 7. A secondary analysis of MAESTRO-NASH data showed modest HbA1c reductions in diabetic participants, but whether this translated into differential histological benefit is unpublished 8. The American Diabetes Association's 2024 Standards of Care now mention resmetirom as a treatment option for patients with MASH and T2D, though without ethnicity-specific guidance 16.

Monitoring Recommendations for Hispanic Patients on Resmetirom

Given the gaps in ethnicity-specific data, a structured monitoring approach is warranted for Hispanic patients initiating resmetirom therapy. This is not a deviation from standard care. It is an adaptation to missing evidence.

Liver-Specific Monitoring

The AASLD practice guidance for MASLD recommends noninvasive assessment of fibrosis at baseline and during treatment 17. For Hispanic patients on resmetirom, consider obtaining baseline FibroScan (vibration-controlled transient elastography) or MRI-PDFF and repeating at 6 and 12 months. A failure to achieve at least a 30% relative reduction in MRI-PDFF by 12 months should prompt reassessment of dose adequacy or alternative diagnoses 17.

Thyroid Function Surveillance

Resmetirom's mechanism targets THR-beta selectively, sparing THR-alpha-mediated cardiac effects. The prescribing information requires TSH monitoring before initiation and periodically thereafter 8. Hispanic patients have higher background rates of thyroid autoimmunity (particularly Hashimoto's thyroiditis in women), which may complicate TSH interpretation during treatment 18. Baseline free T4 and TPO antibody measurement can help distinguish drug effect from underlying thyroid disease.

Lipid Panel Tracking

Resmetirom reduced LDL-C by approximately 13% to 16% in MAESTRO-NASH 7. This effect is relevant because Hispanic patients with MASH frequently have atherogenic dyslipidemia. Repeat fasting lipid panels at 3 and 6 months can confirm whether the expected LDL-C reduction is occurring. Absence of LDL-C lowering at 3 months may signal suboptimal drug exposure, potentially related to CYP3A4/5 rapid metabolism.

What Needs to Happen: The Evidence Gap Ahead

Three specific data needs stand between current practice and confident use of resmetirom in Hispanic patients.

Post-Marketing Diversity Requirements

The FDA's accelerated approval of resmetirom includes a requirement for confirmatory trials 8. Advocacy groups and the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) have called for these trials to include pre-specified enrollment targets for Hispanic participants 19. The MAESTRO-NASH-OUTCOMES trial (NCT06256900), evaluating long-term hepatic outcomes, represents the next opportunity to generate ethnicity-stratified data.

Pharmacogenomic Substudy Design

A dedicated pharmacokinetic substudy in Hispanic participants, genotyped for CYP3A4, CYP3A5, CYP2C8, and PNPLA3 variants, could resolve whether dose adjustments are needed. The FDA's 2022 guidance on pharmacogenomic data in drug labeling provides a framework for such studies 20.

Real-World Evidence Collection

Until prospective trial data arrive, real-world registries tracking resmetirom outcomes by self-reported ethnicity and ancestry would provide early signals. The TARGET-NASH observational study has collected data on MASH treatment patterns across diverse populations and could serve as a platform for post-marketing surveillance 21.

Practical Prescribing Points for Clinicians

Hispanic patients qualifying for resmetirom (biopsy-confirmed MASH, fibrosis F2-F3) should receive the standard weight-based dose: 80 mg daily for body weight <100 kg, 100 mg for ≥100 kg. There is no current evidence supporting ethnicity-based dose modification. The following adjustments to standard workflow are reasonable:

Obtain baseline MRI-PDFF or FibroScan, TSH, free T4, TPO antibodies, and fasting lipid panel before starting therapy. Repeat MRI-PDFF at 6 months; if hepatic fat reduction is <30% relative from baseline, consider checking resmetirom trough levels if available through specialty labs. Screen for CYP3A4 inducers (rifampin, carbamazepine, phenytoin) that could reduce drug exposure. Document PNPLA3 genotype if available from prior testing, as this may inform future dosing guidelines once data mature.

Resmetirom prescribing should not be delayed in Hispanic patients based on the absence of ethnicity-specific trial data. The drug met its primary endpoints across the enrolled population, and withholding it from a group at highest MASH risk would widen existing treatment disparities. Monitor closely, report outcomes to registries, and advocate for inclusive confirmatory trials.

The AASLD recommends repeat noninvasive fibrosis assessment at 12 months for all patients on MASH-directed pharmacotherapy 17. For Hispanic patients, this benchmark should be treated as mandatory, not optional.