99mTc-MIBI heart scintigraphy is used to evaluate cardiac mitochondrial dysfunction in patients with cardiomyopathy [
24],[
25]. In addition,
99mTc-MIBI leg scintigraphy has been used to detect mitochondrial dysfunction in progressive supranuclear palsy patient skeletal muscle [
26].
99mTc-MIBI, a lipophilic cationic myocardial perfusion agent, is considered to be retained in the mitochondria by the higher membrane potential, and the loss of mitochondrial membrane potential results in rapid washout of
99mTc-MIBI from the myocyte [
14]. In the present study, we showed that rapid washout of
99mTc-MIBI indicating mitochondrial dysfunction was observed in the liver of MCD-fed mice. The MCD-fed rodent model is widely used as a NASH model because of its similarity with human NASH pathology. Rodents fed MCD diet showed mitochondrial dysfunction such as reduction of the activity of mitochondrial respiratory chain and hepatic ATP depletion [
27]. These findings suggest that the cause of rapid washout of
99mTc-MIBI is the loss of mitochondrial membrane potential since
99mTc-MIBI is retained to the mitochondrial membrane in response to membrane potential. Rogers et al. showed that prolonged exposure of preadipocytes to fatty acid led to mitochondrial dysfunction such as decrease of ATP content and reduction of mitochondrial inner membrane potential [
28]. We also confirmed the decrease of ATP content in the liver of MCD-fed mice. Therefore, our and previous findings indicate that the rapid clearance of hepatic MIBI might be due to mitochondrial dysfunction including reduction of ATP content in MCD-fed mice. Regarding magnetic resonance imaging (MRI), it has been reported that
Tmax and
T1/2 after injection of gadolinium-ethoxybenzyl-diethylenetriamine penta-acetic acid (Gd-EOB-DTPA) were significantly prolonged in a NASH rat model [
29]. Furthermore,
Tmax and
T1/2 after Gd-EOB-DTPA injection significantly correlated with the fibrosis rate [
30]. It is well known that organic anion-transporting polypeptide (oatp) 1 mediates the uptake of Gd-EOB-DTPA and multidrug resistance-associated protein 2 (mrp2) mediates biliary excretion of Gd-EOB-DTPA in rats [
31],[
32]. These reports suggest that the elimination of
99mTc-MIBI also might be influenced by transporter expression in NASH pathology since
99mTc-MIBI passively diffuses into hepatocytes and the biliary excretion is mediated by P-glycoprotein (P-gp) [
33]. However, Canet et al. recently reported that the liver protein expression of P-gp did not alter in a MCD-fed mouse [
34]. This report suggests that the role of efflux transporter via P-gp is considered to be small for
99mTc-MIBI washout in this MCD-fed mouse.
In the present study, the most important finding is to show the negative correlation between
99mTc-MIBI clearance (
T1/2) and NAFLD activity score including steatosis and inflammation score. Steatosis and inflammation were histologically observed at 2 and 4 weeks after MCD diet in this study. The MCD diet also seemed to aggravate the inflammation at 4 weeks compared with that at 2 weeks after feeding, although the severity of steatosis was not different between 2 and 4 weeks. These data suggest that the
99mTc-MIBI washout is likely to correlate with pathology including both steatosis and inflammation in MCD mice. Masuda et al. [
19] have reported that hepatic
99mTc-MIBI uptake is correlated with the NAFLD activity score in a clinical study. These non-clinical and clinical findings suggest that hepatic
99mTc-MIBI SPECT imaging might be useful for evaluating NASH progression pathology such as mitochondrial dysfunction. However, further study will be needed to clarify the relation between mitochondrial membrane potential and
99mTc-MIBI binding activity.