Discussion
Here we report that the liver-selective THR-ß agonist resmetirom, administered at a dose that does not influence body weight or food intake, has a profound effect on hepatomegaly, hepatic steatosis, liver injury, circulating cholesterol and the histological NAFLD activity score in a diet-induced obese and biopsy-confirmed mouse model of advanced NASH with fibrosis. Our study was performed as an intervention (therapeutic) study in DIO-NASH mice with established metabolic disease also comprising obesity, hypercholesterolemia and insulin resistance, reflecting clinical characteristics of patients with NASH.
Our results are in remarkable agreement with a recently reported phase 2 trial of resmetirom in patients with NASH in which, after 36 weeks of treatment, a forty percent reduction in hepatic fat content measured by magnetic resonance imaging was observed that was accompanied by a significant improvement in circulating liver transaminases, total and LDL cholesterol. Furthermore, resmetirom responders that had lower hepatic fat fractions at 36 weeks also exhibited significantly reduced NAFLD activity scores (Harrison, Bashir, et al, 2019).
Several THR-ß agonists have been investigated in animal models of metabolic disease with fatty liver (reviewed in Sinha, Bruinstroop, et al, 2019). Sobetirome (GC-1) and eprotirome (KB2115), for example, reduced hepatic steatosis and liver triglycerides in ob/ob mice (Martagon, Lin, et al, 2015). Notably, at the selected doses, treatment also resulted in significant body weight loss, and ob/ob mice do not develop manifest NASH or hepatic fibrosis under the investigated conditions. Similarly, both compounds, administered over a period of ten days, led to strong reductions in liver triglyceride content in Sprague-Dawley rats on a high-fat diet (Vatner, Weismann, et al, 2013) that, however, did not develop NASH. Sobetirome also caused a marked decrease in hepatic and circulating triglycerides and serum ALT in rats fed a high-fat, methionine- and choline-deficient diet (Perra, Simbula, et al, 2008). However, treatment was accompanied by weight loss, and the presence of NASH was not investigated. MB07344/VK2809, a THR-β agonist prodrug selectively taken up by the liver and converted into the active principle, reduced hepatic triglycerides and serum lipids in C57BL/6 mice on a high-fat diet and in Zucker diabetic fatty rats (Erion, Cable, et al, 2007; Cable, Finn, et al, 2009), in the absence of NASH and fibrosis. Thus, there is evidence from several models of metabolic disease for an improvement in liver fat and circulating lipids. Our results add to his body of evidence by showing that THR-β agonism can reduce hepatic steatosis and liver injury in a diet-induced obese model combining metabolic syndrome with advanced NASH and fibrosis.
Multiple preclinical models of NAFLD have been described that vary in their translatability to the human situation (Hansen, Feigh, et al, 2017). Our data indicate that with respect to THR-β agonism, data obtained in biopsy-confirmed DIO-NASH mice have a good human translatability. So far, it is not established to which extent resmetirom is able to also hold or to reverse fibrosis in late-stage NAFLD. In the recent phase 2 trial of resmetirom in patients with NASH and predominantly mild fibrosis, no significant improvement was observed (Harrison, Bashir, et al, 2019). It is unclear whether this results from lack of efficacy or whether the treatment duration of 36 weeks was too short to elicit an observable improvement in hepatic fibrosis. Similar to the population investigated in this trial, our model shows mild fibrosis (approximately 60% F2, 40% F3, see fig. 4d) and no significant improvement in fibrosis score was seen with resmetirom. However, reduced expression of markers of fibrogenesis in resmetirom vs. vehicle-treated DIO-NASH animals indicates an antifibrotic effect of THR-β agonism that may translate into a more robust reduction in fibrosis upon longer duration of treatment. We also speculate that longer pre-feeding with the AMLN diet might further aggravate advanced fibrosis in DIO-NASH mice and could be a useful model for preclinical evaluation of the compound with respect to anti-fibrotic efficacy.
Of note, although our whole genome expression analysis indicated activation of the canonical THR-β pathway, the data did not explain the strong reduction of hepatic lipids by resmetirom as there was no clear down-regulation of fatty acid synthesis genes or stimulation of genes associated with fat oxidation. Thus, the anti-steatotic effect of the compound in this model is not due to transcriptional effects. We speculate that THR-β agonism redirects metabolic fluxes by post-transcriptional and allosteric mechanisms.
An important limitation of our study is the restriction to a single dose level. The dose of 3 mg/kg was selected based on a previous study in DIO mice over 23 days where this dose led to ~60% reduction in circulating cholesterol levels (Kelly, Pietranico-Cole, et al, 2014). Whereas at the chosen dose of resmetirom showed a marked improvement in hepatic and systemic metabolic health in the absence of obvious adverse effects, additional studies investigating multiple doses will be required to, e.g., establish the maximal tolerated or minimally or maximally effective doses of resmetirom for the treatment of NASH.