Discussion and conclusions
Despite significant advancements in the treatment of IPF, with two drugs
(PFD and BIBF) approved for its management, the challenge of treating
pulmonary fibrotic diseases remains formidable. Regrettably, specific
targeted therapy for pneumoconiosis, one of the most prevalent global
occupational diseases, is still lacking. In light of this predicament,
this study aimed to address this issue by employing a drug repurposing
strategy. Specifically, the efficacy of monotherapy with PFD or BIBF, as
well as their combination, was evaluated in a murine model representing
advanced-stage silicosis. We found that the combination therapy yields
superior efficacy compared to monotherapy, with no significant
difference in efficacy between high and low dosages, suggesting that
low-dose combination therapy holds great potential for clinical
application. Furthermore, the underlying mechanisms of their therapeutic
effects were explored, aiming to provide novel insights into
pneumoconiosis treatment.
Several preclinical studies have also investigated the therapeutic
effects of PFD on silicosis. For instance, Jingwen Guo et al. examined
the administration of PFD on the first day of silica-induced pulmonary
fibrosis in a rat model and observed improvements in inflammation and
fibrotic status after 14 or 28 days (Guo et al., 2019). Furthermore,
Zhu-Jie Cao et al. employed PFD in 2 stages, from day 1 to day 28 and
from day 14 to day 42 following silica exposure in a murine model, and
found that PFD could treat silicosis by inhibiting STAT3 phosphorylation
to reduce IL-17A secretion(Cao et al., 2022). Additionally, another team
evaluated the therapeutic effects of PFD in a rat model of silicosis at
3 periods, specifically spanning 1-14, 28, and 56 days of silica
exposure, and suggested that PFD might alleviate inflammation and
fibrosis by inhibiting macrophage polarization through the JAK2/STAT3
pathway (Tang et al., 2022). These findings represented an initial and
promising validation of the effectiveness of PFD in a rodent model of
silicosis. However, the evidence for PFD’s therapeutic potential in
advanced stages of pneumoconiosis remains insufficient. This gap in
knowledge arises from the fact that pneumoconiosis patients often seek
medical treatment only when they exhibit significant symptoms in the
advanced stages of fibrosis. Therefore, it is imperative and practically
significant to evaluate the therapeutic effects of PFD on late-stage
fibrosis of silicosis. In this study, PFD administration was initiated 6
weeks after the induction of silicosis in mice, a time point
characterized by extensive fibrosis (Zhujie Cao et al., 2020).
Remarkably, comprehensive and systematic evaluations revealed the
effectiveness of PFD in mitigating the progression of late-stage
silicosis.
On the other hand, several studies have also delved into the potential
of BIBF in the treatment of silicosis. Researchers conducted a study
using silica-induced pulmonary fibrosis mice at 3 different time
intervals: 0-30, 10-30, and 20-30 days. The administration of BIBF
during these periods yielded notable reductions in the accumulation of
inflammatory factors and collagen, indicating its potential to
counteract these pathogenic processes (Wollin et al., 2014). The finding
aligned with our own observations in late-stage silicosis mice, where
BIBF exhibited anti-inflammatory and anti-fibrotic effects.
Interestingly, we also discovered that in mice with severe fibrosis in
the advanced stage, PFD seemed to exhibit a comparative advantage over
BIBF, as suggested by lung function, inflammation, and fibrotic
manifestations. This result may be attributed to PFD’s superior
anti-inflammatory efficacy, which holds paramount significance in the
pathogenesis and progression of silicosis (Leung et al., 2012). In
conclusion, in terms of treatment efficacy, we highly recommend the use
of high-dose PFD as a novel strategy for the treatment of
pneumoconiosis.
Our research indicates that the combined administration of PFD and BIBF
for the treatment of silicosis yields better therapeutic outcomes
compared to using either drug alone. Importantly, this combination
therapy does not pose a significant increase in safety risks when
compared to monotherapy. These results are in line with clinical trials
that have investigated the use of the PFD and BIBF combination in the
treatment of IPF (Vancheri et al., 2018). Consequently, we recommend the
adoption of the low-dose combination therapy in the clinical management
of occupational pneumoconiosis, such as silicosis.
Despite the existence of some literature assessing the therapeutic
effects of PFD and BIBF in silicosis treatment, research regarding the
mechanistic insights of these two interventions remains limited,
warranting further comprehensive investigations. In this study, we
conducted a multi-faceted exploration of the potential mechanisms
underlying the efficacy of PFD and BIBF in silicosis treatment,
employing transcriptomic and metabolomic analyses at both gene and
functional levels. We have observed that PFD and BIBF jointly target
immune-related antigen processing and presentation pathways and
metabolism of substances encompassing steroid, purine, pyrimidine,
glycerophospholipid, and glutathione. Additionally, PFD specifically
suppresses proteasomes (Baker et al., 2014) and p53 (Wang et al., 2015)
elevated in pulmonary fibrosis, and activates platelet and regulation of
lipolysis in adipocytes. BIBF, on the other hand, focuses on inhibiting
MAPK and Hippo signaling pathways while increasing peroxisome and
circadian rhythm. Among these, the MAPK-mediated signaling pathway plays
a critical role in cellular proliferation, differentiation, migration,
and metabolism (Arthur & Ley, 2013; Qian et al., 2016). The Hippo
pathway negatively regulates the transcriptional activity of YAP/TAZ to
participate in various physiological processes such as cell
proliferation, apoptosis, and differentiation in multicellular organisms
(Badouel et al., 2009). Consistently, research has also reported that
the circadian control of the NRF2/glutathione pathway plays a pivotal
role in tackling pulmonary fibrosis (Pekovic-Vaughan et al., 2014) and
mutation of the core clock protein REVERBα could inhibit myofibroblast
activation and collagen secretion (Cunningham et al., 2020). Recently,
there is evidence supporting the notion that the loss of REV-ERBα
exacerbates fibrotic response by promoting the expression of collagen
and lysyl oxidase (Wang et al., 2023). Overall, all these pathways are
considered as candidate pharmacological targets for the treatment of
pulmonary fibrosis, suggesting the potential of BIBF treating silicosis.
Additionally, the combination therapy targeting steroid biosynthesis
with PFD and BIBF highlights the importance of this pathway as a
therapeutic target for silicosis.
In summary, our study demonstrated that both PFD and BIBF, either used
alone or in combination, showed promising therapeutic effects in
advanced silicosis with severe fibrosis. The low-dose combination
therapy exhibited superior efficacy while maintaining a safety profile
comparable to monotherapy, thus suggesting it as a recommended future
clinical treatment approach for silicosis. Additionally, through
transcriptomic and metabolomic analyses, we unveiled the multifunctional
effects of PFD and BIBF by targeting crucial signaling pathways and
metabolic processes involved in the progression of pulmonary fibrosis.
This mechanistic insight may pave the way for the future clinical
application of these drugs and overcome some potential hurdles.