Fibroblast activation protein (FAP) is a serine protease upregulated at sites of tissue remodeling and cancer that represents a promising therapeutic and molecular imaging target. In prostate cancer, studies of FAP expression using tissue microarrays are conflicting, such that its clinical potential is unclear. Furthermore, little is known regarding FAP expression in benign prostatic tissues. Here we demonstrated, using a novel iterative multiplex IHC assay in standard tissue sections, that FAP was nearly absent in normal regions, but was increased consistently in regions of proliferative inflammatory atrophy (PIA). In carcinoma, FAP was expressed in all cases, but was highly heterogeneous. High FAP levels were associated with increased pathological stage and cribriform morphology. We verified that FAP levels in cancer correlated with CD163+ M2 macrophage density. In this first report to quantify FAP protein in benign prostate and primary tumors, using standard large tissue sections, we clarify that FAP is present in all primary prostatic carcinomas, supporting its potential clinical relevance. The finding of high levels of FAP within PIA supports the injury/regeneration model for its pathogenesis and suggests that it harbors a protumorigenic stroma. Yet, high levels of FAP in benign regions could lead to false positive FAP-based molecular imaging results in clinically localized prostate cancer.

AMDM is a paid consultant/advisor to Merck and Cepheid and has received research funding from Janssen and Myriad. SY receives research funding to his institution from Bristol-Myers Squibb and Celgene Janssen and Cepheid and has served as a consultant for Cepheid. He owns founders equity in Brahm Astra Therapeutics and Digital Harmonic. MCM is a paid consultant to Clovis and Exelixis. WNB has received a monetary gift from Imago Biosciences a subsidiary of Merck & Co. Inc.; sponsored research funding from Transcenta Holding and owns founders equity in ABIDA Bio, LLC

This work is supported by NIH/NCI U54 CA274370 (AMDM and SY) NIH/ National Cancer Institute (NCI) Specialized Programs of Research Excellence (SPORE) in Prostate Cancer grant P50CA58236 (AMDM), NIH/NCI grant U01 CA196390 (AMDM and SY), R01 CA255259-04(WNB), R01 CA279993-01A1 (WNB), U.S. Department of Defense Prostate Cancer Research Program (PCRP), W81XWH-18-2-0015 (AMDM), NIH/NIBIB P41 EB024495 (MP), The Johns Hopkins Sidney Kimmel Comprehensive Cancer Center Oncology Tissue Services Laboratory is supported by NIH/NCI grant P30 CA006973 and The Patrick C. Walsh Prostate Cancer Research Fund at Johns Hopkins (AMDM and SY).

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