MASH therapies: FXR, THRβ & ACC inhibitors compared

Three distinct molecular pathways into MASH

Metabolic Dysfunction-Associated Steatohepatitis (MASH) is defined by the co-occurrence of hepatic steatosis, inflammation, ballooning hepatocytes, and progressive fibrosis — and no single molecular target drives all four simultaneously. The three leading first-in-class agents each enter the disease at a different upstream node: FXR agonists through bile acid nuclear signalling, THRβ agonists through mitochondrial metabolic reprogramming, and ACC inhibitors through direct enzymatic blockade of the lipogenic pathway.

FXR agonists: bile acid-centric regulation of lipid and immune signalling

Farnesoid X receptor (FXR) is a nuclear receptor highly expressed in the liver and intestine. Obeticholic acid (OCA) activates FXR to suppress de novo lipogenesis (DNL) by downregulating SREBP-1c and the transcription of lipogenic genes including FASN and ACC. Simultaneously, FXR agonism enhances fatty acid oxidation (FAO) via PPARα and FGF15/19 signalling, and inhibits CYP7A1 — the rate-limiting enzyme in bile acid synthesis — reducing hepatotoxic bile acid accumulation. On the inflammatory axis, FXR antagonises NF-κB nuclear translocation, suppressing TNF-α, IL-1β, and IL-6 production, while shifting Kupffer cell polarisation toward anti-inflammatory M2 phenotypes. Anti-fibrotic effects are direct: FXR activation inhibits TGF-β1/Smad signalling and collagen deposition in hepatic stellate cells (HSCs), and reduces caspase-3-driven hepatocyte apoptosis by lowering bile acid toxicity.

THRβ agonists: mitochondrial metabolism as the therapeutic lever

Resmetirom selectively activates thyroid hormone receptor beta (THRβ) in hepatic mitochondria, producing a distinct metabolic profile. DNL is suppressed through downregulation of both SREBP-1c and ChREBP, reducing ACC and FASN expression. FAO is simultaneously boosted via upregulation of CPT1A, PGC-1α, and ACOX1, enhancing mitochondrial β-oxidation and ketogenesis. Resmetirom also induces LDL receptor expression, lowering circulating LDL-C and triglycerides — a safety advantage over FXR agonists. Inflammation is attenuated through improved mitochondrial efficiency that reduces reactive oxygen species (ROS) and attenuates NLRP3 inflammasome activation and IL-18 release. Anti-fibrotic effects include direct HSC suppression via reduced TIMP-1 and collagen I/III expression, and hepatoprotection through Akt/ERK-mediated hepatocyte regeneration.

ACC inhibitors: enzymatic blockade at the lipogenic chokepoint

Firsocostat inhibits acetyl-CoA carboxylase (ACC), the enzyme that catalyses malonyl-CoA synthesis — a key hub for DNL. By depleting malonyl-CoA, Firsocostat both reduces fatty acid synthesis and, critically, disinhibits CPT1A (which malonyl-CoA normally suppresses), thereby stimulating mitochondrial β-oxidation. This dual mechanism also lowers VLDL synthesis and hepatic lipid export. Anti-inflammatory and anti-fibrotic effects are indirect: reduced intrahepatic diacylglycerols (DAGs) and ceramides diminish PKCε/JNK activation and TGF-β-driven HSC activation. This indirect fibrosis pathway is a key mechanistic gap compared with FXR and THRβ agonists, which act on HSCs more directly.

What Phase 2 and Phase 3 trials actually show

Each agent has generated distinct clinical evidence that defines both its therapeutic ceiling and its tolerability constraints — constraints that directly inform the combination strategy rationale.

Obeticholic acid’s REGENERATE Phase 3 trial showed fibrosis improvement of at least one stage in 23% of patients at 18 months. The key tolerability constraint is pruritus, affecting 51% of patients, alongside LDL-C elevations. Resmetirom’s MAESTRO-NASH Phase 3 trial demonstrated MASH resolution in 26% and fibrosis improvement in 24% of patients at 52 weeks, with a more favourable safety profile — transient diarrhoea was the primary adverse event. Firsocostat reduced liver fat by 28% as measured by MRI-PDFF and showed reductions in the fibrosis biomarker PRO-C3 in Phase 2 trials, but hypertriglyceridaemia affected 45% of patients, representing its most significant clinical liability.

The pattern across all three agents is consistent: meaningful but incomplete efficacy against a disease that requires simultaneous improvement in steatosis, inflammation, and fibrosis. None achieves resolution rates above 26% as a monotherapy, and each carries a distinct safety liability. This is the clinical foundation for the combination strategy rationale examined in the sections below.

Why combinations are mechanistically justified

The mechanistic case for combining these agents rests on three independent lines of logic: complementary coverage of MASH’s pathological axes, synergistic lipid-depleting effects, and the ability of one agent’s mechanism to neutralise another’s side effect profile.

“FXR agonists lower triglycerides, directly countering ACC inhibitor-induced hypertriglyceridaemia — a side-effect neutralisation that is mechanistically predicted and clinically observed.”

Complementary coverage of the three pathological axes

No single agent addresses steatosis, inflammation, and fibrosis with equal potency. FXR agonists offer the broadest anti-inflammatory and anti-fibrotic profile through direct NF-κB and TGF-β1/Smad pathway suppression, but their lipid-lowering is bile acid-centric and their cholesterol effects are adverse. THRβ agonists excel at mitochondrial metabolic optimisation and carry the cleanest safety profile, but their fibrosis suppression is less direct than FXR agonists. ACC inhibitors provide the most targeted DNL blockade but have no direct anti-fibrotic mechanism and worsen triglycerides. Together, the three classes cover every major pathological driver of MASH without redundancy — a rare property in a multifactorial disease, as noted in research published through Nature‘s liver disease literature.

Synergistic lipid depletion: two routes to the same hepatic target

The ACC inhibitor + THRβ agonist pairing is mechanistically elegant: Firsocostat blocks malonyl-CoA synthesis (cutting fatty acid production upstream), while Resmetirom simultaneously upregulates CPT1A, PGC-1α, and ACOX1 (maximising fatty acid disposal downstream). The result is a dual attack on hepatic lipid accumulation — reduced input and increased output — that neither agent achieves alone. Preclinical data show Resmetirom also reverses ACC inhibitor-induced hypertriglyceridaemia in rodent models, a pharmacological interaction consistent with Resmetirom’s LDL receptor induction mechanism.

Side-effect neutralisation as a combination benefit

The FXR + ACC inhibitor combination illustrates a different rationale: OCA’s bile acid-mediated lipid effects attenuate Firsocostat-induced hypertriglyceridaemia, while Firsocostat’s direct DNL suppression compensates for OCA’s incomplete steatosis coverage. Similarly, Resmetirom’s LDL-lowering activity offsets the LDL-C elevation associated with FXR agonism. These are not speculative interactions — they are mechanistically predicted from each drug’s known pharmacology and, in the case of OCA + Firsocostat, clinically confirmed in a Phase 2b trial, as documented in registries maintained by NIH.

Competitive pipeline: combination trial evidence

The clinical pipeline has moved from theoretical synergy to controlled trial data for at least two of the three pairwise combinations, with the third supported by preclinical evidence and patent filings indicating active industry interest.

FXR agonist + ACC inhibitor: OCA + Firsocostat (Phase 2b)

The most advanced combination evidence comes from Phase 2b trial NCT03449446, which tested OCA plus Firsocostat in MASH patients. The combination achieved a 48% reduction in liver fat versus 25% with monotherapy, and a 25% reduction in the PRO-C3 fibrosis marker versus 15% with monotherapy. Critically, OCA attenuated Firsocostat-induced hypertriglyceridaemia — the mechanistically predicted lipid safety benefit. Limitations remain: pruritus persisted in 33% of combination-arm patients, and fibrosis benefits appeared to plateau, suggesting that direct anti-fibrotic agents may need to be added to achieve histological endpoints required by regulators such as the FDA.

THRβ agonist + ACC inhibitor: Resmetirom + Firsocostat

Preclinical and Phase 1 data show that Resmetirom reverses ACC inhibitor-induced hypertriglyceridaemia in rodent models, consistent with its LDL receptor induction mechanism. Human trials remain pending, but the theoretical synergy in simultaneous DNL blockade (Firsocostat) and FAO maximisation (Resmetirom) represents the most metabolically complete pairing of the three combinations. The absence of overlapping safety signals — Resmetirom’s primary adverse event is transient diarrhoea, not dyslipidaemia — makes this combination pharmacologically attractive for steatosis-predominant MASH phenotypes.

FXR agonist + THRβ agonist: OCA + Resmetirom

No clinical trials have yet been initiated for this pairing. However, patent filings suggest active industry interest in combining the broad anti-inflammatory and anti-fibrotic coverage of FXR agonism with the mitochondrial metabolic optimisation of THRβ agonism. The rationale is broad MASH pathway coverage — lipid metabolism, inflammation, and fibrosis — with complementary safety profiles: Resmetirom’s LDL-lowering activity directly offsets OCA’s LDL-C elevation.

Beyond the core three: GLP-1 RAs and ASK1 inhibitors

The combination pipeline extends beyond these three mechanistic classes. Semaglutide (GLP-1 receptor agonist) combined with Resmetirom is in Phase 2 (NCT04944992), targeting weight-driven steatosis reduction alongside hepatic metabolic reprogramming. ASK1 inhibitor Selonsertib combined with Firsocostat showed additive fibrosis reduction in Phase 2 trials, suggesting that upstream stress kinase inhibition can complement the metabolic effects of ACC inhibition — an approach tracked by organisations including WHO in their global liver disease burden assessments.

Safety hurdles, biomarker gaps, and regulatory design

Four structural challenges constrain the path from mechanistic rationale to approved combination regimens — each requiring resolution before combinations can become the standard of care in MASH.

Managing additive safety signals

ACC inhibitor combinations require active triglyceride management, particularly in patients with baseline dyslipidaemia. While FXR co-administration attenuates this effect in clinical data, the 33% pruritus rate persisting in the OCA + Firsocostat combination arm indicates that FXR-related tolerability issues are not fully resolved by combination design. Triple therapy approaches (FXR + THRβ + ACCi) risk additive toxicity across all three profiles simultaneously, requiring careful dose optimisation and patient selection.

Non-invasive biomarker validation

PRO-C3 (a collagen III synthesis biomarker) and MRI-PDFF (liver fat quantification) are used as surrogate endpoints in combination trials, but both require further validation before regulatory bodies will accept them as substitutes for liver biopsy-based histological assessment. This validation gap extends trial timelines and increases the cost of generating combination efficacy data.

Regulatory endpoint requirements

The FDA requires histological endpoints for MASH approval — specifically fibrosis improvement of at least one stage or MASH resolution confirmed by liver biopsy. Demonstrating additive histological benefit in combination trials requires large patient populations and long study durations, significantly increasing the complexity and cost of combination development programmes compared with monotherapy trials.

Phenotype-driven patient stratification

The mechanistic differences between agents suggest that combination selection should be driven by individual patient disease phenotype. Steatosis-predominant patients — where hepatic lipid accumulation is the primary driver — may benefit most from ACC inhibitor plus THRβ agonist combinations that maximally deplete hepatic fat. Fibrosis-centric cases, where stellate cell activation and collagen deposition dominate, are more likely to require FXR agonist-anchored regimens with direct TGF-β1/Smad suppression. Developing the diagnostic tools and biomarker panels to implement this stratification at scale remains an unresolved challenge for the field, as highlighted in WIPO‘s patent landscape analyses of liver disease diagnostics.