Dose-Time-Dependent Cellular Cytotoxicity of Astatine-211-Labeled FAP-Targeting Radiopharmaceuticals

Fibroblast activation protein (FAP), which is overexpressed in cancer-associated fibroblasts (CAFs) is an attractive stromal target for a-therapy. We previously reported moderate anti-tumor effects of ²¹¹At-labeled FAP inhibitors (FAPI) in a PANC-1 xenograft mouse model, where insufficient tumor accumulation and the absence of FAP expression in tumor cells limited therapeutic efficacy. In this study, we conducted in vitro experiments using FAP-negative PANC-1 cells to elucidate the cytotoxicity of ²¹¹At-labeled FAPI in comparison with free ²¹¹ At. We found that, whereas free ²¹¹ At showed only limited cytotoxicity, ²¹¹At-labeled FAPI exerted significant cytotoxic effects. Systematic variation of activity concentrations and exposure durations revealed that cytotoxicity depended primarily on sustained pericellular exposure rather than instantaneous dose. These findings demonstrate that effective cytotoxicity of CAF-targeted ²¹¹At radiopharmaceuticals can be achieved without direct tumor cell targeting and provide a mechanistic basis for optimizing stromal-targeted a -therapy toward clinical translation.