P12.13.B FIBROBLAST ACTIVATION PROTEIN AS A POTENTIAL THERANOSTIC TARGET IN BRAIN METASTASES OF DIVERSE SOLID TUMOURS

Abstract BACKGROUND Brain metastases (BM) are a common complication of oncologic disease associated with dismal prognosis and there is an urgent need for novel therapeutic approaches. Fibroblast activation protein (FAP), a transmembrane serine protease, is an emerging theranostic target in cancer due to its frequent upregulation in the tumour microenvironment. FAP protein itself and/or FAP+ cells were shown to promote cancer progression in several extracranial malignancies. However, very little is known about the expression of FAP and the role of FAP+ cells in the unique microenvironment of BM. In this study, we quantified FAP expression and characterized FAP+ cells in BM of various origin. MATERIAL AND METHODS FAP enzymatic activity and protein concentration were determined in samples of BM (n=58) and control non-tumorous brain tissue (n=12) by an enzymatic assay using a specific fluorogenic substrate, western blot, and ELISA. Expression of FAP and the characteristics of FAP+ cells were analysed by IHC (n=112). Expression of collagen was quantified by Masson´s trichrome staining (n=106) and collagen I concentration was measured by ELISA (n=56). RESULTS FAP expression was significantly higher in BM in comparison to non-tumorous brain tissue, both at the protein and enzymatic activity level. BM with higher FAP expression displayed higher levels of collagen I. FAP was predominantly confined to stromal cells expressing markers typical of cancer-associated fibroblasts and residing in collagen rich regions. FAP expression on tumour cells in a portion of BM was also noted , mainly in BM originating from lung, breast, and renal cancer, melanoma, and sarcoma. Increased FAP expression and the presence of FAP+ stromal cells were common features of BM irrespective of their origin. CONCLUSION We are the first to provide a detailed analysis of FAP expression in BM of diverse origin and characterise FAP+ cells in their microenvironment. The frequent upregulation of FAP and its presence on both stromal and tumour cells support the use of FAP as a promising theranostic target in BM. Funding: This research was funded by the Charles University grant no. 342522 and Program Cooperatio, research area ,,Oncology and Haematology"; Ministry of Health of the Czech Republic - grant NU22-03-00318; Ministry of Education, Youth and Sports of the Czech Republic - LM2023053 of EATRIS-CZ and project “Center of Tumor Ecology” (CZ.02.1.01/0.0/0.0/16_019/0000785) - Operational Program Research, Development and Education; project National Institute for Cancer Research (Programme EXCELES, ID Project No. LX22NPO5102) by the European Union - Next Generation EU.