INTRODUCTION
Idiopathic pulmonary fibrosis
(IPF) is a lung disease of unknown etiology with a poor prognosis that
is characterized by chronic development of severe fibrosis, resulting in
a honeycomb lung (Kim, Collard, &
King, 2006; Raghu et al., 2015). Steroids and immunosuppressive drugs
have long been used to treat IPF, but in many cases, these drugs do not
show therapeutic efficacy against IPF progression (Luppi, Cerri, Beghe,
Fabbri, & Richeldi, 2004; Raghu et al., 2015; Walter, Collard, & King,
2006). The primary etiology of IPF is thought to be chronic pulmonary
fibrosis triggered by chronic injury to airway and alveolar epithelial
cells. Therefore, pirfenidone and nintedanib, antifibrotic agents that
have been shown to significantly improve the reduction of
forced vital capacity (FVC) in
large clinical trials, are being used to treat IPF in clinical practice
(Noble et al., 2011; Raghu et al., 2015; Richeldi et al., 2014).
However, in some cases, these drugs have not shown efficacy and have
been reported to induce adverse effects such as elevation of liver
damage markers, diarrhea, and indigestion (Noble et al., 2011; Richeldi
et al., 2014). Thus, safer drugs eliciting a therapeutic effect equal to
or greater than that of these two approved drugs are necessary.
Although the exact cause of IPF is unknown, it is thought to be
triggered by damage to the lung epithelium as a result of increased
oxidative stress and the repair and remodeling processes, such as
collagen synthesis, that are induced to manage this damage. In other
words, this process is overstimulated, resulting in abnormal wound
repair and remodeling characterized by collagen deposition, which leads
to the development and exacerbation of pulmonary fibrosis (Kinnula &
Myllarniemi, 2008; Sheppard, 2006). The cells that play the greatest
role in this fibrosis-promoting process are myofibroblasts.
Peribronchial and perivascular fibroblasts transdifferentiate (activate)
into myofibroblasts in response to various stimuli, especially
TGF-β1, and accumulate
extracellular matrix components, especially collagen fibers, which are
involved in fibrosis (Hinz et al., 2007; Kisseleva & Brenner, 2008).
Furthermore, the ”apoptosis paradox” is also a possible mechanism of
abnormal fibrosis in IPF patients. Apoptosis is preferentially observed
in alveolar epithelial cells in the lungs of IPF patients, while little
apoptosis occurs in fibroblasts. This produces a relative imbalance
resulting in increased fibroblasts in the lungs of IPF patients, which
is thought to be involved in the pathogenesis of IPF (Maher et al.,
2010; Thannickal & Horowitz, 2006). Thus, it is important to identify
compounds that inhibit transdifferentiation of fibroblasts into
myofibroblasts or inhibit activation of myofibroblasts. In addition,
compounds that are not toxic to alveolar epithelial cells but exert
their effects preferentially on lung fibroblasts are promising
candidates for IPF therapy.
On the basis of these requirements, we implemented an innovative
research strategy (drug repositioning) to identify and develop new IPF
therapeutics by screening drugs currently in clinical use to treat other
diseases (Mizushima, 2011; Pushpakom et al., 2019). The major advantage
of this strategy is that the clinical safety of the drugs screened is
already understood, and the risk of unexpected adverse effects in humans
can be greatly reduced when these drugs are applied to treat other
diseases (Mizushima, 2011; Pushpakom et al., 2019). Using this strategy,
we screened drugs not only for IPF but also chronic obstructive
pulmonary disease, functional dyspepsia, and irritable bowel syndrome
from a library of approved drugs, identified effective drugs for each
disease, and analyzed the mechanisms by which these drugs exert their
efficacy (Asano et al., 2017; Sugizaki et al., 2019; K. Tanaka et al.,
2013; K. I. Tanaka et al., 2017). Recently, other research groups have
used this strategy to develop novel therapeutics for coronavirus
infection 2019, and candidate drugs have been discovered, including the
influenza virus treatment drug remdesivir and the antiparasitic drug
ivermectin (Alam et al., 2021). Therefore, we suggest that this strategy
is useful for discovering new candidates for treatment of human
diseases.
In our previous study, we screened compounds capable of more potently
inhibiting the growth of lung fibroblasts (LL29 cells) than that of lung
alveolar epithelial cells (A549 cells) and identified idebenone, which
has previously been used clinically as a brain metabolic stimulant, from
a library of medications already in clinical use. In addition,
intratracheal administration of idebenone to mice inhibited
bleomycin (BLM)-induced pulmonary
fibrosis and decreased FVC (Sugizaki et al., 2019). Furthermore, in our
previous screening, we found that, in addition to idebenone, the central
muscle relaxant eperisone also acts preferentially on lung fibroblasts.
No studies have been conducted on eperisone to determine its effects on
fibroblasts or pulmonary fibrosis. Therefore, in this study, we
investigated the effect of eperisone, which preferentially induces
fibroblast cell death, on BLM-induced pulmonary fibrosis. In addition,
we examined its adverse effects by analyzing plasma markers and the
gastrointestinal mucosal status when eperisone was administered to
BLM-induced pulmonary fibrosis model mice.