Evaluation of Serum Prolidase as a Biomarker and the Effects of Nintedanib in an Experimental Rat Model of Silicosis

Background/Objectives: Silicosis is a progressive occupational lung disease characterized by persistent inflammation and irreversible pulmonary fibrosis. Reliable circulating biomarkers for the early detection of fibrotic remodeling and monitoring of disease progression remain limited. Prolidase, a key enzyme involved in collagen turnover and extracellular matrix remodeling, has emerged as a potential biomarker in fibrotic disorders. Therefore, this study aimed to evaluate the potential role of serum prolidase as a biomarker for fibrotic remodeling and treatment monitoring in an experimental rat model of silicosis. Methods: Male Wistar rats were divided into three groups: control, silicosis, and silicosis plus nintedanib. Silicosis was induced by intratracheal crystalline silica instillation. Nintedanib was administered by gastric gavage at 50 mg/kg/day from day 10 to day 30. Lung tissues were evaluated histopathologically using hematoxylin and eosin and Masson’s trichrome staining. Serum prolidase, fibroblast growth factor-2 (FGF-2), platelet-derived growth factor (PDGF), and transforming growth factor-beta (TGF-β) levels were measured by enzyme-linked immunosorbent assay (ELISA). Results: Histopathological injury scores were significantly increased in the silicosis group and were attenuated by nintedanib treatment. Serum prolidase levels were significantly higher in both silica-exposed groups than in controls (p = 0.017), but did not differ significantly between untreated and nintedanib-treated animals. TGF-β, FGF-2, and PDGF levels were also significantly elevated following silica exposure. Serum prolidase showed a strong positive correlation with TGF-β (r = 0.69, p < 0.001). ROC analysis demonstrated good diagnostic performance for prolidase (AUC = 0.877), while the combined Prolidase + TGF-β model further improved discrimination (AUC = 0.969). The internally validated multivariable biomarker panel maintained high diagnostic accuracy after leave-one-out cross-validation (LOOCV) (AUC = 0.892). Conclusions: Serum prolidase may serve as a potential circulating biomarker of silica-associated fibrotic remodeling. Although nintedanib improved histopathological lung injury, persistently elevated prolidase levels suggest ongoing extracellular matrix remodeling rather than short-term treatment response. Further longitudinal experimental and clinical studies are warranted to validate its diagnostic and prognostic utility.