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.