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.