Usefulness of FAP[alpha] assessment in bronchoalveolar lavage as a marker of fibrogenesis: results of a preclinical study and first report in patients with idiopathic pulmonary fibrosis

Background Fibroblast activation protein-[alpha] (FAP[alpha]) is a marker of activated fibroblasts that can be selectively targeted by an inhibitor (FAPI) and visualised by PET/CT imaging. We evaluated whether the measurement of FAP[alpha] in bronchoalveolar lavage fluids (BALF) and the uptake of FAPI by PET/CT could be used as biomarkers of fibrogenesis. Methods The dynamics of lung uptake of .sup.18F-labeled FAPI ([.sup.18F]FAPI-74) was assessed in the bleomycin mouse model at various time points and using different concentrations of bleomycin by PET/CT. FAP[alpha] was measured in BALFs from these bleomycin-treated and control mice. FAP[alpha] levels were also assessed in BALFs from controls and patients with idiopathic pulmonary fibrosis (IPF). Results Bleomycin-treated mice presented a significantly higher uptake of [.sup.18F]FAPI-74 during lung fibrinogenesis (days 10 and 16 after instillation) compared to control mice. No significant difference was observed at initial inflammatory phase (3 days) and when fibrosis was already established (28 days). [.sup.18F]FAPI-74 tracer was unable to show a dose-response to bleomycin treatment. On the other hand, BALF FAP[alpha] levels were steeply higher in bleomycin-treated mice at day 10 and a significant dose-response effect was observed. Moreover, FAP[alpha] levels were strongly correlated with lung fibrosis as measured by the modified Aschroft histological analysis, hydroxyproline and the percentage of weight loss. Importantly, higher levels of FAP[alpha] were observed in IPF patients where the disease was progressing as compared to stable patients and controls. Moreover, patients with FAP[alpha] BALF levels higher than 192.5 pg/mL presented a higher risk of progression, transplantation or death compared to patients with lower levels. Conclusions Our preclinical data highlight a specific increase of [.sup.18F]FAPI-74 lung uptake during the fibrotic phase of the bleomycin murine model. The measurement of FAP[alpha] in BALF appears to be a promising marker of the fibrotic activity in preclinical models of lung fibrosis and in IPF patients. Further studies are required to confirm the role of FAP[alpha] in BALF as biomarker of IPF activity and assess the relationship between FAP[alpha] levels in BALF and [.sup.18F]FAPI-74 uptake on PET/CT in patients with fibrotic lung disease. Keywords: Idiopathic pulmonary fibrosis, Fibroblast activation protein, Bronchoalveolar lavage, FAPI, PET scan, Biomarker